Spectra of the mixed-valence complexes [(bpy)2ClOs(L)Ru(NH3)5]4+ (L = 4,4‘-bipyridine, pyrazine; bpy = 2,2‘-bipyridine) are highly solvent dependent. They provide oxidation state specific spectral markers which show that oxidation states for L = 4,4‘-bpy are OsIII−RuII in solvents of donor number (DN) < 14. In solvents of high donor number (DN > 15) they are OsII−RuIII. The isomers coexist at donor numbers 14−15 and the distribution between them tuned in mixtures of acetonitrile and propylene carbonate demonstrating that intramolecular electron transfer can be induced by varying the solvent. Both OsIII−RuII and OsII−RuIII display broad, solvent-dependent intervalence transfer (IT) bands in the near infrared (NIR). There are two isomers of [(bpy)2ClOs(pz)Ru(NH3)5]4+ as well. In one, which is dominant in solvents of DN < 22, the oxidation states are [(bpy)2ClOsIII(pz)RuII(NH3)5]4+. This is shown by the appearance of dπ → dπ marker bands for OsIII (at 4200 and 5800 cm-1 in CD3CN), ν(bpy) OsIII bands in the mid-IR, and the pattern of UV−visible bands. For this isomer a narrow, structured IT band is observed at ∼8100 cm-1 which is nearly solvent independent in solvents of DN ≥ 11.9. The second isomer, [(bpy)2ClOsII(pz)RuIII(NH3)5]4+, is dominant at DN > 24. It has IT bands in the NIR which are broad and solvent dependent. It is concluded that there is significant through-pyrazine electronic coupling between OsIII and RuII in [(bpy)2ClOsIII(pz)RuII(NH3)5]4+, but localized oxidation states, and a residual solvent−vibrational barrier to intramolecular electron transfer continue to exist. The exchanging electron is in a dπ(RuII) orbital largely orthogonal to the bridge. In [(bpy)2ClOsII(pz)RuIII(NH3)5]4+ electronic coupling is much less. Its properties are consistent with those of Class II in the Robin and Day mixed-valence classification scheme. The two isomers coexist in trimethyl phosphate (DN = 23.0) and formamide (DN = 24.0). From the temperature dependence of the mixed-valence equilibrium in trimethyl phosphate, [(bpy)2ClOsIII(pz)RuII(NH3)5]4+ ⇌ [(bpy)2ClOsII(pz)RuIII(NH3)5]4+, K(297K) = 2.8, ΔH° = −8.2 ± 2 kcal/mol, and ΔS° = −27 ± 6 cal mol-1 deg-1. Because of the difference in the extent of electronic delocalization between the two isomers, intramolecular electronic transfer is a complex event involving coupled electronic and nuclear motions. There is no simple relationship between optical and thermal electron transfer.