Reactive p-block cations stabilized by weakly coordinating anions

The chemistry of the p-block elements is a huge playground for fundamental and applied work.

Abstract

The chemistry of the p-block elements is a huge playground for fundamental and applied work. With their bonding from electron deficient to hypercoordinate and formally hypervalent, the p-block elements represent an area to find terra incognita. Often, the formation of cations that contain p-block elements as central ingredient is desired, for example to make a compound more Lewis acidic for an application or simply to prove an idea. This review has collected the reactive p-block cations (rPBC) with a comprehensive focus on those that have been published since the year 2000, but including the milestones and key citations of earlier work. We include an overview on the weakly coordinating anions (WCAs) used to stabilize the rPBC and give an overview to WCA selection, ionization strategies for rPBC-formation and finally list the rPBC ordered in their respective group from 13 to 18. However, typical, often more organic ion classes that constitute for example ionic liquids (imidazolium, ammonium, etc.) were omitted, as were those that do not fulfill the – naturally subjective – “reactive”-criterion of the rPBC. As a rule, we only included rPBC with crystal structure and only rarely refer to important cations published without crystal structure. This collection is intended for those who are simply interested what has been done or what is possible, as well as those who seek advice on preparative issues, up to people having a certain application in mind, where the knowledge on the existence of a rPBC that might play a role as an intermediate or active center may be useful.

Introduction

Main group chemistry continues to reside at the heart of fundamental as well as applied chemistry. As such, recent years have seen an enormous growth of concepts that shed new light on hitherto undiscovered, or more correctly, underdeveloped areas of main group chemistry. Thus, the availability of stable singlet carbenes ¹ as strong donors offered tremendous new perspectives as did the establishment of the frustrated Lewis pairs (FLP) concept ² or the systematic investigation of (often low valent) cationic mixed main group-transition metal salts. ^3–5 In the framework of those approaches, next to other fundamental ⁶ and applied questions, ^7–10 also the stabilization or use of reactive p-block cations (rPBC) with weakly coordinating anions (WCAs) was one focus that led to fascinating new rPBC. This review gives a comprehensive overview on recent rPBC developments since about 2000, but also cites all-time classics in the field. It also includes the fascinating class of transition metal substituted rPBC for which the assignment of the positive charge to one specific moiety is often not clear.

Scope of this review

Many of the p-block elements have relatively high ionization potentials and electronegativities. Thus, most of the stable examples base on delocalization and other electronic or steric effects. In addition, rPBC are often very electrophilic and/or oxidizing. Therefore, chemically stable and inert weakly coordinating anions (WCAs) and solvents are needed to access their salts. These ingredients allowed the syntheses of a large number of fundamentally interesting rPBC of the groups 13–18 in the condensed phase. We discuss typical synthesis routes, give a brief overview of the WCAs, and describe the rPBC ordered according to their main group as well as cation class. However, typical, often more organic ion classes that constitute for example ionic liquids (imidazolium, ammonium, etc.) were omitted, as were those that do not fulfill the – naturally subjective – “reactive”-criterion of the rPBC. As a rule, we only included rPBC with crystal structure and only rarely refer to important cations published without crystal structure.

Handling of substance classes with recent reviews

Some of the substance classes, which fit into this review were just recently and sometimes very comprehensively reviewed (cf. our contribution describing the advances in the synthesis of homopolyatomic cations of the non-metals since 2000 ¹¹ ). To reduce the overlap, we decided to give an overview on general aspects such as WCAs in Table 1 and include a short table with relevant reviews for each main group at the beginning of each main group chapter and only list the compounds in these cases. Therefore, we mainly list, but do not describe the cations of this category in the chapters of their corresponding element. Nevertheless, the scope of this review is rather large, which in any case precludes extensive discussions and mainly serves as an overview on what is known.

Table 1. General reviews with focus on WCAs.

Year	Topic	Title	Ref.
1993	WCAs	The search for larger and more weakly coordinating anions	15
1998	WCAs	Carboranes: a new class of weakly coordinating anions for strong electrophiles, oxidants, and superacids	16
2004	WCAs	Noncoordinating anions—fact or fiction? A survey of likely candidates	13 and 12
2006	WCAs	Chemistry with weakly-coordinating fluorinated alkoxyaluminate anions: gas phase cations in condensed phases?	14, 15 and 17
2006	WCAs	Chemistry of the carba-closo-dodecaborate(–) anion, [CB11H12]–	18
2008		π-Complexation of post-transition metals by neutral aromatic hydrocarbons: the road from observations in the 19th century to new aspects of supramolecular chemistry	19
2013	WCAs	Weakly coordinating anions: halogenated borates and dodecaborates	20
2013	WCAs	Weakly coordinating anions: fluorinated alkoxyaluminates	21
2013	WCAs	Weakly coordinating anions: highly fluorinated borates	22
2015	WCAs	Taming the cationic beast: novel developments in the synthesis and application of weakly coordinating anions (Publication in progress by IK)	23

Reactive p-block cations

The rPBC in this article need a WCA as counterion and, therefore, we first briefly describe typical WCAs and give some advice on their selection before turning to typical ionization and synthetic procedures for rPBC preparations. Thereafter, the ordering of the cation classes for the individual sections is described, and finally the rPBC are grouped according to the main group of the relevant cationic entry. In addition, first applications emerged for rPBC salts and will be highlighted in the respective cation sections.

WCA overview

Because of their potential in fundamental and applied chemistry, ^12–15 a great variety of different WCA types are currently known (Fig. 2) and was frequently reviewed (Table 1).

Fig. 2. Some of the weakly coordinating anions discussed in this review.

But which out of the multitude of published WCAs shown in Fig. 2 should be used for a given problem…? Is there one best WCA that fulfills all needs…?

Clearly holds: the more reactive the rPBC are, the more demanding is the task for the anions, to meet the requirements for a successful stabilization in the condensed phase. Some of this reactivity may be dampened kinetically by the use of suitable bulky ligands, e.g. for the silylium ions. However, there is not one ultimate WCA that fulfills all requirements to allow for use with all in here described rPBC. Typically, rPBC follow at least one of the following classifications:

• Being a strong electrophile, thus having a strong tendency to coordinate an anion or solvent. Silylium ions SiR₃ ⁺ are good examples for this. This coordination is often the entrance towards an anion degradation by heterolytic cleavage of a bond in the WCA.

• Being a strong oxidant, thus needing anions and solvents compatible with this need. Halogen and noble gas cations are typical examples.

• Being a weakly bound complex, in which the interesting main group particle can easily be displaced by anion or solvent, just as in many metal–non-metal clusters. This includes protonated, weakly basic molecules that tend to pass the proton to more basic and more coordinating anions or solvents.

Thus, the demand for very weak coordination behaviour is only medium for several very oxidizing cations, but the necessity of the WCA being stable against oxidation is a prerequisite of highest importance. For example, the typical counterions of group 16 to 18 rPBC are fluorometallates like [MF₆]^– or [M₂F₁₁]^– (M = As, Sb) compatible with (i) the oxidizing power of the cation and (ii) the typically used super acid solvents. However, despite the fact that fluoroantimonates allow for the synthesis of tremendously oxidizing cations like [Xe₂]⁺, they fail to stabilize the extreme electrophiles [SiR₃]⁺ and form F-SiR₃ and antimony pentafluoride. On the other hand, with some steric protection at the silylium ion, already the [B(C₆F₅)₄]^– WCA suffices to stabilize for example the [Si(Mes)₃]⁺ cation. By contrast, and due to the aromatic system, [B(C₆F₅)₄]^– is not compatible with the only mildly oxidizing [NO]⁺ or [NO₂]⁺ cation. Some thoughts that allow for the selection of a suitable WCA for a given problem may be summarized by the triangle shown in Fig. 1.

Fig. 1. Triangle delineating the independent demands of a rPBC that lead to different mixtures of the WCA properties necessary for its successful stabilization.

With Fig. 1 in mind, a personal selection of the “best WCAs” includes [1-H-CB₁₁Me₅Br₆]^–, ²⁴ [1-Et-CB₁₁F₁₁]^–, ²⁵ [CB₁₁(CF₃)₁₂]^–, ²⁶ [Sb₄F₂₁]^–, ²⁷ [Sb(OTeF₅)₆]^–, ²⁸ [Al(OR^PF)₄]^–, ^29–31 [B(C₆F₅)₄]^{–  32–34} and [B(CF₃)₄]^–. ³⁵ A recent noteworthy addition overcoming the frequent disorder of the also towards fluoride abstraction less stable [B(Ar^CF₃ )₄]^– anion is the [B(Ar^Cl)₄]^– WCA. ³⁶

Other aspects that will influence the choice, are the synthetic availability of the entire WCA class, or the specific starting material necessary to ionize the system of interest. In this respect, most of the WCAs known so far also do have disadvantages: the carborates are hard to synthesize and have often low yields. [CB₁₁(CF₃)₁₂]^– is even explosive, as is the LiC₆F₅ intermediate needed for the [B(C₆F₅)₄]^– synthesis. In addition, starting materials such as solvent free Ag⁺ salts or [NO]⁺, [NO₂]⁺ are not accessible as salts of [B(C₆F₅)₄]^–. Anions with multiple –CF₃ groups often tend to disorder in the solid state, which sometimes makes it hard to solve or refine the crystal structure. The problems associated with the refinement of structures containing the [Al(OR^PF)₄]^– WCA even led to the development of the software tool DSR. ³⁷ It allows for the simplified refinement of such disordered structures and is now implemented with standard programs like OLEX2. ³⁸

Therefore, the search for new useful anions is still in progress. With the amminated chloroborate cluster anion [1-Me₃N-B₁₂Cl₁₁]^– another promising candidate that refined earlier ideas by S. Strauss et al., ³⁹ was just recently presented by Jenne et al. in 2014. ⁴⁰ The positive charge of the ammonium function leads to an overall –1 charge and makes it possible to use the in 30 g scale accessible –B₁₂Cl₁₁ cluster residue. Important starting materials M⁺[1-Me₃N-B₁₂Cl₁₁]^– (M⁺ = Na⁺, [HNMe₃]⁺, [HNOct₃]⁺, [NO]⁺, [CPh₃]⁺, [N ⁿ Bu₄]⁺, [Et₃Si]⁺) have been described facilitating the application. ^40,41 More details on typical WCA starting materials to introduce a counterion into the given system can be found in the synthesis section below as well in the numerous WCA reviews cited in Table 1.

Synthesis routes to reactive main group cation salts

At the beginning, each proposal to prepare a target-rPBC needs to consider the choice of the WCA as delineated in the preceding section, as well as the available starting materials, ionization method and reaction medium.

WCA starting materials

A suitable starting material, should be accessible in good yields and contain a useful cation that typically acts as either a strong oxidant (e.g. [O₂]⁺, ⁴² [NO]⁺, ²⁹ [NO₂]⁺, ⁴³ N(arene)₃ ^{+  44} ) a halide (e.g. Li⁺, ⁴⁵ Na⁺, Ag^{+  46} ), hydride- or alkyl-abstractor ([CPh₃]^{+  47} ), a Brønsted acid ([H(OEt₂)₂]⁺, ⁴⁸ [H(NMe₂Ph)]⁺) or a metal cation, if a simple metal complex is desired as product (e.g. Cu^{+  49,50} ) (Table 2). Neutral Lewis acids for bond heterolysis are available in great variety and include the classical simple halides M^IIIX₃ and M^VX₅ (M^III = B, Al, Ga; M^V = P, As, Sb, Bi; X = F, Cl, Br, I; not all combinations useful), the rather fine tunable B(aryl)₃ acids (aryl = fluorinated, ⁵¹ chlorinated ⁵² or fluoroalkylated ⁵³ aromatic residue), or aluminum based systems like Al(C₆F₅)₃ ⁵⁴ and Al(OR^F)₃. ⁵⁵ Also the ion-like R₃Si(WCA) compounds have frequently been used. ^56,57 Recent systematic work analyzed the potency of a given Lewis acid versus fluoride, chloride, hydride and methanide as a base. It includes benchmark Lewis acidity values for a smaller set of simple MX _n acids. ⁵⁸ Neutral Brønsted acids like HF, HNTf₂ and derivatives thereof, ⁵⁹ or combinations of Brønsted and Lewis acids like HBr/nAlBr₃ ^60,61 are suitable for protonations. Novel, and in large quantity available very strong acids like R^HFOSO₃H ⁶² should also be mentioned.

Table 2. Acronym (Acr.) and type of the classified synthesis routes leading to rPBC.

Acr.	Type	Example	Ref.
Com	Complexation reaction		72
Ox	Oxidation reaction; including 1e– and 2e– oxidations.		30 and 31
Lewis	Lewis acid induced halogen bond heterolysis with neutral Lewis acids, including ion-like compounds.		32, 33 and 73
Salt	Salt elimination reaction		74
Hyd	Hydride metathesis reaction with neutral or ionic H–-acceptor	a	76
Alk	Alkyl metathesis reaction with neutral or ionic R–-acceptor		77
Ins	Insertion reaction		78
Prot	Protonation reaction		79
Lig	Ligand exchange reaction		80
Ion	Ionization		81
Other	Other reaction not classified as one of the above	—	—

^a This type of reaction is sometimes referred to as Bartlett–Condon–Schneider (BCS) type hydride transfer reaction. ⁷⁵

Suitable media/solvents

Since the synthesis of reactive ions is aspired, a suitable reaction medium should favorably be polar but not itself be a base or a nucleophile. This often rules out classical polar solvents that are itself good donors such as ethers or nitriles. Often chlorinated solvents CH₂Cl₂ (ε _r = 8.9), 1,2-Cl₂C₂H₄ (ε _r = 10.4) or Cl–Ph (ε _r = 5.7) tend to be good choices that nevertheless are incompatible with strong electrophiles like the silylium ions [SiR₃]⁺. Fluorinated arenes like F–Ph (ε _r = 5.5) and 1,2-F₂C₆H₄ (ε _r = 13.4) are good additions that became cheaper (but not cheap) over the last decade. However, they are incompatible with oxidants like [NO]⁺ or [NO₂]⁺ due to nitration/nitrosation reactions. Especially for non-metal cations, often superacids or SO₂ (ε _r = 16.3), SO₂ClF (ε _r = n.a.) etc. are the solvents of choice. ILs ⁶³ like acidic BMIM[AlCl₄] ^64,65 and others were shown in recent years to be very promising media for rPBC cation synthesis. ^65–67 Especially for group 15 cations, solvent free reactions using Me₃Si-OSO₂CF₃ or MX₃ (M = Al, Ga; X = Cl, Br) were shown to provide quantitative yields of the desired salts. By contrast, several of such reactions do work only incomplete or not at all in solution. ⁶⁸ Similarly, protonations with HBr/nAlBr₃ turned out to be best done solvent free. ^60,61

The recently established concepts of absolute acidity, ⁶⁹ absolute reducity ⁷⁰ and their two-dimensional combination as the protoelectric potential map ⁷⁰ can be used to understand protonation and/or redox chemistry over medium/solvent and even phase boundaries. This also includes ILs and therefore a thermodynamically sound pH definition has been introduced for IL media. ^61,71

Ionization protocols

Overall, we have categorized the rPBC included with the tables in the following sections by an acronym describing the synthetic approach used for their preparation. The synthesis routes are collected, explained and abbreviated in Table 2. Almost all the early approaches to reactive main group cations used halide abstractors as the Lewis acids AsF₅ or SbF₅, which form the conjugated [AsF₆]^– or [SbF₆]^– WCAs through the reaction. The trityl cation is a hydride abstractor, which is especially in case of silanes as starting materials very useful to produce silylium cations. Most of the metal–non-metal complexes were synthesized by complexation of a non-metal molecule (e.g. P₄, S₈, Cl₂, Xe) with a metal salt of a WCA. The coinage metals Cu^I, Ag^I and Au^I with their d¹⁰ electron configuration induce positive charge on the main group elements, stabilize the almost undistorted non-metal clusters, and provide insights in their bonding situation. If the cation is a strong oxidant, it is also possible to oxidize neutral substrates directly to give reactive cations, which are in turn stabilized by the corresponding WCA. An interesting recent addition are the transfer oxidation of e.g. the simple diorganodichalcogenides R₂E₂ (E = S, Se) with the combination of XeF₂ (primary oxidant and source of fluoride) and a Lewis acid. ⁸²

On the representations of the cation chemical structures

Note that the structural diagrams used throughout this review obey a distance criterion for interactions, but not necessarily a 2e2c-understanding of every interaction line. However, at least in the organic residue we attempted to follow an electron precise 2e2c picture. Necessarily, this gets difficult for structures with N-heterocyclic carbenes that formally allow for a neutral dative (imidazolyl) as well as ionic (imidazolium) description (Fig. 3).

Fig. 3. Possible descriptions of NHC-containing structures exemplified for the simple [Cl₂B-I ^t Bu]⁺ cation.

For simplicity, we chose the representation shown in the box in Fig. 3 and adopted similar drawings for related cases throughout. Thus, we only use arrows for relatively weak interactions with the bonding situation in ammine-borane H₃B←:NH₃ being the prototype as suggested by A. Haaland, ⁸³ and later contributions. ⁸⁴ For thoughts on these ongoing discussions, see these recent publications. ⁸⁵ Only if the positive charge can clearly (and not just formally) be attributed to one atom, we assigned the charge to this atom. More common is the case in the box in Fig. 3, in which the charge may be delocalized to quite a series of atoms and therefore we placed the charge at the upper right corner.

Ordering of the cation classes

The rPBC were as far as possible ordered according to accepted cation classes that may either refer to the number of valence electrons (i.e. the onium/enium/inium-series) or to the structure. In each subchapter, we intend to go from homoatomic, to binary and then to more complex cation compositions. The not always consistently used classification according to onium- (8 VE), enium- (6 VE) and inium-cations (4 VE) presents some problems. Note, that the coordination number of a group 14 onium ion may not always be four, as the σ-donation of π-density of a donating double bond may increase the coordination number to 5 as, for example, in the 2-norbornyl cation, a carbonium ion. ⁸⁶ Similar considerations hold for other donor coordinated onium- and enium-ions. Thus, we typically include the coordination number in the cation classification, for example as ligand substituted, (CN = 2). By contrast to these cation assignments, the group 15 to 18 cations were in addition classified by the oxidation state of the central atoms. This is often used for such rPBC. In addition, we included ion-like compounds that were initially defined for silylium compounds with coordinated counterions that structurally have to be addressed as a tight ion-pair but from the reactivity still bear a considerable amount of reactivity related to the free cation, e.g. see the ion like silylium compounds R₃Si(WCA) in Fig. 4. Related cases were published for coordinated aluminum cations, e.g. R₂Al(WCA), and were used in a similar manner. ⁸⁷ For ion-like compounds we keep the notation with the anion in parentheses and no charges written, as in R₃Si(WCA) and R₂Al(WCA). Heteropolyatomic clusters were discussed in the group of their most electropositive element (e.g. [P₃Se₄]⁺ in group 15, but [S₄N₄]²⁺ in group 16).

Fig. 4. Definition of ion-like compounds as exemplified for silylium ions.

In the following chapters we describe the rPBCs of the Group 13 to 18 elements and give selected representative examples for each cation type. However, for reasons of legibility, the full tables that comprehensively cover the rPBC entries of the groups, are collected in landscape format at the end of this document.

Group 13 cations

Traditionally, group 13 chemistry is dominated by compounds in the +III oxidation state. ⁸⁸ Of those, the simple halides are commonly applied as Lewis acid catalysts and initiators (e.g. BF₃) and usually associated with anion formation (e.g. [BF₄]^–). However, discrete trivalent group 13 cations have been found to be more reactive, owing to their greater electrophilicity if paired with coordinative unsaturation. ^10,89 Except for boron, it has become increasingly possible to stabilize group 13 cations in their +I oxidation state, e.g. by employing bulky substituents and/or WCAs ^31,90–92 (for thallium, this is the favored oxidation state due to the inert pair effect ⁹³ ). Featuring a lone pair of electrons and empty p-orbitals, the +I cations are ambiphilic and can function both as Lewis base or acid, thus offering unique reactivities and selectivities in organometallic chemistry, ^94–97 as well as organic ⁹⁸ and polymer ^7,99 syntheses. Overall, different aspects of the chemistry of cationic group 13 compounds were reviewed and these contributions are compiled in Table 3. In this context, this sections intends to give a comprehensive overview of reactive group 13 cations of the larger WCAs since about 2000. Due to the large scope of this chapter, we mainly omit rPBC with the simple halometallate based counterions and only include those in special cases of high relevance.

Table 3. Review articles including cationic group 13 compounds.

Year	Group	Title	Ref.
1985	13	Arene complexes of univalent gallium, indium and thallium	100 and 101
1998	13	Cationic group 13 complexes	102
2004	13	From group 13–group 13 donor–acceptor bonds to triple-decker cations	94
2005	13	Borinium, borenium, and boronium ions: synthesis, reactivity, and applications	89
2007	13	Development of the chemistry of indium in formal oxidation states lower than +III	103
2008	13	Borylene transfer from transition metal borylene complexes	13 and 12
2008	13	Synthesis, characterization, and applications of group 13 cationic compounds	95
2009	13	Highly electrophilic main group compounds: ether and arene thallium and zinc complexes	90
2009	13	Transition metal borylene complexes: boron analogues of classical organometallic systems	104
2010	13	Electron-precise coordination modes of boron-centered ligands	105
2011	13	Coordination chemistry of group 13 monohalides	96
2011	13	New light on the chemistry of the group 13 metals	88
2011	13	The chemistry of the group 13 metals in the +I oxidation state	106
2011	13	Mixed or intermediate valence group 13 metal compounds	107
2011	13	Coordination and solution chemistry of the metals: biological, medical and environmental relevance	108
2012	13	Cationic tricoordinate boron intermediates: borenium chemistry from the organic perspective	109
2012	13	Cyclopentadiene based low-valent group 13 metal compounds: ligands in coordination chemistry and link between metal rich molecules and intermetallic materials	110
2012	13	Low-oxidation state indium-catalyzed C–C bond formation	98
2013	13	1.17-low-coordinate main group compounds – group 13	97
2013	13	Transition metal borylene complexes	5
2013	13	Boron, aluminum, gallium, indium and thallium	111
2015	13	Discrete cationic complexes for ring-opening polymerization catalysis of cyclic esters and epoxides	10

Boron cations

For a long time, boron cations have remained a chemical curiosity due to their redox lability. However and partly owing to the developments in the field of WCAs, more and more boron-based cations are being reported. Overall, the cations can be classified according to the coordination number at boron: i.e., di-, tri-, and tetra-coordinated boron cations are referred to as borinium, borenium and boronium cations. To this day, the boron cations have been most notably reviewed by Nöth (1985; a milestone in cationic boron chemistry), ¹¹² Piers (2005; structural and bonding aspects) ⁸⁹ and Vedejs (2012; reactivties and applications). ¹⁰⁹

Alkyl-/aryl substituted (CN = 2)

To our knowledge, there is only one contribution to this class of compounds: i.e., the recently reported [Mes₂B]⁺ borinium cation with the very good [HCB₁₁Cl₁₁]^–/[B(C₆F₅)₄]^– WCAs. ^32,113 Herein, the boron atom adopts a linear di-coordinated structure and the Mes substituents are aligned orthogonal to each other, allowing for a perfect shielding as well as π-donation into the empty p-orbitals of the highly electrophilic borinium cation (cf. the modelled delocalized molecular orbitals). The [Mes₂B]⁺ cation is likely to become a textbook compound as it is the first borinium cation that does not rely on strongly π-donating heteroatom substituents (cf. the earlier reported [( ^t Bu₃PN)₂B]⁺ cation ¹¹⁴ in the section ligand substituted (CN = 2) in Table 8).

Table 8. Group 13 cations, their counterions (WCA) as well as the synthesis routes. The entries are ordered as follows: (i) from boron- to thallium-based cations, (ii) from unsubstituted, via alkyl/aryl, Cp, arene and ligand to transition-metal substituted cations, (ii) from low to high CNs, (iv) from mono- to multinuclear group 13 complexes. Note that the structural diagrams obey a distance criterion for interactions, but not necessarily a 2e2c-understanding of every interaction line. See comment in the Introduction.

