A comparative study of olefin or acetylene insertion into Ru–H or Os–H of MHCl(CO)(phosphine)2

Abstract

The adduct OsHCl(C₂D₄)(CO)L₂ (L = PⁱPr₃) shows only very slow H/D exchange at 25 °C, but this is easily detectable at 65 °C; no ethyl species is formed in detectable concentration. RuHCl(CO)L₂ shows no detectable C₂D₄ adduct, but Ru–H/C–D exchange at 60 °C is actually faster than for Os. DFT (B3PW91) calculations have been carried out to analyze the relative energies of the isomeric forms that would result from the addition of an alkene or an alkyne to MH(Cl)(CO)(PH₃)₂ (M = Os, Ru). Thus, 18-electron alkyne adducts are compared to the 18-electron vinylidene isomer and to the 16-electron vinyl complex. Similarly, the 18-electron alkene adduct is compared to the 18-electron carbene isomer and to the 16-electron ethyl complex. Two factors are found to control the products formed: (i) the Os complex favors unsaturated π-bonded ligands and an 18-electron count while Ru favors saturated ligands and an unsaturated metal center; (ii) the weaker π bond in the alkyne than in the alkene makes insertion or isomerization of an alkyne thermodynamically more favored than that of an alkene. This results in ethyl complexes being less favored than vinyl complexes in similar experimental conditions. For RuHCl(CO)L₂^′, where L′ is PⁱPr₂[3,5-(CF₃)₂C₆H₃], 1 atm ethylene gives a detectable, colorless ethylene adduct, then also a detectable ethyl complex, all in facile equilibrium.