OSMIUM CONVERTS TERMINAL OLEFINS TO CARBYNES - ALPHA-HYDROGEN MIGRATION REDOX ISOMERS WITH REVERSED STABILITY FOR RUTHENIUM AND FOR OSMIUM

Os(H)(2)Cl2L2 (L = (PPr3)-Pr-i) reacts with propylene or styrene to gi ve equimolar amounts of OsHCl2(CCH2R)L-2 and the hydrogenated olefin; these molecules are isomeric with the ruthenium carbenes RuCl2[C(H)CH2 R]L-2, yet these distinct redox alternatives are thermodynamically pre ferred. Ab initio (B3LYP) calculations show that the unsaturated five- coordinate MCl2(CHMe)(PH3)(2) is more stable than the saturated hexaco ordinate MCl2H(CMe)(PH3)(2) for M = Ru, while the two species are almo st isoenergetic in the case of Os; computationally, it is found that t he osmium hydride carbyne involves a large activation energy (27.2 kca l/mol) to transform unimolecularly to its carbene isomer.