Cation	WCA	Class a	Synthesis	Comment/structure	Ref.
Unsubstituted
In+	[OTf]–	Prot	InCp* + H+[WCA]–	Soluble in organic solvents in contrast to the In(i) halides	182
Tl+	[B(ArCF3 )4]–/[B(C6F5)4]–	Prot	TlOEt + [H(OEt2)2]+[WCA]–	—	196 and 197
Tl+	[B(OTeF5)4]–	Lewis	Tl+[OTeF5]– + B(OTeF5)3 in CH2Cl2/1,2-C2Cl3F3	—	198
Tl+	[Al(ORPF)4]–/[Al(ORHF)4]–/[Al(ORMeF)4]–	Salt	TlF + Li+[WCA]–	—	199 and 200
Alkyl/aryl substituted
[Mes2B]+	[HCB11Cl11]–/[B(C6F5)4]–	Salt	Mes2BF + Et3Si(HCB11Cl11)/[Et3Si(Mes)]+[WCA]–		32 and 113
(R2Al) (R = Me, Et)	[B12Cl12]2–	Alk	R3Al + {[CPh3]+}2[B12Cl12]2–	Ion-like compound	87
(Et2Al)	[CB11H6X6]– (X = Cl, Br)	Alk	Et3Al + [CPh3]+[CB11H6X6]–	Ion-like compound	128
[(2,6-Mes2C6H3)2Al]+	[B(C6F5)4]–	Hyd	(2,6-Mes2C6H3)2AlH + [CPh3]+[WCA]–	Related structure to the [Mes2B]+ cation, though the Mes moieties of the 2,6-Mes2C6H3 substituent additionally shield the aluminum cation	129
[(2,6-Mes2C6H3)2Ga]+	[Li{Al(ORHF)4}2]–	Salt	(2,6-Mes2C6H3)2GaCl + 2Li+[WCA]–	Similar structure as the [(2,6-Mes2C6H3)2Al]+ cation	158
Cyclopentadienyl complexed
[(η5-Cp)2Al]+	[Al(ORPF)4]–	Prot	AlCp3 + [H(OEt2)2]+[WCA]–	—	79
[(η5-Cp′)2Al]+	[B(C6F5)4]–	Alk	Cp′3Al + [CPh3]+[WCA]–	—	131
[(η5-Cp*)2Al]+	[{Ph(Me)B(η5-C5H4)2}ZrCl2]–	Alk	Cp*2AlMe + {Ph(SMe2)B-(η5-C5H4)2}ZrCl2 + [Ph3P N PPh3]+Cl–	—	132
[(η5-Cp*)2Al]+	[MeB(C6F5)3]–	Alk	Cp*2AlMe + B(C6F5)3	—	133
[(η5-Cp)2(Et2O)2Al]+	[Al(ORPF)4]–	Prot	AlCp3 + [H(OEt2)2]+[WCA]–	Et2O can coordinate the [(η5-Cp)2Al]+ cation	79
[Ga2(η5-Cp*)]+	[B(ArCF3 )4]–	Prot	[H(OEt2)2]+[WCA]– + GaCp*		159
[(η1-Cp*)(η3-Cp*)Ga]+	[BF4]–	Prot	Cp*3Ga + HBF4	cf. [B(η5/η1-Cp*)2]+ and [Al(η5/η5-Cp*)2]+	160
[In2(η5-Cp)]+	[Cp3In–Cp–InCp3]–	Com	In+[OTf]– + Cp2Mn in C6H5Me	Inverted sandwich structure (cf. the related [Ga2(η5-Cp*)]+ cation)	183
[In2(η5-Cp*)]+	[B(C6F5)4]–	Prot	[(C6H5Me)H]+[WCA]– + InCp*	Similar structure to the [Ga2(η5-Cp*)]+ cation	185
[(μ-η5-C5Me5)In2(η6-Tol)2]+	[(C6F5)3BO(H)B(C6F5)3]–	Prot, Com	(Cp*In)6 + B(C6F5)3 + H2O·B(C6F5)3		184 and 185
Arene complexed
[Ga(η6-C6H5R) n ]+ (R = F, Me; n = 2, 3)	[Al(ORPF)4]–	Ox	Ga0 + Ag+[WCA]– in arene		31, 91 and 92
[Ga(η6-arene) n ]+ (n = 2, 3)	[Al(ORPF)4]–	Com	[Ga(C6H5F)2–3]+[WCA]– + arene (arene = Mes, p-Xyl, C6Me6)	Bent-sandwich (2 ligands) or tubby coordinated complex (3 ligands)	7
[Ga(η6-DPE)]+	[Al(ORPF)4]–	Com	[Ga(C6H5F)2–3]+[WCA]– + DPE	First structurally characterized bent-sandwich ansa-arene complex	8 and 99
[In(η6-C6H5F) n ]+ (n = 2, 3)	[Al(ORPF)4]–	Ox	In0 + Ag+[WCA]– in C6H5F	Bent-sandwich complex (cf. gallium analogue)	162
[In(η6-o-C6H4F2)2]+	[Al(ORPF)4]–	Salt	InCl + Li+[WCA]– in o-C6H4F2	Bent-sandwich complex (cf. gallium analogue)	186
[Tl(η6-C6Me6)]+	[H2N{B(C6F5)3}2]–	Other	[Tl(C6Me6)2]+ in Et2O + C6H5Me, vacuum	First example of a mono-η6-coordinated thallium complex	204
[Tl(η6-C6H5Me)2]+	[HCB11H5Br6]–	Salt	Cs+[HCB11H5Br6]– + TlF	Bent-sandwich complex (cf. gallium analogue)	201
[Tl(η6-C6H5Me)3]+	[H2N{B(C6F5)3}2]–	Com	[Tl(OEt2)2]+[WCA]– + C6H5Me	Tubby coordinated complex (cf. gallium analogue)	203
[Tl(η6-Mes)2]+	[B(OTeF5)4]–	Lewis, Com	Tl+[OTeF5]– + B(OTeF5)3 in Mes	Tubby coordinated complex (cf. gallium analogue)	202
[Tl(η6-C6Me6)2]+	[H2N{B(C6F5)3}2]–	Com	[Tl(OEt2)3]+[WCA]– + C6Me6	Tubby coordinated complex (cf. gallium analogue)	203
Ligand substituted (CN = 2)
[Cp*B(IMes)]2+	[AlCl4]–	Lewis	Cp*BCl2(IMes) + 2AlCl3		210
[( t Bu3PN)2B]+	[B(C6F5)4]–	Hyd	( t Bu3PN)2BH + [Ph3C]+[WCA]–		114
[ t Bu2MeSi–Al–Si t Bu2–Si t Bu2Me]+	[B(C6F5)4]–	Alk	Al(SiMe t Bu2)3 + [Et3Si]+[WCA]–	Hyperconjugation with a neighboring Si–Si bond	134
[Ga(IR)2]+ (R = Pr, Mes)	[Al(ORPF)4]–	Com	[Ga(C6H5F)2]+[WCA]– + IR		161
[ t Bu2MeSi–Ga–Si t Bu2–Si–Me t Bu2]+	[B(C6F5)4]–	Alk	Ga(SiMe t Bu2)3 + [Et3Si(C6H6)]+[WCA]–	Stabilized by hyperconjugation with a neighboring Si–Si bond	134
[ t Bu3Si–Ga–Si t Bu3]+	[Al(ORPF)4]–	Salt	( t Bu3Si)2GaCl + Ag+[WCA]–	Linear arrangement	163
[Ga(P t Bu3)2]+	[Al(ORPF)4]–	Com	[Ga(C6H5F)2]+[WCA]– + P t Bu3		162
[In(Mes2py)(η6-C6H5F)]+	[B(ArCF3 )4]–	Salt, Com	InBr + Na+[WCA]– + Mes2py		187
[In(IPr)2]+	[Al(ORPF)4]–	Com	[In(C6H5F)2]+ + IPr		161
[In(Mes2py)2]+	[B(ArCF3 )4]–	Salt	In+Br– + Na+[WCA]– + 2Mes2py		187
[Tl(1,2-Cl2C2H4)]+	[B(OTeF5)4]–	Lewis, Com	Tl+[OTeF5]– + B(OTeF5)3 in 1,2-C2H4Cl2		198
Ligand substituted (CN = 3)
[BMes2(IMe)]+	[OTf]–	Salt	Mes2BF + [Me3Si]+[OTf]– + [Ag(IMe)2]+[Ag2I3]–		211
[BCl2(I t Bu)]+	[B(ArCl)4]–	Salt	BCl3(I t Bu) + Na+[WCA]–		74
[{(PPh3)2C}BH2]+	[HB(C6F5)3]–	Hyd			118 and 119
[BMes2(DMAP)]+/[B(ArN)2(DMAP)]+ ArN = 4-(Me2N)-2,6-Me2-C6H2)	[OTf]–	Salt	Mes2BF + Me3Si-OTf + ArN 2BF + DMAP		57
[B(SubPc)]+(Sub = C24H12N6)	[HCB11Me5Br6]–	Salt	B(SubPc)Cl + Et3Si(HCB11Me5Br6)		115
[CatB(O PEt3)]+	[HCB11H5Br6]–	Salt, Com	Ag+[WCA]– + CatBBr + OPEt3		116
[(CatB)(PNP)PdH]+	[B(ArCF3 )4]–/[CB11H12]–	Other	[(PNP)Pd(THF)]+[WCA]– + CatBH		117
[ArN(C( CH2)NAr)(C(Me)NAr)AlH]+ (Ar = DIPP)	[B(C6F5)4]–	Hyd	ArN(CMeNAr)2 + AlH3·NMe2Et + [Ph3C]+[WCA]–		135
[{HC(CMeNAr)2}AlMe]+ (Ar = DIPP)	[B(C6F5)4]–/[MeB(C6F5)3]–	Alk	{HC(CMeNAr)2}AlMe2 + [CPh3]+[WCA]–/B(C6F5)3		136
[Ga(η6-C6H5F)2(DTBMP)]+	[Al(ORPF)4]–	Com	[Ga(η6-μC6H5F) n ]+[WCA]– + DTBMP (n = 2, 3)		164
[Ga(pyrazine)3]+/[{Ga(μ-pyrazine)2-(η1-pyrazine)}+]∞	[Al(ORPF)4]–	Com	[Ga(C6H5F)]+[WCA]– + pyrazine (n = 2, 3)		164
[Ga(PPh3)3]+	[Al(ORPF)4]–	Com	[Ga(C6H5Me)2]+[WCA]– + PPh3		31, 91 and 92
[(iPr2-ATI)InMe]+	[B(C6F5)4]–	Other	Thermolysis of [{iPr2-ATI(CPh3)}InMe2]+[WCA]–		190
[{ArN CPh}2(NC5H3)In]+ (Ar = 2,4- t Bu2C6H3, 2,5- t Bu2C6H3, 2,6-Et2C6H3, 2,6-iPr2C6H3,)	[OTf]–	Com	In+[WCA]– + bis(imino)pyridine ligand		188 and 189
[In(PPh3)3]+	[Al(ORPF)4]–	Com	[In(C6H5F) n ]+[WCA]– + 3 PPh3 (n = 2, 3)	Trigonal pyramidal (cf. gallium analogue)	162
[Tl(Mes2py)(η6-C6H5R)2]+ (R = F, Me)	[B(ArCF3 )4]–	Salt, Com	TlCl + Na+[WCA]– + Mes2py in C6H5R		187
[Tl(timtmb tBu)]+	[OTf]–	Com	Tl+[WCA]– + timtmb tBu		205
[{ArN CPh}2(NC5H3)Tl]+ (Ar = 2,6-Et2C6H3, 2,5- t Bu2C6H3)	[OTf]–	Com	Tl+[WCA]– + bis(imino)pyridine ligand		206
Ligand substituted (CN = 4)
[{(PPh3)2C}BH2(DMAP)]+	[HB(C6F5)3]–	Com	[{(PPh3)2C}BH2]+[WCA]– + DMAP		118 and 119
[BH2(PR2H)2]+ (R = t Bu, Cy, Ph)	[B(ArCF3 )4]–	Salt	[BH2(PR2H)]+Br– + Na+[WCA]–		120
[(1-MIM)2(9BBN)]+	[B(ArCF3 )4]–	Com	[PMAF–9BBN)]+[WCA]– + 1-MIM		212
[Me2Al(OEt2)2]+	[MeB(C12F9)3]–	Alk	AlMe3 + B(12F9)3 in Et2O	—	77
[Me2Al(THF)2]+	[{Me2Si(NDIPP)2}2Zr2Cl5]–	Alk	Al2Me6 + {Me2Si(NDIPP)2}ZrCl2(THF)2	—	143
[Me2Al(NPhMe2)2]+	[B(C6F5)4]–	Prot, Com	Al2Me6 + [HNMe2Ph]+[WCA]–	—	144
[H2Al(NMe3)2]+	[(AlH)8(CCH2 t Bu)6]2–	Other	t Bu CLi + AlH3·NMe3 + ClAlH2·NMe3 + [ t BuCH2(Bzl)NMe2]+Cl–	—	145
[(Pytsi)AlMe]+	[MeB(C6F5)3]–	Alk	(Pytsi)AlMe2 + B(C6F5)3		137
[H2C{hpp}2AlMe2]+	[BPh4]–	Prot	[{hpp}H2C{hpp}H]+[WCA]– + AlMe3		138
[{H2C C(BOX-Me2)2}Al-(Me)2]+	[B(C6F5)4]–	Hyd	{BOX-Me2}Al(Me)2 + [CPh3]+[WCA]–		139
[{BOX-Me2}Al(Me)(NMe2Ph)]+	[MeB(C6F5)3]–	Alk	{BOX-Me2}Al(Me)2 + B(C6F5)3 in NMe2Ph		139
[{6-(CH2NMe2)-2-CPh3-4-Me-C6H2O}Al(iBu)(NMe2Ph)]+	[HB(C6F5)3]–	Hyd	{6-(CH2NMe2)-2-CPh3-4-Me-C6H2O}Al-(iBu)2 + B(C6F5)3 + NMe2Ph		213 and 214
[{HC(CPhNSiMe3)2}-Al(Do)Me]+ (Do = Et2O, THF)	[B(C6F5)4]–/[MeB(C6F5)3]–	Prot/Alk, Com	{HC(CPhNSiMe3)2}AlMe2 + [HNMe2Ph]+[WCA]– + Et2O/B(C6F5)3 + THF		215
[(ArN)C(Me)CHPPh2(NAr)AlMe(OEt2)]+ (Ar = DIPP)	[B(C6F5)4]–	Alk, Com	(ArN)C(Me)CHPPh2(NAr)MMe2 + [Ph3C]+[WCA]– in Et2O		216
[(iPr2-ATI)Al(Et)(Do)]+ (Do = ClPh, NCMe)	[B(C6F5)4]–	Alk, Com	(iPr2-ATI)AlEt2 + [CPh3]+[WCA]– in PhCl/ + MeCN		140 and 141
[(SchNMe2)AlMe]+	[BPh4]–	Salt	(SchNMe2)AlMeCl + Na+[WCA]–		142
[{η2-O,P-(2-PPh2-4-Me-6- t Bu-C6H2O)}2Al]+	[MeB(C6F5)3]–	Alk	{η2-O,P-(2-PPh2-4-Me-6- t Bu-C6H2O)}2AlMe + B(C6F5)3		217
[{H2C C(BOX-Me2)2}Ga-(Me)2]+	[B(C6F5)4]–	Hyd	{BOX-Me2}Ga(Me)2 + [CPh3]+[WCA]–		165
[{BOX-Me2}Ga(Me)]+	[MeB(C6F5)3]–	Alk	{BOX-Me2}Ga(Me)2 + B(C6F5)3 in NMe2Ph		165
[(iPr2-ATI)Ga(Me)(ClPh)]+	[B(C6F5)4]–	Alk, Com	(iPr2-ATI)GaMe2 + [CPh3]+[WCA]– in PhCl		140
[{1,2-(NiPr)2-5-CPh3-cyclohepta-3,6-diene}InMe2]+	[B(C6F5)4]–	Other	(iPr2-ATI)InMe2 + [Ph3C]+[WCA]–		190
[(iPr2-ATI)In(Me)(NMe2Ph)]+	[B(C6F5)4]–	Prot	(iPr2-ATI)InMe2 + [HNMe2Ph]+[WCA]–		190
[Tl(OEt2)4]+	[H2N{B(C6F5)3}2]–	Prot	TlOEt + [H(OEt2)2]+[WCA]– in Et2O		203
Ligand substituted (CN = 5)
[{SalenCF3 }Al(OEt2)]+	[MeB(C6F5)3]–	Alk	{SalenCF3 }AlMe + B(C6F5)3 in Et2O		147
[Ga(η1-C3H5)2(THF) n ]+ (n = 2, 3)	[B(C6F5)4]–/[B(ArCl)4]–	Prot	Ga(η1-C3H5)3(THF) + [HNMe2Ph]+[WCA]–		166
[In(CH2SiMe3)2(THF)3]+	[B(C6F5)4]–	Prot	In(CH2SiMe3)3 + [HNMe2Ph]+[WCA]– in THF		191
[Tl(NPPh)2(η6-C6H5Me)]+ NPPh = 2,5-bis(2-pyridyl)-1-phenylphosphole	[Al(ORPF)4]–	Com			218
Ligand substituted (CN ≥ 6)
[DoAl(MeOH)2]+ (Do = Salen, Acen)	[BPh4]–	Salt, Com	DoAlCl + Na+[WCA]– + MeOH		148 and 149
[Salpen( t Bu)Al(THF)2]+	[BPh4]–	Salt, Com	Salpen( t Bu)AlCl + Na+[WCA]– + THF		150 and 151
[(SchNMe2)Al(OPh)-(THF)2]+	[BPh4]–	Com	[(SchNMe2)AlPh]+[WCA]– + O2 in THF		142
[GaH(THF)4(OTf)]+	[Ga(THF)4(OTf)2]–	Prot	GaCp* + HOSO2CF3 in THF		167
[GaIII{(bipy)3}˙]2+	[Al(ORPF)4]–	Com			17
[Ga([18]crown-6)(η6-/η1-C6H5F)2]+	[Al(ORPF)4]–	Com	[Ga(η6-C6H5F)]+[WCA]– + [18]crown-6 (n = 2, 3)		168
[In([18]crown-6)]+	[OTf]–	Com	In+[WCA]– + [18]crown-6	No coordinated solvent, but a strong anion–cation interaction: cf. In–O = 227.2 pm and 278.5 pm (sum of the van der Waals radii 345 pm)	192 and 193
[In([18]crown-6)(η6-/η1-C6H5F)2]+	[Al(ORPF)4]–	Com	[In(η6-C6H5F) n ]+[WCA]– + [18]crown-6 (n = 2, 3)	Similar structure to the gallium analogue (see above)	168
[In([15]crown-5)2]+	[OTf]–	Com	In+[WCA]– + [15]crown-5	Sandwich complex	194
[{HC(3,5-Me2pz)3} n Tl]+ (n = 1, 2)	[PF6]–	Com	Tl+[WCA]– + HC(3,5-Me2pz)3		219
[Tl([18]crown-6)]+	[H2N{B(C6F5)3}2]–	Com	[Tl(C6H5Me)2]+[WCA]– + [18]crown-6	Similar to gallium analogue, yet featuring significant Tl–F interactions to two counteranions	203
Transition-metal substituted
[(FP*)(BMes)]+	[B(ArCF3 )4]–	Salt	(FP*)(BMes)Br + Na+[WCA]–		220
[CpFe(CO)(PCy3)-(BNCMes2)]+	[B(ArCl)4]–	Salt	CpFe(CO)(PCy3)(B(Cl)-NCMes2) + Na+[WCA]–		221
[CpM(CO)(R){B(NCy2)}]+ (M = Fe, Ru; Do = CO, PMe3, PPh3)	[B(ArCF3 )4]–	Salt	CpM(CO)(R){B(NCy2)Cl} + Na+[WCA]–		12 and 124
[(Cy3P)2(MeCN)Pt(B O)]+	[B(ArCF3 )4]–	Salt, Com	(Cy3P)2Pt(B O)(Br) + Ag+[WCA]– + MeCN		121
[{(OC)5Mn}2(μ-B)]+	[B(ArCF3 )4]–	Salt	{(OC)5Mn}2(μ-BBr) + Na+[WCA]–		222
[(FP’)2(μ-B)]+	[B(ArCF3 )4]–	Salt	(FP′)2B(Cl) + Na+[WCA]–		222
[Fc(NC5H2Me2)BPh]+	[Al(ORPF)4]–	Salt			223
[(FP){B(NiPr2)(OPPh3)}]+	[B(ArCF3 )4]–	Com	[(FP)B(NiPr2)]+[WCA]– + Ph3PO		224
[(FP)B{N(iPr)(CMe2)}(Do)]+ (Do = Ph2C O, Me2C NiPr)	[B(ArCF3 )4]–	Com, other	[(FP) B NiPr2]+[WCA]– + Do		225
[(FP)B(NCy2)(Do)]+ (Do = C5H4PPh3, 4-Pic)	[B(ArCF3 )4]–	Com	[(FP) B NCy2]+[WCA]– + Do		12
[CpRu(CO)2{B(NCy2)-(4-Pic)}]+	[B(ArCF3 )4]–	Salt, Com	CpRu(CO)2{B(NCy2)Cl} + Na+[WCA]– + 4-Pic		124
[(FP*)B(Cl)(LB)]+ (Do = 3,5-lutidine, PMe3, IMe)	[B(ArCl)4]–	Salt	(FP*)B(Cl2)(Do) + Na+[WCA]–		226
[(FP*)B(nacnac)]+	[B(ArCF3 )4]–	Salt, other			227
[(FP)C(NCy)2BNR2]+ (R = iPr, Cy)	[B(ArCF3 )4]–	Ins	[(FP)(BNR)2]+[WCA]– + RN C NR (substoichiometric)		228
[(H)(PNP)Pd(BCat)]+	[B(ArCF3 )4]–	Other	[(BCat)(PNP)Pd(BCat)]+[WCA]– + H2O		117
[(R3P)2Pt{B(Fc)Br}]+ (R = iPr, Cy)	[B(ArCF3 )4]–	Salt	(R3P)2Pt(Br){B(Fc)Br} + Na+[WCA]–		229 and 230
[(Cy3P)2Pt{B(X)X′}]+ (X = Br; X′ = ortho-tolyl, t Bu, NMe2, Pip, Br; XX′ = (NMe2)2, CatB)	[B(ArCF3 )4]–/[B(C6F5)4]–	Salt	(Cy3P)2Pt(Br){B(X)X′} + Na+/K+[WCA]–		231
[(Cy3P)2Pt(Br){B(NC5H4-4-R)X}]+ (R = Me, X = NMe2, Pip, Br; R = t Bu, X = Pip)	[B(ArCF3 )4]–	Salt	(Cy3P)2Pt(Br){B(Br)-(NC5H4-4-R)X} + Na+[WCA]–		125
[(Cy3P)2Pt{B(Br)(NMe2)}-(NCMe)]+	[B(ArCF3 )4]–/[B12Cl12]2–	Com/salt, Com	(Cy3P)2Pt{B(Br)(NMe2)} + NCMe/(Cy3P)2Pt{B(Br)(NMe2)}Br + {Na+}2[WCA]2– + MeCN		230
[(Cy3P)2Pt(BCl2)]+	[B(ArCF3 )4]–	Salt	(Cy3P)2Pt(BCl2)Cl + Na+[WCA]–		230
[Cp*Ru(PiPr3)(BH2Mes)]+	[B(C6F5)4]–	Salt	Cp*Ru(PiPr3)-(BH2MesCl) + Li+[WCA]–·2.5OEt2		232
[(PMAF)2BH2]+	[B(C6F5)4]–	Hyd, Com	PMAF–BH3 + [CPh3]+[WCA]– + PMAF		212
[Rh(PPh3)2(κ1,η-PPh2BH2·PPh3)]+	[B(ArCF3 )4]–	Salt, Com	ClRh(PPh3)3 + Na+[WCA]– + H3B·PPh2H		120
[FcBMe(bipy)]+	[PF6]–	Salt, Com	FcBBrMe + bipy + [NH4]+[WCA]–		126
[FcB(Pic)3]2+	[B(ArCF3 )4]–	Salt, Com	Br2BFc + 2Na+[WCA]– + 3Pic		122
[(FP){C(NCy)2B-(NCy)2CNR2}]+ (R = iPr, Cy)	[B(ArCF3 )4]–	Ins	[(FP){B(NR2)}]+ + CyN C NCy		124 and 228
[(dppe)Cp*FeGaI]+	[B(ArCF3 )4]–	Salt	(dppe)Cp*FeGaI2 + Na+[WCA]–		233
[(FP*)2Ga]+	[B(ArCF3 )4]–	Salt	(FP*)2GaCl + Na+[WCA]–		169
[(FP*)2Ga(4-Pic)]+	[B(ArCF3 )4]–	Salt, Com	(FP*)2GaCl + Na+[WCA]– + 4-Pic		170
[(FP*)Ga(Mes)(dtbpy)]+	[B(ArCF3 )4]–	Salt, Com	(FP*)Ga(Mes)I + Na+[WCA]– + dtbpy		234
[(FP*)Ga(phen)(Y)]+ (Y = Cl, S p Tol)	[BPh4]–	Salt, Com/Lewis	2(FP*)GaCl2 + Na+[WCA]– + phen/[(FP*)Ga(phen)(Cl)]+ + Me3SiS p Tol		171
[(FP)Ga(OEt2){(NCy)2-C t Bu}]+	[B(ArCF3 )4]–	Salt, Com	(FP)Ga(Cl){(NCy)2C t Bu} + Na+[WCA]– in Et2O		235
[(FP)2Ga(bipy)]+	[Cl2Ga(FP)2]–	Lewis, Com	2ClGa(FP)2 + bipy		236
[InPt(PPh3)3]+	[B(ArCF3 )4]–	Com	In+[WCA]– + Pt(PPh3)4		179 and 180
[(phen)2In-Ag(η3-C6H5F)]2+	[Al(ORPF)4]–	Com			17
[(FP*)2In]+	[B(ArCF3 )4]–	Salt	(FP*)2InCl + Na+[WCA]–		195
[(FP*)2In(THF)]+	[B(ArCF3 )4]–	Com	[(FP*)2In]+[WCA]– + THF		195
[Tl(η5-FeCp2)]+	[H2N{B(C6F5)3}2]–	Others	[Tl(η6-C6H5Me)3]+[WCA]– + 2.2FeCp2		204
[Tl2(η5-FeCp2)3]2+	[H2N{B(C6F5)3}2]–	Com	[Tl(η6-C6H5Me)2]+[WCA]– + FeCp2		203
[Tl{(η5-As5)FeCp*}3]+	[FAl{OC6F10(C6F5)}3]–	Salt, Com			176
[{P(Ph2)CH2ox}(Cl)(Tl)Pt-CH2Ph}]+	[PF6]–	Other	Tl+[WCA]– + {P(Ph2)CH2ox}Pt(Cl)-CH2Ph}		208
Multinuclear
[{IPr(H2B)}2(μ-H)]+	[HB(C6F5)3]–	Hyd	IPr + B(C6F5)3		118 and 119
[{Me3N(H2B)}2(μ-H)]+	[B(C6F5)4]–	Hyd	Me3N–BH3 + [CPh3]+[WCA]–		237
[B4H2(μ-hpp)4]2+	[HB(C6F5)3]–	Hyd, Com	[HB(μ-hpp)]2 + B(C6F5)3		123
[[{6-(CH2NMe2)-2-CPh3-4-Me-C6H2O}Al(R)]2]2+ (R = C6H13)	[B(C6F5)4]–	Com	[{6-(CH2NMe2)-2-CPh3-4-Me-C6H2O}Al(iBu)(BrPh)]+[WCA]– + 1-hexene		152
[{2-(CH2Do)-6-R-C6H3O}AlMe({2-(CH2Do)-6-R-C6H3O}AlMe2)]+ (R = Ph, t Bu; Do = NMe2, NC4H8, NC5H10)	[MeB(C6F5)3]–	Alk	{2-(CH2Do)-6-R-C6H3O}AlMe2 + B(C6F5)3		238 and 214
[{MeC(NR)2}2Al2Me3]+ (R = iPr, Cy)	[B(C6F5)4]–/[MeB(C6F5)3]–	Alk, Com	{MeC(NR)2}AlMe2 + [CPh3]+[WCA]–/B(C6F5)3		156
[AlEt(μ-η2,η1-iPr2-ATI)-(μ-Et)AlEt2]+	[B(C6F5]–	Com	[(iPr2-ATI)Al(Et)]+[WCA]–+AlEt3		141
[{(iPr2-ATI)AlMe}2(μ-Me)]+	[B(C6F5)4]–	Alk, Com	(iPr2-ATI)AlMe2 + [CPh3]+[WCA]–		141 and 239
[{(iPr2-ATI)Al(μ-OiPr)}2]2+	[B(C6F5)4]–	Com	[(iPr2-ATI)Al(Et)]+[WCA]– + acetone		141 and 153
[Me2Al(μ-OSi(R123)3)2Al-Me(NMe2Ph)]+ (R1, R2 = Me; R3 = Me, t Bu)	[B(C6F5)4]–	Prot	Me2Al(μ-OSiR3)2AlMe2 + [HNMe2Ph]+[WCA]–		154
[{(iPr2-ATI)Al-(μ-C C t Bu)}2]2+	[B(C6F5)4]–	Com	[(iPr2-ATI)Al(Et)]+[WCA]– + tert-butyl acetylene		141 and 153
[{(tacn)AlMe}2]2+	[MeB(C6F5)3]–	Alk	[(tacn)AlMe2]2 + B(C6F5)3		240
[{(OSSO)Al}2]2+	[MeB(C6F5)3]–	Alk	(OSSO)AlMe + B(C6F5)3		155
[(η6-C6H5F)Ga-(μ-η6-m-TP)2-Ga(η6-C6H5F)]2+	[Al(ORPF)4]–	Com	[Ga(η6-C6H5F) n ]+[WCA]– + m-TP (n = 2, 3)		8 and 99
[{ t BuC(NiPr)2}GaMe-{ t BuC(NiPr)2}GaMe2]+	[B(C6F5)4]–	Alk, Com	{ t BuC(NiPr)2}GaMe2 + [Ph3C]+[WCA]–		156
[{(iPr2-ATI)GaMe}2(μ-OH)]+	[B(C6F5)4]–	Other	[(iPr2-ATI)Ga(Me)(NMe2Ph)]+[WCA]– + H2O		172
[{(Salomphen)Ga}(μ-Cl)]+	[BPh4]–	Salt	(Salomphen)GaCl + Na+[BPh4]–		241
[(BuGa)4(μ-OH)6]2+	[HCB11Br6Me5]–	Other	[(2,6-Mes2C6H3)GaBu]+ [WCA]– + H2O		173
[{(PPh3)3In}2(μ-PPh3)]2+	[Al(ORPF)4]–	Com	[In(C6H5F) n ]+[WCA]– + PPh3 (n = 2, 3)	One PPh3 moiety functions as a bridge between both InI cations	162
[In3(bipy)5–6]3+	[Al(ORPF)4]–	Com			17
[In4(Do)6]4+ (Do = bipy, phen)	[Al(ORPF)4]–	Com			17
[In4{(CpMo(CO)2)2P2}8]4+	[Al(ORPF)4]–	Com	[In(o-C6H4F2)2]+[WCA]– + {CpMo(CO)2}2(P2)		186
[{{ArN CPh}2(NC5H3)Tl}2(μ-η6-C6H5R)]2+ (Ar = 2,6-Et2C6H3, 2,5- t Bu2C6H3; R = H, Me)	[OTf]–	Com	2[(ArN CPh)2(C5H3N)Tl]+[WCA]– + C6H5R		206
[Tl2(NPPh)4]2+ NPPh = 2,5-bis(2-pyridyl)-1-phenylphosphole	[Al(ORPF)4]–	Com			218
[Tl(β-triketimine)2]2+(R = Me, t Bu)	[B(ArCF3 )4]–	Com	Tl+[WCA]– + β-triketimine		14
[Tl2({CpMo(CO)2}2)6]2+	[Al(ORPF)4]–	Com	{CpMo(CO)2}2(P2) + Tl+[WCA]–		207
[Tl3F2Al(ORHF)3]+	[Al(ORHF)4]–	Salt, other	TlF + 2Li+[WCA]–		200
[Tl4(μ-OH)2]2+	[H2N{B(C6F5)3}2]–	Other	[Tl(OEt2)2]+[WCA]– + H2O		203
[{Tl(OR)4(μ-Cl)2}+] n	[PF6]–	Other	RuCl2(DMeOPrPE)2 + Tl+[WCA]–	“Arrested” chloride abstraction yielding a one-dimensional coordination polymer	209
Multinuclear transition-metal substituted
[(CpFe(CO){B(NCy2)})2-(μ-dmpe)]2+	[B(ArCF3 )4]–	Com, salt	(FP){B(NCy2)}Cl + dmpe + Na+[WCA]–		124
[(BCat)(PNP)Pd(BCat)]+	[B(ArCF3 )4]–/[CB11H12]–	Other	[(PNP)Pd(THF)]+[WCA]–/(PNP)Pd(CB11H12) + CatB–BCat		117
[(Cy3P)2{Pt(BBr)}2-(μ-C6H4)]2+	[B(C6F5)4]–	Salt	{(Cy3P)2Pt(Br)(BBr2)}2-(μ-C6H4) + K+[WCA]–		125
[{(Cy3P)2PtB}2(μ-O)2]2+	[Al(ORPF)4]–	Salt	(Cy3P)2BrPt(B O) + Ag+[WCA]–		242
[{(bipy)(Me)B}2(μ-Fc)]2+	[PF6]–	Salt, Com	Fc(BBrMe)2 + 2bipy + [NH4]+[WCA]–		126 and 127
[{(FP)Ga(Mes)}2(μ-Cl)]+	[B(ArCF3 )4]–	Salt, Com	(FP)Ga(Mes)(Cl) + Na+[WCA]–		195
[{{FeCp(CO)2}Ga{(NCy)2-C t Bu}}2(μ-OH)]+	[B(ArCF3 )4]–	Other	[FpGa(OEt2)-{(NCy)2C t Bu}]+[WCA]– + H2O		235
[{Ga(P5FeCp*)3}+] n	[Al(ORPF)4]–	Com			176
[{(DDP)(THF)Ga}2Au]+	[B(ArCF3 )4]–	Salt, Com	{(DDP)Ga}2AuCl + Na+[WCA]– in THF		174
[{(THF)(DDP)GaZn(THF)}2-(μ-Cl)2]2+	[B(ArCF3 )4]–	Salt, Com	(DDP)(Cl)GaZn(Cl)(THF)2 + Na+[WCA]– in THF		175
[{In{η5-E5)FeCp*}3}+] n (E = P, As)	[Al(ORPF)4]–	Com		One-dimensional coordination polymer/similar structure to [{Ga(P5FeCp*)3}+] n	176
[{Tl{(η5-E5)FeCp*}3}+] n (E = P, As)	[Al(ORPF)4]–	Com		One-dimensional coordination polymer/similar structure to [{Ga(P5FeCp*)3}+] n	207 and 176
ECp* substituted (E = Al, Ga)
[Rh(COD)(AlCp*)3]+	[B(ArCF3 )4]–	Com	[Rh(COD)2]+[WCA]– + 3AlCp*		157
[Cp*Fe(GaCp*)3]+	[B(ArCF3 )4]–	Com	[Fe(MeCN)6]2+{[WCA]–}2 + 4GaCp*		243
[Cp*Co(GaCp*)3]2+	[B(ArCF3 )4]–	Ox, Com	[Co(MeCN)6]2+{[WCA]–}2 + 4GaCp*		243
[Cu(GaCp*)4]+	[B(ArCF3 )4]–	Com	[Cu(MeCN)4]+[WCA]– + 4GaCp*		243
[Zn(GaCp*)4]2+	[B(ArCF3 )4]–	Prot, Com	ZnMe2 + [H(OEt2)2]+[WCA]– + 4GaCp*		175
[Zn2(GaCp*)6]2+	[B(ArCF3 )4]–	Other	Zn2Cp*2 + [Ga2Cp*]+[WCA]– mechanism unclear		244
[Rh(COD)(GaCp*)3]+	[B(ArCF3 )4]–	Com	[Rh(COD)2]+[WCA]– + 3GaCp*		157
[{Rh(NBD)(PCy3)-(GaCp*)2}]+	[B(ArCF3 )4]–	Com	[Rh(NBD)(PCy3)2]+[WCA]– + 2GaCp*		157
[Pt(H)(GaCp*)4]+	[B(ArCF3 )4]–	Prot	Pt(GaCp*)4 + [H(OEt2)2*]+[WCA]–		180
[(Ga)Ru(PCy3)2(GaCp*)2]+	[B(ArCF3 )4]–	Other	Ru(PCy3)2(GaCp*)2(H)2 + [Ga2Cp*]+[WCA]–		177
[(Ga)Ni(GaCp*)4]+	[B(ArCF3 )4]–	Other	Ni(GaCp*)4 + [FeCp2]+[WCA]–		178
[(Ga)Pt(GaCp*)4]+	[B(ArCF3 )4]–	Com	Pt(GaCp*)4 + [Ga2Cp*]+[WCA]–		179 and 180
[(Cp*Ga)4Rh{Ga(Me)}]+	[B(ArCF3 )4]–	Prot	(Cp*Ga)4Rh-(η1-Cp*GaMe) + [H(OEt2)2]+[WCA]–		181
[(Cp*Ga)4Rh{Ga(Me)-(py)}]+	[B(ArCF3 )4]–	Com	[(Cp*Ga)4Rh(GaMe)]+[WCA]– + py		181
[Ru(COD)(H)(GaCp*)3]+	[B(ArCF3 )4]–	Com	[Ru(COD)(H)(DMH)3]+[WCA]– + 3GaCp*		245
[Ru(GaCp*)4-{η3-(CH2)2C(Me)}]2+	[B(ArCF3 )4]–	Prot	Ru(GaCp*)3(TMM) + [H(OEt2)2]+[WCA]– TMM = η4-C(CH2)3		177
[{Ru(GaCp*)3-[(CH2)2C{CH2(μ-Ga)}]}2]+	[B(ArCF3 )4]–	Com	Ru(GaCp*)3(TMM) + [Ga2Cp*]+[WCA]–		177
[{(GaCp*)4Pt}{Pt(H)-(GaCp*)3}(μ-Ga)]2+	B(ArCF3 )4]–	Prot, Com	Pt(GaCp*)4 + [H(OEt2)2*]+[WCA]–		180
[Pt3(GaCp*)6(μ-Ga)]+	[B(ArCF3 )4]–	Other	Pt(GaCp*)4 + [FeCp2]+[WCA]– (substoichiometric)		178

^a Classification according to the introduction (Table 2).

Ligand substituted (CN = 3)

Tricoordinate borenium cations are not as electron deficient as the borinium cations and therefore more stable. Nonetheless, the cations can only be isolated in the solid state if chelating (e.g. phthalocyanine ¹¹⁵ and catecholborane ^116,117 ) or strongly σ-donating ligands (e.g. N-heterocyclic carbene I ^t Bu ⁷⁴ or hexaphenylcarbodiphosphorane ^118,119 ) are applied. For the synthesis and reactivity of the [BCl₂(I ^t Bu)]⁺ cation, the nature of the WCA is crucial. Hence and though the cation can be prepared in the presence of [AlCl₄]^–, [OTf]^– or [B(Ar^Cl)₄]^–, only the latter allows for a structure with no notable cation–anion contact. This leads to an increased reactivity of the [BCl₂(I ^t Bu)]⁺[B(Ar^Cl)₄]^– salt. ⁷⁴

Ligand substituted (CN = 4)

Some of the tetra-coordinated boronium cations directly derive from the corresponding borenium cations: i.e., the tricoordinate [PMAF–9BBN]⁺ cation (only stable in solution, as monitored by ¹¹B NMR spectroscopy) reacts with 1-MIM in a ligand exchange and addition reaction to form the tetra-coordinated [(1-MIM)₂(9BBN)]⁺ cation (Fig. 5). ²¹²

Fig. 5. Synthesis of the [(1-MIM)₂(9BBN)]⁺ boronium cation via a borenium cation precursor.

For the isolation of the discrete [BH₂(PR₂H)₂]⁺ cation, the nature of the WCA is again essential: compared to [OTf]^–, [B(Ar^CF₃ )₄]^– features no hydrogen bond with the cation, thus allowing for increased reactivities. ¹²⁰

Transition-metal substituted

The number of transition-metal substituted boron cations is much higher than the one of related alkyl-/aryl- or heteroatom substituted compounds. Numerous contributions have been made by Braunschweig and Aldridge and both authors recently reviewed the chemistry of transition-metal borylene complexes. ^3,5,104 The d-orbitals of the transition-metals allow for stabilizing σ- and π-interactions with the orbitals of boron (Fig. 6) and of all the ligands the FeCp(CO)₂/FeCp′(CO)₂/FeCp*(CO)₂ substituents protrude: e.g., various linear borinium cations, such as [CpFe(CO)₂B(NCy₂)]⁺, and borenium cations derived thereof, such as [(CpFe(CO)₂)B(NCy₂)(4-Pic)]⁺, have been isolated. ¹²

Fig. 6. (a) Orbital interaction between borylenes and transition-metal fragments; (b) and (c) exemplarily selected transition-metal substituted borinium cations.

Another notable substance class are the cationic T-shaped platinum boryl complexes that are usually accessible via salt metathesis reactions: e.g., [(Cy₃P)₂(MeCN)Pt(B O)]⁺ can be synthesized by reacting (Cy₃P)₂Pt(B O)(Br) with the halide abstracting reagent Ag⁺[B(Ar^CF₃ )₄]^–. ¹²¹ Employing a ferrocenyl ligand on the other hand, Braunschweig et al. were able to isolate a rare example of a structurally characterized boron dication: [FcB(Pic)₃]²⁺ (Fig. 7). ¹²²

Fig. 7. Synthesis of the boron dication [FcB(Pic)₃]²⁺ via bromide abstraction and subsequent complexation.

Multinuclear

Due to the pronounced electron deficiency of boron there are not many contributions to the field of cationic multinuclear boron-based rPBC. The neutral diborane [HB(μ-hpp)]₂ complex however, is an excellent precursor for hydride abstractions and via unexpected boron–boron coupling reactions the unprecedented tetraborane dication [B₄H₂(μ-hpp)₄]²⁺ was isolated (Fig. 8). ¹²³

Fig. 8. The tetraborane dication [B₄H₂(μ-hpp)₄]²⁺. The bonding properties in the rhomboid B₄ core of the product can be described as two B–B units connected by 3c-2e bonds, sharing a short diagonal.

Multinuclear transition-metal substituted

Compared to their mononuclear congeners, both the ligands and coordination modes in the multinuclear borinium, borenium and boronium cations are very similar: (i) linear in the [(CpFe(CO){B(NCy₂)})₂(μ-dmpe)]²⁺ complex, ¹²⁴ (ii) trigonal-planar in the [(Cy₃P)₂{Pt(B(Br))}₂(μ-Ph)]²⁺ dication ¹²⁵ and (iii) tetrahedral in [{(bipy)(Me)B}₂(μ-Fc)]²⁺. ^126,127 The aggregation usually occurs via bi-functional ligands like dmpe or via the transition-metal ligand itself (Fig. 9).

Fig. 9. Dicationic (a) [(CpFe(CO){B(NCy₂)})₂(μ-dmpe)]^{2+  124} and (b) [{(bipy)(Me)B}₂(μ-Fc)]^{2+  126,127} complexes.

