Large Kinetic Isotope Effects for the Protonolysis of Metal-Methyl Complexes Are Not Reliable Mechanistic Indicators

Earlier work on protonolyses of several palladium and platinum methyl complexes (with release of methane) had suggested the possibility that observation of an unusually large kinetic isotope effect, consistent with significant contributions from quantum mechanical tunneling, might be diagnostic of a mechanism involving direct protonation of the metal–methyl bond, as opposed to one proceeding via a metal hydride intermediate. By extension of these measurements to a wider set of complexes, we find no support for the proposed correlation.