Deficiencies of the ferricinium-ferrocene redox couple for estimating transfer energies of single ions

The difference between the partial molal entropies of ferrocene and ferricinium\((\bar S_{{\text{Fc}}}^{\text{0}} - \bar S_{{\text{Fc}}^{\text{ + }} }^{\text{0}} )\) has been determined in nine solvents from the temperature dependence of the formal potential of the ferricinium-ferrocene redox couple using a nonisothermal electrochemical cell arrangement in order to probe possible structural reasons for the limitations of the ‘ferrocene assumption’ for estimating the transfer thermodynamics of single ions between different solvents. In contrast to the uniformly small positive values of\((\bar S_{{\text{Fc}}}^{\text{0}} - \bar S_{{\text{Fc}}^{\text{ + }} }^{\text{0}} )\) predicted by the Born model, the experimental quantities varied widely from small or even negative values in hydrogen-bonded solvents (−5 to 3 e.u.) to substantially larger values (11–14 e.u.) in dipolar aprotic media. These variations appear to arise chiefly from additional solvent ordering in the vicinity of the ferricinium cation compared to the ferrocene molecule which is enhanced in the aprotic solvents. The variations in\((\bar S_{{\text{Fc}}}^{\text{0}} - \bar S_{{\text{Fc}}^{\text{ + }} }^{\text{0}} )\) between water and a number of nonaqueous solvents provide a predominate contribution to the differences between the free energies of single ion transfer calculated using the ferrocene and alternative extrathermodynamic assumptions.