A Survey of Ligand Effects Upon the Reaction Entropies of Some Transition Metal Redox Couples

The reaction entropies ∆S°_rc of a number of transition metal redox couples of the form M(III)/(II) in aqueous solution have been determined using nonisothermal electrochemical cells in order to explore the effect of varying the ligand structure upon the nature of the ion-solvent interactions. Examinations of six aquo couples of the form M(OH_2)_n&3+/2+ with varying metal M yielded ∆S°_rc values in the range 36-49 eu. In order to scrutinize the effect of replacing aquo with ammine and simple anionic ligands, Ru(III)/(II) couples were employed since the relative substituion inertness of both oxidation states allowed ∆S°_rc to be determined using cyclic voltammetry. The stepwise replacement of aquo by ammine ligands results in substantial reductions in ∆S°_rc which are attributed to the smaller extent of ligand-solvent hydrogen bonding for ammine compared with aquo ligands. Substitution of both aquo and ammine by anionic ligands also results in substantial reductions in ∆S°_rc. A number of M(III)/(II) couples containing chelating ligands were also examined. Sizable differences in ∆S°_rc were found between Co(III)/(II) couples and the corresponding Ru(III)/(II) and Fe(III)/(II) couples. Suggested explanations are differences in ligand conformation and electron delocalization effects. The possible contribution of outer-sphere solvent structuring effects to the large reorganization energies observed for electron exchange of aquo complexes is noted. The validity of the assumptions required for the estimation of ∆S°_rc from nonisothermal cell measurements is discussed