The compounds HM(CO)(4)SnPh3, M = Os (10), Ru (11) are activated in the presence of Pt(PBu3t)(2) and Pd(PBu3t)(2) toward the insertion of PhC2H into the M-H bond. The compounds PtOs(CO)(4)(SnPh3)(PBu3t)[mu-HCC(H)Ph], 12, and PtOs(CO)(4)(SnPh3)(PBu3t)[mu-H2CCPh], 13, were obtained from the reaction of 10 with PhC2H in the presence of Pt(PBu3t)(2). Compounds 12 and 13 are isomers containing alkenyl ligands formed by the insertion of the PhC2H molecule into the Os-H bond at both the substituted and unsubstituted carbon atoms of the alkyne. Both compounds contain a Pt(PBu3t) group that is bonded to the osmium atom and a bridging alkenyl ligand that is pi-bonded to the osmium atom. The reaction of 11 with PhC2H in the presence of Pt(PBu3t)(2) yielded the products PtRu(CO)(4)(SnPh3)(PBu3t)[mu-HC2(H)Ph], 14, and PtRu(CO)(4)(SnPh3)(PBu3t)[mu-H2C2Ph], 15, which are also isomers similar to 12 and 13. The reaction of 11 with PhC2H in the presence of Pd(PBu3t)(2) yielded the product PdRu(CO)(4)(SnPh3)(PBu3t)[mu-H2C2Ph], 16. Compound 16 contains a Pd(PBu3t) group bonded to the ruthenium atom and a bridging H2C2Ph ligand that is pi-bonded to the palladium atom. Compound 10 reacted with Pt(PBu3t)(2) in the absence of PhC2H to yield the compound PtOs(CO)(4)(SnPh3)(PBu3t)(mu-H), 17. Compound 17 is a Pt(PBu3t) adduct of 10. It contains a Pt-Os bond with a bridging hydrido ligand. Compound 17 reacted with PhC2H to yield 12. Compound 12 reacted with PhC2H to yield the compound PtOs(CO)(3)(SnPh3)(PBu3t)[mu-HCC(Ph)C(H)C(H)Ph], 18. Compound 18 contains a bridging 2,4-diphenylbutadienyl ligand, HCC(Ph)C(H)C(H)Ph, that is pi-bonded to the osmium atom and sigma-bonded to the platinum atom. Fenkse-Hall molecular orbitals of 17 were calculated. The LUMO of 17 exhibits an empty orbital on the platinum atom that appears to be the most likely site for PhC2H addition prior to its insertion into the Os-H bond.