Planar Heptacoordinate Halogens

Abstract

Planar hypercoordinate halogens remain challenging because of the high electronegativity of halogens. Through a systematic exploration of 200 X©M₇E₇ ^– combinations (X = F–At; M = Be–Ba, Zn–Hg; E = O–Po), we identify four global minima featuring planar heptacoordinate halogens: Br©Zn₇O₇ ^–, I©Zn₇O₇ ^–, At©Zn₇O₇ ^–, and At©Cd₇O₇ ^–. These systems have large HOMO–LUMO gaps, considerable singlet–triplet separations, and stability up to 1000 K in molecular dynamics simulations. Bonding analyses (WBI, AdNDP, EDA–NOCV, and IQA) indicate that the central halogen behaves as an anion confined within a covalently supported M₇O₇ ring, with interactions dominated by electrostatics and minor covalent contributions. Magnetic-response and EDDB analyses do not support aromatic stabilization. Stability instead arises from the combination of a rigid metal–oxide framework and electrostatic confinement. A size-dependent stabilization pattern emerges. Zn-based rings stabilize Br, I, and At, whereas the larger and softer Cd-based framework stabilize only At. Rather than introducing a distinct bonding mechanism, these systems extend the electrostatic confinement mechanism previously established at lower coordination numbers, revealing both its persistence and limitations at n = 7. These results show that planar halogen hypercoordination can be sustained up to n = 7 only when the cavity size and ring rigidity simultaneously satisfy a narrow electrostatic confinement window, thereby defining the stability limits of the oxide-ring families examined here.

Introduction

Planar hypercoordinate halogens constitute an emerging area of main-group chemistry, with recent theoretical studies beginning to identify the conditions required to stabilize these motifs. − The conceptual basis of planar hypercoordination can be traced to the development of planar tetracoordinate carbon chemistry, − which established that planar coordination patterns can be extended to other elements and higher coordination numbers under suitable electronic and geometric conditions. − Halogens introduce an additional constraint: their high electronegativity and localized p orbitals limit effective in-plane delocalization, favoring terminal or bridging coordination modes over extended planar interactions. Recent work shows that rigid oxide rings can host halogen anions as planar hypercoordinate centers, including F©Zn₅O₅ ^– and Cl©Zn₆O₆ ^–. , In these systems, stabilization does not arise from in-plane covalency but from a combination of electrostatic confinement, multicenter interactions, and geometric compatibility between the halogen and the cavity defined by the ring. So, these systems exhibit size-selective behavior. The smaller Zn₅O₅ framework accommodates fluorine as a planar pentacoordinate center; expansion to the Zn₆O₆ ring destabilizes F©Zn₆O₆ ^–, resulting in a higher-order saddle point due to the limited interaction of the undersized F atom with the larger cavity. Conversely, larger halogens such as Br are also destabilized in Zn₆O₆ due to size mismatch, whereas chlorine provides the balance of size and polarizability required to stabilize a planar hexacoordinate minimum. This size dependence raises a more fundamental question: does the electrostatic confinement mechanism that stabilizes planar hypercoordinate halogens persist as the coordination number increases, or does it break down beyond a certain geometric limit? Addressing this question requires moving beyond isolated examples to evaluate whether the previously identified size-matching principle remains valid as the coordination number increases.

Neutral (MO)_n clusters (M = Zn, Cd; n ≤ 7) provide a natural platform for this purpose, − as they adopt planar ring geometries whose size increases systematically with n. In particular, Zn₇O₇ represents the largest member of this series that retains a planar structure, providing an opportunity to examine whether expansion of the cavity enables stabilization of heavier halogens in higher coordination environments.

The ability of the host framework to sustain planarity is itself a limiting factor. While Zn₅O₅ and Zn₇O₇ correspond to global minima and Zn₆O₆ lies close in energy to competing structures, previous exploration indicates that Zn₈O₈ no longer retains a planar global minimum, suggesting that the structural integrity of the oxide ring imposes an intrinsic upper bound.