Aluminum cations

Among the group 13 cations, the lower- and higher-coordinated derivatives of aluminum have been of significant interest as they feature increased Lewis acidities and ligand labilities, thus allowing for higher catalytic activities compared to their neutral analogs. ⁹⁵ While Atwood (1998) ¹⁰² and Dagorne (2008) ⁹⁵ have given a good overview on cationic aluminum species from a fundamental perspective, Sarazin and Carpentier (2015) ¹⁰ recently reviewed various discrete cationic aluminum complexes that are able to catalyze ring-opening polymerizations.

Alkyl or aryl substituted

The synthesis of di-coordinated alkyl complexes of aluminum [R₂Al]⁺ (R = Me, Et, 2,6-Mes₂C₆H₃) is only viable, if extremely weakly coordinating anions (e.g. borate ⁸⁷ and carboranes ¹²⁸ ) and/or bulky substituents ¹²⁹ are applied. In the case of “[Me₂Al]⁺” and “[Et₂Al]⁺”, the Lewis acidity of the aluminum cations is so significant that the latter feature distinct contacts to the corresponding WCAs and should therefore be described as ion-like compounds (Fig. 10). However, preliminary investigations showed that ion-like (Et₂Al)₂B₁₂Cl₁₂ is a very active initiator for the cationic polymerization of isobutylene. ¹³⁰

Fig. 10. The ion-like (Me₂Al)₂B₁₂Cl₁₂ salt. ⁸⁷ For clarity, all BCl moieties of the perchlorinated closo-dodecaborate that feature no contact to the “[Me₂Al]⁺” cation have been omitted.

The [(2,6-Mes₂C₆H₃)₂Al]⁺ cation on the other hand, is a discrete and therefore almost linear di-coordinate aluminum cation that features no contact to the WCA [B(C₆F₅)₄]^–. ¹²⁹ The occurrence of the highly Lewis acidic aluminum cation is attributable to the intrinsic stabilization effect of the 2,6-Mes₂C₆H₃ ligand: i.e., bending of the flanking Mes-moieties towards the aluminum center.

Cyclopentadienyl complexed

This class of compounds is to some extent related to the just mentioned alkyl substituted [(2,6-Mes₂C₆H₃)₂Al]⁺ complex. Hence, the Cp ligands are η⁵- but not σ-bonding, and allow for the synthesis of discrete aluminum cations with different WCAs as counterions: [(η⁵-Cp)₂Al]⁺[Al(OR^PF)₄]^–, ⁷⁹ [(η⁵-Cp′)₂Al]⁺[B(C₆F₅)₄]^{–  131} and [(η⁵-Cp*)₂Al]⁺ [MeB(C₆F₅)₃]^–. ^132,133 Moreover, the salts offer insights into the relationship between the nucleophilicity of Cp, Cp′ and Cp*, the corresponding WCAs and the resultant Lewis acidities and reactivities of the aluminum cations: i.e., with increasing nucleophilicity of the Cp ligands (Cp < Cp′ < Cp*) the WCAs can be less coordinating ([MeB(C₆F₅)₃]^– > [B(C₆F₅)₄]^– > [Al(OR^PF)₄]^–). The more interacting anions induce decreased Lewis acidities and lower reactivities of the aluminum cations for the initiation of olefin polymerizations: [(η⁵-Cp)₂Al]⁺ > [(η⁵-Cp′)₂Al]⁺ > [(η⁵-Cp*)₂Al]⁺. ⁷⁹

Ligand substituted (CN = 2)

The above mentioned σ-coordinated [R₂Al]⁺ complexes are either stabilized by intermolecular interactions with the corresponding WCAs or intramolecularly by two bulky terphenyl ligands. Within this context, Sekiguchi et al. were able to contribute another cationic di-coordinated, yet differently intramolecularly stabilized aluminum species: the [ ^t Bu₂MeSi–Al–Si ^t Bu₂–SiMe ^t Bu₂]⁺ cation. ¹³⁴ As supported by the solid-state structure and theoretical calculations, the stabilizing element is a σ–π hyperconjugation of the aluminum cation and the neighboring Si–Si σ bond (Fig. 11).

Fig. 11. Synthesis of the [ ^t Bu₂MeSi–Al–Si ^t Bu₂–MeSi ^t Bu₂]⁺ cation via demethylation and subsequent migration of a ^t Bu₂MeSi group.

Ligand substituted (CN = 3)

Tricoordinate aluminum cations are a bit less electrophilic than their di-coordinated congeners but nevertheless still very reactive. The few examples that have been reported, require chelating and sterically demanding β-diketiminate ligands, thus allowing for the successful synthesis of cationic [(β-diketiminate)Al–H]^{+  135} and [(β-diketiminate)Al–Me]^{+  136} complexes, respectively (Fig. 12).

Fig. 12. (a) The [(β-diketiminate)Al–H]⁺ cation derives from the reaction of a N-imidoylamidine ligand with AlH₃·NMe₂Et and [Ph₃C]⁺[B(C₆F₅)₄]^–. ¹³⁵ (b) The [(β-diketiminate)Al–Me]⁺ cation is formed by reacting the neutral precursor (β-diketiminate)Al(Me)₂ with the demethylating reactants [CPh₃]⁺[B(C₆F₅)₄]^– and B(C₆F₅)₃, respectively. ¹³⁶ .

Ligand substituted (CN = 4)

In their recent review on group 13 cations, Dagorne and Atwood state that “four-coordinate cations are most common … as they incorporate an electronically saturated metal center”. ⁹⁵ In all compounds the aluminum cations are coordinated in a tetrahedral fashion with at least one coordination site being occupied by a heteroatom (N, O, P). Moreover, the vast majority of aluminum cations are incorporated into heterocycles, which derive from chelating ligands, such as Pytsi, ¹³⁷ hpp, ¹³⁸ BOX, ^{139 i}Pr₂-ATI ^140,141 and SchNMe₂. ¹⁴² The usual synthesis routes are alkyl or hydride abstractions. On the other hand, there are a few examples where aluminum is coordinated by four discrete ligands: [Me₂Al(OEt₂)₂]⁺, ⁷⁷ [Me₂Al(THF)₂]⁺, ¹⁴³ [Me₂Al(NPhMe₂)₂]^{+  144} and [H₂Al(NMe₃)₂]^{+  145} (cf. Fig. 13 for the complex synthesis of the [H₂Al(NMe₃)₂]⁺ cation and the in situ generation of the corresponding WCA).

Fig. 13. Salt metathesis and hydroalumination reactions lead to the formation of the weakly coordinating carbaalanate cluster that allows for the synthesis of two equivalents of the [H₂Al(NMe₃)₂]⁺ cation. ¹⁴⁵ .

Ligand substituted (CN ≥ 5)

As mentioned in the previous sub-chapter, chelating ligands are of significant importance in terms of stabilizing cationic highly coordinated (CN ≥ 5) aluminum cations. Of all the different chelates, the Salen derivatives ¹⁴⁶ protrude, allowing for the synthesis of distorted square pyramidal/octahedral aluminum cations that interact with one ¹⁴⁷ or two ^148–151 equivalents of Lewis base, such as Et₂O and THF (Fig. 14).

Fig. 14. Octahedral or quadratic-pyramidal coordinated [SalenAl(Do) _n ]⁺ cations (Do = Et₂O, THF) with n = 1,2.

Multinuclear

A common structural motif of dicationic and dinuclear aluminum cations are the often centrosymmetric [Al₂O₂]-rhomboids ^{141,152–154} as seen in the recently reported [{(OSSO)Al}₂]²⁺ cation (Fig. 15). ¹⁵⁵

Fig. 15. In the [{(OSSO)Al}₂]²⁺ cation one aluminum atom is coordinated in a trigonal-bipyramidal and the other in a distorted-square-pyramidal fashion. The cationic species is a potential catalyst for the ring opening polymerization of propylene oxide. ¹⁵⁵ .

On the other hand, there are various dinuclear, yet singly charged aluminum cations in which the latter usually feature different coordination modes. Notable contributions to this field of research have been made by Jordan et al., such as the cationic aluminum aminotroponiminate ¹⁴¹ and amidinate ¹⁵⁶ complexes in Table 8.

AlCp* substituted

The coordination chemistry of low-valent group 13 organyls such as AlCp* to transition-metals is a growing field in inorganic chemistry, though more contributions were reported using the heavier homologue GaCp* (see below). Nonetheless, Fischer et al. were able to isolate the cationic [Rh(COD)(AlCp*)₃]⁺ complex by reacting [Rh(COD)₂]⁺ with three equivalents of AlCp*. ¹⁵⁷

Gallium cations

As mentioned above, gallium in its +I oxidation state is thermodynamically unstable and usually disproportionates into the metal and the +III ions. Notable contributions to the field of reactive gallium cations therefore allow for the stabilization of the +I oxidation state of gallium. ³¹

Alkyl or aryl substituted

The isolation of the linear di-coordinated [(2,6-Mes₂C₆H₃)₂Ga]⁺ cation ¹⁵⁸ was performed by Wehmschulte et al. as a test run for the above mentioned structurally related [(2,6-Mes₂C₆H₃)₂Al]⁺ cation. ¹²⁹ Hence, the bowl-shaped terphenyl substituents are potential ligands in terms of shielding highly electrophilic cations. Moreover, both syntheses were only possible due to the presence of very good WCAs, such as the [Li{Al(OR^HF)₄}₂]^– and [B(C₆F₅)₄]^– anion.

Cyclopentadienyl complexed

Partial protolysis of GaCp* with [H(OEt₂)₂]⁺[B(Ar^CF₃ )₄]^– yields the bipyramidal double-cone structured [Ga₂(η⁵-Cp*)]⁺ cation. ¹⁵⁹ The latter can be seen as a GaCp*-substituted “naked” Ga⁺ cation, thus reacting as a gallium(i) source with ligands such as DDP (Fig. 16).

Fig. 16. The [Ga₂(η⁵-Cp*)]⁺[B(Ar^CF₃ )₄]^– salt cleanly reacts as a gallium(i) source with ligands such as DDP.

The coordination mode of the Cp* ligands in the [(η¹-Cp*)(η³-Cp*)Ga]⁺ cation on the other hand differs. ¹⁶⁰ Hence, the originally expected η⁵,η⁵-ferrocene-like structure that was also observed for the aluminum analogue is likely perturbed by the more interacting [BF₄]^– counterion.

Arene complexed

Cationic arene complexes of univalent gallium are known for more than 30 years and Schmidbaur et al. have made notable contributions to this field of research. ^100,101 Yet, the reported compounds feature strong cation–anion interactions and are labile towards com- and disproportionations. More recently, Krossing et al. developed a simple oxidative route to [Ga(η⁶-arene) _n ]⁺ complexes of the weakly coordinating [Al(OR^PF)₄]^– anion with n = 2, 3 (Fig. 17). ^31,91,92 The arene complexes have proven to be a powerful starting material for further gallium(i) chemistry (e.g. various ligand exchange reactions) but also highly efficient catalyst for the polymerization of isobutylene. ^7,8,99

Fig. 17. Oxidative access to [Ga(η⁶-arene) _n ]⁺ complexes with n = 2, 3(R = F, Me).

Ligand substituted (CN = 2)

The arene ligands in the [Ga(η⁶-arene) _n ]⁺ cations with n = 2, 3 can be substituted by electron-richer analogues. In addition, σ-donating ligands such as carbenes IR (R = Pr, Mes) ¹⁶¹ or phosphines P ^t Bu₃ ¹⁶² can also be applied, yielding bent [Ga(IR)₂]⁺ and [Ga(P ^t Bu₃)₂]⁺ complexes (cf. the stereoactive electron lone pair at the gallium(i) cation). Another notable di-coordinated gallium(iii) cation is the linear [ ^t Bu₃Si–Ga–Si ^t Bu₃]⁺ complex (Fig. 18), which could be isolated in the presence of the [Al(OR^PF)₄]^– WCA, but not the simple [GaCl₄]^– anion. ¹⁶³

Fig. 18. Molecular structure of the [ ^t Bu₃Si–Ga–Si ^t Bu₃]⁺ cation. A. Budanow, T. Sinke, J. Tilmann, M. Bolte and M. Wagner, Two-coordinate gallium ion [ ^t Bu₃Si–Ga–Si ^t Bu₃]⁺ and the halonium ions [ ^t Bu₃Si–X–Si ^t Bu₃]⁺ (X = Br, I): sources of the supersilyl cation [ ^t Bu₃Si]⁺, Organometallics, 2012, 31, 7298–7301. Data from this reference were used to draw this figure and the hydrogen atoms were omitted for clarity. ¹⁶³ .

Ligand substituted (CN = 3)

This class of tricoordinate gallium(i) cations again derives from the above mentioned [Ga(η⁶-arene) _n ]⁺ cations with n = 2, 3. The coordination mode for the gallium(i) cations is trigonal-pyramidal due to the stereoactive lone pair electrons. Besides sterically less demanding phosphines, N-heterocylic arenes like pyrazine and DTBMP (a σ-, and not a π-donating ligand, proving its perception of being non-nucleophilic wrong) were also applied as potential ligands. ¹⁶⁴ Due to the bifunctionality of pyrazine, both the monomeric [Ga(pyrazine)₃]⁺ complex and the one-dimensional coordination polymer [{Ga(μ-pyrazine)₂(η¹-pyrazine)}⁺]_∞ were isolated (Fig. 19).

Fig. 19. (a) Monomeric [Ga(pyrazine)₃]⁺ complex and (b) one-dimensional coordination polymer [{Ga(μ-pyrazine)₂(η¹-pyrazine)}⁺]_∞. The propagation of the polymer into the second dimension was not possible as each cationic strand is surrounded by strands of the corresponding [Al(OR^PF)₄]^– anions. ¹⁶⁴ .

Ligand substituted (CN = 4)

Using the BOX ligand, Dagorne et al. isolated tetra-coordinate neutral gallium complexes. ¹⁶⁵ The latter were easily ionized by applying [CPh₃]⁺[B(C₆F₅)₄]^– or B(C₆F₅)₃ in NMe₂Ph. Interestingly, the trityl cation functions as hydride and B(C₆F₅)₃ as methyl abstracting reactant (Fig. 20).

Fig. 20. Hydride vs. methyl abstraction of neutral BOX ligated gallium complexes. ¹⁶⁵ .

Ligand substituted (CN ≥ 5)

Cationic penta- and hexa-coordinated gallium complexes are synthesized via protonation ^166,167 or complexation. ¹⁶⁸ Within this context, the [Ga([18]crown-6)(C₆H₅F)₂]⁺ complex is of special interest as the gallium(i) cation features no contact to the corresponding [Al(OR^PF)₄]^– anion and the C₆H₅F ligands coordinate in different fashions (Fig. 21). ¹⁶⁸

Fig. 21. The [Ga([18]crown-6)(η⁶-/η¹-C₆H₅F)₂]⁺ cation. The η⁶- and η¹-coordination modes could be an indication for a stereoactive lone pair on the side of the weaker and only η¹-coordinated C₆H₅F molecule. ¹⁶⁸ .

As the N-heterocyclic arenes are potential ligands for univalent gallium (see above), Krossing et al. additionally reacted the chelating bipy with the [Ga(η⁶-C₆H₅F)₂]⁺ complex. Instead of witnessing a simple ligand exchange reaction, they isolated the paramagnetic and distorted octahedral [Ga^III{(bipy)₃}˙]²⁺ complex due to the non-innocence of the bipy ligand. ¹⁷ This is reminiscent to transition metal chemistry where for example the [Ru^III{(bipy)₃}˙]²⁺ complex features similar bonding.

Transition-metal substituted

Similar to the transition-metal substituted boron cations, CpFe(CO)₂ (FP) and Cp*Fe(CO)₂ (FP*) are the most important ligands in terms of stabilizing di-, tri- and tetra-coordinated gallium cations: cf. the [(FP*)₂Ga]⁺, ¹⁶⁹ [(FP*)₂Ga(4-Pic)]⁺, ¹⁷⁰ [(FP*)Ga(phen)(Y)]⁺ (Y = Cl, S ^p Tol) ¹⁷¹ cations (Fig. 22).

Fig. 22. (a) The linear di-coordinated cation [(FP*)₂(μ-Ga)]⁺ derives from a salt metathesis of (FP*)₂GaCl and Na⁺[B(Ar^CF₃ )₄]^–. The Fe–Ga–Fe moiety features a significant π bonding component (population analysis). ¹⁶⁹ (b) The [(FP*)₂(μ-Ga)(4-Pic)]⁺ cation is an addition product of [(FP*)₂Ga]⁺ and 4-Pic and the second structurally characterized complex containing a cationic tricoordinate gallium centre. ¹⁷⁰ (c) Applying the chelating phen ligand, Ueno et al. isolated the tetra-coordinated [(FP*)Ga(phen)(Y)]⁺ (Y = Cl, S ^p Tol) cations, i.e. the first transition-metal complex with a thiolate group on the gallium atom. ¹⁷¹ .

Multinuclear

There are not many contributions to this field of research and some cationic multinuclear gallium complexes are a product of hydrolysis. ^172,173 Two remarkable examples however are the dinuclear [(η⁶-C₆H₅F)Ga(μ-η⁶-m-TP)₂-Ga(η⁶-C₆H₅F)]²⁺ complex ^8,99 in which the gallium(i) cations are solely π-coordinated by arene ligands as well as the σ-coordinated amidinate-bridging [{ ^t BuC(NⁱPr)₂}GaMe{ ^t BuC(NⁱPr)₂}GaMe₂]⁺ cation. ¹⁵⁶

Multinuclear transition-metal substituted

A notable class of contributions are the β-diketiminate/THF coordinated gallium cations that can be bridged by a gold atom ¹⁷⁴ or a {ZnClTHF}₂-rhomboid. ¹⁷⁵ Reaction of Cp*Fe(η⁵-P₅) with the [Ga(o-C₆H₄F₂)₂]⁺ complex on the other hand resulted in aggregation and the formation of a cationic one-dimensional coordination polymer (Fig. 23). ¹⁷⁶

Fig. 23. Reaction of [Ga(o-C₆H₄F₂)₂]⁺ and Cp*Fe(η⁵-P₅) results in aggregation and formation of a cationic one-dimensional coordination polymer.

GaCp* substituted

As of today, GaCp* is a widely used ligand concerning cationic transition-metal complexes, thus leading to an enormous variety of cationic gallium species. This area of research has been intensively reviewed by Fischer et al. ¹¹⁰ and we would like to refer to the multiple entries in Table 8 of this review. Yet, some of the compounds also include “naked” and bridging gallium atoms: e.g. [(Ga)Ru(PCy₃)₂(GaCp*)₂]⁺, ¹⁷⁷ [(Ga)M(GaCp*)₄]⁺ (M = Ni, ¹⁷⁸ Pt ^179,180 ), [(Cp*Ga)₄Rh{Ga(Me)}]⁺, ¹⁸¹ [(Cp*Ga)₄Rh{Ga(Me)(py)}]⁺, ¹⁸¹ [{Ru(GaCp*)₃-[(CH₂)₂C{CH₂(μ-Ga)}]}₂]⁺, ¹⁷⁷ [{(GaCp*)₄Pt}{Pt(H)-(GaCp*)₃}(μ-Ga)]²⁺. ¹⁸⁰ Contrary to GaCp* (a strong σ-donor and weak π-acceptor, cf. similarity to the boron related compounds in Fig. 6), the “naked” gallium cations function as pure acceptor ligands, with significant components of σ- and π-symmetry contributing to the M–Ga linkages. ^179,180

Indium cations

Compared to the lighter homologue gallium, well-defined indium(i) halides exist, though they are practically insoluble in organic solvents. The synthesis of In⁺[OTf]^– by Macdonald et al. as a soluble alternative is therefore an important development concerning the indium(i) chemistry. ¹⁸²

Cyclopentadienyl complexed

Using the just mentioned In⁺[OTf]^– salt as starting material and reacting it with manganocene, the inverse sandwich complex [In₂(η⁵-Cp)]⁺ was successfully synthesized. ¹⁸³ Interestingly, the counterion is the complex [Cp₃In^III–Cp–In^IIICp₃]^– ion, deriving form a partial oxidation of the starting material. The formation of the mixed valence species seems to be preferred over an alternative indium(ii) species. Reacting InCp* (a hexamer in the solid state) with a mixture of B(C₆F₅)₃ and H₂O·B(C₆F₅)₃, the first indium-based triple-decker cation [(η⁶-Tol)In(μ-η⁵-C₅Me₅)In(η⁶-Tol)]⁺ was formed. ¹⁸⁴ Reducing the size of the counterion from [(C₆F₅)₃BO(H)B(C₆F₅)₃]^– to [B(C₆F₅)₄]^– on the other hand, results in the formation of the inverse sandwich complex [In₂(η⁵-Cp*)]⁺ in which the indium(i) cations are not capped by toluene molecules but rather interact with the [B(C₆F₅)₄]^– anions (Fig. 24). ¹⁸⁵

Fig. 24. Reducing the size of the counterion from [(C₆F₅)₃BO(H)B(C₆F₅)₃]^– to [B(C₆F₅)₄]^– “squeezes” the toluene molecules from the triple-decker cation, yielding the inverse sandwich complex [In₂(η⁵-Cp*)]⁺.

Arene complexed

By reacting elemental indium with Ag⁺[Al(OR^PF)₄]^–, Krossing et al. expanded the above mentioned oxidative route to gallium(i) salts towards the synthesis of [In(arene) _n ]⁺ complexes with n = 2, 3. ¹⁶² Identical compounds can also be synthesized by using the salt metathesis reactions of Scheer et al., with insoluble InCl as starting material. ¹⁸⁶

Ligand substituted (CN = 2)

These [In(arene) _n ]⁺ complexes with n = 2, 3 are an ideal starting material for further indium(i) chemistry: e.g. the arene ligands can be substituted for N-heterocyclic carbenes such as IPr. ¹⁶¹ Salt metathesis reactions on the other hand are still very important: i.e., using the isosteric and isoelectronic terphenyl Mes₂py ligand, Aldridge et al. were able to isolate mixed-leptic [In(Mes₂py)(η⁶-C₆H₅F)]⁺ (both σ- and π-coordinated) and homo-leptic [In(Mes₂py)₂]⁺ complexes (only σ-coordinated, though the flanking mesityl rings of the Mes₂py ligands also partly π-coordinate). ¹⁸⁷ The latter features an indium(i) cation wholly encapsulated by two Mes₂py ligands and remarkably long In–N distances, which the authors explain with an energy mismatch between the (low lying) pyridine ligand donor and (high energy) metal acceptor orbitals.

Ligand substituted (CN = 3)

Besides complexation and ligand exchange reactions of In⁺[OTf]^– and [In(arene) _n ]⁺ (n = 2, 3) with ligands such as bis(imino)pyridines ^188,189 and PPh₃, ¹⁶² tricoordinate indium cations can also be isolated by thermolysis of [{ⁱPr₂-ATI(CPh₃)}InMe₂]⁺, a cationic tetra-coordinated indium precursor (Fig. 25, conversion of (a) to (b)). ¹⁹⁰

Fig. 25. Formation of tetra- and tricoordinate cationic diimine substituted indium complexes: (a) [{ⁱPr₂-ATI(CPh₃)}InMe₂]⁺, (b) [(ⁱPr₂-ATI)InMe]⁺ and (c) [(ⁱPr₂-ATI)In(Me)(NMe₂Ph)]⁺. For each complex, the counterion is [B(C₆F₅)₄]^–.

Ligand substituted (CN = 4)

The cationic diimine [{ⁱPr₂-ATI(CPh₃)}InMe₂]⁺ complex was synthesized by reacting the neutral (ⁱPr₂-ATI)InMe₂ precursor with the ionizing [Ph₃C]⁺[B(C₆F₅)₄]^– salt (Fig. 25). ¹⁹⁰ Surprisingly, the latter does not function as methyl abstracting reactant but rather adds to the C5 carbon of (ⁱPr₂-ATI)InMe₂. Reacting (ⁱPr₂-ATI)InMe₂ with the protonating [HNMe₂Ph]⁺[B(C₆F₅)₄]^– on the other hand, results in CH₄ formation and the labile adduct [(ⁱPr₂-ATI)In(Me)(NMe₂Ph)]⁺ (Fig. 25). ¹⁹⁰

Ligand substituted (CN ≥ 5)

Compared with the lighter homologues aluminum and gallium, indium shows a tendency to expand its coordination sphere. ^95,102 Protonolysis of the neutral In(CH₂SiMe₃)₃ complex in THF therefore yields a penta-coordinated indium cation: [In(CH₂SiMe₃)₂(THF)₃]⁺. ¹⁹¹ Moreover, In⁺[OTf]^{–  192,193} and [In(arene) _n ]^{+  168} (n = 2, 3) can be reacted with the crown ether [18]crown-6, yielding cationic indium complexes with similar structures to the gallium congener (cf. Fig. 21) and strong anion–cation interactions in the case of the [OTf]^– anion. Reacting In⁺[OTf]^– with [15]crown-5 on the other hand, the sandwich complex [In([15]crown-5)₂]⁺ was isolated. ¹⁹⁴

Transition-metal substituted

The class of cationic transition-metal substituted indium compounds very much relates to the related gallium structures: i.e., the [InPt(PPh₃)₃]⁺ complex with a “naked” Pt-substituted indium cation ^179,180 as well as the di- and tricoordinate [(FP*)₂In]⁺ and [(FP*)₂In(THF)]⁺ complexes. ¹⁹⁵ Reacting the chelating phen ligand with the [In(C₆H₅F)₂]⁺ complex in the presence of silver salt, Krossing et al. isolated the silver bound indium dication ¹⁷ [(phen)₂In–Ag(η³-C₆H₅F)]²⁺ that is related to the [InPt(PPh₃)₃]⁺ complex. ^179,180 In this complex the tetragonal-pyramidal [In(phen)₂]⁺ cation reacts as a Lewis basic donor (cf. the stereoactive 5s lone pair at indium), while the [Ag(η³-C₆H₅F)]⁺ complex is the corresponding Lewis acidic acceptor.

Multinuclear

For the synthesis of multinuclear indium cations, the [In(arene) _n ]^{+  168} complexes with n = 2, 3 are a powerful starting material. Hence a dicationic [{(PPh₃)₃In}₂(μ-PPh₃)]²⁺ complex in which one PPh₃ ligand bridges both indium(i) cations was isolated. ¹⁶² Applying the non-innocent and chelating bipy and phen ligands on the other hand, Krossing et al. surprisingly isolated the first cationic tri- and tetra-nuclear indium clusters: [In₃(bipy)_5–6]³⁺ and [In₄(Do)₆]⁴⁺ (Do = phen, bipy) (Fig. 26). ¹⁷ This result very much differs from the above mentioned synthesis of the paramagnetic [Ga^III{(bipy)₃}˙]²⁺ complex and can be attributed to the higher redox-stability of indium compared to gallium. In addition and to our knowledge, these are the first higher charged clusters that have been reported to this day: i.e., for cluster formations usually reductive syntheses are applied, yielding neutral and anionic clusters.

Fig. 26. Cationic (a) [In₃(bipy)_5–6]³⁺ and (b) [In₄(Do)₆]⁴⁺ (Do = bipy, phen) complexes synthesized via ligand exchange reactions and aggregations.

Thallium cations

In contrast to the lighter homologues, thallium's thermodynamic most stable oxidation state is +I. Hence, various syntheses of unsubstituted thallium(i) cations of different WCAs have been reported: i.e., the protonation of TlOEt using [H(OEt₂)₂]⁺[B(Ar^CF₃ )₄]^–/[B(C₆F₅)₄]^–, ^196,197 the Lewis acid base reaction of Tl⁺[OTeF₅]^– and B(OTeF₅)₃ ¹⁹⁸ and the salt metathesis of TlF and Li⁺[Al(OR^HF/PF)₄]^–. ^199,200 The thallium(i) salts are relatively stable (cf. the silver congeners decompose upon exposure to light) and mainly used as reactants to introduce WCAs (e.g. salt metathesis reactions). Tl⁺[Al(OR^HF)₄]^– could only be isolated, if the precursors were applied in an exact 1 : 1 stoichiometry. An excess of TlF however, led to the formation of the cationic multinuclear [Tl₃F₂Al(OR^HF)₃]⁺ complex. ²⁰⁰

Arene complexed

Various cationic thallium(i) arene complexes have been reported. While the di- and tricoordinate [Tl(η⁶-arene) _n ]⁺ complexes (arene = C₆H₅Me, n = 2, 3; ²⁰¹ Mes, n = 2; ²⁰² C₆Me₆, n = 2 ²⁰³ ) are structurally related to the lighter homologues, C₆Me₆ additionally allows for the first mono-coordinated [Tl(η⁶-C₆Me₆)]⁺ complex (DFT calculations gave a remarkable Tl–C₆Me₆ π-bonding energy of 163 kJ mol^–1). ²⁰⁴

Ligand substituted (CN = 2)

Reacting Tl⁺[OTeF₅]^– with B(OTeF₅)₃ in 1,2-dichloroethane, the solvent functions as chelating ligand, thus forming the five-membered TlCl₂C₂-ring in [Tl(1,2-Cl₂C₂H₄)]⁺ (after silver and ruthenium, thallium was at that time the third reported metal atom coordinated by a simple chlorocarbon). ¹⁹⁸ By contrast, from CH₂Cl₂ the “naked” Tl⁺[B(OTeF₅)₄]^– salt was isolated.

Ligand substituted (CN = 3)

Similar to the lighter homologues gallium and indium, the [Tl(η⁶-C₆H₅R)₂]⁺ (R = F, Me) bent-sandwich complexes can interact with N-heterocyclic ligands such as Mes₂Py, thus forming tricoordinate [Tl(Mes₂py)(η⁶-C₆H₅R)₂]⁺ complexes. ¹⁸⁷ On the other hand tri-dentate chelating ligands like timtmb ^{tBu  205} and bis(imino)pyridines ²⁰⁶ can be applied to isolate tricoordinate thallium(i) cations (Fig. 27).

Fig. 27. Both the (a) [Tl(timtmb ^tBu)]⁺ and (b) [{ArN CPh}₂(NC₅H₃)Tl]⁺ complexes derive from Tl⁺[OTf]^– and are synthesized via complexation reactions of the corresponding ligands. In addition, the inverted sandwich structure (c) [{{ArN CPh}₂(NC₅H₃)Tl}₂(μ-η⁶-C₆H₅R)]²⁺ (Ar = 2,6-Et₂C₆H₃, 2,5- ^t Bu₂C₆H₃; R = H, Me) was isolated.

Ligand substituted (CN = 4)

The protonation of TlOEt with [H(OEt₂)₂]⁺[H₂N{B(C₆F₅)₃}₂]^– in Et₂O yielded the tetrahedral coordinated cationic [Tl(OEt₂)₄]⁺ complex, which shows no contact to the corresponding WCA. ²⁰³

Ligand substituted (CN ≥ 5)

2,5-Bis(2-pyridyl)-1-phenylphosphole (NPPh) exhibits a rich coordination chemistry towards thallium(i) cations and dependent on the nature of the solvents and WCAs, different structures were obtained: i.e., reacting Tl⁺[Al(OR^PF)₄]^– with NPPh in CH₂Cl₂/C₆H₅Me, the tetra-coordinated and C₆H₅Me-capped [Tl(NPPh)₂(η⁶-C₆H₅Me)]⁺ complex formed, whereas in CH₂Cl₂/n-pentane the dinuclear and dicationic [Tl₂(NPPh)₄]²⁺ was isolated (Fig. 28). If Tl⁺[PF₆]^– was applied as starting material a coordination polymer with strong cation–anion interactions was formed.

Fig. 28. Solvent effects on the formation of cationic thallium(i) complexes of NPPh. (a) If toluene is applied, the solvent-stabilized penta-coordinated [Tl(NPPh)₂(η⁶-C₆H₅Me)]⁺ complex forms. (b) If non-coordinating n-pentane is applied two [Tl₂(NPPh)₂]⁺ cations aggregate via their phenyl substituents, forming the dinuclear and dicationic [Tl₂(NPPh)₄]²⁺ complex.

An even higher coordinated thallium cation is the [Tl([18]crown-6)]⁺ complex, which features a similar structure as the [18]crown-6 complexes of the lighter homologues gallium and indium. ²⁰³

Transition-metal substituted

Reacting the above mentioned [Tl(η⁶-C₆H₅Me) _n ]⁺ complexes (n = 2, 3) with FeCp₂, Sarazin et al. were able to isolate the [Tl₂(FeCp₂)₃]⁺{[H₂N{B(C₆F₅)₃}₂]^–}₂ salt that contains the mono- and di-coordinated [Tl(FeCp₂) _n ]⁺ complexes with n = 1, 2 in a 1 : 1 ratio. ²⁰³ Increasing the amount of FeCp₂ from 1 to 2.2 equivalents, only the [Tl(FeCp₂)]⁺ complex was isolated. ²⁰⁴ In contrast to the lighter homologues, the reaction of Cp*Fe(η⁵-As₅) with Tl⁺[PF₆]^– and Li⁺[FAl{OC₆F₁₀(C₆F₅)}₃]^– did not result in aggregation and formation of a cationic one-dimensional coordination polymer, but rather yielded the pseudo-trigonal-planar [Tl{(η⁵-As₅)FeCp*}₃]⁺ complex. ¹⁷⁶ Performing a similar chemistry in the presence of the very good WCA [Al(OR^PF)₄]^– however, one-dimensional polymers were isolated (cf. Fig. 23), proving the importance of the WCA. ²⁰⁷ Reacting the neutral Pt(CH₂Ph)Cl(PCH₂-ox) complex with Tl⁺[PF₆]^–, Braunstein et al. did not isolate any chloride abstraction product but a “trapped” thallium(i) cation: the cationic [{P(Ph₂)CH₂ox}(Cl)(Tl)Pt-CH₂Ph}]⁺ complex. ²⁰⁸ Herein, the ligand functions as a chelate and interacts with thallium via a Pt–Tl bond and a η⁶-benzyl coordination (Fig. 29).

