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) have been obtained using (1)H NMR for the self-exchange reaction of the (eta(5)-C(5)(CH(3))(5))(2)Fe](+) hexafluorophosphate and tetrafluoroborate with (eta(5)-C(5)(CH(3))(5))(2)Fe] in acetone-d(6) (deltaV(ex) = -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 CD(2)Cl(2), but even that produced only a minor reduction in k(ex) and so had a negligible effect on deltaV(ex) ( = -6.4 +/- 0.2 cm(3) mol(-)(1) with PF(6)(-)). In all cases, deltaV(ex) 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 approximately 5-fold increase in k(ex) on going from (eta(5)-C(5)H(5))(2)Fe](+/0) to (eta(5)-C(5)(CH(3))(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.