Selective solvation effects on mixed-valency species in mixed solvents

A simple binding-site model is developed to describe the behavior and optical response of binuclear mixed-valent metal complexes in mixed solvents. A free energy expression is constructed, including energy terms due to electron delocalization, to solvent molecules in the first solvation shell, and to interactions between solvent molecules, both in the bulk and in the first shell. An entropy term includes effects of parititioning the solvent species between bulk solvent and solvation shells. Minimization of the free energy yields an equilibrium condition; the resulting equations are precisely those of the two-sublattice model for magnetism. Conditions for symmetric solvation or for asymmetric solvation (with a corresponding blue shift in the intervalence-transfer absorption band) are developed, as a function of solvent composition and temperature. A phase diagram is given; at low temperatures, asymmetric solvation will almost always occur.