Fig. 29. The first fully characterized metal–metal bonded Tl–Pt–Cl complex. If Ag⁺[OTf]^– and Ag⁺[BF₄]^– is applied, the expected chloride abstraction takes place. ²⁰⁸ .

Multinuclear

Some of the cationic multinuclear thallium complexes have already been mentioned in the text above. A further example is the [{Tl(β-triketimine)}₂]²⁺ complex that features Tl–η⁶-aryl and weak thallophilic interactions, allowing to overcome the Coulomb repulsion of both cations (cf. Fig. 28). ¹⁴ The reaction of the P _n -ligand {CpMo(CO)₂}₂(P₂) with Tl⁺[Al(OR^PF)₄]^–, yields the dicationic [Tl₂({CpMo(CO)₂}₂)₆]²⁺ complex that features a distorted Tl₂P₄ ring (Fig. 30). ²⁰⁷

Fig. 30. Formation of the [Tl₂({CpMo(CO)₂}₂)₆]²⁺ complex.

Reacting RuCl₂(DMeOPrPE)₂ with Tl⁺[PF₆]^– an “arrested” chloride abstraction occurs. ²⁰⁹ In the resultant one-dimensional coordination polymer, the thallium(i) cations are coordinated in an unusual octahedral fashion with a stereoactive 6s² lone pair at thallium.

Group 14 cations

Already in 1887, Henderson described the synthesis of trityl malonate starting from triphenylmethyl bromide and ethylic sodiomalonate ²⁴⁶ and 15 years later, Bayer and Villiger realized that the yellow color of a solution of triphenylmethane in concentrated sulfuric acid is the result of the formation of a carbocation. ²⁴⁷ Despite these early discoveries, it took another 63 years until the structure of this cation could be determined. ²⁴⁸ While the first structure determination succeeded with [ClO₄]^–, the structure of the trityl cation is nowadays known with several different anions (e.g. ref. 249) and it has become a common reagent for the generation of various other cations. Only one earlier structure determination of a carbocation was published: the structure of triphenylcyclopropenium perchlorate in 1963. ²⁵⁰ Since then, many rPBCs of group 14 were synthesized and characterized. Carbocations have drawn a lot of interest, due to their role as intermediates in organic chemistry. Silylium ions are more electrophilic and more reactive than their carbon analogues, so that the structural characterization of a truly free silylium ion was only achieved in 2002 ²⁴ and is still in the focus of interest. But also the heavier elements of group 14 were subject to extensive research and today a multitude of interesting rPBCs are known, part of which have been reviewed in the articles included with Table 4.

Table 4. Review articles including cationic group 14 compounds.

Year	Title	Ref.
1995	Modern approaches to silylium cations in condensed phase	251
2005	Cations of group 14 organometallics	252
2005	Carbon, silicon, germanium, tin and lead	253
2010	Silylium ions in catalysis	254
2010	H+, CH3 +, and R3Si+ carborane reagents: when triflates fail	255
2011	N-heterocyclic carbene analogues with low-valent group 13 and group 14 elements: syntheses, structures, and reactivities of a new generation of multitalented ligands	256
2013	Catenated compounds – group 14 (Ge, Sn, Pb)	257
2015	Cations and dications of heavier group 14 elements in low oxidation states	258

Carbon

Homopolyatomic and cluster cations

In group 14, carbon is the only element for which homopolyatomic cations are known in condensed phase. While [C₇₆]⁺, ⁴⁴ and [C₆₀]⁺, ²⁵⁹ are already known for more than ten years, there is only one more recently published compound of that class. In [C₆₀]²⁺([AsF₆]^–)₂, ²⁶⁰ the cations build a 1D polymeric structure, in which the [C₆₀]²⁺ cations are connected alternatingly by single C–C bonds and four-membered carbon rings. Along with the before mentioned [C₆₀]⁺, the protonated buckminsterfullerene [HC₆₀]⁺ was published ²⁵⁹ and by oxidation of the [C₅₉N]₂ dimer, [C₅₉N]⁺ was synthesized and structurally characterized. ²⁶¹

Carbonium ions

As mentioned before, the classification according to onium-, enium and inium-cations is not always consistent and in literature, the term carbonium ion is often used to describe what is mostly a carbenium ion. A prototype for a carbonium ion is the 2-norbornyl cation, whose structure has been controversially discussed. In 2013, 49 years after its first preparation under stable ion conditions, ²⁶² its structure could be determined by scXRD. ⁸⁶ This finally provided a crystallographic proof that the 2-norbornyl cation adopts the non-classical structure ( Fig. 31). It remains the only structurally characterized non-classical carbonium ion. Substituted relatives exhibit distinctly distorted structures that are better classified as carbenium ions. ²⁶³

Fig. 31. Non-classical vs. classical structure of the 2-norbornyl cation.

Carbenium ions

The first simple structurally characterized alkyl cation, was the tert-butyl cation with [Sb₂F₁₁]^– as the counterion, ²⁶⁴ and later also with [HCB₁₁Me₅Cl₆]^–. ²⁶⁵ In the same publication, two more carbocations with slight variations in the alkyl chains were presented (Fig. 32). ²⁶⁵ Recent additions include the super-acidic room temperature ionic liquid [(CH₃)₃C]⁺[Al₂Br₇]^{–  266} and an additional structure of the tert-butyl cation with the [HCB₁₁Cl₁₁]^– anion. ²⁶⁷ In 2000, ion-like (CH₃)₂CF(AsF₆) was the first structural characterized example of a fluorinated carbocation and was published together with a higher substituted variant. ²⁶⁸ In both compounds, each cation is stabilized by two stronger contacts to the anion. The higher substituted [(m-CF₃C₆H₄)(C₆H₅)CF]⁺ derivative, contains the less coordinating [As₂F₁₁]^– anion in the structure with only weak interaction between the ions. ²⁶⁸ With [HCB₁₁I₁₁]^–, two more fluoro-substituted carbocations and one with fluorine substituted aryl residues could be isolated (see Fig. 32). ⁵⁶ Apart from [CF₃]⁺, all [CX₃]⁺ cations are now synthesized and structurally characterized (see Fig. 32). First, [CI₃]⁺[Al(OR^PF)₄]^– was published in 2003 ²⁶⁹ and shortly after [CCl₃]⁺ and [CBr₃]⁺ with [Sb(OTeF₅)₆]^– as the counterion. ²⁷⁰ In addition, the latter was used to stabilize related [C(OTeF₅)₃]⁺. ²⁷⁰ Later, also [CCl₃]⁺ and [CBr₃]⁺ were synthesized with the [Al(OR^PF)₄]^– and the [(R^PFO)₃Al–F–Al(OR^PF)₃]^– counterions. ²⁷¹ In all of those compounds containing [CX₃]⁺ cations, still some weak interactions between cation (mainly halogen atoms) and anion exist. These interactions are weaker between [Br-C(SBr)₂]⁺ and the mentioned alkoxyaluminate, due to delocalization of the charge. ¹¹⁷ Although comparable, far stronger interaction between cation and anion was found in [(MeO)(MeS)CSH]⁺[SbF₆]^–. ²⁷² However, there is no close contact between the carbon atom and the fluorine atoms of the anion. Instead, the anion forms hydrogen bonds to the thiol group of the cation.

Fig. 32. Structurally characterized carbenium ion salts.

In 2004, the structure of the benzonorbornenyl cation was published, with an intramolecular stabilization of the cationic center by the aromatic ring. ²⁷³ Intermolecular stabilized carbenium cations are known of the [CI₃]⁺ with the weak bases PX₃ (X = Cl, Br, I) and AsI₃ (Fig. 33). ²⁷⁴ Only two related vinyl cations are known (see Fig. 33). ^275,276 Both are β-substituted by two silyl groups, which help to stabilize the positive charge.

Fig. 33. Structurally characterized ligand-stabilized carbenium ions and vinyl cation salts.

Delocalized (cyclic) carbocations

Only shortly after the first structural characterization of an alkyl cation, the first structure determination of an arenium ion – [C₆Me₇]⁺[AlCl₄]^– – was published. ²⁷⁷ To date, more structurally characterized arenium ions with several WCAs are known (Fig. 34).

Fig. 34. Protonated and methylated structurally characterized arenium ion salts.

An exception is the radical cation [C₆F₆]˙⁺ in the solid state structures with [Sb₂F₁₁]^– and [Os₂F₁₁]^–: it yields two different forms. ²⁷⁸ One cation can be described as a quinoidal cation and the other as a bisallyl cation (see Fig. 35) and both are separated by a barrier of around 13 kJ mol^–1 according to calculations.

Fig. 35. Lewis structures of the canonical forms of the quinoid and the bisallyl cationic form of [C₆F₆]˙⁺.

Shortly after the publication of the radical cation of the hexafluorobenzene, some more related structures were presented. Among them, the other perhalogenated benzene radical cations ^279,280 and some partially and mixed substituted analogs, including the [C₆F₅–C₆F₅]˙^{–  280,281} (Fig. 36). The only other example displaying both a quinoidal and a bisallyl cationic form is [2,4,6- ^t Bu₃C₆H₂NH₂]˙⁺. ²⁸² At 123 K, this cation adopts the bisallylic structure but upon heating, a transition to the quinoidal form occurs.

Fig. 36. Structurally characterized substituted benzene radical cation salts.

A different type of delocalized cations are the allyl cations amongst which the cyclopropenyl cations take a special position. Already since 1986, two examples, [(Cy)₃C₃]⁺ and [(Cy)₂(Ph)C₃]⁺, are known ²⁸³ and in the same year, an allyl cation stabilized by an hydroxyl group has been published (Fig. 37). ²⁸⁴ In 2002, the structure of [C₅Me₅H₂]⁺ was determined although it was by mistake addressed as an [C₅Me₅]⁺ cation, probably due to its unexpected formation during the reaction of C₅Me₅H with [Ph₃C]⁺. ²⁸⁵ Finally a silyl stabilized allyl cation was characterized, which formed via an interesting mechanism that starts with the formation of a silylium cation (Fig. 37). ²⁸⁶

Fig. 37. Structurally characterized delocalized cation salts.

Ion-like carbon compounds

As mentioned before, in the analog structure of [(m-CF₃-C₆H₄)(Ph)CF]⁺ with [AsF₆]^– instead of [AsF₁₁]^–, stronger interactions to the anions are present. ²⁶⁸ The same applies to the related Me₂CF(AsF₆). ²⁶⁸ Also known is the ion-like (Me₂CH)(HCB₁₁Me₅Br₆), which displays a covalent C–Br distance of about 210 pm. ²⁸⁷ Along with the latter, the preparation of H₃C(HCB₁₁Me₅Br₆) and H₃C(HCB₁₁Me₅Cl₆) was reported, but no structural data from XRD was presented. In 2010, the strongly methylating ion-like Me₂B₁₂Cl₁₂ was structurally characterized with a C–Cl bond length of 182 pm. ²⁸⁸

Silicon

Silylium ions (CN = 3)

Silylium ions are certainly amongst the most electrophilic cations known and thus exhibit an enormous Lewis acidity. Most of them are either stabilized by bulky ligands, or display a strong interaction with the corresponding WCA and have therefore to be categorized as ion-like compounds. In addition, the first claimed “stable silyl cation” [Et₃Si]⁺ in 1993 contained a coordinating toluene ligand – a feature typical for many silylium ions. ²⁸⁹ In order to obtain a truly tricoordinate silylium ion without stabilization through the anion or an additional ligand, bulkier substituents were needed. Hence, the first structurally characterized compound featuring a free silylium ion was [Mes₃Si]⁺[HCB₁₁Me₅Br₆]^{–  24} and in 2013, ([Pemp₃Si]⁺)₂[B₁₂Cl₁₂]^2– was published (Fig. 38). ²⁹⁰ The latter was afterwards also synthesized and characterized with [Al(OR^PF)₄]^–. ²⁹¹ In all three structures, the cation has no closer contacts to the anion.

Fig. 38. Structurally characterized tricoordinate silylium ions.

Delocalized cyclic cations

Despite the early characterization of cyclopropenyl cations, the first example for a comparable silicon ion was published only in 2000 (Fig. 39, left). ²⁹² In this compound, however, it is not the three-membered silicon-ring with a delocalized π-system but rather a silicon butterfly with one Si–Si–σ-bond stabilizing the positive charge. A direct equivalent of a cyclopropenyl cation was finally published in 2005 (Fig. 39). ²⁹³ One more example is known with the positive charge being partially delocalized over four silicon atoms. ²⁹⁴ An example for a Si(ii) cation with 6π-aromaticity provides the silyliumylidene-like species introduced by Driess et al. (Fig. 39, right). ²⁹⁵ This compound is stabilized by delocalization so that, although produced through protonation with [H(OEt₂)₂]⁺[B(C₆F₅)₄]^–, no ether molecule remains coordinated to the cation.

Fig. 39. Delocalized cyclic silicon centered cations.

Ligand-stabilized silicon cations

Already in 1983, pyridine stabilized [Me₃Si]⁺ was reported. ²⁹⁶ Yet, this compound is stable to such an extent that Br^– and I^– are sufficient as anions and that it can be prepared just by reacting Me₃SiX with pyridine. Many of these [R₃Si-L]⁺ ions stabilized by different σ-donors are known: with nitriles, ^297–299 pyridine, ^296,300 water, ³⁰¹ o-dichlorobenzene, ⁸¹ sulfur dioxide ⁸¹ and bipyridine. ³⁰² Even though 2,6-bis(2,6-difluorophenyl)phenyldimethylsilylium ion has no additional ligand acting as a σ-donor, the cationic center is stabilized by one fluorine of each 2,6-difluorophenyl-substituent (Fig. 40). ³⁰³ With the stronger stabilizing DMAP, the dication [Me₂Si(dmap)₂]²⁺ has been synthesized. ³⁰²

Fig. 40. Structurally characterized silylium ions stabilized by σ-donors.

[R₃Si-L]⁺ ions with π-donor ligands L = arenes like the before mentioned [Et₃Si(C₇H₈)] are less stabilized than those with σ-donors (Fig. 42). ²⁸⁹ Several different arene adducts of [Me₃Si]⁺ were reported by Schulz and Villinger et al. (Fig. 42). ³⁰⁴ As can be seen in Fig. 41, some of these compounds are coordinated by a second arene molecule binding in an η⁶-fashion to the proton ipso to the silylium center. This shows that these arena adducts are also very strong cationic Brønsted acids.

Fig. 42. Structurally characterized silylium ions stabilized by internal or external π-donors. Ar = benzene, toluene, ethylbenzene, n-propylbenzene, and iso-propylbenzene, o-xylene, m-xylene, p-xylene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, mesitylene.

Fig. 41. Molecular structure of [Me₃Si(C₈H₁₀)·(C₈H₁₀)]⁺. The weak interaction between the stabilized cation and the adjacent ethylbenzene is indicated by the dashed bonds. M. F. Ibad, P. Langer, A. Schulz and A. Villinger, J. Am. Chem. Soc., 2011, 133, 21016–21027. Data from this reference were used to draw this figure.

Comparable to the before mentioned 2,6-bis(2,6-difluorophenyl)phenyldimethylsilylium ion without any additional ligand, a 2,6-diarylphenyldimethylsilyl cation is existing, which is stabilized by intramolecular π-donation (Fig. 42). ⁷⁶

Compounds of the type [R₃Si–X–SiR₃]⁺ have to be treated as a special case of ligand stabilization. The first example of this type is the initially as [Et₃Si]⁺ misinterpreted [Et₃Si–H–SiEt₃]⁺, ³⁰⁵ whose structure determination has been published about two years ago. ³⁰⁶ [Me₃Si–H–SiMe₃]^{+  81} is also known as well as the analogous [Me₃Si–X–SiMe₃]⁺ compounds with X = F, Cl, Br, I ³⁰⁷ and trifluoromethanesulfonate. ³⁰⁸ The X-bridged species are typically addressed as halonium ions, but it appears more reasonable to address them as ligand-stabilized silylium ions (see Fig. 43).

Fig. 43. Canonical structures of the halogen-bridged bis-silylium ions.

Calculations state that the positive charge is still located at the silicon atoms and F, Cl and Br are negatively charged. ³⁰⁷ Only in the case of iodine, a small positive charge is located at the bridging atom. ³⁰⁷ Additionally, bissilylated pseudohalonium cations [Me₃Si–X–SiMe₃]⁺ with X = CN, OCN, SCN, and NNN are known. ³⁰⁹ Of these, only in [(Me₃Si)₂NNN]⁺ both silyl groups are attached to the same atom, ³⁰⁹ so that the structure of the cation is analog to the protonated hydrogen azide ³¹⁰ (see Fig. 44 and Table 5 for [H₂N₃]⁺[SbF₆]^–). Some more examples with bridged SiR₃-groups, in which both groups are connected with each other, are known (Fig. 44). ^311–315

Fig. 44. Structurally characterized bridged bissilylium ion salts.

Table 5. Review articles including cationic group 15 compounds.

Year	Title	Ref.
2004	Homoatomic cages and clusters of the heavier group 15 elements. Neutral species and cations	394
2008	Catena–phosphorus cations	392
2011	Homo- and heteroatomic polycations of groups 15 and 16. recent advances in synthesis and isolation using room temperature ionic liquids	66
2012	Multiple-charged P1-centered cations: perspectives in synthesis	395
2013	Catenated phosphorus compounds	391
2013	Recent advances in the syntheses of homopolyatomic cations of the non-metallic elements C, N, P, S, Cl, Br, I and Xe	11
2013	Catenated compounds – group 15 (As, Sb, Bi)	396
2014	Interpnictogen cations: exploring new vistas in coordination chemistry	393
2014	The chemistry of cationic polyphosphorus cages – syntheses, structure and reactivity	397
2015	Coordination chemistry of homoatomic ligands of bismuth, selenium and tellurium	398 and 399

A special case of intramolecular ligand stabilization can be observed in [FcSiMe ^t Bu]⁺. ³¹⁶ Here the silicon is dipped towards the iron atom due to two 3c2e bonds between C _ipso , Si and Fe and C′ _ipso , Si and Fe, respectively (Fig. 45). ³¹⁶

Fig. 45. Molecular structure of [FcSiMe ^t Bu]⁺. K. Müther, R. Fröhlich, C. Mück-Lichtenfeld, S. Grimme and M. Oestreich, J. Am. Chem. Soc., 2011, 133, 12442–12444. Data from this reference were used to draw this figure.

More intramolecular σ-donor stabilized silylium ions are known: [RSi(R′)₂]⁺ or [RSi(R′)(R′′)]⁺ with R being a pincer ligand can be seen as an extra class of ligand-stabilized silicon cations. In 2009, several silylium ions with OCO and SCS pincer ligands were published by Jutzi et al. (Fig. 46). ³¹⁷

Fig. 46. Structurally characterized silylium ion salts with intramolecular stabilization by pincer ligands.

All before mentioned ligand-stabilized silicon cations contain an inter- or intramolecularly by additional donor atoms stabilized [R₃Si]⁺ cation. Two more different types of ligand-stabilized silicon cations were published with silicon in oxidation state +IV. Both were synthesized by oxidation of silicon(ii) cations through elemental sulfur (Fig. 47). ^318,319 These cations containing subvalent silicon are very rare and most of the known examples are bearing a cyclopentadienyl substituent (see Cyclopentadienyl substituted cations). However, with well stabilizing ligands, two [LSiCl]⁺ cations were synthesized (Fig. 47). ^318,320 Both are prepared just by adding the chelating ligand to NHC·SiCl₂. The NHC ligand is being replaced by L and yields the [LSiCl]⁺ cation with chloride as the anion. This shows that the silicon cationic center is largely stabilized by coordination. By using well stabilizing NHCs, it was possible to generate an [(L)(L′)SiI]⁺[I]^– and even the dication [L₃Si]²⁺([I]^–)₂. ³²¹ In addition, two related silicon(ii) monocations [RSi(L) _n ]⁺ were structurally characterized in which the residue R is not a halogen atom (Fig. 48). ³¹⁹

Fig. 47. Structurally characterized ligand-stabilized silathionium cations.

Fig. 48. Structurally characterized ligand-stabilized cations of subvalent silicon.

Cyclopentadienyl substituted cations

So far, two cyclopentadienyl substituted silicon cations without any further stabilization through additional ligands were structurally characterized. First [(C₅Me₅)Si]⁺ with [B(C₆F₅)₄]^– was published in 2004 ³²² and two years later, the synthesis and characterization of [(C₅ ⁱPr₅)Si]⁺[Al(OR^PF)₄]^– was presented. ³²³ However, the latter structure determination was of poor quality and did not allow to obtain any exact structural parameters. Additionally, two ether stabilized [(C₅Me₅)Si]⁺ cations are known (Fig. 49). ³²⁴

Fig. 49. Structurally characterized cyclopentadienyl substituted silicon cation salts with and without additional ligands.

Ion-like silylium compounds

In alkylsilylium ion salts without an additional stabilizing ligand, interactions between the cation and the corresponding WCA can be observed. The first structurally characterized R₃Si(WCA) was the iso-propyl substituted compound in 1993. ³²⁵ Today, at least one example with the most common alkyl substituents ^t Bu, ⁱPr, Et and Me is known. ^{81,297,326–330} Additionally, with ^t Bu₂MeSi(CB₁₁H₆Br₆) one mixed substituted ion-like silylium compound was published. ³²⁸ Another example might be Fc₃Si(OTf). However, its Si–O interaction is with 175 pm in the range of a normal Si–O bond. ³³¹

Germanium, tin and lead

Cluster cations

As for silicon, no homopolyatomic cations comparable to the fullerenium ions are known for germanium, tin and lead. Nevertheless, one example of a germanium cluster exists (Fig. 50). ³³² The cluster is composed of ten germanium atoms, of which seven bear substituents. The remaining three unsubstituted germanium atoms carry the positive charge, which is evenly distributed.

Fig. 50. Lewis structure of the 5-iodo-2,4,6,8,9,10-hexakis(tri-tert-butylsilyl)heptacyclo[4.4.0.0^1,3.0^2,5.0^3,9.0^4,7.0^8,10]decagerman-7-ylium ion.

Enium ions

Just as for silicon, enium ions of Ge, Sn and Pb need substituents with a high steric demand to shield the cationic center. The first example of the heavier elements of group 14 – [ ⁿ Bu₃Sn]⁺[CB₁₁Me₁₂]^– – does have, as expected, interactions between cation and the [CB₁₁Me₁₂]^– WCA. ³³³ At about the same time, Lambert et al. and Sekiguchi et al. published the first examples of free enium ions of germanium and tin. While Lambert relied on bulky aryl substituents to synthesize [(Tipp)₃Sn]⁺, ³⁴ Sekiguchi deployed silyl groups and managed to produce [( ^t Bu₂MeSi)₃Ge]^{+  334} and [( ^t Bu₂MeSi)₃Sn]⁺, ³³⁵ all with [B(C₆F₅)₄]^– as their counterpart (Fig. 51). Although enium ions with aryl substituents have always been under the first examples for carbon, silicon and tin, it kept lacking an example for germanium until in 2009 [Ge({2,6-O ^t Bu}₂C₆H₃)₃]⁺[Al(OR^PF)₄]^– was synthesized and characterized. ³³⁶ However, the cationic center is stabilized by contacts to the oxygen atoms of the tert-butoxy residues at 286 and 288 pm. ³³⁶ More recently, a mixed substituted enium ion of tin has been published (Fig. 51). ³³⁷ Examples for lead are still missing and the only formally [R₃Pb]⁺ containing ion-like substance Et₃Pb(HCB₁₁H₅Br₆) has like its Si, Ge and Sn analogs stronger interactions between the ions. ³³⁸

Fig. 51. Structurally characterized enium ions of germanium and tin.

Delocalized cyclic cations

The germanium compound [Ge₃(Si ^t Bu₃)₃]⁺, has been published long before the first silicon analog of a cyclopropenyl cation. ³³⁹ Although it is known with a few different anions, ^340,341 it is still the only example of delocalized cyclic cations of the heavier elements of group 14 (similar to Fig. 39).

Ligand-stabilized

Far less ligand-stabilized cations of Ge, Sn and Pb in oxidation state +IV are known than of Si. [Me₃Sn(OPPh₃)₂]⁺[(MeSO₂)₂N]^– and [Ph₃Sn(OPPh₃)₂]⁺[(MeSO₂)₂N]^– were synthesized already in 1994 ³⁴² and six years later, the [ ^t Bu₃E(NC- ^t Bu)]⁺ cations were synthesized with E = Ge and Sn, but only for the germanium compound the crystal structure is known. ²⁹⁸ In addition, together with the analogous silicon complex, [Me₂Ge(bipy)(OTf)]⁺[OTf]^– has been published. ³⁰² Interesting is however, that the corresponding substances with DMAP coordinating to germanium and the ones with DMAP or bipyridine coordinating to tin have to be described as ion-like, since in all of them both [OTf]^– anions do have close contacts to the cationic center. ³⁰² As already stated for silicon, symmetrical compounds of the type [R₃E–X–ER₃]⁺ are somewhat special since the positive charge is evenly distributed and it is not possible to speak of an cation and a ligand anymore. Contrary to silicon, only one cation belonging to this type is known for the heavier homologues (Fig. 52). ³⁴³ Additionally, for germanium and tin ligand-stabilized dimeric cations are known, both synthesized by oxidation of their E(ii) precursors through elemental sulfur (Fig. 52). ^344,345

Fig. 52. Ligand-stabilized cations of germanium, tin and lead in oxidation state IV.

Norbornyl cations with the heavier group 14 elements were classified in here as ligand-stabilized cations, although one may address them as onium ions. Although the heavier norbornyl cation analogues were all published – also with silicon – no crystal structure could be determined. ³⁴⁶ However, by addition of acetonitrile to the norbornyl cations, the stronger σ-donor replaces the weaker π-donating C C double bond. An exception is the plumbanorbornyl cation, which gets coordinated by acetonitrile additionally and remains coordinated by the alkene (scXRD). ³⁴⁶ A comparable π-stabilization as in the norbornyl cations can be found in the 1,4,5-trigermabicyclo[2.1.0]pent-2-en-5-ylium ion, in which the cationic center is coordinated intramolecularly by a C C double bond. ³⁴⁷ Another unique π-stabilization can be observed in bis(cyclopentenemethyl)plumbylium. ³⁴⁸ This cation is intramolecularly stabilized by the C–C double bonds of the two cyclopentene substituents (Fig. 52).

Hard to classify are two germanium cations stabilized by a monoanionic bidentate bis(NHC)borate ligand (Fig. 53). ³⁴⁹ Both originate from the attempt to synthesize a germanium dication stabilized by the before mentioned ligand through the reaction of LGeH with [Ph₃C]⁺[B(C₆F₅)₄]^–. Instead of delivering the desired germanium dication, two different products were obtained. In one, instead of abstracting the hydride, the trityl cation attacks the lone pair of the Ge(ii) cation, forming the adduct. In the other, the hydride is indeed abstracted by the trityl cation, but the resulting germanium dication is coordinated by unreacted starting material.

Fig. 53. Germanium cations stabilized by a monoanionic bidentate bis(NHC)borate ligand.

Apart from those examples, the ligand-stabilized cations of the heavier group 14 elements are in oxidation state +II. Already as early as 1989, [Sn([15]crown-5)₂]²⁺ has been published along with its crystal structure. ³⁵⁰ This cation is accessible directly through the reaction of SnCl₂ with two equivalents of the crown ether, which is why [SnCl₃]^– serves as the counterion. In this or a similar fashion it has been possible to synthesize a portfolio of different crown ether complexes of tin(ii) and lead(ii). ^351–353

To isolate the first related Ge(ii) compound, better stabilizing ligands were needed. By employing NHC ligands, a germanium dication was isolated (Fig. 54). ³⁵⁴ The germanium center is highly stabilized by its ligands, and – although iodides are the counterions – only weak interactions between the ions are present. Another germanium containing dication was synthesized with the encapsulating cryptand[2.2.2], ³⁵⁵ and a few years later the analogous tin complex. ³⁵⁶ Today, quite a few different crown ether complexes of germanium are known as well (Table 9). ³⁵⁷ By using other well stabilizing chelating N-donor ligands, it was also possible to isolate [(L)Ge]²⁺ cations. ³⁵⁸

Fig. 54. Structurally characterized dicationic compounds of germanium and tin.

Table 9. Overview on structurally characterized cations of group 14.