Here, we examine whether this stabilization mechanism remains viable at higher coordination numbers by exploring X©M₇E₇ ^– systems (X = F–At; M = Be–Ba, Zn–Hg; E = O–Po), designed to map the interplay between cavity size, host rigidity, and halogen identity. Our results identify four global minima featuring a planar heptacoordinate halogen (p7X) center: Br©Zn₇O₇ ^–, I©Zn₇O₇ ^–, At©Zn₇O₇ ^–, and At©Cd₇O₇ ^–, representing the highest planar coordination number reported to date for a halogen center. These findings indicate a size-dependent selection mechanism: Zn-based rings stabilize Br, I, and At, whereas the larger and softer Cd-based framework stabilizes only At. This study evaluates the extent to which the oxide-ring motif can sustain planar hypercoordination as the cavity expands.

Methods

The potential energy surfaces (PESs) were explored using the AUTOMATON program. An initial screening was carried out at the ωB97X-D/SDDAll − level, followed by reoptimization of low-energy structures at the ωB97X-D/def2-TZVPP level with harmonic frequency analysis to confirm their nature as minima or saddle points. All geometry optimizations, frequency calculations, and wave function stability analyses were performed using Gaussian 16 C.02. Final energies were obtained at the DLPNO–CCSD­(T) − /CBS , level using the ORCA 6.0 package.

Thermal stability was evaluated through Born–Oppenheimer molecular dynamics (BOMD) simulations at 300, 600, and 1000 K over 50 ps using the PBE0 functional and Stuttgart–Dresden effective core potentials, as implemented in the deMon2K program. ,

Chemical bonding was characterized using complementary approaches. Wiberg bond indices (WBI) and natural population analysis (NPA) were obtained with NBO 7.0, while Adaptive Natural Density Partitioning (AdNDP) was performed with Multiwfn 3.8. Real-space energy decomposition was analyzed through Interacting Quantum Atoms − (IQA) at the PBE0-D3/def2-TZVPP level using the AIMAll program, and interaction energies were further examined with energy decomposition analysis using natural orbitals for chemical valence (EDA–NOCV) as implemented in ADF (2024). ,

Magnetic response was evaluated through magnetically induced current densities (MICD) computed with GIMIC at the ωB97X-D/def2-TZVPP level using the gauge-including atomic orbital (GIAO) formalism. Electronic delocalization was quantified using the Electron Density of Delocalized Bonds , (EDDB) method at the same level. Full computational details are provided in the ESI.

Potential Energy Surface Screening

The identification of p7X candidates followed the workflow outlined in Scheme . A total of 200 D _7h -derived X©M₇E₇ ^– combinations (X = F–At; M = Be–Ba, Zn–Hg; E = O–Po) were optimized at the ωB97X-D/def2-TZVPP level in both singlet and triplet multiplicities. Of these, only eight retain the planar D _7h arrangement as true minima (Tables S1–S2): Br©Zn₇O₇ ^–, I©Zn₇O₇ ^–, At©Zn₇O₇ ^–, and At©Cd₇O₇ ^– from the group-12 series, together with Cl©Be₇O₇ ^–, Br©Mg₇O₇ ^–, I©Mg₇O₇ ^–, and At©Mg₇O₇ ^– from the alkaline-earth series. The remaining 192 optimized D _7h structures exhibit one or more imaginary frequencies and therefore correspond to saddle points at this level. This sharp selectivity is not solely determined by halogen size but also by the intrinsic stability of the host skeleton. Only oxide-based rings (E = O) produce D _7h minima, and only with specific metal–halogen size combinations. While the global minima of the bare Zn₅O₅ and Zn₇O₇ rings adopt D _5h and D _7h symmetries, respectively, and Zn₆O₆ is nearly degenerate with alternative structures, the global minimum of Zn₈O₈ is not planar. This loss of planar stability in Zn₈O₈ prevents further extension of the electrostatic confinement mechanism beyond n = 7.

1. Workflow for Identifying Planar Heptacoordinate Halogen (p7X) Candidates and Their Global Minima from D _7h X©M₇E₇ ^– Starting Structures.

