Roles of pi-alkyne, hydride-alkynyl, and vinylidene metal species in the conversion of alkynes into vinylidene: New theoretical insights

The transformation of acetylene into vinylidene, as promoted by the metal f ragment [(pp(3))Co](+) [pp(3) = P(CH2CH2PPh2)(3)], is unimolecular and feat ures the hydride-acetylide species as an intermediate. The paper describes a detailed ab initio study of the reaction, in particular with regard to th e step involving 1,3-H shift. The best computational results are obtained b y mimicking the pp(3) Ligand with actual ethylenic chains rather than with single PH3 molecules. The keypoints along the two-step reaction path (pi-ac etylene, hydride-acetylide, and vinylidene complexes, as well. as intermedi ate transition states) have been optimized for Co-I and Rh-I derivatives at the MP2 level. For the fragment [(pp(3))Co](+), the barrier associated wit h transformation of the hydride-acetylide intermediate to vinylidene (20.6 kcal/mol) is easier to surmount compared to that for reversion to the react ants (28.6 kcal/mol). The situation is reversed for the analogous Rh-I syst em, with the initial pi-acetylene adduct being slightly more stable. Althou gh higher in energy, the hydride-acetylide species is the experimentally de tected product of the reaction of acetylene with the fragment [(pp(3))Rh](). The salient chemical aspects of the 1,3-H shift are discussed in terms o f perturbation theory arguments. Parallel EHMO calculations, which have pro vided a relatively good consistency with the ab initio results, allow the p roposal of an orbital rationale for the mode of migration of the hydride li gand along the substantially Linear Co-C-alpha-C-beta grouping.