Reactions of Os(CO)4(η2-C2H2) with Phosphines: Substitution and Facile CO Insertion into Osmium−Alkyne Bonds. Combined Synthetic and Kinetic Studies

The reactions of Os(CO)4(η2-C2H2) (1) with PMe3 and PtBu3 are shown to yield an unexpectedly rich and diverse number of products. With excess PMe3 at 0 °C, substitution of CO results in the mono(phosphine) (2) and bis(phosphine) (3) complexes as well as the CO-inserted osmacyclobutenone Os(CO)2(PMe3)2{η1:η1-C(O)C2H2} in two isomeric forms (4a,b). At ambient temperature, compound 3 transforms into the hydrido−acetylide species Os(CO)2(PMe3)2(H)(C2H) (7), whereas 2 gives the bimetallic flyover bridged compound Os2(CO)5(PMe3)2{μ-η1:η1:η2-H2C2C(O)C2H2} (6), identified on the basis of detailed multinuclear NMR spectroscopic studies. With excess PtBu3 at 0 °C, the only isolated product is the doubly CO inserted osmacyclopentene-2,4-dione Os(CO)3(PtBu3)(η1:η1-C(O)C2H2(O)C) (5), characterized further by X-ray crystallography. The rates of substitution of various phosphines on 1 to give the monophosphine complex are consistent with a dissociative mechanism, as found previously for the hexafluorobutyne analogue of ...