Hydride-carbyne to carbene transformation in an osmium-acetate-bis(triisopropylphosphine) system:: Influence of the coordination mode of the carboxylate and the reaction solvent

The bis-solvato hydride-allenylidene complex [OsH(CCCPh2)(CH3CN)(2)((PPr3)-Pr-i)(2)]BF4 (1) reacts with acetic acid to give the hydride-carbyne [OsH(kappa(2)-O2CCH3)(CCHCPh2)((PPr3)-Pr-i)(2)]BF4 (2), which in 1,2-dichloroethane under reflux is stable and does not evolve into its five-coordinate carbene isomer. In acetonitrile at room temperature, complex 2 is in equilibrium with [OsH{kappa(1)-OC(O)CH3}(CCHCPh2)(CH3CN)((PPr3)-Pr-i)(2)]BF4 (4; Delta H degrees = -6.4 +/- 0.3 kcal center dot mol(-1), Delta S degrees = -22.9 +/- 1.1 eu). At 353 K, complex 4 is transformed into the carbene [Os{kappa(1)-OC(O)CH3}(CHCHCPh2)(CH3CN)(2)((PPr3)-Pr-i)(2)]BF4 (5; 40%), which is obtained in high yield (88%) by reaction of [Os(CHCHCPh2)(CH3CN)(3)((PPr3)-Pr-i)(2)][BF4](2) with sodium acetate in 2-propanol. The hydride-carbyne to carbene transformation is analyzed by DFT calculations.