Alkyne dichotomy and hydrogen migration in binuclear cyclopentadienylmetal alkyne complexes.

The structures and energetics of the binuclear cyclopentadienylmetal alkyne systems CpMCR (M = Ni, Co, Fe; R = Me and NMe) have been investigated using density functional theory. For the CpMC(NMe) (M = Ni, Co, Fe) systems the relative energies of isomeric tetrahedrane CpM(alkyne) structures having intact alkyne ligands and alkyne dichotomy structures CpM(CNMe) in which the C[triple bond, length as m-dash]C triple bond of the alkyne has broken completely to give separate MeNC units depending on the central metal atoms. For the nickel system CpNiC(NMe) as well as the related nickel systems CpNi(MeCNMe) and CpNiCMe the tetrahedrane structures are clearly preferred energetically consistent with the experimental syntheses of several stable CpNi(alkyne) complexes. The tetrahedrane and alkyne dichotomy structures have similar energies for the CpCoC(NMe) system whereas the alkyne dichotomy structures are significantly energetically preferred for the CpFeC(NMe) system. The potential energy surfaces for the CpM(MeCNMe) and CpMCMe systems (M = Co, Fe) are complicated by low-energy structures in which hydrogen migration occurs from the alkyne methyl groups to one or both alkyne carbon atoms to give CpM(CHNMe) and CpM(CHMe) derivatives with bridging metalallylic ligands, CpM(CH[double bond, length as m-dash]C[double bond, length as m-dash]CHNMe) and CpM(CH[double bond, length as m-dash]C[double bond, length as m-dash]CHMe) with bridging allene ligands, as well as CpM(CH[double bond, length as m-dash]CH-CNMe) and CpM(CH[double bond, length as m-dash]CH-CHMe) with bridging vinylcarbene ligands. For the CpMCMe (M = Co, Fe) systems migration of a hydrogen atom from each methyl group to an alkyne carbon atom can give relatively low-energy CpM(CH[double bond, length as m-dash]CH-CH[double bond, length as m-dash]CH) structures with a bridging butadiene ligand. Five transition states have been identified in a proposed mechanism for the conversion of the CpCo/MeC[double bond, length as m-dash]CNMe system to the cobaltallylic complex CpCo(CHNMe) with intermediates having agostic C-H-Co interactions and an activation energy barrier sequence of 13.1, 17.0, 15.2, and 12.0 kcal mol.