Solvent, Ligand, and Ionic Charge Effects on Reaction Entropies for Simple Transition-Metal Redox Couples

The dependence of the reaction entropies, ASo,, for simple M(III/II) redox couples (M = Ru, Fe, Os, Cr) upon the nature of the ligands and the solvent is examined with a view toward correlating ASo, with simple physical parameters. For couples containing ammine, ethylenediamine, polypyridine, cyclopentadiene, or pseudohalide ligands, ASo, in a given solvent is found to correlate well with (Zo: Zrd2), where Z,, and Zd are the charge numbers of the oxidized and reduced forms, and with l l r , where r is the effective radius of the redox couple. This suggests that specific ligand-solute interactions do not provide a predominant contribution to ASo, for these systems, although this effect is probably important for aquo redox couples in water. The dependence of ASo, upon the solvent correlates reasonably well with the solvent "acceptor number" and other solvent polarity parameters. This is rationalized in terms of a contribution to ASo, arising from disruption of the surrounding solvent structure by the charged solute. The predictive as well as interpretative virtues of such semiempirical correlations of reaction entropies are pointed out.