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

The adduct OSHCl(C2D4)(CO)L-2( L=(PPr3)-Pr-i) shows only very slow H/D exch ange at 25 degreesC, but this is easily detectable at 65 degreesC; no ethyl species is formed in detectable concentration. RuHCl(CO)L-2 shows no detec table C2D4 adduct, but Ru-H/C-D exchange at 60 degreesC 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 additio n of an alkene or an alkyne to MH(Cl)(CO)(PH3)(2) (M=Os, Ru). Thus, 18-elec tron alkyne adducts are compared to the 18-electron vinylidene isomer and t o the 16-electron vinyl complex. Similarly, the 18-electron alkene adduct i s compared to the 18-electron carbene isomer and to the 16-electron ethyl c omplex. Two factors are found to control the products formed: (i) the Os co mplex favors unsaturated pi -bonded ligands and an 18-electron count while Ru favors saturated ligands and an unsaturated metal center; (ii) the weake r pi bond in the alkyne than in the alkene makes insertion or isomerization of an alkyne thermodynamically more favored than that of an alkene. This r esults in ethyl complexes being less favored than vinyl complexes in simila r experimental conditions. For RuHCl(CO)L'(2), where L' is (PPr2)-Pr-i[3,5- (CF3)(2)C6H3], 1 atm ethylene gives a detectable, colorless ethylene adduct , then also a detectable ethyl complex, all in facile equilibrium.