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The reaction entropies lSo,, of a number of transition metal redox couples of the form M ( I I I ) / ( 11) in aqueous solution have bcen determined using nonisothermal electrochemical cells in order to explore the effect of varying the ligand structure upon the nature of the ion-solvent interactions. Examination of six aquo couples of the form M ( O H Z ) , , ~ + ' ~ + with varying metal M yielded ASo,, values in the range 36-49 eu. In order to scrutinize the effect of replacing aquo with ammine and simple anionic ligands, R u ( l I I ) / ( I I ) couples were employed since the relativc substitution inertness of both oxidation states allowed AS',, to be determined using cyclic voltammetry. The stepwise replacement of aquo by ammine ligands results in substantial reductions in ASo,, 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 resulis in substantial reductions in ASo,,. A number of M ( I I I ) / ( I I ) couples containing chelating ligands were also examined. Sizable differcnces in ASo,, were found between Co(lll)/(ll) couples and the corresponding R u ( l l l ) / ( l l ) and Fe(lll)/(ll) 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 ASo,, from nonisothermal cell measurements is discussed.

A survey of ligand effects upon the reaction entropies of some transition metal redox couples