Os(H)(2)NO((PPr3)-Pr-i)(2)(+): Mechanism and energetics of a room temperature reversible ethyl/Hydridoethylene equilibrium and contrasting double insertion of acetylene

cis,trans-Os(H)(2)(NO)L-2(+) (L = (PPr3)-Pr-i) rapidly adds ethylene to giv e an eta (2)-ethylene complex with C2H4 trans to one hydride and cis to the other. One hydride and the four ethylene protons are exchanging reversibly above -20 degreesC, with a measured DeltaG(double dagger) (20 degreesC) of 15 kcal/mol. DFT calculations on the rearrangement reveal bond making and breaking, as well as migration of the nonexchanging hydride. Nitrosyl bendi ng is not involved. Use of acetylene leads to insertion into both Os-H bond s; the resulting Os(CH=CH2)(2)(NO)L-2(+) was fully characterized as having inequivalent vinyl groups and undergoes facile vinyl ligand site exchange i n solution.