130
J. Phys. Chem. 1982, 8 6 , 130-135
scopic dielectric-constant values are not the same as the microscopic values near the reactants. It has been observed in conductivity studies of several salts in acetonitrile to which various amounts of TCNE had been added that the mobility of differents anions (ClO;, PFG-,AuC1;) were affected differently by TCNE, probably because the microscopic values of the static dielectric constant and/or of the viscosity were not the same as the macroscopic ~a1ues.l~ Assuming that the Marcus weak-interaction conditions apply, the lack of dependence of the Fe(Cp),O?+exchange rate on solvent dielectric properties indicates that AGO* is about the same for different solvents, possibly because it is close to zero for all solvents, as it must be for the TCNE-TCNE- exchange reaction in acetonitrile. If AGO* N 0 for the Fe(Cp)2-Fe(Cp)z+exchange reaction and if KPZ -10" M-' s-l, as is usually assumed,2J0w, AGi* would need to be -6 kcal mol-' to account for the observed value of k fz 5 X lo6 M-' s-l in many solvents at 25 "C. The difference in the Cp-Cp distance for Fe(Cp), and Fe(Cp),+ of -0.06 l i 1 8 should result in a small but appreciable AGi* value, not zero as was a ~ s u m e d .Also, ~ removal of solvent molecules from reactants to bring them into contact could cause w, to have an appreciable value
+
pr1
(18) Haaland, A. Acc. Chem. Res. 1979, 12, 415.
even though there is no Coulombic repulsion between reactants. This effect should be present for all outersphere electron transfer reactions for which reactant contact is necessary, but, when both reactants are charged, Coulombic effects should dominate. The necessity for redistribution of charge during electron exchange between Fe(CpIz and Fe(Cp)z+,because of the different orders of molecular orbitals 1evels,l8could be a deterrent to electron exchange and might account for the observed negative values of AS*, an indication that the transmission coefficient, K, is less than unity.3 The enthalpies of activation, AH*,calculated from the observed small temperature dependences (E,,, Table 11) are consistent with values for the model (calculated as described in ref 3), but the entropies of activation, AS*, are generally somewhat more negative than calculated, indicating that KPZis