Potential energy surface (PES) searches on these eight candidates using the AUTOMATON program identified four of these candidates as global minima: Br©Zn₇O₇ ^–, I©Zn₇O₇ ^–, At©Zn₇O₇ ^–, and At©Cd₇O₇ ^–. In all cases, the next-lowest isomers correspond to distorted three-dimensional structures, either through displacement of the halogen from the cavity center or partial collapse of the M–O ring. At the DLPNO–CCSD­(T) − /CBS , //ωB97X-D/def2-TZVPP level, the energy separations from these competing structures are 8.5 (Br©Zn₇O₇ ^–), 9.9 (I©Zn₇O₇ ^–), 8.8 (At©Zn₇O₇ ^–), and 6.1 kcal/mol (At©Cd₇O₇ ^–), confirming the planar p7X arrangement as the thermodynamically preferred motif (Figures S1–S4). For the At-containing systems, relative energies were also evaluated, including scalar relativistic effects through the zeroth-order regular approximation (ZORA) at the ωB97X-D3/TZ2P-ZORA level on ωB97X-D/def2-TZVPP geometries (Figures S3–S4). In all cases, the energetic ordering among the located isomers is preserved, supporting the global-minimum assignments for At©Zn₇O₇ ^– and At©Cd₇O₇ ^–.

Three of the four global minima are based on the Zn₇O₇ framework, whereas At©Cd₇O₇ ^– contains Cd instead of Zn. This substitution is associated with the larger cavity of Cd₇O₇. The neutral Cd₇O₇ ring is itself a global minimum at the ωB97X-D/def2-TZVPP level, lying 1.1 kcal/mol below the next isomer (Figure S5), in line with the known tendency of group-12 metal oxides to form planar ring structures. , This result indicates that the anion-in-neutral-ring motif is preserved across the Zn and Cd series, while also suggesting that changes in metal identity provide an additional degree of freedom for stabilizing larger halogens. This sharp reduction (from 200 initial combinations to only four global minima) indicates that planar heptacoordination is not a general outcome of cavity expansion, but rather a highly constrained regime where the electrostatic confinement mechanism operates at its limit.

The electronic structure further supports the intrinsic stability of the identified p7X systems (Table S4). All four species exhibit large HOMO–LUMO gaps (7.5–8.9 eV), substantial singlet–triplet separations (41–58 kcal/mol), and T₁ diagnostics (0.011–0.015) well below the single-reference threshold (0.02). Harmonic vibrational analyses yield no imaginary frequencies, with the lowest positive frequencies lying in the 4–15 cm^–1 range, indicating shallow minima at this level. These results indicate that Br©Zn₇O₇ ^–, I©Zn₇O₇ ^–, At©Zn₇O₇ ^–, and At©Cd₇O₇ ^– correspond to electronically stable, single-reference ground states containing p7X centers.

The D _7h structures are minima at the ωB97X-D/def2-TZVPP level. Of the tested functionals (Table S3), B3PW91-D3 and ωB97X-D, consistently yield positive frequencies for all four systems. In contrast, PBE0-D3, BP86-D3, and TPSSh ,,, produce small imaginary frequencies (|ν| ≤ 14 cm^–1) for Br©Zn₇O₇ ^– and At©Cd₇O₇ ^–. These low-magnitude imaginary frequencies reveal that the classification of these shallow structures is sensitive to the functional and numerical treatment.

Thermal stability of the planar heptacoordinate structures was evaluated through BOMD simulations at 300, 600, and 1000 K over 50 ps. These simulations probe whether the D _7h arrangement persists under thermal fluctuations or relaxes toward lower-symmetry configurations. The RMSD profile of Br©Zn₇O₇ ^– (Figure ) remains well confined across all temperatures, with mean values of 0.29, 0.37, and 0.47 Å at 300, 600, and 1000 K, respectively. I©Zn₇O₇ ^– (Figure S6) exhibits slightly lower RMSD values (0.25, 0.36, and 0.39 Å), corresponding to smaller amplitude structural fluctuations. In both systems, the RMSD increases smoothly with temperature but shows no drift over time, indicating that thermal motion leads to vibrational distortion rather than structural rearrangement.

1.

Root-mean-square deviation (RMSD, Å) as a function of simulation time (0–50 ps) for Br©Zn₇O₇ ^– from Born–Oppenheimer molecular dynamics (BOMD) simulations at the PBE0/SDD level at 300, 600, and 1000 K.

