[en] The thermodynamics of mixed-valence isomers - the two limiting charge distributions in a mixed-valence complex - are reviewed and used as a basis for calculating the electronic coupling (H_AB). Infrared spectroscopy is used to determine equilibrium constants between the two mixed-valence isomers in the strongly coupled adiabatic limit. Equilibrium constants in the diabatic (H_AB=0) limit are independently estimated from electrochemical data for corresponding symmetric complexes. Analysis of these data provide direct measurements of ΔΔG⁰=ΔG₀⁰-ΔG₁⁰, the difference in thermodynamic driving force for electron transfer in the diabatic and strongly coupled adiabatic limits which it is shown can be interpreted within the framework of appropriate potential energy surfaces for ET to estimate H_AB