The valence-localized decacyanodiruthenium(III,II) analogue of the Creutz–Taube ion. Completing the full d5/d6 triad [(NC)5M(μ-pz)M(CN)5]5−, M=Fe,Ru,Os; pz=pyrazine

The new ruthenium(II) complex ion [(NC)5Ru(μ-pz)Ru(CN)5]6− has been prepared and studied as the hexakis(tetraethylammonium) salt. Although spectroelectrochemical experiments in dichloromethane were affected by adsorption as they were in other solvents, the compound could be oxidized via two reversible one-electron steps with a mixed-valent pentaanionic intermediate. In comparison with the iron and osmium analogues, the system [(NC)5Ru(μ-pz)Ru(CN)5]5− is distinguished by less negative redox potentials, a smaller comproportionation constant Kc of only 104.7, a very broad (Δν1/2=4200 cm−1) symmetrical IVCT band at 1760 nm (ε=2600 M−1 cm−1) conforming with the Hush model for weakly coupled mixed-valent systems, the absence of an EPR signal even at 3.5 K and the appearance of infrared bands (CN, pyrazine ring vibrations) indicating localized valence on the time scale of 10−12 s. Together with the high MLCT energies these results suggest a weaker metal–pyrazine interaction for the ruthenium system in comparison with the Fe and Os analogues reported previously. In relation to the Creutz–Taube ion, the substitution of ammine ligands by non-innocent cyanide ions attenuates the metal–metal interaction across the π accepting pyrazine bridge.