Solvent, ligand, and ionic charge effects on reaction entropies for simple transition-metal redox couples

The dependence of the reaction entropies for simple M(III)/(II) redox couples, with M = Ru, Fe, Os, Cr, upon the nature of the ligands and the solvent is examined with a view towards correlating the entropies with simple physical parameters. For couples containing ammine, ethylenediamine, polypyridine, cyclopentadiene or psuedohalide ligands reaction entropy in a given solvent is found to correlate well with a function of the charge numbers of the oxidized and reduced forms, and with 1/r, where r is the effective radius of the redox couple. This suggests that shortrange ligand-solute interactions do not provide a predominant contribution to reaction entropy for these systems, although this effect is probably important for aquo redox couples in water. The dependence of reaction entropy upon the solvent correlates reasonably well with the solvent acceptor number and other solvent polarity parameters.