The At-containing cluster (Figure S7) displays larger RMSD amplitudes, particularly at 1000 K. The larger RMSD amplitudes are consistent with softer vibrational modes and, in the case of At©Cd₇O₇ ^–, with the greater flexibility of the Cd–O framework. Despite these larger fluctuations, the structural deviations remain bounded, and no transition to alternative geometries is detected. Across all trajectories, no bond cleavage, isomerization, or ring collapse occurs. The halogen remains centered within the cavity, and the metal–oxygen ring retains its integrity. These results indicate that the heptacoordination and the integrity of the metal–oxide rings are retained throughout the 50 ps simulations, even at 1000 K, although appreciable instantaneous deviations from planarity occur.

Structure and Bonding

The four global minima (Figure ) share a common structural feature: long X–M distances (3.50–3.52 Å for Zn; 3.89 Å for Cd) that exceed the sums of Pyykkö covalent radii, together with very small WBI_X–M values (0.01–0.02). These descriptors indicate that covalent interactions between the central halogen and the surrounding metal atoms are weak.

2.

(a) Structure of D _7h planar heptacoordinate halogen systems, X©M₇O₇ ^– (X = Br, I, At; M = Zn, Cd), with selected NPA atomic charges |e|. (b) Structural and bonding descriptors: X–M, M–M, and M–O distances (Å) and Wiberg bond indices (WBI).

In contrast, the M₇O₇ ring displays short M–O distances (1.77–1.78 Å for Zn; 1.96 Å for Cd) and relatively large WBI_M–O values (0.34–0.40), indicating a covalently connected ring. Metal–metal separations are considerably longer (3.03–3.06 Å for Zn; 3.38 Å for Cd), with WBI_M–M values close to zero (0.006–0.007), excluding M–M bonding. So, these features define a preorganized, covalently connected ring that remains structurally intact upon incorporation of the central halogen. The charge distribution reinforces this separation of roles. NPA analysis yields a halide-like center (q _X ≈ −0.9 |e|), electropositive metal sites (q_M ≈ +1.6 |e|), and electronegative oxygen atoms (q _O ≈ −1.6 |e|). This arrangement (a negative center surrounded by a positively charged inner shell and a negative outer periphery) corresponds to electrostatic confinement of the halogen within the cavity defined by the M₇O₇ ring, consistent with an anion-in-neutral-ring motif.

To examine the bonding pattern further, AdNDP analysis was performed for Br©Zn₇O₇ ^– as a representative case (Figure ). The Zn₇O₇ ring is described by seven O 2s lone pairs (ON ≈ 1.99 |e|), 14 2c–2e Zn–O σ bonds (ON ≈ 1.99 |e|), and seven 3c–2e Zn–O–Zn π bonds (ON ≈ 1.99 |e|), matching the covalent ring inferred from WBI values. The central halogen retains four 1c–2e lone pairs (ON ≈ 1.97–1.99 |e|), with no delocalized bonding involving the metal centers, while each Zn atom preserves five 3d lone pairs (ON ≈ 1.99 |e|). The same bonding pattern is found for I©Zn₇O₇ ^–, At©Zn₇O₇ ^–, and At©Cd₇O₇ ^– (Figures S8 and S9), indicating that the halogen remains electronically localized and does not participate in multicenter bonding with the ring.

3.

Adaptive Natural Density Partitioning (AdNDP) analysis of Br©Zn₇O₇ ^– at the ωB97X-D/def2-TZVPP level. Occupation numbers (ON) are given in |e|.

This description is further supported by the energy decomposition analysis (EDA–NOCV) performed for all four systems (Tables and S5). Among the tested fragmentation schemes, the X^–(singlet) + M₇O₇(singlet) partition yields the smallest orbital interaction term, providing the most consistent representation of the interaction. The attractive interaction is dominated by the electrostatic component, with ΔV _elstat accounting for 76–79% and ΔE _oi contributing 21–24%.

