ISOMERIC HYDRIDO VINYLIDENE, MH(HALIDE) (C=CH2)L-2, AND ETHYLIDYNE, M(HALIDE)(C-CH3)L-2 (M = OS, RU L EQUALS PHOSPHINE), ARE ENERGETICALLY SIMILAR BUT NOT INTERCONVERTING/

It is shown that there is no deuterium-isotope incorporation in the vi nylidene CHPh group of RuDX(CCHPh)((PBu2Me)-Bu-t)(2) (X = Cl, I) on th e time scale of 24 h at 25 degrees C, which indicates that the (unobse rved) isomeric alkylidyne species RuX(CCHDPh)((PBu2Me)-Bu-t)(2) is not readily accessible under these conditions. Ab initio (B3LYP) calculat ions on the model MHCl(CCH2)(PH3)(2) (M = OS, Ru) show the ethylidyne isomer MCl(CCH3)(PH3)(2) to be close in energy (2.5, -1.1 kcal mol(-1) for Os and Ru, respectively) and effectively inaccessible at 25 degre es C due to the high energy (54.2, 45.5 kcal mol(-1) for Os and Ru, re spectively) of the transition state for such a 1,3-migration. In contr ast, simple rotation of the CCH2 group of MHCl(CCH2)(PH3)(2) by 180 de grees around the C-C axis is calculated to have a low barrier (8.1, 4. 3 kcal mol(-1) for Os and Ru, respectively), indicating this to be the mechanism for isomerization observed in OsHCl(CCHPh)((PPr3)-Pr-i)(2).