Table 4. Probe ligands that form 1 : 2 ML₂ complexes with metal ions at pH 7.0.

Ligand	Metal	Probe	ελnm (M−1 cm−1)	β2 (M−2)	Refs
Par1			ε415 ∼ 37 800		[78]
	Fe(II)	FeII(Par)2	ε705 = 18 600	n.d.	[78]
	Co(II)	CoII(Par)2	Δε514 = 50 000 2	n.d.	[79]
			Δε508 = 51 300 2	n.d.	[27]
	Ni(II)	NiII(Par)2	Δε500 = 52 000 2	n.d.	[79]
	Zn(II)	ZnII(Par)23	Δε500 = 66 000 2	2.0 × 1012	[75]
			Δε492 = 60 800 2	4.7 × 1011	[27]
Tar1			ε470 = 24 800		[78]
	Fe(II)	FeII(Tar)2	ε720 = 19 000 4	4.0 × 1013 5	[78]
	Fe(III)	FeIII(Tar)2	Δε540 = 46 500 2	4.0 × 1021 6	[78]
	Ni(II)	NiII(Tar)2	Δε535 = 38 000 2	4.3 × 1015	[6]
Fs7	Cu(I)	CuI(Fs)2	ε484 = 6700	5.0 × 1013	[33]
Fz7	Cu(I)	CuI(Fz)2	ε470 = 4320	1.3 × 1015	[33]
Bca7	Cu(I)	CuI(Bca)2	ε562 = 7900	1.6 × 1017 8	[8]
			Δε358 = 42 900 2		[36]
				5.0 × 1017 8	[24]
Bcs7	Cu(I)	CuI(Bcs)2	ε483 = 13 000	6.3 × 1019 8	[8]
				6.3 × 1020 8	[24]

¹Spectral properties of Par and Tar are highly pH-dependent, see refs. [27,78,102];

²Extinction coefficient corresponds to change in absorbance with respect to ligand only;

³When working at micromolar-range concentrations, Par must be in excess to prevent dissociation of Zn(II) and formation of 1 : 1 Zn^IIPar complexes;

⁴Value is pH-dependent and may be estimated by a relationship of ε₇₂₀ = (4.14 pH–10) mM⁻¹cm⁻¹, see ref. [78];

⁵Values at pH 7.2, 7.4 and 7.6 are also available, see ref. [78];

⁶Calculated from the Nernst equation based on experimentally determined reduction potential (∼314 mV) of the Fe^III(Tar)₂/Fe^II(Tar)₂ redox couple [78];

⁷Concentrations can be calibrated via titration of Cu(I) into respective ligand solutions as described in refs. [33,103];

⁸Somewhat different values for Cu^I(Bca)₂ and Cu^I(Bcs)₂ have been reported, primarily due to a selection of different $E_{\mathrm{aq}}^{\mathrm{o}}$ value for the aqueous Cu²⁺/Cu⁺ redox couple.