ELECTRONIC COUPLING IN MIXED-VALENCE BINUCLEAR RUTHENIUM AMMINE COMPLEXES AS PROBED BY AN ELECTROCHEMICAL METHOD AND AN EXTENSION OF MULLIKEN THEORY OF DONOR-ACCEPTOR INTERACTIONS

An electrochemical approach to assessing the degree of electronic coupling in mixed-valence binuclear complexes is outlined, The method relies on the comparison of electrochemical potential shifts induced at both the directly and indirectly perturbed metal sites when a ligand substitution process is carried out at one site, e.g., [symmetric] (NH3)5Ru-L(br)-Ru(NH3)5(4+/5+/6+) --> [asymmetric]L(NH3)5Ru-L(br)-Ru(NH3)5(4+/5+/6+), where the bridging ligand L(br) is either pyrazine or 4-cyanopyridine and the perturbing ligand 1- is a substituted pyridine. It is found that the degree of electronic coupling in these systems is at least three times that which would be predicted based solely on spectroscopic measurements. The stabilization energy due to electron delocalization in these complexes can be accounted for with near-quantitative accuracy. It is also shown how the Mulliken analysis of the data allows for an estimation of the Wolfsberg-Helmholz constant K, which can be used in the calculation of off-diagonal matrix elements for molecular donor-acceptor interactions.