Density functional study of alkyne to vinylidene rearrangements in [(Cp)(PMe3)2Ru(HCCR)]+ (R = H, Me)

The alkyne to vinylidene isomerization in [(Cp)(PMe3)(2)Ru(HCdropCH)](+) and [(Cp)(PMe3)(2-)Ru(HCdropCMe)](+) + has been investigated by density functional calculations. For both systems, the potential energy surface for the two possible isomerization mechanisms, ie., through a 1,2-hydrogen shift or through an oxidative addition to a hydrido-alkynyl intermediate, has been analyzed by a gradient-corrected DFT approach. The vinylidene complexes have been found more stable than the corresponding alkyne complexes, 13.1 and 10.4 kcal mol(-1), respectively, and are the thermodynamically most stable species on the potential energy surfaces of the two systems. The 1,2-hydrogen shift, proceeding via an eta(2)-(C-H)-coordinated alkyne intermediate, is the energetically most favorable path for both ethyne and propyne isomerizations, with highest energy barriers of 26.8 and 18.6 kcal mol(-1), respectively. However, while the higher energy barrier computed for the oxidative addition rules out such a process in the propyne rearrangement (29.0 vs 18.6 kcal mol-1), the barriers for the 1,2-hydrogen shift and for the oxidative addition are almost comparable in the case of the ethyne rearrangement (26.8 vs 31.7 kcal mol(-1)), so that the oxidative addition process might become competitive. For the inverse vinylidene to propyne rearrangement we calculate an overall activation enthalpy and entropy of 25.5 kcal mol(-1) and -3.0 cal K-1 mol(-1), respectively, in excellent agreement with the experimental values of 26.8 +/- 0.7 kcal mol(-1) and -4.9 +/- 1.9 cal K-1 mol(-1).