Hydrogen Migration in Transition Metal Alkyne and Related Complexes

The electronic and structural characteristics of the reaction interconverting a l‐alkyne complex into a vinylidene via a 1,2 hydrogen shift are examined. In a mononuclear system, initial slippage of the alkyne to an η 1 geometry is indicated. The subsequent step is shown to be analogous to the isomerization of a methylvinyl cation. We conclude that an alternative route involving a hydrido‐acetylide species will be of much higher energy. A concerted shift in binuclear and trinuclear systems is ruled out, based on the loss of a strong bonding interaction between the alkyne and the metallic piece, in the transition state. In trinuclear systems, a mechanism for the isomerization is suggested involving prior oxidative addition of the CH bond across a metal‐metal bond. The metallic piece in this case assists the transformation. The discussion is extended to other reactions featuring hydrogen shifts; these include the intramolecular formation of a binuclear vinylidene from a 1,2‐hydrido‐acetylide complex, the isomerization of a binuclear μ‐alkylidyne into a μ‐vinyl geometry and the transfer of a bridging hydrogen onto a capping hydrocarbon fragment in trinuclear cluster complexes.