For the Ru-III/II couple in [(bpy)(2)ClOs(4,4’-bpy)Ru (NH3)(5)](3+) (bpy = 2,2’-bipyridine; 4,4’-bpy = 4,4’bipyridine), E(1/2) varies linearly with the donor number (DN) of the solvent with a slope of -26 +/- 3 mV/DN unit ranging from nitromethane to dimethylsulfoxide. For the Os-III/II couple, the variation is -3 +/- 1 mV/DN unit. Plots of Delta E(1/2) = E(1/2)(2) - E(1/2)(1) VS DN (E(1/2) is the half wave potential for the first or second wave by cyclic voltammetry) undergo a change in slope at DN similar to 14 where there is a change in oxidation states in the mixed-valence form from Os-III-Ru-II to Os-II-Ru-III By extrapolation of these data, Delta G degrees for the mixed-valence equilibrium, [(bpy)(2)ClOsIII-(4,4’-bpy)Ru-II(NH3)(5)](4+) reversible arrow [(bpy)(2)ClOSII(4,4’-bpy)RU(III)(NH3)(5)](4+), varies from +5.8 kcal/mol in nitromethane to -7.5 kcal/mol in dimethyl sulfoxide. It differs from Delta E(1/2) by up to similar to 20% even though it has sometimes been assumed in the literature that Delta E(1/2) = -Delta G degrees. For [(bpy)(2)ClOs(pz)Ru(NH3)(5)](3+) (pz = pyrazine) both Os-III/II and RU(III/II) couples are significantly solvent dependent for solvents of DN < 24. In these solvents oxidation states in the mixed-valence form are Os-III-Ru-II. The slopes of E(1/2) VS DN plots are -21 +/- 4 mV/DN unit (RU(III/II)) and -8 +/- 4 mV/DN unit (Os-III/II). At DN > 24 the oxidation states switch to Os-II-RU(III) and the solvent dependence reverts to being largely in Ru-III/II There is evidence in the electrochemical data, in comparisons between [(bpy)(2)ClOs(pz)Ru(NH3)(5)](3+) and [(bpy)(2)ClOs(4,4’-bpy)RU(NH3)(5)](3+) for significant through-bridge electronic coupling in [(bpy)(2)ClOsII(pz)Ru-III(NH3)(5)](4+) but not in I(bpy)2ClOs"(pz)Ru"’(NH3)51"+. The difference in behavior is caused by extensive H-bonding to the solvent at -RU(III)(NH3)(5)](3+) in [(bpy)(2)ClOsII(pz)Ru-III(NH3)(5)](4+) This mixes solvent character into d pi(Ru-III) which decreases electronic coupling across the bridge.