FeII-to-CoIII charge-transfer transitions in methylene-bridged metallocene salts

Four compounds have been studied which contain ferrocene or octamethylferrocene donors linked by CH2 bridges to cobaltocenium or pentamethylcobaltocenium acceptors. The electronic spectra of these compounds cannot be accounted for by superposition of those of the constituent metallocenes; in each case, a low-energy solvatochromic absorption is observed, the energy of which parallels the difference in electrode potentials between the iron and cobalt centers. This absorption is assigned to Fe-II-to-Co-III charge transfer; analysis using Hush theory gives delocalization parameters V and alpha in the ranges 46-130 meV and 0.019-0.069, respectively, indicating much stronger coupling between the metal centers than in structurally similar ferrocene/ferrocenium systems.