Hydrogen-Atom Transfer Reactions of Transition-Metal Hydrides

Many reactions of transition-metal hydrides involve H* transfer. With olefins such transfer gives a radical cage, from which escape and collapse lead to product formation. Inverse isotope effects, second-order kinetics independent of ligand concentration, and CIDNP are diagnostic for this mechanism. Many other reactions of transition-metal hydrides occur by radical chain mechanisms, in which H* is abstracted by carbon-centered radicals or by metal radicals. Reasonably accurate values are now available for the M-H bond strengths of most of the common hydrides, and these values help rationalize the known H* transfer reactions of these hydrides. While the rates of certain H* transfer reactions have been measured by radical clock methods, the measurement of H* transfer rates to a substituted trityl radical has provided the first general comparison of the H* donor abilities of the various hydrides. These relative H* transfer rates are significantly affected by steric factors.