Vibronic Participation of the Bridging Ligand in Electron Transfer and Delocalization:  New Application of a Three-State Model in Pyrazine-Bridged Mixed-Valence Complexes of Trinuclear Ruthenium Clusters

The importance of the bridging ligand in mediating electronic communication and delocalization between charge centers in near-delocalized mixed-valence complexes is investigated in the specific context of pyrazine-bridged, hexanuclear ruthenium complexes that display electron-transfer rates on the order of 10 11 −10 12 s -1, as observed by partial coalescence of infrared bands. It is shown that a three-state vibronic model explicitly including the electronic and vibrational participation of the bridging ligand is needed to account for infrared and resonance Raman evidence of vibronic activity in symmetric modes of the bridging pyrazine ligand. The specific molecular orbital and normal-mode basis for this application of the three-state model is presented using experimental observations and density functional calculations on model trinuclear ruthenium clusters. An important conclusion of the application of this model is that infrared activity of symmetric bridging ligand modes in mixed-valence complexes is not a reliable indicator of electronic asymmetry on the vibrational time scale. The three-state model is shown to qualitatively reproduce the “tunability” of electronic communication observed in these complexes. The central importance of such a direct consideration of the bridging ligand in similar near-delocalized or delocalized complexes (like the well-known Creutz−Taube ion) is re-emphasized in this study.