1. EDA-NOCV Results for X©Zn₇O₇ ^– (X = Br, I, At) using X^– and Zn₇O₇ as Interacting Fragments at the PBE0/TZ2P-ZORA Level.

energy terms	interaction	interaction energies Br – (singlet) + Zn 7 O 7 (singlet)	interaction energies I – (singlet) + Zn 7 O 7 (singlet)	interaction energies At – (singlet) + Zn 7 O 7 (singlet)
ΔE int		–67.2	–61.9	–61.7
ΔE Pauli		26.1	48.6	61.5
ΔE attr		–93.3	–110.5	–123.2
ΔV elstat		–70.7 (75.8%)	–86.4 (78.1%)	–96.8 (78.6%)
ΔE oi		–22.6 (24.2%)	–24.1 (21.9%)	–26.4 (21.4%)
ΔE oi(1)	Br–/I–/At– (4/5/6p x ) → Zn7O7 donation	–6.7 (29.6%)	–7.7 (31.8%)	–8.6 (32.6%)
ΔE oi(2)	Br–/I–/At– (4/5/6p y ) → Zn7O7 donation	–6.7 (29.6%)	–7.7 (31.8%)	–8.6 (32.6%)
ΔE oi(3)	Br–/I–/At– (4/5/6s) → Zn7O7 donation	–2.0 (8.8%)	–1.6 (6.7%)	–1.1 (4.2%)
ΔE oi(rest)		–7.2 (32.0%)	–7.2 (29.7%)	–8.1 (30.7%)

^aAll energy values are given in kcal/mol.

^bThe percentage contribution with respect to the total attraction is given in parentheses.

^cThe percentage contribution with respect to the total orbital interaction is given in parentheses.

Within the Zn₇O₇ series, the electrostatic contribution increases from Br^– (75.8%) to I^– (78.1%) to At^– (78.6%), indicating a progressive shift toward a more electrostatic interaction as the halogen becomes larger and more polarizable. Decomposition of ΔE _oi (Figures S10–S12) identifies two dominant degenerate X^–(p _x , p _y ) → M₇O₇ donor channels [ΔE _oi(1) = ΔE _oi(2) = −6.7 to −8.6 kcal/mol, 59–65%], together with a weaker X^–(singlet) → M₇O₇ contribution [ΔE _oi(3) = −1.1 to −2.0 kcal/mol, 4–9%], while the remaining interaction is distributed among higher-order terms [ΔE _oi(rest) = −7.2 to −8.1 kcal/mol, 30–32%]. These results indicate that the central halogen is best described as an anion confined within a covalently supported M₇O₇ ring, with the interaction dominated by electrostatics and only a modest covalent contribution arising from localized lone-pair donation.

To resolve the interaction hierarchy at the atom-pair level, Interacting Quantum Atoms − (IQA) analysis was performed (Figure ). This approach partitions the interaction energy into Coulombic (classical electrostatic) and exchange–correlation (covalent) components, allowing a direct comparison of the nature of X–M, M–O, and M–M interactions within the same framework.

4.

Interacting Quantum Atoms (IQA) energy decomposition for D _7h X©M₇O₇ ^– systems (X = Br, I, At; M = Zn, Cd). V _IQA is the total interatomic interaction energy, decomposed into Coulombic (V _C ) and exchange–correlation (V _XC ) components (kcal/mol).

The X–M interactions are stabilizing and predominantly Coulombic, with V _C accounting for 87–92% of the total interaction and V _XC contributing the remaining 8–13%. This partitioning is consistent with the predominantly electrostatic character indicated by the EDA–NOCV analysis. In contrast, the M–O interactions show a significantly larger exchange–correlation contribution (V _C = 64–69%; V _XC = 31–36%), indicating a higher covalent character within the M₇O₇ ring. Metal–metal interactions remain repulsive and exhibit negligible exchange–correlation contributions, reinforcing the absence of direct M–M bonding. The IQA delocalization indices provide a complementary real-space measure of electron sharing. The values follow the same hierarchy: δ­(X–M) = 0.08–0.12, δ­(M–O) = 0.84–0.89, and δ­(M–M) and δ­(X–O) ≈ 0.04–0.07. Slightly lower values are obtained for At©Cd₇O₇ ^–, consistent with its longer interatomic distances.

Because each X–M pair corresponds to a direct stabilizing interaction, the central halogen satisfies the IUPAC definition of coordination number and is therefore heptacoordinate. This interaction pattern does not introduce a new bonding mechanism but rather represents the continuation of the electrostatic confinement regime previously identified in lower-coordinate systems. Its persistence at n = 7 indicates that the coordination limit is not determined by the emergence of new bonding modes, but by geometric and electrostatic constraints imposed by the host.

