Self-exchange reaction kinetics of metallocenes revisited: Insights from the decamethylferricenium-decamethylferrocene reaction at variable pressure

Rate constants k(ex) and volumes of activation DeltaV(ex)(double dagger) have been obtained using H-1 NMR for the self-exchange reaction of the [(eta(5)-C-5(CH3)(5))(2)Fe](+) hexafluorophosphate and tetrafluoroborate with [(eta(5)-C-5(CH3)(5))(2)Fe] in acetone-d(6) (DeltaV(ex)(double dagger) = -8.6 +/- 0.3 cm(3) mol(-1)), dichloromethane-d(2), and (semiquantitatively) in acetonitrile-d(3). Under the experimental conditions, ion pairing was significant only in CD2Cl2, but even that produced only a minor reduction in k(ex) and so had a negligible effect on DeltaV(ex)(double dagger) ( = -6.4 +/- 0.2 cm(3) mol(-1) with PF6-). In all cases, DeltaV(ex)(double dagger) is negative and consistent with a simple two-sphere activation model, rather than with that of Weaver et al. (Nielson, R. M.; McManis, G. E.; Safford, L. K.; Weaver, M. J. J Phys. Chem. 1989, 93, 2152) in which the barrier crossing rate is limited by solvent dynamics. Similarly, the similar to5-fold increase in k(ex) on going from [(eta(5)-C5H5)(2)Fe](+/0) to [(eta(5)-C-5(CH3)(5))(2)Fe](+/0) in acetone can be explained with the two-sphere model on the basis of the effects of reactant size on the solvent reorganization energy, without reference to solvent dynamics.