Hydrogen‐Atom Transfer Reactions of Transition‐Metal Hydrides

Abstract

Abstract 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.