Acetylene to vinylidene rearrangements on electron rich d6 metal centers:: a density functional study

The acetylene to vinylidene isomerization on several Ru(II)d(6) metal fragments with different electron richness of the metal center has been investigated by means of density functional theory calculations. We considered the [(eta(5)-C5Me5)Ru(dippe)](+), [(eta(5)-C5Me5)Ru(dmpe)](+), [(eta(5)-C5H5)Ru(PMe3)(2)](+), [(eta(6)-C6Me6)(PMe3)ClRu](+), [(eta(5)-C5H5)Ru(CO)(PPh3)](+) and [(eta(6)-C6H6)(PMe3)ClRu](+), species which are quite common in the chemistry of cationic Ru(II) complexes and span a wide range of electron-richness. For each of the considered fragments, the minima on the potential energy surfaces for the two possible isomerization mechanisms, i.e. through a direct 1,2-hydrogen shift or through a hydrido-alkynyl intermediate, have been localized. A linear correlation has been found between the C=C stretching frequencies of the vinylidene complexes, as an estimate of the electron richness, and the stability of the corresponding hydrido-alkynyl intermediates. For the most electron-rich among the considered fragments, [(Cp*)(dippe)Ru(HCCH)](+), the hydrido-alkynyl species has been found essentially isoenergetic with the alkyne complex (only 1.9 kcal mol(-1) higher), in agreement with the experimental evidence showing for this system an equilibrium between these two species. For the same [(Cp*)(dippe)Ru](+) fragment, a detailed analysis of the reaction profiles for the two possible acetylene rearrangement pathways has been performed. Our results show that once the eta(2)-C-H coordinated acetylene intermediate is accessed, the system can easily evolve towards a hydrido-alkynyl intermediate, this process being kinetically favored with respect to the direct 1,2-shift leading to the vinylidene product.