Electrochemical and spectral probes of metal/ligand orbital mixing in Ru(NH sub 3 ) sub 4 (bpy) sup 2+ and Ru(NH sub 3 ) sub 4 (phen) sup 2+

An electrochemical variational method has been used to examine experimentally the extent of metal/ligand orbital mixing and electronic coupling in complexes of the type Ru{sup 11}(NH{sub 3}){sub 4}L{sup 2+} (L = 2,2{prime}-bipyridine or 1,10-phenanthroline). From the experiments, strong localization of d{pi} electrons at the ruthenium metal center is indicated. Nevertheless, some mixing of d{pi}(Ru) and {pi}*(bpy; phen) is detected, implying a significant degree of delocalization (ca. 25%). These results differ substantially from those derived from metal-to-ligand charge-transfer (MLCT) oscillator strength measurements. The latter indicate less than 2% delocalization. The difference is particularly surprising in view of the fact that both approaches derive from Mulliken charge-transfer theory and the two are employed at similar (but not identical) levels of approximation. The origin of the discrepancies is unclear, but may be related to the choice of charge-transfer distance which must necessarily be made in the optical analysis. Independent estimates of electronic delocalization, based on partial oxidation-state markers in vibrational Raman spectra, corroborate the electrochemical findings.