The elongated dihydrogen complex [Os{C6H4C(O)CH3}(eta(2)-H-2)(H2O)((PPr3)-Pr-i)(2)]BF4 (1) reacts with 1,1-diphenyl-2-propyn-1-ol and 2-methyl-3-butyn-2-ol to give the hydride-hydroxyvinylidene-pi-alkynol derivatives [OsH{=C=CHC(OH)R-2}{eta(2)-HC&3bond; CC(OH)R-2}((PPr3)-Pr-i)(2)]BF4 (R = Ph (2), Me (3)), where the pi-alkynols act as four-electron donor ligands. Treatment of 2 and 3 with HBF4 and coordinating solvents leads to the dicationic hydride-alkenylcarbyne compounds [OsH(&3bond; CCH=CR2)S-2((PPr3)-Pr-i)(2)][BF4](2) (R = Ph, S = H2O (4), CH3CN (5); R = Me, S = CH3CN (6)), which in acetonitrile evolve into the alkenylcarbene complexes [Os(=CHCH=CR2)(CH3CN)(3)((PPr3)-Pr-i)(2)][BF4](2) (R = Ph (7), Me (8)) by means of a concerted 1,2-hydrogen shift from the osmium to the carbyne carbon atom. Treatment of 2-propanol solutions of 5 with NaCl affords OsHCl2(&3bond; CCH=CPh2)((PPr3)-Pr-i)(2) (10), which reacts with AgBF4 and acetonitrile to give [OsHCl-(&3bond; CCH=CPh2)(CH3CN)((PPr3)-Pr-i)(2)]BF4 (11). In this solvent complex 11 is converted to [OsCl(=CHCH= CPh2)(CH3CN)(2)((PPr3)-Pr-i)(2)]BF4 (12). Complex 5 reacts with CO to give [Os(=CHCH= CPh2)(CO)(CH3CN)(2)((PPr3)-Pr-i)(2)][BF4](2) (15). DFT calculations and kinetic studies for the hydride-alkenylcarbyne to alkenylcarbene transformation show that the difference of energy between the starting compounds and the transition states, which can be described as eta(2)-carbene species (Os=C(R)H), increases with the basicity of the metallic center. The X-ray structures of 4 and 7 and the rotational barriers for the carbene ligands of 7, 8, and 12 are also reported.