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.The great redox flexibility of the transition elements permits the possible existence of redox tautomers, or isomers, and the question of the comparative stabilities of these. Conceivable alternative structures (i.e., involving significant movement of atoms) in a number of cases include M(η 2 -EE′) vs M(E)(E′), where E, E′ ) O, S, NR, PR, CR 2 . Some particularly striking examples of redox isomers are represented by dihydride/dihydrogen tautomers existing in thermal equilibrium (eq 1). 1 Equally remarkable are the examples where change of solvent alters the stable form from L n Cu 2 (µ-O 2 ) to L n -Cu 2 (µ-O) 2 . 2 Isomers which differ by hydrogen migration represent fundamental transformations of organometallic chemistry (eq 2 and 3), and a hydrogen migration to N 2 (eq 4) has been reported with conversion from solution to the solid phase. 3 Conversion from a hydride/ vinylidene complex to a carbyne has been reported on

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