Cation	Anion	Class. a	Synthesis	Comment	Ref.
Homopolyatomic and cage cations
[C76]+	[HCB11H5Br6]–	Ox	C76 + [Ar3N]+[WCA]–		44
[C60]+	[HCB11H5Cl6]–	Ox	C60 + [Ar3N]+[WCA]–		259
[C60]2+	[AsF6]–	Ox	C60 + 3 AsF5	Polymeric	260
[HC60]+	[HCB11H5Cl6]–	Prot	C60 + H(WCA)		259
[C59N]+	[Ag(HCB11H5Cl6)2]–	Ox	(C59N)2 + 2[HBPC]˙+[WCA]–		261
	[B(C6F4H)4]–	Other			332
Onium ions
	[Al2Br7]–	Lewis			86
Enium ions
[(CH3)3C]+	[HCB11Me5Cl6]–	Hyd	n BuH + Me+[WCA]– or t BuH + Me(WCA)		265
[(CH3)3C]+	[Al2Br7]–	Lewis	t BuBr + 2 AlBr3		266
[(CH3)3C]+	[HCB11Cl11]–	Other	Thermal decomposition of [Et2Cl][CHB11Cl11]		267
	[HCB11Me5Br6]–	Hyd			265
	[HCB11Me5Br6]–	Hyd			265
	[HCB11I11]–	Lewis	p-CH3-C6F4-CF3 + Et3Si(WCA) + PhF		56
	[HCB11Cl11]–	Lewis	p-CH3-C6F4-CF3 + Et3Si(WCA) + PhF		56
	[As2F11]–	Lewis	C6H5CF3 + AsF5	Excess AsF5	268
	[HCB11I11]–	Lewis	p-F-C6H4CF3 + Et3Si(WCA) + PhF		56
	[HCB11I11]–	Lewis	CH3CF3 + Et3Si(WCA) + PhF		56
[CI3]+	[Al(ORPF)4]–	Salt	CI4 + Ag+[WCA]–		269
[CCl3]+	[Sb(OTeF5)6]–	Ox	CCl4 + [XeOTeF5]+[WCA]–		270
[CCl3]+	[Al(ORPF)4]–, [(RPFO)3Al-F-Al(ORPF)3]–	Salt	CCl4 + Ag+[WCA]–		271
[CBr3]+	[Sb(OTeF5)6]–	Ox	CBr4 + [XeOTeF5]+[WCA]–		270
[CBr3]+	[Al(ORPF)4]–, [(RPFO)3Al-F-Al(ORPF)3]–		CBr4 + Ag+[WCA]–		271
[C(OTeF5)3]+	[Sb(OTeF5)6]–	Ox	CBr4 + [XeOTeF5]+[WCA]–		270
	[Al(ORPF)4]–	Ox	CS2 + [AsBr4]+[WCA]– and CS2 + Br2 + Ag+[WCA]–		117
	[B(C6F5)4]–	Hyd			275
	[B(C6F5)4]–	Hyd			276
[Mes3Si]+	[HCB11Me5Br6]–	Other	Mes3Si(CH2CH CH2) + Et3Si(WCA)		24
[Pemp3Si]+	[B12Cl12]2–	Hyd	2Pemp2MeSiH + [Ph3C]2 +[WCA]–		290
[Pemp3Si]+	[Al(ORPF)4]–	Hyd	1.5Pemp2MeSiH + [Ph3C]+[WCA]–		291
	[B(C6F5)4]–	Ox			334
	[Al(ORPF)4]–	Salt	(Ar)3GeBr + Ag+[WCA]–	R = O t Bu	336
	[B(C6F5)4]–	Ox			335
[(Tipp)3Sn]+	[B(C6F5)4]–	Other	(allyl)(Tipp)3Sn + [E]+[WCA]–	[E]+ not exactly defined, likely [Et3Si(C6H6)]+ or comparable	34
	[B(C6F5)4]–	Com, Lig		R = Et, iPr	337
Delocalized (cyclic) cations
[C6H7]+	[HCB11Me5Br6]–	Prot	C6H6 + H(WCA)		386
	[HCB11H5Br6]–	Prot	C6H6 + H(WCA)		387
	[HCB11H5Br6]–	Prot	C6Me2H4 + H(WCA)		387
	[HCB11H5Br6]–	Prot	C6Me3H3 + H(WCA)		387
	[B(C6F5)4]–	Prot	Et3Si(WCA) + HCl + C6(Me)5H	C–H···F–C interactions	388
	[HCB11H5Br6]	Prot	C6Me6 + H(WCA)		387
[C6Me7]+	[AlCl4]–	Other	C6Me6 + CH3Cl + AlCl3		277
[C6F6]˙+	[Sb2F11]–	Ox	C6F6 + [O2]+[WCA]–	Crystallized out of HF	278
[C6F6]˙+	[Os2F11]–	Ox	C6F6 + OsF6 + SbF5	In HF	278
[C6Cl6]˙+	[Sb2F11]–	Ox	C6F6 + SbF5		279
[C6Br6]˙+	[As2F11]–	Ox	C6Br6 + [O2]+[AsF6]– + HSO3F		280
[C6I6]˙+	[AsF6]–	Ox	C6I6 + AsF5	In HF	279
[C6I6]˙+	[SbF6]–	Ox	C6I6 + SbF5	In HF	279
[C6I6]˙+	[OTf]–	Other	[C6I6][AsF6] + HOTf		279
[C6HF5]˙+	[AsF6]–	Ox	C6HF5 + [O2]+[WCA]–		279
	[SbF6]–	Ox	C6H2F4 + [O2]+[WCA]–		279
	[AsF6]–	Ox	C6H3F3 + [O2]+[WCA]– + AsF5		279
	[SbF6]–	Ox	C6H3F3 + [O2]+[Sb2F11]–	In HF	279
	[Sb2F11]–	Ox	C6F5(CF3) + [O2]+[WCA]–	In HF	280
	[Sb2F11]–	Ox	C6F4(CF3)2 + SbF5	In HF	280
	[Sb2F11]–	Ox	C6H2Cl4 + [O2]+[WCA]–		280
	[Sb3F16]–	Ox	[C6F5–C6F5] + [O2]+[WCA]– + SbF5		280
	[Nb2F11]–	Ox			281
	[SbF6]–	Ox			282
[Cy3C3]+	[SbF6]–	Salt	[Cy3C3]Cl + Ag+[WCA]–		283
[Cy2PhC3]+	[BF4]–	Lewis	[Cy2(Ph)C3]F + BF3		283
	[SbCl6]–	Prot			284
	[HCB11Me5Br6]–	Hyd	ArMe2SiH + [Ph3C]+[WCA]–		286
	[B(C6F5)4]–	Other			292
	[B(C6F4R)4]–	Other		R = 4-SiMe2 t Bu	293
	[Zr2Cl7Cp*2]–	Lewis			294
	[B(C6F5)4]–	Prot			295
	[B(C6H5)]–, [B(ArCF3 )4]–, [B(C6F4R)4]–	Other		R = 4-Si(Me)2( t Bu)	339–341
Ligand-stabilized cations
	[Sb2F11]–	Lewis			273
[I3C-PX3]+	[Al(ORPF)4]–	Comp	[CI3]+[WCA]– + PX3	X = Cl, Br, I	274
[I3C-PI3]+	[(RPFO)3Al-F-Al(ORPF)3]	Comp	[CI3]+[WCA]– + PI3		274
[I3C-AsI3]+	[Al(ORPF)4]–	Comp	[CI3]+[WCA]– + AsI3		274
	[HCB9H4Br5]–	Hyd	iPr3SiH + [Ph3C]+[WCA]– + MeCN		297
	[B(C6F5)4]–	Other	t Bu3Si–Si t Bu3 + 2[Ph3C]+[WCA]– + t BuCN		298
	[B(C6F5)4]–	Com		+ MeCN	299
[Fc3Sipy]+	[B(ArCF3 )4]–	Lig	[Fc3Si(THF)]+ + py		300
	[HCB11H5Br6]–	Ion	t Bu3Si(WCA) + H2O		301
	[HCB11Cl11]–	Ion	iPr3Si(WCA) + C6H4Cl2		81
	[HCB11Me5Br6]–	Ion	Et3Si(WCA) + SO2		81
	[OTf]–	Ion	Me2Si(OTf)2 + bipy		302
	[B(C6F5)4]–	Hyd	Me2ArSiH + [Ph3C]+[WCA]–		303
	[OTf]–	Ion	Me2Si(OTf)2 + 2DMAP		302
	[B(C6F5)4]–	Hyd	Et3SiH + [Ph3C]+[WCA]–		289
[Me3Si(Ar)]+	[B(C6F5)4]–	Lig	[Me3SiHSiMe3]+[WCA]– + arene	Ar = benzene, toluene, ethylbenzene, n-propylbenzene, and iso-propylbenzene, o-xylene, m-xylene, p-xylene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, mesitylene	304
	[B(C6F5)4]–	Hyd			76
[Me3SiHSiMe3]+	[HCB11HCl11]–	Hyd	2Me3SiH + [Ph3C]+[WCA]–		81
[Et3SiHSiEt3]+	[B(C6F5)4]–	Hyd	2Et3SiH + [Ph3C]+[WCA]–		306
[Me3SiXSiMe3]+	[B(C6F5)4]–	Ion	Me3SiX + Me3Si(WCA)	X = F, Cl, Br, I	307
[Me3Si-CN-SiMe3]+	[B(C6F5)4]–	Ion	Me3SiCN + Me3Si(WCA)		309
[Me3Si-OCN-SiMe3]+	[B(C6F5)4]–	Ion	Me3SiOCN + Me3Si(WCA)		309
[Me3Si-SCN-SiMe3]+	[B(C6F5)4]–	Ion	Me3SiSCN + Me3Si(WCA)		309
[(Me3Si)2NNN]+	[B(C6F5)4]–	Ion	Me3SiNNN + Me3Si(WCA)		309
	[B(C6F5)4]–	Ion	Me3Si(OTf) + Me3Si(WCA)		308
	[B(C6F5)4]–	Hyd			311
	[B(C6F5)4]–	Other			311
	[B(C6F5)4]–	Hyd		Ar = Tol	312
	[B(C6F5)4]–	Other	(Me3Si)3CSiMePhH + [Ph3C]+[WCA]–		313
	[B(C6F5)4]–	Other			314
	[B(C6F5)4]–	Other			314
	[B(C6F5)4]–	Other		X = Cl, Br	315
	[B12Cl12]2–	Hyd	2FcMe t BuSiH + [Ph3C]2 +[WCA]–		316
	[OTf]–	Lewis			317
	[OTf]–	Lewis			317
	[OTf]–	Lewis			317
	[OTf]–	Lewis			317
	[OTf]–	Lewis			317
	Cl–	Ion			318
	Cl–	Ox			318
	[OTf]–	Ox			319
	Cl–	Ion			320
	I–	Ion			321
	[OTf]–	Ox			319
		Other			319
	I–	Ion			321
	[B(C6F5)4]–	Ox	t Bu3Ge–Ge t Bu3 + 2[Ph3C]+[WCA]– + t BuCN		298
	[OTf]–	Ion	Me2Ge(OTf)2 + bipy		302
	[B(C6F5)4]–	Hyd			343
	[AlCl4]–	Ox			344
	[B(C6F5)4]–	Ox or Com			349
	[B(C6F5)4]–	Hyd			349
	I–	Ion			354
	[OTf]–	Ion, Lig			355
[Ge([12]-crown-4)2]2+	[GeCl3]–	Ion	GeCl2·dioxane + [12]crown-4		357
[Ge([12]-crown-4)2]2+	[OTf]–	Ion	GeCl2·dioxane + [12]crown-4 + 2Me3Si(OTf)		357
[GeCl([15]-crown-5)]+	[GeCl3]–	Ion	2GeCl2·dioxane + [15]crown-5		357
[Ge(OTf)([15]-crown-5)]+	[OTf]–	Ion	GeCl2·dioxane + [15]crown-5 + 2Me3Si(OTf)		357
[GeCl([18]-crown-6)]+	[GeCl3]–	Ion	2GeCl2·dioxane + 1.5[18]crown-6		357
	[GeCl3]–	Ion			360
	[GeCl3]–	Ion			361
	[AlCl4]–	Lewis			344
	[B12Cl12]2–	Com			344
	[HO{B(C6F5)3}2]–	Lewis			369
	[B(C6F5)4]–	Prot		Protonated by acidified benzene	365
	[B(C6F5)4]–	Prot			365
	[SbF6]–	Salt			80
	[SbF6]–	Lig			80
	[SbF6]–	Lig			80
	[Al(ORPF)4]–	Salt			366
	[Al(ORPF)4]–	Lig			366
	[B(C6F5)4]–	Lewis			347
[Me3Sn(OPPh3)2]+	[(MeSO2)2N]–	Ion	Me3SnN(SO2Me)2 + 2OPPh3		342
	[B(Ph)4]–	Ox		L = DMAP	345
[Sn([15]crown-5)2]2+	[SnCl3]–	Ion	2SnCl2 + 2[15]crown-5		350
[Sn([12]crown-4)2]2+	[OTf]–	Ion	Sn(OTf)2 + 2[12]crown-4		352
[Sn([15]crown-5)2]2+	[OTf]–	Ion	Sn(OTf)2 + 2[15]crown-5		352
[Sn([18]crown-6)(OTf)]+	[OTf]–	Ion	Sn(OTf)2 + [18]crown-6		352
[Sn([18]crown-6)(F)]+	[PF6]–	Other	[Sn([18]crown-6)]2+[WCA]2 – + KF		353
	[OTf]–	Ion	Sn(OTf)2 + cryptand[2.2.2] or 2SnCl2 + cryptand[2.2.2] + 4Me3Si(OTf)		356
	[SnCl3]–	Ion	2SnCl2 + cryptand[2.2.2]		356
	[SnBr3]–	Ion	2SnCl2 + cryptand[2.2.2] + 4Me3SiBr		356
	[B(C6F5)4]–	Com			346
	[B(C6F5)4]–	Other			348
	[SnCl3]–	Salt			389
	[GeBr3]–	Ion	GeBr2 + bipy	Bulk product is GeBr2(bipy)	359
	[GeCl3]–	Ion	GeCl2·dioxane + pmdta		359
	Cl–	Ion			363
	Cl–	Ox			363
	Cl–	Ion			364
	[GeCl3]–	Ion	GeCl2·dioxane + Me4-cyclam		358
	[GeBr3]– + Br–	Ion	GeBr2 + Me3-tacn		358
	[SnCl3]–	Ion			360
	[SnCl3]–	Ion			361
	[SnBr3]–	Ion		Ar = 2,5- t Bu2(C6H3)	362
	[SnCl3]–	Ion		Ar = 2,5- t Bu2(C6H3) and 2,6-Me2(C6H3)	362
	[Al(ORPF)4]–	Salt			366
	[Al(ORPF)4]–	Lig			366
	[MeB(C6F5)3]–	Lewis			367
	[SbF6]–	Salt			368
	[B(Ph)4]–	Salt			345
	[B(ArCF3 )4]–	Salt			345
	[B(C6F5)4]–	Other	(Tipp)2Sn + [Et3Si(C7H8)]+[WCA]–		371
[Pb(NO3)([12]-crown-4)2]+	[Pb(NO3)3([12]crown-4)]–	Ion	Pb(NO3)2 + [12]crown-4		351
[Pb([15]-crown-5)2]2+	[Pb(NO3)3([15]crown-5)]–	Ion	Pb(NO3)2 + 2[15]crown-5		351
[Pb(benzo-[15]-crown-5)2]2+	[Pb(NO3)3(benzo-[15]crown-5)]–	Ion	Pb(NO3)2 + benzo-[15]crown-5		351
	[B(C6F5)4]–	Salt			367
	[B(C6F5)3(CH3)]–	Lewis			370
Cyclopentadienyl substituted cations
	[B(C6F5)4]–	Prot	(Me5C5)2Si + [Me5C5H2]+[WCA]–		322
	[Al(ORPF)4]–	Prot	(Me5C5)(iPr5C5)Si + [H(OEt2)2]+[WCA]–		323
	[B(C6F5)4]–	Com	[(Me5C5)Si]+[WCA]– + dme		324
	[B(C6F5)4]–	Com	[(Me5C5)Si]+[WCA]– + [12]crown-4		324
	[BF4]–	Prot	(C5Me5)2Ge + H(WCA)		376
	[SnCl3]–	Lewis	(C5Me5)GeCl + SnCl2		377
	[BF4]–	Prot	(C5Me5)2Ge + H(WCA)		373
	[B(C6F5)4]–	Salt	(C5Me5)SnCl + Li+[WCA]–		375
	[B(C6F5)4]–	Salt	(C5Me5)PbCl + Li+[WCA]–		375
	[Ga(C6F5)4]–	Other	(C5Me5)2Sn + Ga(C6F5)3		378
	[B(C6F5)4]–	Com	[(C5Me5)Sn]+[WCA]– + (C5Me5)2Sn		375
	[B(C6F5)4]–	Com	[(C5Me5)Pb]+[WCA]– + (C5Me5)2Pb		375
Ion-like compounds
Me2(B12Cl12)		Salt	[Li]2 +[WCA]– + 2.2MeF + 2.6AsF5	1 : 1 mixture with [Li]2 +[WCA]–	288
(Me2CH)(HCB11Me5Br6)		Lewis	(H3C)2CHCl + (H3C)(HCB11H5Br6)		287
Me2CF(AsF6)		Lewis	(H3C)2CF2 + AsF5		268
(m-CF3-C6H4)(Ph)CF(AsF6)		Lewis	C6H5CF3 + AsF5		268
Me3Si(HCB11F11)		Hyd	Me3SiH + [Ph3C]+[WCA]–		326
Me3Si(C2H5CB11F11)		Hyd	Me3SiH + [Ph3C]+[WCA]–		326
Me3Si(FAl(ORPF)3)		Other	Ag+[Al(ORPF)4]– + Me3SiCl or AlEt3 + 3HORPF + Me3SiF		327
Et3Si(HCB11H5Br6)		Hyd	Et3SiH + [Ph3C]+[WCA]–		328
Et3Si(HCB11Cl11)		Hyd	Et3SiH + [Ph3C]+[WCA]–		81
iPr3Si(HCB11H5Br6)		Hyd	iPr3SiH + [Ph3C]+[WCA]–		325
iPr3Si(HCB11H5Cl6)		Hyd	iPr3SiH + [Ph3C]+[WCA]–		329
iPr3Si(HCB11H5I6)		Hyd	iPr3SiH + [Ph3C]+[WCA]–		329
iPr3Si(HCB9H4Br5)		Hyd	iPr3SiH + [Ph3C]+[WCA]–		297
t Bu3Si(HCB11H5Br6)		Hyd	t Bu3SiH + [Ph3C]+[WCA]–		328
t Bu3Si(FAl(ORPF)3)		Other	t Bu3SiI + 15 ( t Bu)3SiF + Ag+[Al(ORPF)4]–		330
t Bu2MeSi(HCB11H5Br6)		Hyd	t Bu2MeSiH + [Ph3C]+[WCA]–		328
Fc3Si(OTf)		Prot			331
Et3Ge(HCB11H5Br6)		Hyd	Et3GeH + [Ph3C]+[WCA]–		338
Et3Sn(HCB11H5Br6)		Lewis	Et3SnCl + (Et)3Si(WCA)		338
n Bu3Sn(CB11Me12)		Ox	n Bu6Sn2 + 2 CB11Me12		333
Et3Pb(HCB11H5Br6)		Lewis	Et3PbCl + (Et)3Si(WCA)		338
Transition-metal substituted cations
	[B(C6F5)4]–	Salt			380
	[PF6]–	Salt			379
	[BF4]–	Com			381
		Salt			382
	[CB11H12]–	Salt		L = THF	382
	[ClO4]–	Ion			383
	[ClO4]–	Ion			383
	[W(CO)3Cp]–	Prot			384
	[AlBr4]–	Lewis			385
	[B(ArCl)4]–	Salt			385
{(Cy3P)2Pt}2Sn(AlBr4)2		Lewis			385
	[AlCl4]–	Lewis			385
	[B(ArCl)4]–	Salt			385
	[AlCl4]–	Salt			385
{(Cy3P)2Pt}2Pb(AlCl4)2		Lewis			385

The autoionization reaction used for the preparation of many of the crown ether complexes has also been applied to synthesize most of the structurally characterized [(L)EX]⁺[WCA]^– compounds of germanium and tin (Fig. 55). ^{344,359–362,389} With even stronger donating ligands, comparable salts [(L)GeCl]⁺Cl^– were prepared. ^363,364 These compounds are strongly stabilized so that even halides are sufficient as anions (Fig. 56). Related [RE(L)]⁺ cations with the residue R not being an halogen atom are also known. In these cations, the residue is capable to stabilize the cationic center by an additional σ- or π-donation (Fig. 57). ^{80,345,365–368} In case of bulky residues it was possible to work without an additional ligand and to obtain the free [RE]⁺ cations (Fig. 57). ^365,366,369 For lead, one additional [RE(L)]⁺ cation is known with R being a bulky aryl ligand and with a toluene molecule coordinating to the lead atom. ³⁷⁰

Fig. 55. Ligand-stabilized cations [(L)EX]⁺ of germanium and tin.

Fig. 56. Ligand-stabilized cations [(L)GeCl]⁺ with chloride as their counterion.

Fig. 57. Structurally characterized [RE(L)]⁺ and [RE]⁺ cations of germanium, tin and lead.

A rather special case is [Sn(C₇H₈)₃]²⁺, in which a tin(ii) cation is coordinated by three toluene molecules. ³⁷¹ Although lots of arene complexes of tin(ii) are known, almost all of them do still have strong interactions to the anions, mostly halides and/or [AlCl₄]^– (see for some examples ref. 19). An exception is the Sn(ii) complex with [2.2.2]paracyclophane. ³⁷² Only one of the two [AlCl₄]^– ions is coordinated to the tin atom, the other one does not have interactions with the cation. However, [Sn(C₇H₈)₃]²⁺([B(C₆F₅)₄]^–)₂ is the first example of a tin(ii) complex with independent arenes and without additional stabilization by the anion (Fig. 58).

Fig. 58. Molecular structure of [Sn(C₇H₈)₃]²⁺. A. Schäfer, F. Winter, W. Saak, D. Haase, R. Pöttgen and T. Müller, Chem. – Eur. J, 2011, 17, 10979–10984. Data from this reference were used to draw this figure.

Cyclopentadienyl substituted cations

The tin analog of the [(C₅Me₅)Si]⁺ cation was already published in 1979, ³⁷³ about 25 years before the silicon compound was characterized by XRD. This is due to the fact, that [(C₅Me₅)Sn]⁺ could be synthesized as its [BF₄]^– salt, which is not possible in case of [(C₅Me₅)Si]⁺ because of its instantaneous decomposition. ³⁷⁴ In [(C₅Me₅)Sn][BF₄] are still some stronger interactions present between the fluorine atoms and tin. In 2005, the structure of the [(C₅Me₅)Sn][B(C₆F₅)₄] was determined in which these interactions are a lot weaker ³⁷⁵ and with the same anion, [(C₅Me₅)Pb]⁺ was synthesized and structurally characterized. ³⁷⁵ The sole exception is germanium, whose [(C₅Me₅)Ge]⁺ was only characterized by XRD with [BF₄]^{–  376} and [SnCl₃]^{–  377} as its counterion and not with any larger WCA. In addition, interesting triple-decker cations are known for tin and lead: [({Me₅C₅}Sn)₂(μ-Me₅C₅)]⁺ was first synthesized and structurally characterized with the [Ga(C₆F₅)₄]^– anion, ³⁷⁸ its structure was subsequently published with [B(C₆F₅)₄]^– together with the analogous lead compound (Fig. 59). ³⁷⁵

Fig. 59. Structurally characterized cyclopentadienyl substituted cations of germanium, tin and lead.

Ion-like compounds of germanium, tin and lead

As for silicon, alkyl substituted enium ions of the heavier group 14 elements without any additional ligand need stabilizing interactions with the WCA. However, far less examples are known for E = Ge, Sn and Pb, although already in 2000, the first example was published with ⁿ Bu₃Sn(CB₁₁Me₁₂). ³³³ The other known examples are the Et₃E(HCB₁₁H₅Br₆) compounds already mentioned before. ³³⁸

Transition-metal substituted cations of germanium, tin and lead

Other than for silicon, more transition metal coordinated cations are known for the heavier elements of group 14, especially for tin. Via a salt elimination reaction, the complex cation [(dppe)₂W Sn-C₆H₃-2,6-Mes₂]⁺ was synthesized with [PF₆]^– as its counterion in which the W–Sn–C angle is close to 180°. ³⁷⁹ A similar germanium compound was published one year after, in 2004. In [(MeCN)(dppe)₂W Ge-(η¹-Cp*)]⁺, the germanium is substituted by a Cp* and the tungsten atom is coordinated additionally by an acetonitrile molecule. ³⁸⁰ As WCA serves [B(C₆F₅)₄]^– in this case. A new complex cation featuring a Sn–Pt bond was published in 2010. In trans-[Pt(Me)(SnCl₂)(2-PyPPh₂)₂][BF₄], the tin atom is pentacoordinated and adopts a trigonal-bipyramidal geometry. ³⁸¹ Along with that, comparable compounds were synthesized with the remaining group 10 metals, but no crystal structure determination was performed. By using an OCO-pincer ligand, a chromiumpentacorbonyl coordinated tin(ii) cation was synthesized. Two variants were published, {2,6-(MeOCH₂)₂C₆H₃}(H₂O)SnCr(CO)₅(OTf), in which the tin is coordinated additionally by a water molecule and [{2,6-(MeOCH₂)₂C₆H₃}(THF₂)SnCr(CO)₅]⁺[CB₁₁H₁₂]^–, in which the tin is hexacoordinated with two THF molecules complementing the coordination sphere. ³⁸² The former has indeed no contact between the triflate and the tin atom, but a strong hydrogen bond between the coordinated water molecule and the anion is existing, with an O–O distance of about 261 pm. Two more chromiumpentacorbonyl coordinated tin(ii) cations were published in 2013, both also with a pincer-type ligand (Fig. 60). ³⁸³ The same ligand was used to prepare the [RSn{W(CO)₃Cp}₂]⁺, with R = R = 4- ^t Bu-2,6-{P(O)(OⁱPr)₂}₂C₆H₂) and [W(CO)₃Cp]^– as its counterion. ³⁸⁴ Recently, new platinum-coordinated cations of tin and lead were published. Starting from (Cy₃P)₂Pt(SnBr₂), [{(Cy₃P)₂Pt-SnBr}₂]⁺ was synthesized with two different anions. ³⁸⁵ The analogous dimeric lead cation [{(Cy₃P)₂Pt-PbCl}₂]⁺ was accessible by using (Cy₃P)₂Pt(PbCl₂) as a starting material. ³⁸⁵ Through further reaction with AlX₃, {(Cy₃P)₂Pt}₂Sn(AlBr₄)₂ respectively {(Cy₃P)₂Pt}₂Pb(AlCl₄)₂ were synthesized. ³⁸⁵ However, in both dications some interactions between the ions are present. Additionally, the dimeric lead cation was also synthesized with iodine instead of chlorine. ³⁸⁵

Fig. 60. Structurally characterized transition metal substituted cations of germanium, tin and lead.

Group 15 cations

Of all the pnictogen elements, especially phosphorus has a rich cation chemistry. The analogy between CR₄ and [PR₄]⁺ displays the possibility of creating a large variety of cationic phosphorus frameworks (Fig. 61).

Fig. 61. Analogy between cationic phosphorus atom and carbon.

Over the last decades a multitude of catenated phosphorus cations were synthesized. The classical phosphino-phosphonium cation (Fig. 62, left), which can be synthesized through halide abstraction from PR₂Cl and formal insertion/coordination of the resulting [PR₂]⁺ (see section “Oxidation state +III” below) into a R₂P–R bond/to PR₃ stands for an entire substance class of compounds typically containing organic residues R. ³⁹⁰ However, we refer the interested reader to the multitude of recent reviews especially on these cations, ^391,392 the analogous interpnictogen cations (Fig. 63) ³⁹³ and other types of cationic pnictogen compounds (Table 5).

Fig. 62. Examples for catenated phosphorus cations in the formal phosphino-phosphonium or diphosphonium form. Also cyclic versions are available.

Fig. 63. Example for a catenated interpnictogen cation.

Homopolyatomic cations

Except for the long-known bismuth cations, all homopolyatomic pnictogen cations were synthesized in the last 16 years. In late 1999 the third all-nitrogen molecule [N₅]⁺ – besides N₂ and N₃ ^– – was prepared through a reaction of [N₂F]⁺[AsF₆]^– and HN₃. ⁴⁰⁰ The obtained compound [N₅]⁺[AsF₆]^– is explosive but an anion exchange led to the more stable [N₅]⁺[SbF₆]^– and to the crystal structure of [N₅]⁺[Sb₂F₁₁]^–. ⁴⁰¹ In 2004 the reduction of SbCl₃ with [Ga]⁺[GaCl₄]^– in a GaCl₃–benzene solution led to the square-antiprismatic arachno-[Sb₈]²⁺ Wade cluster cation. ⁴⁰² Recently also the first – formally electron precise and Zintl type – phosphorus cation [P₉]⁺ was synthesized through the reaction of P₄ and the nitrosyl salt of the [Al(OR^PF)₄]^– WCA. ²⁹ By contrast, bismuth has a rich cation chemistry. The first structure of a bismuth cation was measured already in 1962. Most of them were synthesized through high temperature solid state reactions. The newer room temperature approaches are based on ionic liquids. ⁴⁰³ Normally the clusters formed are badly soluble, but there is evidence that the use of very weakly coordinating anions like [Al(OR^PF)₄]^– can lead to [Bi _n ]⁺ clusters, which are soluble in solvents like CH₂Cl₂ or SO₂. ⁴⁰⁴ Besides the lighter noble gases and fluorine, only arsenic has still no homopolyatomic cation. Yellow arsenic (As₄), which has now a relatively stable storage form (see section “Metal–pnictogen complexes”) might be a good starting point for a future synthesis (Fig. 64).

Fig. 64. Examples for homopolyatomic pnictogen cations (only for bismuth, several entries are known).

Metal–pnictogen complexes

There are still only a few complexes with pnictogen modifications as ligands in the literature. Early examples of tetrahedro-P₄ complexes like (PPh₃)₂Rh^ICl(η²-P₄) are better viewed as phosphide complex (PPh₃)₂Rh^III(P₄ ^2–). By contrast, the d¹⁰-metal cation Ag⁺ is ideal for the stabilization of the non-metallic clusters and the electronic structure of the ligand stays relatively unaffected (see also chapter chalcogen cations). In 2001, the WCA [Al(OR^PF)₄]^– made it possible to crystallize the [Ag(P₄)₂]⁺ complex and later through salt metathesis with CuI also the copper complex [Cu(P₄)₂]⁺ was accessible. In 2012 the gold complex was obtained as [GaCl₄]^– salt and completed the whole series [M(P₄)₂]⁺ (M = Cu, Ag, Au). Recently light-stable (!) [Ag(As₄)₂]⁺[Al(OR^PF)₄]^– was synthesized, which finally serves as a good storage form of yellow arsenic (As₄). As such, As₄ is both thermally and photochemically unstable. The salt made it possible to transfer the As₄ tetrahedron to gold in [Ph₃PAu(As₄)]⁺ and opens new possibilities in the synthesis of arsenic complexes (Fig. 65). ⁴⁰⁵

Fig. 65. Molecular structure of [Ag(As₄)₂]⁺[Al(OR^PF)₄]^–. C. Schwarzmaier, M. Sierka, M. Scheer, Angew. Chem., Int. Ed., 2013, 52, 858–861. C. Schwarzmaier, M. Sierka and M. Scheer, Angew. Chem., 2013, 125, 891–894. Data from this reference were used to draw this figure. The disorder of the anion was omitted for clarity.

Diazonium cations and heavier homologues

There is a multitude of crystal structures of different cluster or cluster-like cations that contain pnictogen atoms. ^{391–393,397} We decided to give an overview and to list parent (model)-compounds like [N₂Ph]⁺ and [N₂Mes]⁺ in case of diazonium cations ⁴⁰⁶ as examples for the entire diazonium substance classes. The heavier homologues of the diazonium cations [RNPn]⁺ (Pn = P, As) need sterically demanding groups like Mes* (2,4,6- ^t Bu₃C₆H₂) to protect the highly reactive triple bonds (Fig. 66). ^407,408

Fig. 66. The diazonium cation and its heavier homologues. R e.g. Mes*.

Cluster and cage cations

The reaction of phosphorus halides PX₃ with halide abstractors led to very reactive carbene-analogous [PX₂]⁺ cations (see chapter “Phosphenium ions”), which are able to formally insert in the X₂P–X bonds of a second equivalent to form [P₂X₅]⁺ clusters or in the P–P bond of white phosphorus to produce [P₅X₂]⁺ for instance. Insertion in P₄S₃ or halide abstraction from P₄S₃I₂ followed by rearrangements led to the phosphorus–sulphur cluster cations [P₅S₂X₂]^{+  134} and [P₇S₆I₂]⁺. ²³⁴ Ref. 134 contains investigations on the nature of this formal insertion reaction, which is not as simple as thought and rather follows a concerted, orbital controlled mechanism (Fig. 67).

Fig. 67. Typical examples for phosphorus cations.

The binary group 15 and 16 cations have also a strong tendency to form clusters. The newer examples like the antimony-chalcogen cations [Sb₁₀Se₁₀]²⁺ and [Sb₇Te₈]⁵⁺ were synthesized in ionic liquids or GaCl₃ melts. ^409,410 Very recently [P₃Se₄]⁺, the first binary P–Se-cation was characterized by six different groups with three different approaches. ⁴¹¹ It is accessible from solution, but also through solid state syntheses. ⁴¹² In 2004 the synthesis of the sulphur- and selenium-bismuth cations from a chloroaluminate melt completed the series of the heterocubane cluster cations [Bi₄Ch₄]⁴⁺ (Ch = S, Se, Te) (Fig. 68). ^413,414

Fig. 68. Examples for binary pnictogen containing cations.

(4n + 2)π-cations

The pnictogen cations with planar delocalized π-systems can be described as (pseudo-)aromatic systems. The four-membered rings were all synthesized through halide abstraction from the neutral rings with two halogen atoms. The five-membered As–N ring was prepared through cycloaddition of the highly reactive [AsNMes*]⁺, which reacts as dienophile with the 1,3 dipole tritylazide N₃CPh₃ (Fig. 69).

Fig. 69. Heteroatomic, cationic aromatic 6π-systems containing pnictogen atoms.

π*–π*-complexes

Like the chalcogen compounds, the pnictogen cations containing π*–π*-interactions can be described as dimers of chalcogen radicals, whose half-occupied interacting orbitals have π*-character. The interannular π*–π*-bonds between the “monomers” are relatively weak. They were synthesized through halide abstraction from the chlorides of the monomers (Fig. 70). ⁴¹⁵

Fig. 70. Arsenic cations containing π*–π*-interactions.

Radical cations

Radical cations of pnictogens can be obtained through direct oxidation of Pn₂ fragments with stabilizing ligands like N-heterocyclic carbenes (NHC) or cyclic alkylaminocarbenes (CAAC). As one-electron-oxidants the trityl salt of [B(C₆F₅)₄]^– was used (Fig. 71). ^208,416,417

Fig. 71. Cationic phosphorus radicals stabilized by NHCs or CAACs.

Bulky arylphosphines and -diphosphines (Fig. 72) can also be oxidized to their radical cations, if the cation is stabilized by a WCA. ^418,419

Fig. 72. Phosphorus radical cations.

The delocalization of the single electron over a ring system also leads to stabilization. The four-membered radical cation ring systems with different pnictogen atoms (Fig. 73) were obtained through direct oxidation with silver and nitrosyl salts of WCAs. ⁴²⁰

Fig. 73. Cyclic pnictogen radical cations.

Formal oxidation state +I

In some compounds like [P₃Ph₆]^{+  421} the oxidation state of the central pnictogen “P⁺” can be described as +I, which is for example supported by the unusual high field shift in ³¹P-NMR of the central phosphorus atom in these cations (–210 to –270 ppm). ⁴²² In case of the ligand-stabilized arsenic cation [AsDppDIMPY]⁺ this is also supported by the synthesis: (DppDIMPY = [α,α′-{2,6-ⁱPr₂PhNC(Me)}₂(C₅H₃N)]). The reduction of AsCl₃ with SnCl₂ led to a cation with a planar carbenoid-like structure. ⁴²³ Under the same conditions with a different ligand an arsa-carbenoid of type [As(NR)₂C₂H₂]⁺ was obtained (Fig. 78). This displays the difficulty of a clear assignment of oxidation states in such systems (Fig. 74).

Fig. 78. Examples for pnictogen carbenoids.

Fig. 74. Phosphorus and arsenic cations in formal oxidation state +I.

Phosphenium ions (oxidation state +III)

The chemistry of the highly reactive phosphenium ions [P(R/Y)₂]⁺ (Fig. 75) was part of many studies in the past. The stability increases with the π-donor-ability of the substituent and the Lewis acidity with a stronger negative inductive effect (FIA: [P(NH₂)₂]⁺ < [PCl(NH₂)]⁺ < [PCl₂]⁺). ⁴²⁴

Fig. 75. Structurally characterized simple phosphenium ions.

For most of the reactive phosphorus cations, the decomposition is normally accompanied by the formation of strong P–X bonds (X = F, Cl…). This makes it necessary to use weakly coordinating anions stable against electrophilic cations. For the homoleptic halogen substituted cations, extremely weak anions are needed. The first examples of the less, but still highly reactive mixed amino-halogen substituted phosphenium cations were published already in 1976. Through the use of a halide-abstractor (MCl₃, M = Al, Ga, Fe) is was possible to prepare [P(NR₂)Cl]⁺ (R = Me, Et, ⁱPr) (X = [AlCl₄]^–) but no crystallographic data was obtained. It was not until 2012 that the first crystal structure of a halogen and a pseudohalogen mono-substituted phosphenium cation was determined. The structures of [P(NR₂)X]⁺ (R = TMS; X = Cl, N₃, NCO, NCS) and (R = ⁱPr; X = Cl, N₃) were determined by scXRD. All cations were stabilized with the [GaCl₄]^– anion. Especially the azidophosphenium compound turned out to be a versatile starting material for further chemistry, and made it possible to derive more complex phosphor-centered cations like iminophosphorane-substituted-phosphonium salts [ⁱPr₂NPNP(Cl)₂NR₂]⁺[GaCl₄]^– [R = ⁱPr, SiMe₃] (Fig. 76) – for instance through the reaction with the corresponding chlorophosphane R₂NPCl₂.

Fig. 76. Iminophosphorane-substituted phosphorus cation.

Miscellaneous cations in oxidation state +III

There are also some examples of the heavier homologues in oxidation state +III (Fig. 77). They were typically synthesized through halide abstraction with Lewis acids.

Fig. 77. Pnictogen cations with pnictogen atoms in formal oxidation state +III.