Although the interaction between the halide and the surrounding ring is predominantly electrostatic, the stability of planar hypercoordinate halogens is not determined by electrostatics alone. The geometric match between the size of the halide and the cavity generated by the metal–oxide ring also plays a critical role. The results indicate that the size selectivity in the Zn_nO_n series is controlled by the match between the halide size and the cavity generated by the metal–oxide ring. The Zn–O distances in the isolated rings remain almost unchanged from Zn₅O₅ to Zn₇O₇ (1.76–1.77 Å), so the cavity grows mainly because the ring contains more Zn–O units. The center-to-Zn distance therefore increases from 2.54 Å in Zn₅O₅ to 3.08 Å in Zn₆O₆ and 3.63 Å in Zn₇O₇. When these cavity sizes are compared with the six-coordinate Shannon radii of the halides (F^– = 1.33 Å, Cl^– = 1.81 Å, Br^– = 1.96 Å, and I^– = 2.20 Å), all planar global minima fall within a narrow cavity-to-anion radius ratio of ca. 1.65–1.91, including F©Zn₅O₅ ^– (1.91), Cl©Zn₆O₆ ^– (1.70), and Br©Zn₇O₇ ^– (1.85), and I©Zn₇O₇ ^– (1.65), whereas systems outside this range become unstable or lose planarity. For example, if the cavity is too large for the halide, as in F©Zn₆O₆ ^– (2.32) or Cl©Zn₇O₇ ^– (2.01), the electrostatic confinement becomes too weak and in-plane imaginary modes appear. If the halide is too large for the cavity, as in Br©Zn₆O₆ ^– (1.57), the anion is displaced out of the plane. Thus, planar halogen hypercoordination can be sustained to n = 7 only when cavity size and ring rigidity jointly satisfy the requirements of a narrow confinement window.

This geometric window is necessary, but it is not sufficient. The framework must also be rigid enough to keep the halide confined. The Zn-based rings adapt moderately upon halide insertion: Zn–O bonds elongate by only 1.0–1.3%, whereas Zn–Zn distances contract by 3.0–5.4%, consistent with a ring that can adjust while maintaining confinement. Although the cavity-to-anion radius ratio of I©Cd₇O₇ ^– falls within the range found for several planar global minima, this structure remains a shallow saddle point. Compared with the Zn–O interactions, the Cd–O interactions are less stabilizing according to the IQA analysis, despite retaining appreciable covalent character. Moreover, insertion of I^– produces only small structural changes in the Cd₇O₇ ring (+0.5% in Cd–O and −2.3% in Cd–Cd), indicating a weaker structural response of the host framework. Thus, the apparently favorable size match is not accompanied by sufficiently strong host–guest coupling to retain I^– within the molecular plane. In contrast, the larger At^– anion, with an estimated radius of ca. 2.3 Å, interacts more effectively with the wider Cd₇O₇ cavity and forms the global-minimum structure. These results show that geometric matching alone is insufficient; planar confinement also requires sufficiently strong interactions between the confined anion and the host framework.

Electronic Structure

The electronic structure supports the stability of the planar heptacoordinate motif. All four clusters exhibit large HOMO–LUMO gaps (7.5–8.9 eV; Figure ). The HOMO is predominantly localized on the central halogen (64–70%), whereas the LUMO is centered on the metal oxide framework (66–71%). This spatial separation indicates distinct regions for electron removal and electron addition: detachment primarily involves the confined halide, whereas electron attachment is associated with the M₇O₇ ring.

5.

HOMO, LUMO, and HOMO–LUMO gaps for D _7h X©M₇O₇ ^– (X = Br, I, At; M = Zn, Cd).

This distribution of frontier orbitals is reflected in the electron-binding strength. The first vertical detachment energies (VDEs; Table S7), computed at the OVGF − (5.24–6.39 eV; pole strengths 0.90–0.91) and TD-ωB97X-D (5.50–6.32 eV) levels, exceed the electron affinities of all halogen atoms. These values place the clusters within the superhalogen regime. Within the Zn-based series, both methods follow the trend Br > I > At. Replacing Zn by Cd produces a modest decrease in VDE, as the softer Cd–O framework reduces the electrostatic confinement of the excess electron.

