HYDRIDE IS NOT A SPECTATOR LIGAND IN THE FORMATION OF HYDRIDO VINYLIDENE FROM TERMINAL ALKYNE AND RUTHENIUM AND OSMIUM HYDRIDES - MECHANISTIC DIFFERENCES

RuHX(H-2)L-2 (L = (PBu2Me)-Bu-t; X = Cl, I) and OsH3ClL2 (L = (PPr3)-P r-i) react (time of mixing) with terminal alkynes in a 1:2 stoichiomet ry to give MHX(=C=CHR)L-2 and RCH = CH2. The reactions of RuHX(H-2)L-2 with PhC=CD lead to RuDX(=C=CHPh)L-2 as the only isotopomers. This re sult is understood in terms of an insertion of the alkyne in the Ru-H bond to make a vinyl, followed by an alpha-hydrogen migration, giving the hydrido/vinylidene. This reaction path has been studied by ab init io (B3LYP) calculations with location of the minima and transition sta tes. In agreement with isotopic labeling experiments, it is shown that the path starting with insertion into the Ru-H,bond is more favorable than a 1,2-migration within the coordinated alkyne ligand. The:Favore d reaction is calculated to be exothermic from the starting compounds to intermediates and to products with the energies of all transition s tates well below that of the separated reactants. The mechanism involv es the formation of a 14-electron eta(1)-vinyl intermediate which, aft er structural changes, gives the 16-electron hydrido/vinylidene produc t. The same general reaction path is calculated to be preferred for Os , but differences occur between the geometries of the vinyl intermedia te (it is eta(2)-vinyl for Os) and in the total number of intermediate s involved.