Influence of Side Chains on Electronic Coupling between Metal Centers in Simple Model Systems

Studies on both small and large molecules, especially proteins, have been reported in which the electron transfer pathways are better described by a through-bond than a direct metal-to-metal distance. The role of the intervening matter in the redox process is difficult to quantify. In the present study, the through-bond distance and driving force between the donor and acceptor are constant in a series of binuclear complexes and only the side chains along the electron transfer pathway are changed in a systematic manner. Correlations are observed between functions of E_op and ∈_max of the transition bands and ΔE_1/2 measured electrochemically, the sum of the Taft σ_I parameters for the substituents, the dihedral angle between the two pseudoaromatic rings in the system, and the HOMO bond energies of the complexes calculated by ZINDO.