Comparative absorption, electroabsorption and electrochemical studies of intervalence electron transfer and electronic coupling in cyanide-bridged bimetallic systems: ancillary ligand effects Original Research Article

Electroabsorption or Stark spectroscopy has been used to evaluate the systems (NC)5MII–CN–RuIII(NH3)51− and (NC)5MII–CN–RuIII(NH3)4py1−, where MII=FeII or RuII. When a pyridine ligand is present in the axial position on the RuIII acceptor, the effective optical electron transfer distance – as measured by the change in dipole moment, |Δμ| – is increased by more than 35% relative to the ammine substituted counterpart. Comparison of the charge transfer distances to the crystal structure of Na[(CN)5Fe–CN–Ru(NH3)4py] · 6H2O reveals that the Stark derived distances are ∼50% to ∼90% of the geometric separation of the metal centers. The differences result in an upward revision in the Hush delocalization parameter, cb2, and of the electronic coupling matrix element, Hab, relative to those parameters obtained exclusively from electronic absorption measurements. The revised parameters are compared to those, which are obtained via electrochemical techniques and found to be in only fair agreement. We conclude that the absorption/electroabsorption analysis likely yields a more reliable set of mixing and coupling parameters.