ANALYSIS OF SUPEREXCHANGE COUPLING IN METALLOCENE-METALLOCENIUM REDOX PAIRS

Using a corresponding orbital approach implemented computationally with the INDO method of Zerner et al., we have evaluated and analyzed the electron-transfer matrix elements (H(if)) for the ferrocene/ferrocenium and cobaltocene/cobaltocenium (Cp2M/(Cp2M)+) electron exchange processes. Taking due account of the relative energies of different orientations of the reactants, we obtain good agreement with estimates of H(if) inferred from experimental kinetic data, as well as a detailed interpretation of the coupling based on a fully specified effective one-electron model. This model reveals distinct electronic mechanisms for the M = Fe and M = Co cases, corresponding, respectively, to superexchange of the "electron" and "hole" type. Variationally defined effective donor and acceptor orbitals are shown to be essentially equivalent to appropriately selected canonical molecular orbitals of the Cp2M/(Cp2M)+ transition-state complex. Departures from a simple one-electron model affect the calculated H(if) values by less than 10% (i.e., the many-electron core overlap integral is greater-than-or-similar-to 0.9) even though the wave functions properly reflect the appreciable degree of electronic relaxation which is known to accompany the oxidation of the Cp2M molecules. Sigma as well as pi-type orbitals of the Cp rings are shown to play a significant role in facilitating the electronic coupling between reactants.