Another example of ligand-stabilized pnictogen cations are the N-heterocyclic carbenoid rings [Pn(NR)₂C₂H₂]²⁺ (Pn = P, As, Sb), which are formally 1,4-diaza-1,3-butadiene complexes of a pnictogen cation in oxidation state +III, but the delocalization of the positive charge supports also a description as a neutral pnictogen atom. In case of the 1,3,2-diazaphospholidinium rings [Pn(NR)₂C₂H₄]⁺ (Pn = P, As) the double bond between C4 and C5 is missing.

Reactive pnictonium cations (oxidation state +V)

The halo-pnictonium cations [PnX₄]⁺ with pnictogen atoms in oxidation state +V (Fig. 79), have a very different presence in the literature. For phosphorus, all four cations (X = F, Cl, Br, I) have been synthesized but for [PF₄]⁺ no crystal structure was determined. A multitude of structures of [PX₄]⁺ with different anions was characterized, but only a few of [AsX₄]⁺ and [SbX₄]⁺ are known. The cations are normally prepared from PnX₃, X₂ and a Lewis acid.

Fig. 79. Classical halo-pnictonium cations in oxidation state +V.

But there are also some newer, highly oxidized cations in the literature: the formal [PnPh₃]²⁺ cations (Fig. 80), which have a strong contact to the anion, serve as useful starting materials for further coordination chemistry of Pn^V compounds. ⁴²⁵ The ligand stabilized formal “PO⁺” cations were prepared through the oxidation of a phosphorus carbenoid (Fig. 80, see also the phosphorus carbenoids above) with the amine-N-oxides Me₃NO and pyO. ⁴²⁶ The charge of the carbene-stabilized formal “[PFPh₂]²⁺”, which was prepared from the carbene-stabilized “[PF₂Ph₂]⁺” through fluoride abstraction is likely partially localised on the strongly bound ligand (Fig. 80). ⁴²⁷

Fig. 80. Examples for recent pnictogen cations in oxidation state +V.

Protonated cations

With the super acidic system HF/MF₅ (M = As, Sb) it is possible to protonate hydrazoic or phosphoric acid for instance and obtain the aminodiazonium [H₂N₃]⁺ and phosphatacidium (tetrahydroxyphosphonium) cation [P(OH)₄]⁺ (Table 10). The structure determinations of the [SbF₆]^– salts revealed the structures of the cations (Fig. 81). ⁴²⁸

Table 10. Overview on selected and structurally characterized pnictogen cations.

Cation	Anion	Class. a	Synthesis	Comment b	Ref.
Homopolyatomic cations
[N5]+	[Sb2F11]–	Other	[N2F]+[SbF6]– + HN3 in aHF		401
[P9]+	[Al(ORPF)4]–	Ox	P4 + [NO]+[Al(OC(CF3)3)4]–	2.5 equiv. of P4, no X-ray	29
[Sb8]2+	[GaCl4]–	Lewis	SbCl3 + Ga+[GaCl4]– in GaCl3/C6H6		402
[Bi2]4+·[Bi9]5+	[Ag3Bi3Br15]3–·6[Br]–	Lewis, Ox	Bi + BiBr3 + Ag	HTS (350 °C)	429
2[Bi5]+·[Bi6]2+	2[IrBi6Br12]–·[IrBi6Br13]2–	Lewis, Ox	Bi + Ir + BiBr3	HTS (1000 °C)	430
[Bi5]3+	[AlCl4]–	Lewis, Ox	Bi + BiCl3 + AlCl3	HTS	431
[Bi5]3+	[AlCl4]–	Lewis, Ox	Bi + BiCl3 + [BMIM]Cl/AlCl3	Ionic liquid based synthesis	403
[Bi5]3+	[AlX4]– (X = Br, I)	Lewis, Ox	Bi + BiX3 + AlX3	HTS (490 °C (I), 520 °C (Br))	432
[Bi5]3+·2SO2	[AsF6]–	Lewis, Ox	Bi + AsF5 in SO2	No X-ray	196
[Bi8]2+	[AlCl4]–	Lewis, Ox	Bi + BiCl3 + AlCl3	HTS	433
[Bi8]2+	[Ta2O2Br7]–	Lewis, Ox	Bi + BiBr3 + TaBr5	HTS (570 °C), traces of H2O	434
[Bi9]5+	4[BiCl5]2–·[Bi2Cl8]2–	Lewis, Ox	Bi + BiCl3	HTS (325 °C), Bi6Cl7	435 and 236
[Bi9]5+	[Bi]+·3[HfCl6]2–	Lewis, Ox	Bi + BiCl3 + HfCl4	HTS	436
[Bi9]5+	[Bi]+·3[NbCl6]2–	Lewis, Ox	Bi + BiCl3 + NbCl5	HTS (550 °C), Nb(v) to Nb(iv) reduction	434
[Bi9]5+	[Sn7Br24]10–	Lewis, Ox	Bi + BiBr3 + Sn	HTS (250 °C), C 4v symmetric	437
Metal–nonmetal-cluster complexes
[Cu(P4)2]+	[Al(ORPF)4]–	Com	CuI + [Ag]+[Al(ORPF)4]– + P4		438
[Cu(P4)]+	[GaCl4]–	Com	CuCl + GaCl3 + P4		439
[Ag(P4)2]+	[Al(ORPF)4]–	Com	Ag+[Al(ORPF)4]– + P4		72
[Ag(P4)]+	[GaCl4]–	Com	AgCl + GaCl3 + P4		439
[Au(P4)2]+	[GaCl4]–	Com	AuCl + GaCl3 + P4		439
[Cp*M(dppe)(P4)]+ (M = Fe, Ru)	[BPh4]–	Salt	[Cp*M(dppe)Cl] + P4 + Na+[BPh4]–		440
[CpOs(PPh3)2(P4)]+	[OTf]–	Salt	[CpOs(PPh3)2Cl] + Ag+[OTf]–		441
[{CpRu(PPh3)2}2(P4)]2+	[OTf]–	Com, salt	[CpRu(PPh3)2Cl] + P4 + Ag+[OTf]–		442
[{CpRu(PPh3)2}-{CpOs(PPh3)2}(P4)]2+	[OTf]–	Com	[CpOs(PPh3)2(P4)]+ + [{CpRu(PPh3)2}]+	Bridging end-on/end-on	441
[Ag(As4)2]+	[Al(ORPF)4]–	Com	Ag+[Al(ORPF)4]– + As4		405
[AuPPh3(As4)]+	[Al(ORPF)4]–	Com, salt	[Ag(As4)2]+[Al(ORPF)4]– + AuPPh3Cl		405
[Cp*Ru-(dppe)(As4)]+	[Al(ORPF)4]–	Com, salt	[Ag(As4)2]+[Al(ORPF)4]– + Cp*Ru-(dppe)Cl		443
[Ag(P4S3) n ]+ (n = 1, 2)	[Al(ORPF)4]–	Com	Ag+[Al(ORPF)4]– + P4S3		444
[Ag2(P4S3)6]+	[Al(ORPF)4]–	Com	Ag+[Al(ORPF)4]– + P4S3		170
[{CpRu(PPh3)2}2(P4S3)]2+	[OTf]–	Com, salt	[CpRu(PPh3)2Cl] + P4S3 + Ag+[OTf]–		442
Clusters, cluster-like and catenated cations
[N2Ph]+	[BF4]–	Other	PhNH2 + NaNO2 in HCl(aq) and Na+[BF4]–	N–N triple bond	445
	[BF4]–	Other	H2NC6(CF3)5 + [NO]+[BF4]–		446
	[OsO2(NO3)2(Mes)]–	Other	[N2Mes]+[NO3]– + [OsO2(NO3)(Mes)]+		447
	[GaCl4]–	Other	Hg(N2(TMS)3)2 + Ag+[GaCl4]– or Bi(N2(TMS)3)2 + GaCl3 + Cl2	LTS (–80 °C)	448
	[AlCl4]–	Lewis	Mes*-NPCl + AlCl3	N–P triple bond	407
	[GaCl4]–	Other	[P(NiPr2)N3]+[GaCl4]– + PCy3		449
	[GaCl4]–	Other	[P(NiPr2)N3]+[GaCl4]– + P(NR2)Cl2	N2 as leaving group	449
	[SbF6]–	Salt	[( t BuIM)NPCl(C(PPh3)2)]+[Cl]– + Ag+[SbF6]–	Dicationic iminophosphorane	450
[P2Ph5]+	[OTf]–	Lewis	Ph2PCl + TMSOTf + PPh3	First homoleptic phosphine-phosphonium	451
[P2Me6]2+	[OTf]–	Lewis	P2Me4 + MeOTf		452
[P2Br5]+	[Al(ORPF)4]–	Salt	PBr3 + Ag+[Al(ORPF)4]–		453
[P2I5]+	AlI4 –	Lewis	PI3 + AlI3		454
[P2I5]+	[Al(ORPF)4]–	Salt	PI3 + Ag+[Al(ORPF)4]–		453
	[AlCl4]–	Lewis	PPh3 + PCl3 + AlCl3	[P(PPh3)2]+	421
	[OTf]–	Lewis	P2Me4 + PMe2Cl + TMSOTf	[(PMe2)3]+	455
	[AlCl4]–	Prot	[P(PPh3)2]+[AlCl4]– + AlCl3 + HCl		456
[P3I6]+	[F(Al(ORPF)3)2]–	Salt	P4 + I2 + Ag+[Al(ORPF)4]–	[(PI2)3]+	457
[PMe(AsMe3)2]+	[OTf]–	Other	MePCl2 + AsMe3 + TMSOTf	[PRL2]+	458
[P4Ph8]2+	[OTf]–	Lewis	PPhCl2 + PPh3 + TMSOTf		459
[P4NO]+	[Al(ORPF)4]–	Other	P4 + [NO]+[Al(ORPF)4]–	Insertion in P4, no X-ray	78
	[GaCl4]–	Lewis	ClP(PMes*)2PCl + GaCl3	Bicyclic phosphine-phosphonium	460
[(PPh)2(PnPh3)2]2+ (Pn = As, Sb)	[AlCl4]–	Lewis	PPhCl2 + PnPh3 + AlCl3		461
[P5Br2]+	[Al(ORPF)4]–	Lewis, other	P4 + PBr3 + Ag+[Al(ORPF)4]–	Insertion in P4	453 and 462
[P5Ph2]+, [P7Ph6]3+	[GaCl4]–	Lewis, other	P4 + Ph2PCl + GaCl3	Different stoichiometries, insertion in P4, HTS (60–70 °C)	68
[P5RCl]+ (R = Me, Et, iPr, Cy, Ph, C6F5)	[GaCl4]–	Lewis, other	P4 + RPCl2 + GaCl3	Insertion in P4	463
[P5R2]+ (R = Me, Et, iPr, Cy, Mes, Dipp)	[GaCl4]–	Lewis, other	P4 + R2PCl + GaCl3	Insertion in P4	464
[P5(NCy2)Cl]+	[GaCl4]–	Lewis, other	P4 + P(NCy2)Cl2 + GaCl3	Insertion in P4	465
	[Al(ORPF)4]–	Salt, other	P4S3 + PX3 + Ag+[Al(ORPF)4]–	Initial insertion in P4S3	134
	[AlCl4]–	Other	[P4(AsPh3)2]2+([AlCl4]–)2 + PPh3	Ligand exchange	466
	[AlCl4]–	Other	AsPh3 + PCl3 + AlCl3		466
	[Al(ORPF)4]–	Salt	P4S3I2 + Ag+[Al(ORPF)4]–		234
	[OTf]–	Other	(AsPh3)(OTf)2 + PCl3 + Ph3As		467
	[AlCl4]–	Lewis, other	P(red) + Se + SeCl4 + BMImCl/AlCl3	Ionic liquid based synthesis, rhombohedral and orthorhombic modification	412
[P3Se4]+	[AlCl4]–	Lewis, other	P3Se4 + Me5C6Br + AlCl3	In solution, orthorhombic modification	412
[P3Se4]+	[Ga2Cl7]–	Lewis, other	PCl3 + SeTMS2 + GaCl3	In solution	412
[P5Se2Ph2]+	[GaCl4]–	Ox	[P5Ph2]+[GaCl4]– + Se	HTS (160 °C)	464
[P5Se2Cy2]+	[GaCl4]–	Ox	[P5Cy2]+ + GaCl3 + Se	HTS (140 °C)	464
Mes*–N As	[GaCl4]–	Lewis	Mes*NAsCl + GaCl3	N–As triple bond	408
[As3N3Ph3Cl2]+	[GaCl4]–	Lewis	N2As2Ph2Cl2 + GaCl3	As3N3 ring	468
[As2Me6]2+	[OTf]–	Other	Me3As + PCl3 + TMSOTf		458
[As2Ph6]2+	[AlCl4]–	Ox	AsPh3 + PCl3 + AlCl3		466
	[AsF6]–	Ox	As4S4 + AsF5 in SO2		469
[As3S4]+	[SbF6]–	Ox	As4S4 + SbF5 in SO2		469
	[AlCl4]–	Ox, Lewis	As + AsCl3 + S + AlCl3	HTS (80 °C)	470
	[SbF6]–	Ox	As + Se + SbF5 in SO2		469
[As3Se4]+	[AlCl4]–	Ox, Lewis	As + AsCl3 + Se + AlCl3	HTS (80 °C)	470
([Sb2Se2]+) n	[AlCl4]–	Ox	Sb + Se + BMImCl/AlCl3	Ionothermal, HTS (160 °C)	67
	[AlCl4]–	Ox, others	Sb + Se + SeCl4 + BMImCl/AlCl3	Ionic liquid based synthesis	409
([Sb2Te2]+) n	[AlCl4]–	Ox, Lewis	Sb + Te + SbCl3 + NaCl + AlCl3	HTS (130 °C), polymeric	15
[Sb7Te8]5+	3[Ga2Cl7]–·2[GaCl4]–/2[Ga2Cl7]–·3[GaCl4]–	Ox, Lewis	Sb + Te + SbCl3 + GaCl3	AlCl3/GaCl3 melt	410
	[MCl4]– (M = Al, Ga)	Ox, Lewis	Sb + Te + SbCl3 + MCl3 + NaCl	AlCl3/GaCl3 melt	410
[Bi2X4]2+ (X = Cl, Br)	[AlX4]–	Lewis	BiX3 + AlX3		471
[Bi4OF2Cl6(C6Me6)4]2+	[Al(ORPF)4]–	Other	[Bi5]+[AsF6]– + Li+[Al(ORPF)4]– + C6Me6	Evidence for soluble [Bi n ] x+ salts, partial decomposition of anion and solvent	404
	[AlCl4]–	Ox, Lewis	Bi + BiCl3 + Ch in AlCl3/NaCl	HTS (130 °C), heterocubane, series Bi4Ch4 4+ (Ch = S, Se, Te) complete	413
	[AlCl4]–	Ox, Lewis	Bi + BiCl3 + Te in AlCl3/NaCl	HTS (130 °C), heterocubane	414
[PdBi10]4+	([BiBr4]–)∞	Ox	Bi2Pd + Bi + Br2	HTS (1000 °C) Pd@[Bi10]4+	472
(4n + 2)π-cations
	[GaCl4]–	Lewis	ClP(μ-NTer)2PCl + GaCl3	P2N2 ring	473
	[N3(GaCl3)2]–	Other	[ClP(μ-NTer)2P]+[GaCl4]– + TMSN3 + GaCl3	P2N2 ring	473
	[GaCl4]–	Other	[Mes*-NAs]+[GaCl4]– + Ph3CN3	Cycloaddition	408
	[GaCl4]–	Lewis	ClAs(μ-NTer)2AsCl + GaCl3	As2N2 ring	474
	[OTf]–	Salt	ClAs(μ-NTer)2AsCl + Ag+[OTf]–	As2N2 ring	474
	[N3(GaCl3)2]–	Lewis	[ClAs(μ-NTer)2As]+[GaCl4]– + TMSN3 + GaCl3	As2N2 ring	474
[As(μ-NTer)2As]2+	[OTf]–	Salt	ClAs(μ-NTer)2AsCl + Ag+[OTf]–	As2N2 ring, 4π system, 2 equiv. of Ag+[OTf]–	474
[ClSb(μ-NTer)2Sb]+	[GaCl4]–	Lewis	ClSb(μ-NTer)2SbCl + GaCl3	Sb2N2 ring	475
[Sb(μ-NTer)2Sb]2+	[OTf]–	Salt	ClSb(μ-NTer)2SbCl + Ag+[OTf]–	Sb2N2 ring, 4π system, 2 equiv. of Ag+[OTf]–	475
[IBi(μ-NTer)2Bi]+	[B(C6F5)4]–	Salt	IBi(μ-NTer)2BiI + [Ag(Tol)3]+[B(C6F5)4]–	Bi2N2 ring	475
[Bi(μ-NTer)2Bi]2+	[OTf]–	Salt	ClBi(μ-NTer)2BiCl + Ag+[OTf]–	Bi2N2 ring, 4π system, 2 equiv. of Ag+[OTf]–	475
π*–π*-complexes
	[GaCl4]–	Lewis	AsS2(CH)2Cl + GaCl3		415
	[MCl4]– (M = Al, Ga)	Lewis	As(NMe)2(CH)2Cl + MCl3		415
Radicals
	[B(C6F5)4]–	Ox	[P(CN(Dipp)C10H18)N(C(N(Dipp))2C2H2)] + [CPh3]+[B(C6F5)4]–	CN(Dipp)C10H18 CAAC, cyclic alkylaminocarbene	416
	[SbF6]–	Ox	[(NTMS)2(PNTMS2)2] + [NO]+[SbF6]–		420
	[X]– = [SbF6]–, [Al(ORPF)4]–	Ox	PTipp3 + Ag+[X]–		418
	[Al(ORHT)4]–	Ox	P2Tipp4 + Ag+[Al(ORHT)4]–		419
	[B(C6F5)4]–	Ox	[P2(NTer)2] + [Ag(Tol)3]+[B(C6F5)4]–		476
	[B(C6F5)4]–	Ox	[P2(C(N(Dipp))2C2H2)2] + [CPh3]+[B(C6F5)4]–	C(N(Dipp))2C2H2 NHC, N-heterocyclic carbene	208
	[B(C6F5)4]–	Ox	[P2(CN(Dipp)C10H18)2] + [CPh3]+[B(C6F5)4]–	NC10H18 = tetramethylamide	477
	[Al(ORPF)4]–	Ox	[P4(NiPr2)4] + [NO]+[BF4]– + Li+[Al(ORPF)4]–		420
	[B(C6F5)4]–	Ox	[AsP(NTer)2] + [Ag(Tol)3]+[B(C6F5)4]–		476
	[GaCl4]–	Ox	[As2(C(N(Dipp))2C2H2)2] + GaCl3		417
	[B(C6F5)4]–	Ox	[As2(NTer)2] + [Ag(Tol)3]+[B(C6F5)4]–		476
Oxidation state +I
	[SbCl5·THF]–	Lewis	AsCl3 + SnCl2 + DppDIMPY	[AsL3]+, DppDIMPY = [α,α′-{2,6-iPr2PhN-C(Me)}2(C5H3N)]	423
Oxidation state +III
[P(NiPr2)2]+	[GaCl4]–	Lewis	P(NiPr2)2Cl + GaCl3	[PX2]+	478 and 479
[P(NCy2)Cl]+	[GaCl4]–	Lewis	P(NCy2)Cl2 + GaCl3	[PX2]+	465
[P(NiPr2)Cl]+	[GaCl4]–	Lewis	P(NiPr2)Cl2 + GaCl3	[PX2]+	449
[P(NTMS 2)Cl]+	[GaCl4]–	Lewis	P(NTMS2)Cl2 + GaCl3	[PX2]+	480
[P(NiPr2)N3]+	[GaCl4]–	Other	[P(NiPr2)Cl]+[GaCl4]– + TMSN3	[PX2]+	449
[P(NTMS2)X]+ (X = N3, NCO, NCS)	[GaCl4]–	Other	[P(NTMS2)Cl]+[GaCl4]– + TMSX	[PX2]+	480 and 481
[P(NTMS2)OTMS]+	[GaCl4]–	Other	[P(NTMS2)Cl]+[GaCl4]– + TMSCNO	[PX2]+	481
[PCp*Cl]+	[Cl(Al(ORPF)3)2]–	Lewis	PCp*Cl2 + PhF·Al(ORPF)3	[PR2]+	482
[PCp*2]+	[Cl(Al(ORPF)3)2]–	Lewis	PCp*2Cl + PhF·Al(ORPF)3	[PR2]+, phosphocenium ion	482
[AsCp*Cl]+	[Cl(Al(ORPF)3]–	Lewis	AsCp*Cl2 + PhF·Al(ORPF)3	[AsRX]+	482
[SbCl2(AsMe3)]+	[OTf]–	Lewis	SbCl3 + AsMe3 + TMSOTf	[SbX2L]+	483
[SbPhCl(AsPh3)]+	[AlCl4]–	Lewis	SbPhCl2 + AsPh3 + AlCl3	[SbRXL]+	483
[BiPh(AsPh3)]2+	[OTf]–	Lewis	BiCl2Ph + AsPh3 + TMSOTf	[BiRL]2+	484
[BiCl(SbPh3)]2+·C6H6	[AlCl4]–	Lewis	BiCl3 + AsPh3 + AlCl3	[BiXL]2+	484
[BiCl2(AsPh3)2]+	[OTf]–	Lewis	BiCl3 + AsPh3 + TMSOTf	[BiX2L2]+	483
[BiCl2(SbPh3)2]+·C7H8	[AlCl4]–	Lewis	BiCl3 + SbPh3 + AlCl3	[BiX2L]+	484
[Bi(N2TMS3)2]+	[GaCl4]–	Lewis	Bi(N2TMS3)2Cl + GaCl3	[BiX2]+	485
	[BF4]–	Salt	PCl(N t Bu)2C2H2 + Ag+[BF4]–	P carbenoid	486
	[PF6]–	Salt	PCl(N t Bu)2C2H2 + Ag+[PF6]–	P carbenoid	487
	[GaCl4]–	Lewis	PCl(NMe)2C2H4 + GaCl3	P carbenoid	479
	[GeCl5]–·[Cl]–	Other	Ge(N t Bu)2C2H4 + PCl3	P carbenoid	488
	[PF6]–	Salt	PCl(N t Bu)2C2H4 + Ag+[PF6]–	P carbenoid	487
	[GaCl4]–	Lewis	PCl(NDipp)2C2H4 + GaCl3	P carbenoid	426
	[SbCl5·THF]–	Lewis	AsCl3 + SnCl2 + (MesN)2C2H2	As carbenoid	423
	[GeCl5]–·[Cl]–	Other	Ge(N t Bu)2C2H4 + AsCl3	As carbenoid	488
	[GaCl4]–	Lewis	As(iPrN)2C10H6Cl + GaCl3	As carbenoid	489
	[AlCl4]–	Lewis	AsCl(HN)SC6H4 + AlCl3	As carbenoid	490
	[AlCl4]–	Lewis	AsClS2C6H3CH3 + AlCl3	As carbenoid	490
	[GaCl4]–	Lewis	AsCl(NMe)2C3H6 + GaCl3	As carbenoid	491
	[OTf]–	Other	Sb(iPrN)2C10H6(NMe2) + HOTf	Not planar through the ligand NHMe2	489
	[Sb2Cl8]2–	Lewis	[Sb( t BuN)2C2H2]+ + SbCl3	Sb carbenoid	492
Oxidation state +V
[NF4]+	[BF4]–, [SbF6]–, [Sb2F11]–	Lewis	NF3 + F2 + BF3 or SbF5		493
[PF4]+	[Sb3F16]–	Lewis	PF5 + SbF5	No X-ray	494
[PCl4]+	[SnCl6]2–	Lewis	PCl5 + SnCl4	For more structures see ref. 495	496
[PBr4]+	[Al(ORPF)4]–	Salt	PBr3 + Br2 + Ag+[Al(ORPF)4]–		453
[PI4]+	[AlCl4]–	Lewis	PI3 + ICl + AlCl3 in CS2		495
[PI4]+	[AlBr4]–	Lewis	PI3 + IBr + AlBr3 in CS2		495
[PI4]+	[AlI4]–	Lewis	PI3 + I2 + AlI3 in CS2		497
[PI4]+	[GaI4]–	Lewis	PI3 + I2 + GaI3 in CS2		495
[PI4]+	[Al(ORPF)4]–	Salt	PI3 + I2 + Ag+[Al(ORPF)4]–		453
[AsCl4]+	[AsF6]–	Lewis, Ox	AsCl3 + Cl2 + AsF5		498
[AsCl4]+	[As(OTeF5)6]–	Lewis, Ox	AsCl3 + ClOTeF5 + As(OTeF5)5		499
[AsBr4]+	[Al(ORPF)4]–	Salt	AsBr3 + Br2 + Ag+[Al(ORPF)4]–		500
[AsBr4]+	[FAs(OTeF5)5]–	Lewis, Ox	AsBr3 + BrOTeF5 + AsF(OTeF5)4		499
[SbCl4]+	[Sb2F11]–	Lewis	SbCl5 + SbF5		501
[SbCl4]+	[Sb(OTeF5)6]–	Ox	Sb(OTeF5)3 + Cl2		502
[SbBr4]+	[Sb(OTeF5)6]–	Ox	Sb(OTeF5)3 + Br2		502
	[GaCl4]–	Ox	[(CH2)2(NDipp)2P]+ + OL	“PO+” cation	426
	[B(C6F5)4]–	Lewis	[(SIMes)PF2Ph2]+ + [Et3Si(Tol)]+[B(C6F5)4]–	“PFPh2 2+” cation	427
[Ph3Pn]2+ (Pn = Sb, Bi)	[OTf]–	Salt	Ph3PnCl2 + Ag+[OTf]–	Strong contact to the anion	425
Protonated cations
[H2N3]+	[SbF6]–	Prot	HN3 + HF/SbF5		310
[P(OH)4]+	[SbF6]–	Prot	H3PO4 + HF/SbF5		428

^a Classification according to the introduction: Lewis = Lewis acid halogen bond heterolysis, Ox = oxidation, Com = complexation reaction, Prot = protonation, other = all other reactions not classified.

^b HTS = high temperature synthesis.

Fig. 81. Structures of [H₂N₃]⁺ and [P(OH)₄]⁺ in their [SbF₆]^– salts.

Group 16 cations

Hundreds of chalcogen cations are known to the literature (see Table 6 for reviews). The relatively strong Ch–Ch– and Ch–X-single bonds (Ch = S, Se, Te; X = F, Cl, Br, I) led to a great diversity of reactive compounds, which include homo- and heteropolyatomic clusters, radical cations and a large number of different [ChX₃]⁺ structures for instance. To avoid the formation of the more stable neutral compounds, weakly coordinating anions are needed to stabilize the reactive chalcogen cations.

Table 6. Review articles including cationic group 16 compounds.

Year	Title	Ref.
2000	Recent advances in the understanding of the syntheses, structures, bonding and energetics of the homopolyatomic cations of groups 16 and 17	503
2003	Homoatomic sulfur cations	504
2004	Cages and clusters of the chalcogens	505
2006	Synthesis, reactions and structures of telluronium salts	506
2011	Homo- and heteroatomic polycations of groups 15 and 16. Recent advances in synthesis and isolation using room temperature ionic liquids	66
2013	Catenated sulfur compounds	507
2013	Catenated compounds – group 16 (Se, Te)	508
2013	Recent advances in the syntheses of homopolyatomic cations of the non-metallic elements C, N, P, S, Cl, Br, I and Xe	11
2013	RCNSSS+: a novel class of stable sulfur rich radical cations	509
2015	Coordination chemistry of homoatomic ligands of bismuth, selenium and tellurium	398 and 399

Homopolyatomic cations

The first observation of homopolyatomic cations were the colored solutions of elemental sulfur, selenium and tellurium in sulfuric acid in the 18th and 19th century. Over the next centuries, the nature of these solutions stayed unclear and it was not before the middle of the 20th century that the use of superacidic media and better analytical methods made it possible to characterize the responsible species. Since then, starting with [O₂]⁺[PtF₆]^– in 1962, ⁵¹⁰ the crystal structures of a multitude of different homopolyatomic chalcogen cations were measured in the last 50 years (Fig. 82). All of them have more or less weakly coordinating anions as counter ions. In some cases, cationic clusters with unusual bonding situations including trans-annular interactions and negative hyperconjugation were found that presented quite a challenge for theory (e.g. the [S₈]²⁺ dication). ^11,503,511 The cations were mostly synthesized under superacidic conditions or through solid state or solvothermal reactions at higher temperature. Either the elemental chalcogen is directly oxidized with strong oxidants like MF₅ (M = As, Sb) or WCl₆ or a combination of the elemental chalcogen, chalcogen halides ChX₄ (e.g. SeCl₄, TeBr₄) and a strong Lewis acid undergo a synproportionation.

Fig. 82. Selected examples for homopolyatomic chalcogen cations.

Metal–chalcogen complexes

The use of very weakly coordinated metal salts M[WCA] (e.g. M⁺ = Cu⁺, Ag⁺) made it possible to obtain complexes with very weak ligands like the elemental modifications of the chalcogen elements. Stable, metastable and hitherto unknown modifications were prepared, for example [Cu(S₁₂)(S₈)]⁺, ⁵¹² [Cu₂Se₁₉]²⁺ (Fig. 83) ⁵⁰ and [Ag₂Se₆]²⁺ (Fig. 84). ⁵¹³ In all such complexes, extensive charge delocalization from the metal cation to the chalcogen ring took place as evidenced by cation–anion contacts as well as accompanying quantum chemical calculations.

Fig. 83. Molecular structure of [Cu₂Se₁₉]²⁺([Al(OC(CF₃)₃)₄]^–)₂·C₆F₄H₂. (a) J. Schaefer, A. Steffani, D. A. Plattner and I. Krossing, Angew. Chem., Int. Ed., 2012, 51, 6009–6012; Angew. Chem., 2012, 124, 6112–6115. Data from this reference were used to draw this figure.

Fig. 84. Molecular structure of [(SO₂)₂Ag₂(Se₆)]²⁺([Sb(OTeF₅)₆]^–)₂. D. Aris, J. Beck, A. Decken, I. Dionne, I. Krossing, J. Passmore, E. Rivard, F. Steden and X. Wang, Phosphorus, Sulfur Silicon Relat. Elem., 2004, 179, 859–863. Data from this reference were used to draw this figure. One anion of the formula unit and the disorder of the anion and SO₂ molecules were omitted for clarity.

Clusters/cluster-like cations

Chalcogens have a strong tendency to form clusters. There are examples for chains, rings and cages with almost every combination of the groups 15 and 16 (Fig. 68 and 85). The clusters often have delocalized charges, positive and negative hyperconjugation, or show pseudo-aromaticity (Fig. 86) or π*–π*-interactions (Fig. 87). The clusters were often synthesized through direct oxidation of neutral clusters like S₄N₄ or mixtures of the elements (e.g. Se and Te) with strong oxidants like MF₅ in SO₂. [NS]⁺, a useful starting material for the syntheses of further rPBC, can be obtained by halide abstraction from trichlorocyclotrithiazene (NSCl)₃ with [Ag]⁺[WCA]^–. ⁵¹⁴

Fig. 85. Selected examples for cationic chalcogen clusters or cluster-like structures.

Fig. 86. Heteroatomic cationic aromatic (6π) systems containing chalcogen atoms. (S₄N₃ ⁺ is a 10π system).

Fig. 87. Selected chalcogen cations containing different π*–π*-interactions.

(4n + 2)π-cations

Some planar cationic conjugated π-systems containing chalcogen atoms can be described as (pseudo-)aromatic systems. The four-membered rings are related to the homopolyatomic cations Ch₄ ²⁺ and were synthesized through direct oxidation of mixtures of the elemental chalcogens instead of the pure elements. The five-membered rings were obtained by cycloadditions of [NS]⁺ and [NS₂]^{+  515} or in case of the selenium containing rings with Se, [Se₈]²⁺ or EtSeCl and [NS]⁺ as starting materials. ^516–518 [S₄N₃]⁺, a 10π-system, was synthesized through the reaction of S₄N₄ with Se₂Cl₂ and is stabilized by the polymeric ([SeCl₅]^–)_∞ anion. ⁵¹⁹

π*–π*-complexes

The chalcogen cations containing π*–π*-interactions can be described as dimers of chalcogen radicals, whose half-occupied interacting orbitals have π*-character. The two [Ch₂I₄]²⁺ cations (Ch = S, Se) have a very similar structure, but include different orbital interactions. In case of [S₂I₄]²⁺ two 2e4c-bonds were formed through the π* of the diatomic molecules ([I₂···S₂···I₂]²⁺). ⁵²⁰ In [Se₂I₄]²⁺ the two delocalized π* orbitals of the “monomer” [SeI₂]˙⁺ are overlapping. ⁵²¹ In the last 10 years, chalcogen systems, which are analogous to the [I₄]²⁺ cation were characterized by scXRD. The isolobality of [Ch₂R₂]⁺˙ and [X₂]⁺˙ leads to the same rectangular structural motif with two long π*–π*- and two short σ-interactions. Overall those cations are typically diamagnetic in the solid state (Fig. 88).

Fig. 88. Selected chalcogen radical cations, which were stabilized by WCAs.