The metal–oxide ring plays a dual role. It defines the cavity that enforces planar heptacoordination and, at the same time, stabilizes the excess electron. Electron binding therefore depends directly on the confinement imposed by the framework.

Electron Delocalization

Electron delocalization was examined from complementary magnetic-response and electron-density perspectives. Magnetically induced current densities (MICD) and the EDDB , descriptors were computed for all four global minima (Figures and S13).

6.

(a) MICD-vector plots of the D _7h I©Zn₇O₇ ^–; (b) Ring-current strengths (RCSs, nA/T) of the X-, M-, and O-centered circuits. In each sequence, the values correspond to Br©Zn₇O₇ ⁻, I©Zn₇O₇ ⁻, At©Zn₇O₇ ⁻, and At©Cd₇O₇ ⁻, respectively.

The net ring current strength is negligible (≤0.1 nA/T), well below the values associated with aromatic systems and lower than that reported for the related phCl cluster Cl©Zn₆O₆ ^– (0.31 nA/T). The current density is dominated by diatropic vortices centered on individual atoms. The magnitude of these local currents follows the sequence X (40.9–54.1 nA/T) > M (26.3–31.4 nA/T) > O (15.5–17.4 nA/T), indicating that the magnetic response is atomic rather than cyclic.

The EDDB analysis provides a complementary picture of electron delocalization. The total population of delocalized electrons (5.86–5.92 |e|) indicates the presence of multicenter interactions within the M₇O₇ framework. However, the corresponding isosurface reveals that this delocalization is spatially fragmented: it is concentrated on oxygen atoms, connected by narrow regions at the metal sites, and only weakly involves the central halogen. The absence of a continuous delocalization pathway across the ring does not support aromatic behavior.

Conclusion

This work identifies planar heptacoordinate halogen centers in four global-minimum structures (Br©Zn₇O₇ ^–, I©Zn₇O₇ ^–, At©Zn₇O₇ ^–, and At©Cd₇O₇ ^–), extending the highest planar coordination number reported for halogens to seven. The planar D _7h arrangement arises as a highly selective solution within the X©M₇O₇ ^– chemical space, requiring both a stable metal–oxide ring and compatibility between the halogen and the cavity size. The resulting systems are dynamically robust and electronically stable within a single-reference description. More broadly, the results indicate that planar halogen hypercoordination remains feasible up to n = 7 only within a narrow confinement window defined jointly by cavity size and framework rigidity.

Bonding analyses define a consistent picture. The M₇O₇ ring forms a covalent scaffold, while the central halogen remains electronically localized and interacts with the ring through electrostatic attraction distributed over the surrounding metal centers. This interaction pattern satisfies the IUPAC definition of coordination number and corresponds to heptacoordination without substantial direct covalent X–M bonding. Rather than introducing a new bonding paradigm, these systems show the persistence of an electrostatic confinement mechanism as the coordination number increases.

Magnetic and electron density analyses exclude aromatic contributions. Stability originates from electrostatic confinement within a rigid cavity rather than cyclic electron delocalization. Zn₇O₇ provides the confinement window required to stabilize Br, I, and At, whereas the larger and softer Cd₇O₇ cavity is compatible only with At. More broadly, the results indicate that planar halogen hypercoordination is retained to n = 7 only within a narrow confinement window jointly defined by cavity size and structural rigidity, rather than through the emergence of new bonding interactions.

This limitation is further reinforced by the instability of larger oxide rings. Within the Zn–O series, Zn₇O₇ is the largest ring considered here that retains a planar global-minimum structure, whereas Zn₈O₈ does not. This result prevents a direct extension of this specific host framework but does not exclude higher coordination numbers in other ring compositions. Thus, the upper coordination limit identified here is not only a consequence of cavity–halogen matching, but also of the structural stability of the host itself.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsphyschemau.6c00103.

• Optimized structures and relative energies of low-lying isomers; vibrational-frequency and electronic-structure data; Born–Oppenheimer molecular dynamics simulations; AdNDP, EDA–NOCV, EDDB, and IQA bonding analyses; vertical detachment energies; and Cartesian coordinates (PDF)

The authors declare no competing financial interest.