Density Functional Study of Butadiyne to Butatrienylidene Isomerization in [Ru(HC≡CC≡CH)(PMe3)2(Cp)]+

The butadiyne to butatrienylidene isomerization in [Ru(HCtCCtCH)(PMe3)2(Cp)]þ has been investigated by density functional calculations. Several possible minima have been identified on the potential energy surface for the coordinated C4H2 moiety, and a few plausible isomerization mechanisms have been analyzed by a DFT approach. The butatrienylidene complex has been found to be more stable than the butadiyne adduct by -13.1 kcal mol-1 in enthalpy and is the thermodynamically most stable species on the potential energy surface. The energetically most favorable isomerization pathway has been found to initially follow the same pathway experimentally and theoretically characterized for the simpler alkyne rearrangement on a d6 metal fragment, i.e. a 1,2-hydrogen shift passing through an agostic intermediate, and leading to a ethynyl vinyl intermediate, for which an activation enthalpy of 23.1 kcal mol-1 (activation free energy of 20.8 kcal mol-1) was found. The isomerization then proceeds through a proton migration from the Cβ to the terminal Cδ atom occurring through deprotonation of the ethynyl vinylidene, leading to a butadiynyl complex which is then reprotonated to the final butatrienylidene product, with an overall activation energy of 17.4 kcal mol-1 (activation free energy of 19.6 kcal mol-1).