FeII-to-CoIII Charge-Transfer Transitions in Methylene-Bridged Metallocene Salts

Abstract

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 α 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.