The five-coordinate hydrido−dihydrogen complex [OsH(η2-H2)(CO)(PiPr3)2]BF4 (1) reacts with methyl propiolate in acetone to (2). The acetone ligand of 2 can be easily displaced by methyl propiolate and 1,1-diphenyl-2-propyn-1-ol to (3) respectively. The structure of 4 has been determined by X-ray diffraction. The geometry around the osmium atom can be rationalized as a distorted octahedron with the phosphine ligands occupying opposite positions. The remaining perpendicular plane is formed by the chelating alkenyl ligand, which acts with a bite angle of 62.8(2)°, the carbonyl group disposed trans to the oxygen atom. Complex 4 reacts with methyllithium to give the alkynyl derivative which is unstable in methanol and quantitatively evolves Complex 6 can be alternatively obtained from the reaction of 4 with NaOCH3. Complex 4 also reacts with NaCl to afford trans-chlorocarbonyl Os{C[C(O)OCH3]CH2}Cl(CCCPh2)(CO)(PiPr3)2 (7) and cis-chlorocarbonyl Os{C[C(O)OCH3]CH2}Cl(CCCPh2)(CO)(PiPr3)2 (8). At 60 °C, toluene solutions of 7 and 8 yield the as a result from the migratory insertion of the allenylidene ligand into the Os−C(alkenyl) bond of 7 and 8. The structure of 9 has been also determined by X-ray diffraction. The geometry around the osmium atom can be described as a distorted octahedron with the phosphine ligands occupying two relative trans positions. The ideal equatorial plane is formed by the chelate allenyl ligand which acts with a bite angle of 78.66(12)°, the carbonyl group disposed trans to the oxygen atom. We thank the DGICYT (Projects PB 95-0806 and PB94-1186, Programa de Promoción General del Conocimiento) for financial support. A.J.E. thanks the EU (Human, Capital and Mobility Programme, CT93-0347) for a grant. Peer reviewed