Os(H) 2 Cl 2 L 2 (L = P i Pr 3 ) reacts with propylene or styrene to give equimolar amounts of OsHCl 2 (CCH 2 R)L 2 and the hydrogenated olefin; these molecules are isomeric with the ruthenium carbenes RuCl 2 [C(H)CH 2 R]L 2, yet these distinct redox alternatives are thermodynamically preferred. Ab initio (B3LYP) calculations show that the unsaturated five-coordinate MCl 2 (CHMe)(PH 3 ) 2 is more stable than the saturated hexacoordinate MCl 2 H(CMe)(PH 3 ) 2 for M = Ru, while the two species are almost isoenergetic in the case of Os; computationally, it is found that the osmium hydrido carbyne involves a large activation energy (27.2 kcal/mol) to transform unimolecularly to its carbene isomer.
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Spivak et al. (1998) studied this question.