Redox-state-dependent preferential solvation of ruthenium(II) and ruthenium(III) ammine complexes. Implications for electron-transfer processes in mixed solvents

Preferential solvation of the asymmetric binuclear complex (bpy)/sub 2/Ru/sup II/Cl(pyz)Ru/sup II/III/(NH/sub 3/)/sub 4/py/sup 3+/4+/ by Me/sub 2/SO in acetonitrile has been studied via UV-vis, near-IR, and electrochemical techniques. It is found that the strong donor Me/sub 2/SO preferentially solvates the mixed-valence 4+ form of the dimer to a greater degree than the fully reduced 3+ form due to the relatively higher Lewis acidity of the ammine protons when coordinated to Ru(III). As one index of this, the equisolvation point of the II,II dimer is found at chi/sub Me2SO/ = 0.104 whereas that for the II,III form is at chi/sub Me2SO/ = 0.003. The degree of solvent re-sorting that must occur upon a change in redox state at the ruthenium ammine moiety as a function of chi/sub Me2SO/ is quantified and found to go through a maximum at chi/sub Me2SO/ approx. = 0.019. It is concluded that a solvent trapping barrier arising from nonequilibrium preferential solvation must be considered in situations involving electron-transfer processes of preferentially solvated solutes.