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

Article abstract of DOI:10.1021/ic030067y

Rate constants k_ex and volumes of activation ΔV_ex? have been obtained using ¹H NMR for the self-exchange reaction of the [(η⁵-C₅(CH₃)₅)₂ Fe]⁺ hexafluorophosphate and tetrafluoroborate with [(η⁵-C₅(CH₃)₅)₂Fe] in acetone-d₆ (ΔV_ex? = -8.6 ± 0.3 cm³ mol^-1), dichloromethane-d₂, and (semiquantitatively) in acetonitrile-d₃. 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 ΔV_ex? (= -6.4 ± 0.2 cm³ mol^-1 with PF₆-). In all cases, ΔV_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 ～5-fold increase in k_ex on going from [(η⁵-C₅H₅)₂Fe]^+/0 to [(η⁵-C₅(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.