Radical cations

The chalcogen elements have a rich radical cation chemistry. Most of the cations contain (pseudo-)aromatic systems or Ch–Ch fragments, over which the unpaired electron is delocalized. Strong one-electron oxidants like [NO]⁺ or XeF₂ combined with a Lewis acid were frequently used to synthesize the cations. There are also examples of diradicals like (CNS₃˙⁺)₂, obtained by the reaction of homopolyatomic sulfur cations (a formal [S₃]⁺ equivalent) and dicyanogen. ^522,523 We also refer the reader to a recently published comprehensive review about [RCNSSS]˙⁺ radical cations and their properties. ⁵⁰⁹

Oxidation state +II

Chalcogen cations [RCh]⁺ in the formal oxidation state +II are only known in combination with stabilizing donor ligands. It was for example possible to stabilize the formal selenium cation [RSe]⁺ with the two amine-arms of a pincer ligand ⁵²⁴ (Fig. 89). Two different Te(ii) cations with the strong donors DMAP and the carbene ⁱPrIM as ligands (Fig. 89) were synthesized with the useful starting material [(Dipp₂BIAN)Te]²⁺([OTf]^–)₂, a base stabilized “Te(OTf)₂”, which was presented in 2009 and can also be understood as a tellurium analogue of a carbene. ⁵²⁵ The thi-, selen- and tellur-irenium cations (in the figure drawn as coordination complexes of [RCh]⁺ ions) were all synthesized with starting materials that contain Ch–Ch bonds like Me₂S₂ ⁸² or [Se₃Me₃]^{+  526} or already oxidized chalcogens like [PhTe]⁺[SbF₆]^{–  526} and alkynes.

Fig. 89. Selected chalcogen cations with chalcogen atoms in the formal oxidation state of +II.

Another example of ligand-stabilized chalcogen cations are the N-heterocyclic carbenoidic rings [R₂C₂N₂Ch]²⁺ (Ch = S, Se, Te), which are formally 1,4-diaza-1,3-butadiene complexes of a chalcogen cation in oxidation state +II, but the delocalization of the positive charge supports a description as a chalcogen in oxidation state +IV (Fig. 90). ⁵²⁷ The carbene-analogues were prepared through complexation of an in situ generated Ch²⁺ dication, which can be obtained through halide abstraction from SCl₂, ⁵²⁸ SeCl₄ ⁵²⁹ or (Dipp₂BIAN)TeI₂ ⁵²⁵ (Dipp₂BIAN = 1,4-(2,6-diisopropyl)phenyl-bis(arylimino)-acenaphthene).

Fig. 90. Carbene-analogous chalcogen dications.

Oxidation state +IV

The [ChX₃]⁺ cations were one of first structurally characterized reactive chalcogen cations (Fig. 91). The first experiments mainly on [TeCl₃]⁺ were published already in the 1950s. ⁵³⁰ Now, a multitude of crystal structures can be found in the literature. Due to the relatively simple vibrational spectra of the four-atomic molecules, they can serve as a probe for the coordination power of the anion. The stronger the secondary interaction is, the weaker are the intramolecular Ch–X bonds and thus they get red-shifted. ⁵³¹ There are also over 60 crystal structures of triorganyltelluronium [TeR₃]⁺ cations with different anions. We decided only to list the “classics” [TePh₃]^{+  532} and [Te(C₆F₅)₃]^{+  533} and to refer to a recently published review about the chemistry and structures of these cations. ⁵⁰⁶ Some newer examples of compounds with chalcogen atoms in oxidation state +IV are the triazidetelluronium cation [Te(N₃)₃]^{+  534} and a fluoride bridged version of [TeCl₃]⁺. ²⁸

Fig. 91. Chalcogen cations with chalcogen atoms in oxidation state +IV. * Every combination but [SI₃]⁺ is known.

Oxidation state +VI

To our knowledge, the only chalcogen cation with a chalcogen atom in oxidation state +VI is [TePh₅]⁺. It was obtained through halide abstraction from TePh₅Cl with silver triflate and crystallized with the classical WCAs [B(C₆F₅)₄]^{–  532} and [B(Ar^CF₃ )₄]^– (Fig. 92). ⁵³⁵

Fig. 92. [TePh₅]⁺, a cation with a chalcogen atom in oxidation state +VI.

Protonated chalcogen cations

The use of super-acidic conditions, makes it possible to obtain the protonated forms of very weakly basic molecules like trifluorosulfonic acid (Fig. 93). ⁵³⁶ In some cases, it is not possible to isolate the neutral form of a molecule, but the conjugated positively charged acid (e.g. carbonic acid ⁵³⁷ ). We decided to list the protonated carbonic acid together with the sulphur species like protonated sulphuric acid in this chapter (Table 11). ⁵³⁸

Fig. 93. Examples for protonated molecules obtained under superacidic conditions.

Table 11. Overview on structurally characterized chalcogen cations.

Cation	Anion	Class a	Synthesis	Comment b	Ref.
Homopolyatomic cations
[O2]+	[PtF6]–	Ox	O2 + PtF6		510
[O2]+	[PtF6]–	Ox	O2 + PtF6	Neutron diffraction	539
[S4]2+	[AsF6]–	Ox	S + AsF5 in SO2		540
[S4]2+·4[S7I]+	[AsF6]–	Ox, Lewis	S + I2 + AsF5 in SO2		540 and 541
[S4]2+	[Sb9F39]2–(= [(SbF6)5(Sb2F4)(Sb2F5)]2–)	Ox	S + SbF5 in SO2	Traces of Br2 were added	542
[S4]2+·AsF3	[AsF6]–	Ox	S + AsF5 + AsF3 in HF	Traces of Br2 were added	543
[S8]2+	[AsF6]–	Ox	S + HF/AsF5		544
[S8]2+	[SbF6]–·[Sb3F14]–(=[(SbF6)2(SbF2)]–)	Ox	S + SbF5 in SO2		542
[S8]2+	[AsF6]–	Ox	[S8]2+([AsF6]–)2 in SO2/SO2ClF		73
[S19]2+	[AsF6]–	Ox	S + AsF5 in SO2/SO2ClF		545
[S19]2+	[SbF6]–	Ox	S + SbF5 in SO2		542
[Se4]2+	[AlCl4]–	Lewis, Ox	Se + SeCl4 + AlCl3		546
[Se4]2+	[MCl6]2– (M = Zr, Hf)	Lewis, Ox	Se + SeCl4 + MCl4	HTS (130 °C)	235
[Se4]2+	[SbF6]–·[Sb4F11]–(=[(SbF6)2(Sb2F5)2]–)	Ox	Se + SbF5 in SO2		547
[Se4]2+	[Sb9F39]2–(=[(SbF6)5(Sb2F4)(Sb2F5)]2–)	Ox	S + Se + SbF5 in SO2		546
[Se4]2+	[MoOCl4]–	Ox	Se + MoOCl4	HTS (190 °C)	548
[Se4]2+	[Mo2O2Cl8]–·[MCl6]– (M = Zr, Hf)	Lewis, Ox	[Se4]2+[Mo2O2Cl8]2– + [Se4]2+[MCl6]2– or Se, SeCl4 + MoOCl4 + MCl4	HTS (120 °C)	549
[Se8]2+	[AlCl4]–	Lewis, Ox	Se + SeCl4 + AlCl3	Crystals from vapour-phase transport	550
[Se8]2+·[Te6]4+·SO2	[AsF6]–	Ox	Se + Te + AsF5 in SO2		551
[Se10]2+	[SbF6]–	Ox	Se + SbF5 in SO2		552
[Se10]2+	[SO3F]–	Ox	Se + AsF5 in SO2		553
[Se10]2+	[Bi4Cl14]–	Lewis, Ox	Se + SeCl4 + BiCl3	HTS (90 °C)	554
[Se17]2+	[WCl6]–	Ox	Se + WCl6		555
[Se17]2+	[NbCl6]–	Lewis, Ox	Se + SeCl4 + NbCl5 in SnCl4	Solvothermal (150 °C)	556
[Se17]2+	[TaBr6]–	Lewis, Ox	Se + SeBr4 + TaBr5 in SiBr4	Solvothermal (150 °C)	556
[Te4]2+	[AlCl4]–[Al2Cl7]–	Lewis, Ox	Te + TeCl4 + AlCl3	HTS (250 °C)	557
[Te4]2+	[SbF6]–	Ox	Te + SbF5 in SO2	Germanium was added to obtain mixed cations	546
[Te4]2+	[WCl6]–	Ox	Te + WCl6	HTS (190 °C)	558
[Te4]2+	[WCl6]–	Ox	Te + WCl6	Traces of Br2 were added, β-mod.	559
[Te4]2+	[Zr2Br10]2–	Lewis, Ox	Te2Br + ZrBr4	HTS (210 °C)	560
[Te4]2+	[HfCl6]–	Lewis, Ox	Te + TeCl4 + HfCl4	HTS (200 °C)	235
[Te4]2+	[MCl6]– (M = Nb, Ta), [TaBr6]–, [Ta2Cl10O]2–	Lewis, Ox	Te + TeCl4 + MCl5	HTS (170 °C)	561
[Te4]2+	[Bi6Cl10]2–[Bi2Br8]2–	Lewis, Ox	Te + TeX4 + BiX3 (X = Cl, Br)	HTS (170 °C)	562
[Te4]2+	[Nb2Cl10O]2–	Lewis, Ox	Te + TeCl4 + NbCl5 + NbOCl3	HTS (200 °C)	563
[Te4]2+	[MoCl4O]–	Ox	Te + MoOCl4	HTS (250 °C)	564
(Te10 2+) n ·Te4 2+	[Bi4Cl16]2–	Lewis, Ox	Te + TeCl4 + BiCl3	HTS (150 °C)	565
[Te6]2+	[MCl6]2– (M = Zr, Hf)	Lewis, Ox	Te + TeCl4 + MCl4	HTS (220 °C)	103
[Te6]2+	[WCl4O]–	Ox	Te + WOCl4	HTS (150 °C)	566
[Te6]2+	[NbCl4O]–	Lewis, Ox	Te + TeCl4 + NbOCl3	HTS (200 °C)	567
[Te6]4+·2AsF3	[AsF6]–	Ox	Te + AsF5 in SO2		568
(Te7 2+) n	[AsF6]–	Other	[Te4]2+([AsF6]–)2 + Fe(CO)5 in SO2	Reduction of Te4 2+	569
(Te7 2+) n	[Be2Cl6]2–	Lewis, Ox	Te + TeCl4 + BeCl2	HTS (250 °C)	565
(Te7 2+) n	[WBr4O]–·[Br]–	Ox	Te + WOBr4/WBr5	HTS (230 °C)	570
(Te7 2+) n	[WCl4O]–·[Cl]–	Ox	Te + WOCl4/WCl5	HTS (150 °C)	571
(Te7 2+) n	[NbCl4O]–·[Cl]–	Ox	Te + TeCl4 + NbOCl3	HTS (225 °C)	101
(Te7 2+) n	[NbBr4O]–·[Br]–	Ox	Te2Br + NbOBr3	HTS (220 °C)	101
[Te8]2+	[WCl6]–	Ox	Te + WCl6	HTS (200 °C)	572
[Te8]2+	[ReCl6]2–	Lewis, Ox	Te + TeCl4 + ReCl4	HTS (230 °C)	573
[Te8]2+	[Bi4Cl14]2–	Lewis, Ox	Te + TeCl4 + BiCl3	HTS (160 °C)	574
[Te8]2+	[U2Br10]2–	Lewis, Ox	Te + TeBr4 + UBr5 in SiBr4	Solvothermal 200 °C	575
[Te8]2+	[Ta4O4Cl16]4–	Lewis, Ox	Te + TeCl4 + TaCl5 + TaOCl3 + [BMIM]+Cl–	Ionic liquid based synthesis	576
[Te8]4+	([VCl4O]2–) n	Ox	Te + VOCl3	HTS (270 °C), cubic	577
Metal–nonmetal-cluster complexes
[Cu(S12)(S8)]+	[Al(ORPF)4]–	Com	Cu+ + S		512 and 578
[Cu(S12)(CH2Cl2)]+	[Al(ORPF)4]–	Com	Cu+ + S		512 and 578
[Ag(S8)2]+	[Al(ORPF)4]–	Com	Ag+ + S		579
[Cu2Se19]2+	[Al(ORPF)4]–	Com	Cu+ + Se(red)		50
([Ag2(Se6)]2+)∞	[AsF6]–	Com	Ag+ + Se		513
[Ag(Se6)]+	[Ag2(SbF6)3]–	Com	Ag+ + Se		513
[Ag2(Se6)(SO2)2]2+	[Sb(OTeF5)6]–	Com	Ag+ + Se(grey)		513
[Ag2(Se6)(SO2)4]2+	[Al(ORPF)4]–	Com	Ag+ + Se(grey)		580
[Ag2Se12]2+	[FAl(OC(C5F10)(C6F5))3]–, [Al(ORPF)4]–)	Com	Ag+ + Se(red)		176 and 581
Clusters/cluster-like
NS+	[AlCl4]+	Lewis	(NSCl)3 + AlCl3		517
[NS2]+	[AlCl4]+	Other	S4N4 + AlCl3		582
[S4N4]2+	[SbCl6]–	Ox	S4N4/S3N3Cl3 + SbCl5 in SO2		583
[S4N4]2+	[Sb3F14]–·[SbF6]–	Ox	S4N4 + SbF5 in SO2		583
[S5N5]+	[SbCl6]–	Lewis	S3N3Cl3 + SbCl5 in SOCl2		584
[S3Cl3]+	[AsF6]–	Other	[SCl3]+[AsF6]– + S8 in SO2		585
[(S2N2C)2]2+	[SbF6]–, [Sb2F11]–	Other	[NS2]+[AsF6]– + (CN)2 in SO2		586
[S4Te4]2+·SO2	[AsF6]–	Ox	Te + Sn + AsF5 in SO2		587
[S3Te3]2+	[AsF6]–	Ox	S + Te + AsF5 in SO2		588
[Se3Cl3]+	[AsF6]–	Other	[SeCl3]+[AsF6]– + Se in SO2		585
[Se6Ph2]2+·2SO2	[AsF6]–	Other	[Se4]2+([AsF6]–)2 + Ph2Se2		589
[Se6I2]2+·2SO2	[AsF6]–	Ox	Te + I2 + AsF5 in SO2 or [SeI3]+[AsF6]– 2 + [Se8]2+([AsF6]–)2		590
[Te6I2]2+	[WCl6]–	Ox	Te + I2 + WCl6	HTS (150 °C)	566
([Te15X4]2+) n	([MOX4]–) n (M = Mo, X = Cl, Br; M = W, X = Br)	Other	Te2Br + MoOBr3, TeCl4 + MoNCl2/MoOCl3, Te + WBr5/WOBr3		591
[Se4Te2]2+	[MF6]– (M = As, Sb)	Ox	Se + Te + MF5 in SO2		588
[Se4Te3]2+	[MOCl4]–	Ox	Se + [Te6]2+([MOCl4]–)2	HTS (190 °C)	592
[Se6Te2]2+·[Se8Te2]2+·2SO2	[AsF6]–	Ox	S + Se + Te + AsF5 in SO2	Heterocubane	593
[Se8Te2]2+	[MF6]– (M = As, Sb)	Ox	Se + Te + MF5 in SO2	Isostructural to Se10 2+	593
[Se8Te2]2+·SO2	[AsF6]–	Ox	Te + [Se8]2+([AsF6]–)2 in SO2		594
(4n + 2)π-cations
[S2N3]+	[Hg2Cl6]2–	Lewis	NSCl + HgCl2		595
[S3N2]2+	[MF6]– (M = As, Sb)	Other	[SN]+[MF6]– + [S2N]+[MF6]– in SO2	Cycloaddition	515
[S4N3]+	([SeCl5]–)∞	Ox	S4N4 + Se2Cl2 in SOCl2	10π-aromatic	519
[S3Se]2+	[Sb3F16]–·3[SbF6]–	Ox	S + Se + SbF5 in SO2	Traces of Br2 were added, contains disordered mixture of [S x Se x–4]2+	596
[SSe2N2]2+	[AsF6]–	Ox	[(SSe2N2)2]2+([AsF6]–)2 + AsF5 in SO2		597
[S2SeN2]2+	[AsF6]–	Other	[NS]+[AsF6]– + [Se8]2+[AsF6]–		516
[S2SeN2Cl]+	[AlCl4]–	Other	[NS]+[AlCl4]– + Se/EtSeCl		517 and 518
[Se2Te2]2+	[Sb3F14]3–·[SbF6]–	Ox	Se + Te + SbF5 in SO2		598
[SeTe3]2+	[Sb3F14]3–·[SbF6]–	Ox	Se + Te + SbF5 in SO2	Contains disordered mixture of [SeTe3]2+, [Te4]2+ and [Se2Te2]2+	598
π*–π*-complexes
	[AsF6]–	Ox	S4N4 + AsF5 in SO2		599
[S6N4]2+	[S2O2F]–, [SO3F]–	Ox	S4N4 + HSO3F in SO2		600
	[AsF6]–	Ox	S + I2 + AsF5 in SO2		520
	[Sb2F11]–	Ox	Se + [I2]+[Sb2F11]– in SO2		521
	[MF6]– (M = As, Sb)	Other	S4N4 + [Se4]2+([MF6]–)2 in SO2		601
	[BF4]–	Ox	Me2Se2 + XeF2 + BF3·OEt2		82
[(MeSe)4]2+	[OTf]–	Ox	Et2Te2 + [NO]+[OTf]–		602
	[OTf]–	Ox	Et2Te2 + [NO]+[OTf]–		82 and 602
Radicals
	[AsF6]–	Ox	S4N4 + [Te4]2+([AsF6]–)2 in SO2		600
	[NTf2]–	Ox	TTF + 0.5XeF2 + TMSNTf2	TTF = Tetrathiafulvalene	82
	[SbCl6]–	Ox	(C8H10)2S2 + SbCl5	Strong transannular interactions	603
	[OTf]–	Ox	(NeoS)2 + [NO]+	Diorgano disulfide–nitrosonium adduct	604
	[FAl(ORPF)3]–	Ox	C12H8S2 + XeF2 + Al(OC(CF3)3)3	C12H8S2 = thianthrene	82
	[Al(ORPF)4]–	Ox	1,8-(SPh)2Nap + [NO]+	S–S-3e–σ-bond Nap = naphthalene	605
	[AsF6]–	Ox	S + AsF5 + (CN)2 in SO2	Traces of Br2 were added	522
(CNS3˙+)2	[SbF6]–, [Sb2F11]–	Ox	S + SbF5 + (CN)2 in SO2	Traces of Br2 were added	523
	[Sb2F11]–	Ox	(C6F5S)2 + SbF5		606
	[As2F11]–	Ox	(C6F5Se)2 + AsF5		606
	[SbF6]–	Ox	(2,6-Mes2C6H3Ch)2 + [NO]+		606
Oxidation state +II
[Me2S-SMe]+	[SbCl6]–	—	—		607
[MeS-S(Me)-SMe]+	[SbCl6]–	Ox, Lewis	S2Me2 + SbCl5	=[S3Me3]+	607
	[OTf]–	Com	S(OTf)2 + (NR)2C2H2	Sulfur carbenoid, oxidation state unclear	528
	[B(C6F5)4]–	Other	[S(NDipp)2C2H2]2+ ([OTf]–)2 + K+[B(C6F5)4]–	Sulfur carbenoid, salt metathesis, oxidation state unclear	528
	[X]– = [SbCl6]–, [BF4]–	Other	[S3Me3]+[X]– + C2H2 t Bu2	Thiiranium ion	608
	[X]– = [BF4]– [PF6]–	Other	[S3Me3]+[X]– + C2 t Bu2	Thiirenium ion	608
[MeSC2 t Bu2]+·CH2Cl2	[CHB11Cl11]–	Ox	Me2S2 + C2 t Bu2 + XeF2 + Me3Si(CHB11Cl11)		82
	[B(C6F5)4]–	Ox	Ph2S2 + C2 t Bu2 + XeF2 + [Me3Si(Tol)]+[B(C6F5)4]–	Thiirenium ion	82
[MeSe-Se(Me)-SeMe]+	[SbCl6]–	Ox, Lewis	Se2Me2 + SbCl5	=[Se3Me3]+	607
	[SbCl6]–	Other	[Se3Me3]+[SbCl6]– + C2 t Bu2	Selenirenium ion	526
[PhSeC2Ad2]+·CH2Cl2	[SbCl6]–	Other	[PhSe]+[SbCl6]– + C2Ad2		526
	[SnCl6]2–	Ox, Lewis	(NDipp)2C2H2 + SnCl2 + SeCl4	Selenium carbenoid	529
	[SbF6]–	Other	[PhTe]+[SbF6]– + C2 t Bu2	Tellurirenium ion	526
	[PF6]–	Other	C6H3(CH2NMe2)2SeMe + t BuOCl + K+[PF6]–		524
	[PF6]–	Ox	(C6H3(CH2)2Ch)2Se + [NO]+[PF6]–		609
	[OTf]–	Other	[(Dipp2BIAN)Te]2+ ([OTf]–)2 + DMAP	Ligand exchange	525
	[OTf]–	Other	[(Dipp2BIAN)Te]2+ ([OTf]–)2 + 2 iPrIM	Ligand exchange, anions coordinating	525
	[OTf]–	Other	[(Dipp2BIAN)Te]2+ ([OTf]–)2 + iPrIM	Ligand exchange, 4 equivalents of iPrIM	525
	[OTf]–	Hal	(Dipp2BIAN)TeI2 + Ag+[OTf]–	Tellurium carbenoid, Dipp2BIAN = 1,4-(2,6-diisopropyl)phenyl-bis(arylimino)acenaphthene	525
Oxidation state +IV
[SF3]+	[BF4]–	Lewis	SF4 + BF3	Crystals through sublimation	610
[SF3]+	[GeF6]2–	Lewis	SF4 + GeF4		611
[SCl3]+	[ICl4]–	Ox, Lewis	S + Cl2 + I2		612
[SCl3]+	[ICl4]–	Ox, Lewis	S + Cl2 + I2	Second modification	613
[SCl3]+	[UCl6]–	Lewis	SOCl2 + UCl5		614
[SCl3]+	[AlCl4]–	Lewis	SCl4 + AlCl3		615
[SCl3]+	[SbCl6]–	Ox, Lewis	As4S4 + SbCl5 in SO2		531
[SCl3]+	[MoOCl4]	Ox	S + MOCl4	HTS (100 °C), large excess of ICl3 was added	616
[SBr1.2Cl1.8]+	[SbCl6]–	Ox, Lewis	S + Br2 + SbCl5 in SO2	Attempt to prepare [SBr3]+[SbCl6]–	531
[SBr3]+	[AsF6]–	Lewis	S + Br2 + AsF5 in SO2		617 and 618
[(SX2)2N]+ (X = F, Cl)	[AsF6]–	Ox	[S2N]+[AsF6]– + X2 in SO2		619
[SeF3]+	[NbF6]–, [Nb2F11]–, [TaF6]–	Lewis	SeF4 + NbF5	First structure with a [ChX3]+ cation (Ch = S, Se, Te; X = F, Cl, Br, I)	620
[SeCl3]+	[AlCl4]–	Lewis	SeCl4 + AlCl3 in SO2Cl2		621
[SeCl3]+	[SbCl6]–	Ox, Lewis	As + Se + SbCl5, or SeCl4 + SbCl5 in SO2	Melt	531
[SeCl3]+	[MoOCl4]–	Others	[Se4]2+[MoOCl4]2– in SOCl2	Decomposition at 150 °C, β-modification	622
[SeCl3]+	[AuCl4]–	Lewis	SeCl4 + AuCl3		623
[SeBr3]+	[AsF6]–	Ox, Lewis	Se + Br2 + AsF5 in SO2	Small amount of AsF3 was added	624
[SeBr3]+	[SbF6]–	Ox, Lewis	[Se4]2+([SbF6]–)2 + Br2 + AsF5 in SO2		624
[SeBr3]+	[AlBr4]–	Lewis	SeBr4 + AlBr3	HTS (150 °C)	625
[SeI3]+	[AsF6]–	Ox, Lewis	[Se4]2+([SbF6]–)2 + I2 in AsF3		617 and 618
[SeI3]+	[SbF6]–	Ox, Lewis	Se + I2 + SbF5 in SO2
[TeF3]+	[Sb2F11]–	Lewis	TeF4 + SbF5		626
[TeF3]+	[SO4]2–	Ox, Lewis	Te + Br2 + AsF5 in SO2		531
[TeCl3]+	[AlCl4]–	Lewis	TeCl4 + AlCl3	Monoclinic	627
[TeCl3]+	[AlCl4]–	Lewis	S7TeCl2 + AlCl3	TeCl3[AlCl4] (triclinic)	531
[TeCl3]+	[AsF6]–	Lewis	TeCl4 + AsF5		531
[TeCl3]+	[SbF6]–	Lewis	TeF4 + SbF5 in CH2Cl2	Solvent decomposition	531
[TeCl3]+	[AuCl4]–	Lewis	TeCl4 + AuCl3		628
[TeCl3]+	[MoCl4O]–	Lewis	TeCl4 + MoOCl3	HTS (180 °C)	629
[TeCl3]+	[MoCl6]2–	Lewis	TeCl4 + MoCl4	HTS (195 °C)	630
[TeCl3]+	[Re2Cl6]–	Lewis, Ox	Te + TeCl4 + ReCl5	HTS (150 °C), β-modification	630
[TeCl3]+	[MoCl6]2–·[Cl]–	Lewis	TeCl4 + MoCl4	HTS (300 °C)	631
[TeCl3]+	[MCl6]– (M = Nb, Ta)	Lewis	TeCl4 + MCl5	α- and β-modification	632
[TeCl3]+	[WCl6]–	Lewis	TeCl4 + WCl6		632
[TeBr3]+	[AsF6]–	Ox, Lewis	Te + Br2 + AsF5 in SO2		624
[TeBr3]+·1/2Br2	[AuBr4]–	Ox, Lewis	Te + Au + Br2	HTS (160 °C)	633
[TeBr3]+	[Zr2Br9]–	Other	[Te4]2+[Zr2Br10]2–	Decomposition above 250 °C	560
[TeBr3]+	[MBr6]– (M = Ta, W)	Lewis	TeBr4 + MBr5		632
[TeI3]+	[AsF6]–	Ox, Lewis	Te + I2 + AsF5 in SO2		634
[TeI3]+	[SbF6]–	Ox, Lewis	Te + I2 + SbF5 in SO2		617 and 618
[TeI3]+	[AlI4]–	Lewis, Ox	Te + I2 + AlI3	HTS (150 °C)	625
[TeI3]+	[MI4]–, (M = Ga, In)	Lewis, Ox	Te + M + I2	HTS (350 °C)	635
[TeI3]+·1/2SO2	[AsF6]–	Ox, Lewis	Te + I2 + AsF5 in SO2	Hemisolvate of [TeI3]+[AsF6]–	636
[TeI3]+	[Al(ORPF)4]–	Salt	TeX4 + Ag+[Al(ORPF)4]–	Good solubility	637
[TePh3]+	[B(C6F5)4]–	Other	[TePh5]+[B(C6F5)4]–	Thermal decomposition at 150 °C	532
[Te(C6F5)3]+	[OTf]–	Other	Te(C6F5)4 + TMSOTf		533
[Te(N3)3]+	[SbF6]–	Ox	[Te4]2+([SbF6]–)2 + KN3 in SO2	Side products may be explosive!	534
[F(TeCl3)2]+	[Sb(OTeF5)6]–	Ox, Lewis	TeBr4 + Ag+[Sb(OTeF5)6]– in SO2ClF		28
	[BF4]–	Ox	(C6H3(CH2ChPh)2)TePh + [NO]+[BF4]–		638
	[OTf]–	Ox	(C6H3(CH2SPh)2)TePh + t BuOCl + O(Tf)2		638
Oxidation state +VI
	[B(C6F5)4]–	Hal	TePh5Cl + Ag+[OTf]– + Li+[B(C6F5)4]–		532
[TePh5]+	[ClO4]–	Hal	TePh5Cl + Ag+[ClO4]–		535
[TePh5]+···MeCN	[B(ArCF3 )4]–	Hal	TePh5Cl + Ag+[OTf]– + [B(ArCF3 )4]–		535
Protonated cations
	[AsF6]–	Prot	OC(OTMS)2 + HF/AsF5		537
	[Ge3F16]4–	Prot	Me2SO + HF/GeF4		639
	[SbF6]–	Prot	OTf2 + HF/SbF5		536
	[SbF6]–	Prot	SO2(OTMS)2 + HF/SbF5		538

^a Classification according to the introduction (Table 2): Com = complexation reaction, Lewis = Lewis acid halogen bond heterolysis, Prot = protonation, Ox = oxidation, other = all other reactions not classified.

^b HTS = high temperature synthesis.

Group 17 cations

The electronegative halogen atoms are strongly oxidizing and have high ionization energies as well as electron affinities. This makes halogen cations good electrophiles, which need very oxidation-resistant WCAs to be stabilized in condensed phases. Concerning the synthesis of these highly reactive compounds, there are many similarities to the noble gas cations. Almost every halogen cation, for which a crystal structure is known, was synthesized through a halide abstraction by a strong Lewis acid like MF₅ (M = As, Sb). The starting materials are normally neutral interhalogen compounds like ClF₃, BrF₅, IF₇, I₂Cl₆, or IBr, and the majority of the obtained cations contains halogen atoms in oxidation state +III, +V and +VII (Table 7).

Table 7. Review articles including cationic group 17 compounds.

Year	Title	Ref.
2000	Recent advances in the understanding of the syntheses, structures, bonding and energetics of the homopolyatomic cations of groups 16 and 17	503
2008	Polyvalent perfluoroorgano- and selected polyfluoroorgano-halogen(iii and v) compounds	640
2013	Recent advances in the syntheses of homopolyatomic cations of the non-metallic elements C, N, P, S, Cl, Br, I and Xe	11

Fluorine cations

Because of the high electronegativity and ionization potential of fluorine, it would be very difficult to oxidize it and obtain actual fluorine cations in the condensed phase. By contrast, in the gas phase this is possible. ⁶⁴¹ Some bulk cationic compounds contain fluorine, but it is very unlikely that the positive charge is actually localized on the fluorine atom. Such compounds like for example the in Fig. 94 shown compound that was published as a formal disilylfluoronium ion are discussed in the chapters of the element, which has a larger positive charge density (here silicon). ⁶⁴²

Fig. 94. Left: Formal disilylfluoronium ion that bears a larger positive charge density at the silicon atoms. Therefore, it is discussed with the silylium cations in the section on group 14 rPBC. Right: The Selectfluor reagent.

It should be mentioned that also electrophilic “F⁺-” or “N–F-” reagents like “Selectfluor” belong to this class of compounds that are very useful for organic transformations and compatible with solvents like CH₂Cl₂. ⁶⁴³

Chlorine, bromine and iodine cations

Homopolyatomic cations

Two homopolyatomic chlorine cations are known in the solid state and its crystal structures of [Cl₃]⁺ and [Cl₄]⁺ were published in 1999 ⁴² and 2000. ⁶⁴⁴ [Cl₂]⁺, which would be the lighter homologue to the known [Br₂]⁺ and [I₂]⁺, was only detected in the gas phase. [Cl₃]⁺ can be synthesized through a reaction of ClF with AsF₅, with the adduct ^δ+Cl–F ^δ– → AsF₅ as intermediate. This leads to a more activated ClF with a more positively charged chlorine atom and results in the formation of [Cl₂F]⁺[AsF₆]^–, which can be described as a formal “Cl⁺” stabilized by a second equivalent of ClF. When elemental chlorine is used instead of a second equivalent of ClF, [Cl₃]⁺ is formed. [Cl₄]⁺, which is a homopolyatomic cation but also a π*–π*-complex (see below: π*–π*-complexes of group 17) can be obtained by direct oxidation of chlorine with the strong one-electron oxidant IrF₆. For bromine, three cations are known. Of those, [Br₂]⁺ was one of the first homopolyatomic cations of the non-metals for which the crystal structure was determined. It was already in 1968 that Edwards et al. stabilized it with the very good [Sb₃F₁₆]^– WCA. ⁶⁶³ The structures of the other two known cations [Br₃]⁺ and [Br₅]⁺ were measured relatively late in 1991. [Br₅]⁺ can be synthesized by oxidation of elemental bromine with the strong oxidant [XeF]^{+  211} and the only measurable crystals of a [Br₃]⁺ salt were obtained from a 20 year old [BrF₂]⁺[AsF₆]^– solution. ⁶⁴⁵ Iodine has five known cations. [I₂]⁺, [I₃]⁺ and [I₅]⁺ are isostructural to the lighter homologues and can all be obtained by oxidation from I₂ with the strong Lewis acids MF₅ (M = As, Sb). [I₄]²⁺ was synthesized through the entropically unfavorable dimerization of the paramagnetic [I₂]⁺˙ radical cation at low temperature and can be described as an rectangular planar diamagnetic π*–π*-complex (see below: π*–π*-complexes of group 17) (Fig. 95).

Fig. 95. Structurally characterized homopolyatomic cation of chlorine, bromine and iodine.

Metal–halogen complexes

The very weak coordination power of the neutral dihalogen molecules X₂ (F₂, Cl₂, Br₂, I₂) made it very difficult to obtain metal–halogen-complexes. Only for diiodine, which is the strongest donor, it was possible to get polymeric [{Ag(I₂)} _n ] ⁿ⁺ cations through the reaction of [Ag]⁺[MF₆]^– (M = As, Sb) and I₂ in liquid SO₂. ⁶⁴⁶ Also one example of a neutral complex with diiodine as ligand is known: [Rh₂(O₂CCF₃)₄(I₂)]·I₂. ⁶⁴⁷ Dibromine and dichlorine complexes remained unknown. However, very recently, ⁶⁴⁸ the use of the very weakly coordinating solvent perfluorohexane and one of the weakest anions [Al(OR^PF)₄]^– led to the isolation of the first dichlorine and dibromine complexes [Ag(X₂)]⁺ (X = Cl, Br, also I) (Fig. 96). Moreover, diiodine turned out to have a rich coordination chemistry and formed three further structures with Ag₂I₂-moiety as well as isolated [Ag₂(I₂)₄]²⁺ as well as [Ag₂(I₂)₆]²⁺ dications well separated from the counterion (Fig. 96). It should be noted that the [Ag(X₂)]⁺ cations are structurally related to the [X₃]⁺ cations and the polymeric [{Ag(I₂)} _n ] ⁿ⁺ cations bear some similarity to [I₅]⁺.

Fig. 96. Structurally characterized metal complexes with dihalogen molecules as ligands.

