Intramolecular and Environmental Contributions to Electrode Half-Reaction Entropies of M(tacn)23+/2+ (M = Fe, Co, Ni, Ru; tacn = 1,4,7-Triazacyclononane) Redox Couples

Electrode half-reaction entropies, ΔS°rc, are measured as a function of solvent and electrolyte type and concentration for four M(tacn)23+/2+ (M = Fe, Ni, Co, Ru; tacn = 1,4,7-triazacyclononane) redox couples that experience different amounts of structural change in conjunction with electron transfer. Metal dependent values of ΔS°rc are observed for these couples and are shown to arise primarily from vibrational and electronic contributions to intramolecular entropy. Vibrational terms become important when frequencies are small and change significantly with a change in oxidation state, as occurs when an increase in number of antibonding electrons weakens metal−ligand bonds upon reduction. Entropy measurements are referenced to the Ru3+/2+ couple, which is characterized by small inner-shell reorganization. Experimentally, mean values of Δ(ΔS°rc)M-Ru = 27 (7), 30 (6), and 69 (14) J mol-1 K-1 obtained from data in seven solvents are observed for M = Fe, Ni, and Co, respectively. The computed sums of vibratio...