Electron transfer and delocalization in mixed-valence complexes

The spectroscopic consequences of electron transfer and delocalization are investigated in mixed-valence complexes with strong interactions between charge sites. Partially coalesced infrared carbonyl stretching absorption bands in thermodynamically “tunable”, pyrazine-bridged mixed-valence “dimers” of trinuclear ruthenium clusters lead to estimated electron transfer rates on the order of 1011–10 12 s−1 and provide unique insight into the phenomenon of thermally activated electron transfer. This unique insight provides a viewpoint from which to reevaluate existing paradigms for the behavior of such near-delocalized complexes.; Complicated, partially coalesced infrared band shapes in “asymmetric” ruthenium cluster mixed-valence complexes are simplified by isotopic substitution and lead to the first experimental determination of distinct, coexisting charge transfer isomers. The low equilibrium constants between distinct charge transfer isomers contradict predictions based on the semi-classical Marcus-Hush model and suggest the need for a better theoretical description.; Strong solvent dependence of the carbonyl band shapes and correlation of estimated electron transfer rates with solvent dipolar relaxation shows direct evidence of the influence of outer-sphere dipolar dynamics on dynamic intramolecular electron transfer.; Observation of strong infrared activity in formally infrared-forbidden symmetric bridging ligand modes and this activity's lack of correlation with coordination asymmetry of the bridging ligand suggest that these modes are activated by vibronic coupling and are not necessarily a reliable indicator of electronic localization as previously thought. Vibronic coupling of symmetric bridging ligand modes to the “intervalence” band shows that the spectroscopic behavior of these particular mixed-valence complexes is best described in terms of a three-state vibronic model which explicitly includes the electronic and vibrational participation of the bridging ligand in electron transfer and delocalization. The specific molecular orbital and normal-mode application of this model to the hexaruthenium complexes is shown.; Further evidence of the importance of the bridging ligand in electronic delocalization is shown in binuclear iron and ruthenium “mixed-valence” complexes linked by conjugated hydrocarbon bridging ligands. These complexes are best characterized as metal-stabilized, organic radicals due to majority occupation of charge on the bridging ligand.