Medium Effects in Weakly Coupled Electron-Transfer Processes: Calculation of Inner-Sphere Reorganizational Energies at Zero Ionic Strength

The solvent dependence of the intervalence transition (IT) band of the very weakly coupled binuclear (2,8-dithiadispiro-[3.1.3.1]decane)decaamminediruthenium(II,III) hexafluorophosphate was studied. No correlation could be discerned with any solvent parameter. A dependence of the optical band of the ruthenium binuclear complex, E_op, on the ionic strength of the medium was observed, with some solvents showing stronger effects than others. When ionic strength studies were performed and the extrapolated values at zero ionic strength were employed, a correlation of E_op⁰ with 1/D_op - 1/D_s was observed. The calculated inner-sphere reorganizational energy, λ_i, was 25 kcal/mol. The calculated slope of the plot of E_op⁰ against 1/D_op - 1/D_s was 20.7 whereas the measured value was only 2.02. These results indicate that the dielectric continuum model is not appropriate for theories used in analyzing IT bands of very weakly coupled mixed-valence complexes.