One of the unique aspects of electron-transfer reactions in mixed solvents is the possibility for unsymmetrical selective solvation of reactants versus products. We have developed a versatile electrochemical method, based on reaction-entropy measurements, to detect its occurrence. The method is illustrated by results for five related redox systems: Ru(NH3)6 3+/ 2+, Ru(NH3)5(py)3+/ 2+, Ru(NH3)4(bpy)3+/2+, Ru(NH3)2(bpy)2 3+/ 2+, and Ru(bpy)3 3+/2+(bpy = 2,2'-bipyridine; py = pyridine), in mixtures of acetonitrile and dimethyl sulfoxide as solvent. The key finding is that a sharp increase (and a more gradual decrease) in reaction entropy occurs over the range of solvent composition where unsymmetrical selective solvation is significant. The dependence of the reaction entropy for Ru(NH3)5(py)3+/ 2+on mixed-solvent composition can be described fairly well by a simple statistical model. There are difficulties, however, in understanding the magnitudes of the observed entropy effects.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry