Cage effects in organotransition metal chemistry: Their importance in the kinetic estimation of bond dissociation energies in solution

Abstract

Following a brief review of the literature of radical cage effects in solution, the available evidence for the operation of solvent cage effects in organotransition metal chemistry is summarized. Kinetic determination of ML bond dissociation energies (BDEs) in solution are examined within this context, emphasizing a comparison of the current gas phase reaction coordinate model vs a model more appropriate for solution work that includes solvent cage effects. The temperature dependence of the cage effect is presented in its proper mathematical form which shows that solution kinetics cannot be connected to BDEs without knowledge of the cage efficiency factor ( F c), except in special cases. It is pointed out that the observed activation enthalpy in solution, Δ H obs ‡(soln), is a composite containing variable amounts of the difference between the activation enthalpy for the cage combination (Δ H c ‡) and that for diffusive separation of the cage pair (Δ H d ‡), depending on the cage efficiency factor ( F c). No constant correction to Δ H obs ‡(soln) can be expected; a better first approximation would be to use the activation enthalpy for viscous flow of the solvent, Δ H η ‡, although this includes the implicit assumption that F c = 1. The text also attempts to analyse critically the available, relevant literature and to note areas requiring further attention. A short section in which the equations are applied to Ni(CO) 4 and PhCH(CH 3)Co(DMG) 2(Base) (and thus NiCO and CoC BDEs, respectively) is included. A brief list of major points is also provided in the Summary.