π*–π*-complexes

The three halogen containing cations that can be described as π*–π* complexes, can be understood as adducts of two homonuclear diatomic (radical) cations. According to its synthesis from chlorine and [O₂]⁺[SbF₆]^–, [Cl₂O₂]⁺˙ can be described as complex of the paramagnetic [O₂]⁺˙ cation with Cl₂. For [Cl₄]⁺˙, which is made from Cl₂ and the strong oxidant IrF₆, an initially formed paramagnetic [Cl₂]⁺˙ could react with another equivalent of Cl₂. In the case of [I₄]²⁺, which was obtained from an [I₂]⁺˙ solution at low temperature, a dimerization of the two paramagnetic [I₂]⁺˙ units to give the [I₄]²⁺ dimer is obvious. All three syntheses have in common that at least one molecule with a half-filled π* orbital as HOMO is involved, for [I₄]²⁺ even both “starting materials”. The formed π*–π* interaction includes two quarter bonds for [Cl₂O₂]⁺˙ (243/241 pm, Σ vdW radii: 327 pm) and [Cl₄]⁺˙ (293 pm, Σ vdW radii: 350 pm) and two half bonds for [I₄]²⁺, where the interaction leads to 2e-4c π*–π* bond (328 pm, Σ vdW radii: 396 pm, Fig. 97). ^11,503

Fig. 97. Structurally characterized halogen cations, which include π*–π*-interactions. Bond lengths: [Cl₂O₂]⁺: O–O 121, Cl···O 241/243, Cl–Cl 191; [Cl₄]⁺: Cl–Cl 194, Cl···Cl 293; [I₄]²⁺: I–I 258, I···I 328 [pm].

Oxidation state +I

The isolated halogens X in this oxidation state would either correspond to a triplet state X⁺ or the triplet dimer [X = X]²⁺ – isoelectronic to the dichalcogens O₂ to Te₂. Neither the monomer, nor the dimer cation are hitherto known in condensed phase. Only one cation type with a formal oxidation state of +I is known: [I(Donor)₂]⁺, in which the electrophilicity of I⁺ is dampened by coordination of neutral donor molecules. Thus, [I₃]⁺[AsF₆]^– reacted with acetonitrile to form the structurally characterized salt [I(NCMe)₂]⁺[AsF₆]^– (Fig. 98). ⁶⁴⁹ The closely related [I(py)₂]⁺[BF₄]^– is a useful reagent in organic chemistry. ⁶⁵⁰ For the latter it is disputable, if this is better assigned as being a pyridinium cation.

Fig. 98. [I(NCMe)₂]⁺ cation in [I(NCMe)₂][AsF₆] with iodine in the formal oxidation state +I. ⁶⁴⁹ .

Oxidation state +III

The known interhalonium cations with the central halogen atoms in oxidation state +III are [ClF₂]⁺, [BrF₂]⁺, [ICl₂]⁺, [IBrCl]⁺ and [IBr₂]⁺ (Fig. 99). All were synthesized by halide abstraction from a neutral interhalogen. Others, like [Cl₂F]⁺ and [IF₂]⁺ are also accessible, but no crystal structures are known. ⁶⁵¹ The cations [I₃Cl₂]⁺ and [I₃Br₂]⁺ can also be understood as interhalonium compounds with two iodine atoms in oxidation state +III and one in –I and are in some way the lighter homologues of the homopolyatomic cation I₅ ⁺. ^652–654 Two recently published examples are the dialkyl chloronium cations [ClR₂]⁺ (R = Me, Et), which were synthesized through alkylation of chloromethane and –ethane, both stabilized by the very good carborate WCA [CHB₁₁Cl₁₁]^– (Fig. 100). The superacid H(CHB₁₁Cl₁₁) was used to generate ion-like methyl and ethyl cations “R⁺” by protonation of the chloroalkanes RCl, which than react with a second equivalent of RCl to the chloronium cations. ⁶⁵⁵

Fig. 99. Structurally characterized halonium cations.

Fig. 100. Molecular structure of [ClMe₂]⁺[CHB₁₁Cl₁₁]^–. E. S. Stoyanov, I. V. Stoyanova, F. S. Tham and C. A. Reed, J. Am. Chem. Soc., 2010, 132, 4062–4063. Data from this reference were used to draw this figure. Bond lengths in [pm].

In case of bromine and iodine, also some new examples were synthesized during the last 20 years. The stable cyclic bromonium and iodonium ions of sterically hindered olefins (Fig. 101) were stabilized by the [OTf]^– anion and can be seen as stable intermediates of the halogenation of olefins. ⁶⁵⁶

Fig. 101. Stable [OTf]^– salts of cyclic bromonium and iodonium ions. ⁶⁵⁶ .

Oxidation state +V

There are also some examples of cations in oxidation state +V. Besides the classical [XF₄]⁺ cations (X = Cl, Br, I), the two oxocations [ClO₂]⁺ and [BrO₂]⁺ as well as since 2008 also one example of a cation with a X^V–carbon bond were structurally characterized. This electrophilic [IF₂(C₆F₅)₂]⁺ cation was synthesized as [BF₄]^– salt through halide abstraction and ligand exchange from C₆F₅IF₄ and C₆F₅BF₂. ⁶⁵⁷ Examples without crystal structures include [OClF₂]^{+  658} and [OBrF₂]⁺ (Fig. 102). ⁶⁵⁹

Fig. 102. Structurally characterized halogen cations with halogen atoms in oxidation state +V. ^{657,660–662} .

Oxidation state +VII

In 2004, with the octahedral complexes of the series [XF₆]⁺ (X = Cl, Br, I) the first structures of cations with oxidation state +VII were published (Fig. 103). All three cations were crystallized with the anion [Sb₂F₁₁]^– and have very weak contact to the anion (Fig. 104). It is also possible to obtain the [AsF₆]^– and [SbF₆]^– salts, but the differentiation between the octahedral [XF₆]⁺ and [MF₆]^– would not be easily done by X-ray crystallography (e.g., for [BrF₆]⁺[AsF₆]^– or [IF₆]⁺[SbF₆]^–). The use of [Sb₂F₁₁]^– allowed for a clear differentiation between cations and anions (Table 12). ⁶⁶⁰

Fig. 103. Structurally characterized halogen cations with halogen atoms in oxidation state +VII. ⁶⁶⁰ .

Fig. 104. Molecular structure of [BrF₆]⁺[Sb₂F₁₁]^–. J. F. Lehmann, G. J. Schrobilgen, K. O. Christe, A. Kornath and R. J. Suontamo, Inorg. Chem., 2004, 43, 6905–6921. Data from this reference were used to draw this figure. Bond lengths in [pm].

Table 12. Overview on all structurally characterized halogen cations.

Cation	Anion	Class a	Synthesis	Comment	Ref.
Homopolyatomic cations
[Cl3]+	[AsF6]–	Lewis	[Cl2F]+[AsF6]– + Cl2 + AsF5		42
[Cl3]+	[X]– = [SbF6]–, [Sb2F11]–, [Sb3F16]–	Other	[Cl2O2]+[X]– + Cl2 in HF at RT		42
[Br2]+	[Sb3F16]–	Ox	Br2 + HSO3F/SbF5/3SO3		663
[Br3]+	[AsF6]–	Other	[BrF2]+[AsF6]– decomposition	Store [BrF2]+[AsF6]– for 20 years (!)	645
[Br5]+	[MF6]– (M = As, Sb)	Ox	[XeF]+[MF6]– + Br2		211
[I2]+	[Sb2F11]–	Ox	I2 + SbF5		15
[I3]+	[AsF6]–	Ox	I2 + AsF5		15
[I5]+	[AsF6]–	Ox	I2 + AsF5		664
[I15]3+	[SbF6]–	Ox	I2 + SbF5		15
Metal–nonmetal-cluster complexes
[Ag(Cl2)]+	[Al(ORPF)4]–	Com	Ag+[Al(ORPF)4]– + Cl2	Only stable at low temperature
[Ag(Br2)]+	[Al(ORPF)4]–	Com	Ag+[Al(ORPF)4]– + Br2
[Ag2(I2) x ]2+ (x = 1, 4, 6)	[Al(ORPF)4]–	Com	Ag+[Al(ORPF)4]– + I2
[Ag(I2)] n n+	[MF6]– (M = Sb, As)	Com	Ag+[MF6]– + I2	First complex with halogen as donor	646
π*–π*-complexes
[Cl4]+	[IrF6]–	Ox	Cl2 + IrF6	Radical, homo-polyatomic cation	644
[Cl2O2]+	[X]– = [SbF6]–, [Sb2F11]–	Com	[O2]+ + Cl2		42
[I4]2+	[MF6]– (M = As, Sb), and [Sb3F14]–/[SbF6]–	Com	2[I2]+	Also homopoly-atomic cation	665
Oxidation state +I
[I(NCMe)2]+	[AsF6]–	Other	[I3]+ + MeCN	[I3]+ as I+ donor	649
Oxidation state +III
[ClF2]+	[AsF6]–	Lewis	ClF3 + AsF5		666
[ClF2]+	[SbF6]–	Lewis	ClF3 + SbF5		667
[ClF2]+	[RuF6]–	Lewis	ClF3 + RuF5		668
[ClMe2]+[ClEt2]+	[CHB11Cl11]–	Prot	H(CHB11Cl11) + RCl (R = Me, Et)	Protonation of RCl	655
[BrF2]+	[SbF6]–	Lewis	BrF3 + SbF5		669
[Br(C6F5)2]+	[BF4]–	Lewis	BrF3 + (C6F5)2BF		670
	[OTf]–	Lewis	C20H28 + Br2 + MeOTf	Stable bromonium ion C20H28 = adamantyli-deneadamantane	656
[ICl2]+	[SbF6]–	Lewis	I2Cl6 + SbF5		671
[IBr2]+	[Sb2F11]–	Lewis	IBr + SbF5		672
[IBr0.75Cl1.25]+	[SbCl6]–	Lewis	IBr + Cl2 + SbF5		672
[I3Cl2]+	[X]– = [SbCl6]–, [AlCl4]–	Lewis, other	I2 + SbCl5		652–654
[I3Br2]+	[SbCl6]–	Lewis, other	I2 + SbCl5		653
[I(C6F5)R]+	[BF4]–	Lewis	I(C6F5)F2 + RBF2 R = C6H5, o-C6H4F, m-C6H4F, p-C6H4F, 2,4,6-C6H2F3, C6F5	Strong interactions with the anions	673
	[OTf]–	Lewis	C20H28 + I2 + Ag+[OTf]–	Stable iodonium ion C20H28 = adamantyli-deneadamantane	656
Oxidation state +V
[ClF4]+	[SbF6]–	Lewis	ClF5 + SbF5		661
[ClO2]+	[SbF6]–	Lewis	ClO2F + HF/SbF5		674
[ClO2]+	[Sb2F11]–	Lewis	ClO2F + 2SbF5	Also an oxidation product of [ClF2]+	675
[ClO2]+	[X]– = [BF4]–, [GeF5]–	Lewis	ClO2F + BF3, GeF4		611 and 676
[ClO2]+	[ClO4]–	Ox	ClO2 + O3	Cl2O6	677
[ClO2]+	[RuF6]–	Lewis Ox	ClO2F + 2RuF5 ClF3 + HF/RuO4		668
[BrF4]+	[Sb2F11]–	Lewis	BrF5 + 2SbF5		662 and 678
[BrO2]+	[SbF6]–	Lewis, other	BrO3F + SbF5	Reduction of BrVII	674
[IF4]+	[SbF6]–	Lewis	IF5 in HF/SbF5		662 and 679
[IF4]+	[Sb2F11]–	Lewis	IF5 + 2HF/SbF5		662 and 680
[I(C6F5)2F2]+	[BF4]–	Lewis	C6F5IF4 + C6F5BF2		657
Oxidation state +VII
[ClF6]+	[Sb2F11]–	Ox, Lewis	ClF5 + F2 + SbF5		660
[BrF6]+	[Sb2F11]–	Ox, Lewis	BrF5 + F2 + SbF5		660
[IF6]+	[Sb2F11]–	Lewis	IF7 + SbF5		660

^a Classification according to the introduction (Table 2): Lewis = Lewis acid halogen bond heterolysis, Ox = oxidation, Com = complexation reaction, Other = all other reactions not classified.

Group 18 cations

Due to the high ionization potential of the noble gases, their cations all need weakly coordinating and very oxidation resistant anions to be stabilized. The history of noble gas compounds started in the early 1960s, when Bartlett obtained “[Xe]⁺[PtF₆]^–” from a reaction of [O₂]⁺[PtF₆]^– and xenon – the noble gas with the lowest ionization potential. Over the next decades the composition of this product stayed unclear. In 2000 a very comprehensive review about the nature of the product was published with the result that it is very likely a [XeF]⁺ salt of a polymeric (weakly coordinating) [PtF₅] _n anion. ⁶⁸¹ All of the hitherto known and structurally characterized noble gas cations are included with Table 13.

Table 13. Cationic noble gas compounds with weakly coordinating anions.

Cation	Anion	Class. a	Synthesis	Comment	Ref.
Homopolyatomic cations
[Xe2]+	[Sb4F21]–	Lewis	[XeF]+/Xe + SbF5	First homopoly-atomic cation	27
[Xe4]+	[Sb x F5x+1]–	Lewis	[XeF]+[Sb2F11]–, SbF5 + Xe	No X-ray	696
Metal–nonmetal-cluster complexes
[AuXe4]2+	[Sb2F11]–	Lewis, Com	AuF3, Xe in HF/SbF5	First xenon metal complex, gold(ii) 2 modifications: triclinic and tetragonal	695 and 697
[trans-AuXe2]2+	[SbF6]–	Com	HAuCl4, XeF2, Xe in HF/SbF5	Gold(ii)	695
[cis-AuXe2]2+	[Sb2F11]–	Com	[AuXe4]2+([Sb2F11]–)2–Xe at RT	Gold(ii)	695
[trans-AuXe2F]2+	[SbF6]– + [Sb2F11]–	Com	Au, XeF2, Xe in HF/SbF5	Gold(iii)	695
[Au2Xe2F]3+	[SbF6]–	Com	Au, XeF2, Xe in HF/SbF5	Gold(ii)	695
[(F3As)AuXe]+	[Sb2F11]–	Com	AuF3, Xe, AsF3 in HF/SbF5		698
[HgXe]2+	[SbF6]– + [Sb2F11]–	Com	HgF2, Xe in HF/SbF5		698
Oxidation state +II
[KrF]+	[MF6]– (M = As, Sb, Bi)	Lewis	KrF2 + MF5	First krypton cation	699
[KrF]+	[AuF6]–	Lewis	KrF2 + Au		682
[Kr2F3]+·KrF2, [Kr2F3]+·1/2KrF2, [Kr2F3]+·[KrF]+	[SbF6]–[SbF6]–[AsF6]–	Lewis	KrF2 + MF5		699
[XeF]+	[Sb2F11]–	Lewis	XeF2 + 2SbF5		687
[XeF]+	[RuF6]–	Lewis	XeF2 + RuF5		700
[XeF]+	[AsF6]–	Lewis	XeF2 + AsF5		701
[XeF]+	[N(SO2F)2]–	Lewis	XeF2 + HN(SO2F)2	First Xe–N bond, strong interaction with the anion	702
[XeF]+·HF	[Sb2F11]–	Lewis	XeF2 + 2SbF5		27
[XeF]+	[X]– = [AsF6]–, [SbF6]–, [Sb2F11]–, [BiF6]–, [Bi2F11]–	Lewis	XeF2 + AsF5, SbF5, BiF5	Better structures for [XeF]+[X]–: [X]– = [MF6]–, [SbF6]–, [Sb2F11]–	691
[Xe2F3]+	[AsF6]–	Lewis	2XeF2 + AsF5	Monoclinic structure	703 and 704
[Xe2F3]+	[AuF6]–	Lewis	XeF2 + AuF5		686
[Xe2F3]+	[MF6]– (M = As, Sb)	Com	[XeF]+ as starting material	[Xe2F3]+[AsF6]– trigonal structure	705
[Xe(N(SO2F)2)]+	[Sb3F16]–	Lewis	3 step synthesis with AsF5 and SbF5		706
[XeC6F5]+	[(F5C6)2BF2]–	Lewis	XeF2 + B(C6F5)3	First Xe–C bond	693
[(MeCN)XeC6F5]+[XeC6F5]+	[BX]– (X = CF3, C6F5) [BX]– (X = CF3, CN)	Lewis, Com	XeF2 + C6F5BF2	Salt metatheses with [XeC6F5]+[BF4]–	707
[(C6F5Xe)2Cl]+	[AsF6]–	Other	[XeC6F5]+ + TMSCl	First Xe–Cl bond	708
[XeOChF5]+ (Ch = Se, Te)	[AsF6]–	Lewis	FXeOChF5 + AsF5	First Xe–O bond	709
[XeOTeF5]+·SO2ClF	[Sb(OTeF5)6]–	Other	Xe(OTeF5)2 + Sb(OTeF3)3	OTeF5 – abstraction	710
[XeCl]+	[Sb2F11]–	Other	[XeF]+ + [Cl]–	Ligand exchange	711
[XeN(H)TeF5]+	[AsF6]–	Lewis	[F5TeNH3]+[AsF6]– + XeF2	First Xe–N(sp3) bond	712
[(F3SN)XeF]+	[AsF6]–	Com	F3SN + [XeF]+	First Xe–N(sp) bond	713
[XeNSF4]+	[AsF6]–	Other	In solid state and HF/BrF5 solution	Rearrangement of [F3SNXeF]+[AsF6]–	714
[(F3SN)XeNSF4]+	[AsF6]–	Com	[F3SNXeF]+ + F3SN		715
[Xe3OF3]+	[MF6]– (M = As, Sb)	Other	H2O + XeF2/[XeF]+	Hydrolysis to XeFOH followed by a reaction with [Xe2F3]+	716
Oxidation state +IV
[XeF3]+	[Sb2F11]–	Lewis	XeF4 + 2SbF5		717
[XeF3]+	[BiF6]–	Lewis	XeF4 + BiF5		718
[XeF3]+	[Sb2F11]–	Lewis	XeF4 + SbF5	Better structure	691
[XeF3]+·HF [H5F4]+·2([XeF3]+·HF) [XeF3]+	[Sb2F11]–[SbF6]·2[Sb2F11]–[SbF6]–	Lewis	XeOF2·xHF + HF/SbF5		719
Oxidation state +VI
[XeF5]+	[PtF6]–	Lewis	XeF6 + PtF5	First xenon cation structure	688 and 689
[XeF5]+	[AsF6]–	Lewis	XeF6 + AsF5		704
[XeF5]+	[SbF6]–	Ox	[XeF]+[SbF6]– + F2 in aHF		692
[XeF5]+	[Sb2F11]–	Other	[XeF5]+[SbF6]–	Crystals from a O2SbF6/XeF5SbF6 mixture	692
[XeF5]+	[RuF6]–	Lewis	XeF6 + RuF5		700
[XeF5]+	[PdF6]2–	Lewis	2XeF6 + PdF4		720
[XeF5]+	[Ti4F19]3–	Lewis	XeF6 + TiF4	XeF6 from XeF2, F2 and UV radiation	721
[XeF5]+	[m-F(OsO3F2)2]–[OsO3F2]–	Lewis	XeF6 + (OsO3F2)∞		690
[XeF5]+	[Cu(SbF6)3]–	Other	[XeF5]+[SbF6]– + Cu+[SbF6]– in aHF	Anion exchange	43
[XeF5]+	[AgF4]–	Lewis	AgF2 + KrF2 + XeF6 in aHF		722
[XeF5]+	[AuF4]–	Lewis	XeF6 + BrF3 AuF3		722
[XeF5]+·XeF2·[XeF5]+·2XeF2 [XeF5]+·1/2XeF2	[AsF6]–[AsF6]–[AsF6]–	Com	XeF2 + [XeF5]+[AsF6]–		723
[XeF5]+·XeOF4	[SbF6]–	Lewis	XeF6 + [H3O]+[SbF6]–		694
[XeF5]+·NO2 +	[SbF6]–	Other	[XeF5]+[SbF6]– + [NO2]+[SbF6]–		43
[Xe2F11]+	[AuF6]–	Lewis	2XeF6 + AuF5		724
[Xe2F11]+	[OsO3F2]–	Lewis	XeF6 + (OsO3F2)∞		690
[XeO2F]+[F(XeO2F)2]+	[MF6]– (M = As, Sb) [AsF6]–	Lewis	XeO2F2 + MF5 (M = As, Sb)	α- and β-modification of [XeO2F]+[SbF6]–	694

^a Classification according to the introduction (Table 2): Lewis = Lewis acid halogen bond heterolysis, Com = complexation reaction, other = all other reactions not classified.

Krypton cations

The only other noble gas besides xenon, for which cations are known, is krypton. The oxidation state +II is the only one known yet. The diversity of the amount of compounds and the number of hitherto realized Kr–X bonds is much smaller. Only two different fluorine-containing cations are known ([KrF]⁺ and [Kr₂F₃]⁺) and some nitrile complexes, which can be described as Kr–N compounds. [KrF]⁺ is a very strong oxidative fluorinating reagent ⁶⁸² and made is possible to synthesize [XeF₅]⁺ from Xe, ⁶⁸³ [ClF₆]⁺ from ClF₅, ⁶⁸⁴ [BrF₆]⁺ from BrF₅ ⁶⁸⁵ and [O₂]⁺ from O₂. ⁶⁸⁶ There are only seven crystal structures of krypton cations in the literature and five of them with the classical [AsF₆]^– and [SbF₆]^– anions.

Xenon cations

The lower ionization potential calls for a richer chemistry of xenon. Thus, since the birth of noble gas chemistry, a multitude of different xenon cations containing the element in the oxidation state II, IV and VI were synthesized.

Homopolyatomic cations

The two known homopolyatomic cations are [Xe₂]⁺ and [Xe₄]⁺. It was only possible to obtain a crystal structure from [Xe₂]⁺, but because of its importance [Xe₄]⁺, which was assigned based on spectroscopic and computational evidence as being stable at higher Xe pressure, is also mentioned.

Fluoroxenon cations and related

The majority of such cations include Xe–F bonds ([XeF _m ]⁺, m = 1, 3, 5; [Xe₂F _n ]⁺, n = 3, 11, Fig. 105), but over the decades a lot of different cations with Xe–X (X = F, Cl, O, N, C) bonds were obtained and are included with Table 13 and in part also with Fig. 106. Some of them were already published in the late sixties, ^687–689 but also recently with [XeF]⁺[SbF₆]^– and [XeF₃]⁺[SbF₆]^– some new structures were presented ^690,691 and in 2015 the crystal structures of [XeF₅]⁺ with the classical WCAs [SbF₆]^– and [Sb₂F₁₁]^– were measured for the first time, ⁶⁹² which shows that the investigation of xenon cations is still in progress.

Fig. 105. Fluoroxenon cations.

Fig. 106. Examples of Xe–X bonds (X = C, Cl, N, O) in cations.

Most of the syntheses use Lewis acids to abstract fluoride from the neutral xenon fluoride (XeF₂, XeF₄, XeF₆). Most structures contain the conjugated weakly coordinating anions of these Lewis acids ([MF₆]^–, M = As, Sb, Au, Ru). Another mentionable approach is the reaction of XeF₂ with the Lewis acid B(C₆F₅)₃, which led to the [F₅C₆Xe]⁺ cation containing the first Xe–C bond and the unsymmetric anion [(F₅C₆)₂BF₂]^–. ⁶⁹³ Through the use of the dioxydifluoride XeO₂F₂ as starting material, it was also possible to obtain the mixed cation [XeO₂F]⁺ or the fluoride bridged [F(XeO₂F)₂]⁺ (Fig. 107). ⁶⁹⁴

Fig. 107. Examples of Xe–O–F cations.

Metal–xenon complexes

There are also some examples of metal–xenon complexes (M = Au^I, Au^II, Au^III and Hg^II), which can be described as part of the stabilization of modifications of main group elements as metal complexes ([M _m (E _n )]⁺). Formally, Xe is isoelectronic to iodide I^–, and thus complex formation appeared to be difficult, but feasible. The syntheses proceed under the superacidic conditions of the systems HF/MF₅ (M = As, Sb). In case of these compounds, the role of the anions can be described as unreactive, but not really weakly coordinating. Most of them have relatively short contacts to the anions (shorter than the sum of the van der Waals-radii) and show typical coordination spheres with the anions included (e.g. square planar trans-Au^IIXe₂(SbF₆)₂) (Fig. 108). ⁶⁹⁵

Fig. 108. Molecular structure of trans-Au^IIXe₂(SbF₆)₂. T. Drews, S. Seidel and K. Seppelt, Angew. Chem., 2002, 114, 470–473; Angew. Chem., Int. Ed., 2002, 41, 454–456. Data from this reference were used to draw this figure.

Only the [AuXe₄]²⁺ dication exists in a truly ionic structure with two [Sb₂F₁₁]^– counterions in the lattice (Fig. 109).

Fig. 109. Molecular structure of tetragonal [Au^IIXe₄]²⁺([Sb₂F₁₁]^–)₂. T. Drews, S. Seidel and K. Seppelt, Angew. Chem., 2002, 114, 470–473; Angew. Chem., Int. Ed., 2002, 41, 454–456. Data from this reference were used to draw this figure.

Conclusion

Sparked by the availability of new WCAs and new WCA starting materials in combination with novel concepts like FLP and others, the number of rPBC exploded over the last one to two decades. Noteworthy additions were found for each p-block element and, despite their quite high moisture and air sensitivity, true applications of rPBC salts emerged.

Where will this lead to over the next one or two decades…? To our understanding the blue sky synthesis of rPBC salts barely accessible with good/novel WCAs in combination with suitable media will continue to function as an “eye-opener” of what is possible. Many surprising discoveries will force us to sharpen our use of bonding concepts or lead to novel applications. It is often the combination of structural knowledge (“Wow, this crazy cation is stable…? I would have never thought so.”) that leads to the right moment of wonder and then inspiration (“Hm, if this cation is really straight forward accessible, one could use its electrophilic/acidic/oxidizing/activating properties in application XY”). In the 21st century, it is our duty as creative scientists to use this potential from fundamentals to the first application. Do not hesitate to really seek for application of your rPBC salt, as rarely others will pick up on these ideas, since the activation barrier for synthesizing a to this application group unknown rPBC is simply too high. So do not give up until you have demonstrated a possible application – yourself or through collaborations – to a level that others will continue. And on the other hand this compilation of rPBC should encourage application based groups to identify interesting cations that may have an application. Contact the people, the chances are very good that through an informal collaboration showing a proof-of-principle new and relevant application areas may be developed.

In this respect, we are looking forward to all the scientific creativity that is breaking loose, and to realize what potentially could be done with the rPBC. This is an integral element of innovation and the justification for preparing blue sky or simply beautiful and esoteric compounds.

Abbreviations

CN

Coordination number

DFT

Density functional theory

ε _r

Relative permittivity of a solvent (static dielectric constant)

FLP

Frustrated Lewis pairs

IL

Ionic liquid

n.a.

Not available

rPBC

Reactive p-block cations

WCA

Weakly coordinating anion

Ar^CF₃

3,5-(CF₃)₂C₆H₃

Ar^Cl

3,5-Cl₂-C₆H₃

9BBN

9-Bora[3.3.1]bicyclononane

bipy

1,2-Bipyridine

BOX

Bis(oxazoline)

CatBH

Catecholborane/1,3,2-benzodioxaborole

COD

1,5-Cyclooctadiene

Cp

C₅H₅

Cp′

C₅Me₄H

Cp*

C₅Me₅

Cy

Cyclohexyl

DDP

2-(DIPP)amino-4-(Dipp)imino-2-pentene

Dipp

2,6-ⁱPr₂-C₆H₃

DMAP

4-Dimethylaminopyridine

DMeOPrPE

1,2-(Bis(dimethoxypropyl)-phosphino)ethane

DMH

1,1-Me₂N₂H₄

Dmp

2,6-Dimethyl-phenyl

dmpe

1,2-Bis(dimethylphosphino)ethane

Do

Donor

DPE

1,2-Diphenylethane

dppe

1,2-Bis(diphenylphosphino)ethane

DTBMP

2,6-Di-tert-butyl-4-methylpyridine

dtbpy

4,4′-Di-tert-butyl-2,2′-bipyridyl

Et

Ethyl

Fc

Ferrocenyl

FP

CpFe(CO)₂

FP′

Cp′Fe(CO)₂

FP*

Cp*Fe(CO)₂

hppH

1,3,4,6,7,8-Hexahydro-2H-pyrimido-[1,2-a]pyrimidine

IMe

1,3-Bis(methyl)imidazol-2-ylidene

IMes

1,3-Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene

ⁱPr

iso-Propyl

ⁱPr₂-ATI

N,N′-Diisopropylaminotroponiminate

IPr

1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene

I ^t Bu

1,3-Bis(tert-butyl)imidazol-2-ylidene 3,5-lutidine 3,5-dimethylpyridine

Me

Methyl

Me₄-cyclam

N,N′,N′′,N′′′-Tetramethyl-1,4,8,11-tetraazacyclotetradecane

Mes

2,4,6-Me₃C₆H₂

Me₃SiS ^p Tol

1-SSiMe₃-4-Me-C₆H₄

Me₃-tacn

N,N′,N′′-Trimethyl-1,4,7-triaza-cyclononane

nacnac

(NMesCMe)₂CH

NBD

2,5-Norbornadiene

NPPh

2,5-Bis(2-pyridyl)-1-phenylphosphole

1-MIM

N-Methylimidazole

m-TP

meta-Terphenyl

OSSO

trans-1,2-Cyclooctanediyl-bridged[OSSO]-type bis(phenolate)

OR^PF

–OC(CF₃)₃

OR^HT

–OC(CH₃)(CF₃)₂

OR^HF

–OC(H)(CF₃)₂

OR^MeF

–OC(CH₃)(CF₃)₂

Ph

–C₆H₅

Phen

1,10-Phenanthroline

4-Pic

4-Methylpyridine

Pip

Piperidyl

PMAF

Pentamethylazaferrocene

pmdta

N,N,N′,N′,N′′-Pentamethyldiethylenetriamine

PNP

Bis(2-ⁱPr₂ P4-Me-phenyl)amido

Py

Pyridine

Pytsi

C(SiMe₃)₂SiMe₂(2-C₅H₄N)

p-Xyl

para-Xylene

R

Typical univalent organic residue

Salen

N,N′-Ethylenebis(2-hydroxyphenyl)imine

Salen^CF₃

N,N′-Ethylenebis(2-hydroxy-2-(CF₃)₂-ethyl)imine

Salomphen

N,N′-(4,5-Dimethyl)phenylene-bis(2-hydroxyphenyl)imine

Salpen

N,N′-Propylenebis(2-hydroxyphenyl)imine

Sch

Tridentate Schiff base

SubPc

Subphthalocyanine

tacn

1,4-ⁱPr₂-1,4,7-Triaza-cyclononane

^t Bu

tert-Butyl

Tf

–SO₂CF₃

THF

Tetrahydrofuran

timtmb ^tBu

1,3,5-{Tris(3-tert-butylimidazol-2-ylideno)methyl}-2,4,6-trimethylbenzene

TMM

η⁴-C(CH₂)₃

Tol

Toluene

Tipp

2,4,6-ⁱPr₃-C₆H₂

X

Halogen

Biographies

Tobias A. Engesser

Dipl.-Chem. Tobias Engesser obtained his intermediate diploma at the Technische Universität Karlsruhe (now KIT) and continued his studies at the Albert-Ludwigs-Universität Freiburg, where he also wrote his diploma thesis about reactive tellurium cations and started his PhD in 2011 at the Institute of Inorganic Chemistry under the supervision of Prof. Ingo Krossing. His research interests are reactive phosphorus cations and gold(i) starting materials, both in combination with weakly coordinating anions. On the basis of his interest for phosphorus compounds he stayed for three months in the group of Prof. Christopher “Kit” Cummins at the Massachusetts Institute of Technology in 2015.

Martin R. Lichtenthaler

Martin R. Lichtenthaler (1986) received his Diploma (2010) and PhD (2015) from the Albert-Ludwigs-Universität Freiburg, Germany. As a member of the Krossing group, he has developed a novel class of highly efficient, main group metal-based olefin polymerization catalysts and discovered unprecedented cationic clusters of univalent indium. After a research stay with the New Technology Office of Merck Ltd. Japan, he is likely to join the University of California, Berkeley as a postdoc in 2016. His research interests are organometallic and polymer chemistry, catalysis, molecular modelling and materials science.

Mario Schleep

Dipl.-Chem. Mario Schleep is a PhD student in the group of Prof. Ingo Krossing at the University of Freiburg, where he received his Diploma in 2012. During his studies, he has undertaken a research stay dedicated to poly-metallic chromium and vanadium clusters in the group of Prof. Eric McInnes at the University of Manchester (England) for two semesters. While dealing with electrolytes for lithium ion batteries during his thesis, his current research focuses on the synthesis of reactive tin(ii) cations stabilized by weakly coordinating anions.

Ingo Krossing

Ingo Krossing studied chemistry in Munich (LMU) and finished his PhD thesis 1997 (with Prof. H. Nöth). From 1997 to 1999, he worked as Feodor Lynen postdoc with Prof. J. Passmore at UNB, Canada. In 1999, he started his independent career as a Liebig- and DFG-Heisenberg-Fellow at the Universität Karlsruhe (TH) (mentor: Prof. H. Schnöckel). 2004 he changed as assistant professor to the Ecole Polytechnic Federale de Lausanne (EPFL), before being appointed Chair of Inorganic Chemistry at the Albert-Ludwigs-Universität Freiburg in 2006. His research interests cover ionic systems from reactive cations to ionic liquids, as well as electrochemical energy storage. With an ongoing ERC Advanced Grant he develops absolute acidity and reducity scales.