Isomeric hydrido/vinylidene, MH(halide) (C=CH2)L2, and ethylidyne, M(halide)(C-CH3)L2 (M = Os, Ru; L equals phosphine), are energetically similar but not interconverting

It is shown that there is no deuterium-isotope incorporation in the vinylidene CHPh group of RuDX(CCHPh)((PBu2Me)-Bu-t)(2) (X = Cl, I) on the time scale of 24 h at 25 degrees C, which indicates that the (unobserved) isomeric alkylidyne species RuX(CCHDPh)((PBu2Me)-Bu-t)(2) is not readily accessible under these conditions. Ab initio (B3LYP) calculations 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 degrees C due to the high energy (54.2, 45.5 kcal mol(-1) for Os and Ru, respectively) of the transition state for such a 1,3-migration. In contrast, simple rotation of the CCH2 group of MHCl(CCH2)(PH3)(2) by 180 degrees 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).