Electrode half-reaction entropies, Delta S degrees(rc) are measured as a function of solvent and electrolyte type and concentration for four M(tacn)(2)(3+/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 Delta S degrees(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 Delta(Delta S degrees(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 vibrational (obtained using octahedral stretching frequencies of M(NH3)(6)(3+/2+) complexes) and electronic entropy differences equal 4, 31, and 69 J mol(-1) K-1 for the same three metals relative to Ru. The unexpectedly large value of Delta(Delta S degrees(rc))(Fe-Ru) is the result of a spin-state equilibrium in solution. Temperature-dependent magnetic susceptibility measurements yield Delta H degrees = 23.8 (1.0)M mol(-1), Delta S degrees = 68.2 (2.8) J mol(-1) K-1, and K-SE = 0.25 (298 K) for conversion of low- to high-spin Fe(tacn)(2)(2+) in D2O. Observation of uniform values of Delta(Delta S degrees(rc))(M-Ru) for each couple in seven solvents indicates that, if inner-and outer-shell reorganizations are coupled during electron transfer, this fact is not reflected in the solvent dependence of Delta S degrees(rc). Ion-pair formation occurs between oxidized complexes and electrolyte anions. Negative and positive contributions to Delta S degrees(rc) result as ion-paired M(tacn)(2)(3+) X- is reduced to dissociated M(tacn)(2)(2+) in H2O when X- = Cl- and ClO4-, respectively.