An Exceedingly Stable Diiron(II,III) Complex Ion [(tz){Fe(CN)5}2]5− with Comproportionation Constants between 108 (in H2O) and 1019 (in CH3CN)

A record equilibrium constant of K(c)=10(19) was found for the comproportionation of the diiron(II,III) complex 1 in acetonitrile. In water the K(c) value is strongly diminished (10(7.9)), but still exceeds that of the Creutz-Taube ion. Intervalence charge transfer bands occur at 2520 nm (in CH(3)CN) and 2250 nm (in D(2)O), and all evidence points to a strongly coupled system with delocalized valences.