Treatment in acetonitrile at -30 degrees C of the hydride-alkenylcarbyne complex [OsH( CCH=CPh2)(CH3CN2)(2)((PPr3)-Pr-i)(2)][BF4](2) (1) with 'BuOK produces the selective deprotonation of the alkenyl substituent of the carbyne and the formation of the bis-solvento hydride-allenylidene derivative [OsH(=C=C=CPh2)(CH3CN)(2)((PPr3)-Pr-i)(2)]BF4 (2), which under carbon monoxide atmosphere is converted into [Os(CH=C=CPh2)(CO)(CH3CN)(2)((PPr3)-Pr-i)(2)]BF4 (3). When the treatment of 1 with 'BuOK is carried out in dichloromethane at room temperature, the fluoro-alkenylcarbyne [OsH F( CCH=Cph(2))(CH3CN) (p(i)Pr(3))(2)]BF4 (4) is isolated. Complex 2 reacts with terminal alkynes. The reactions with phenylacetylene and cyclohexylacetylene afford [Os{(E)-CH=CHR}(=C=C=CPh2)(CH3CN)(2)((PPr3)-Pr-i)(2)]BF4 (R = Ph (5), Cy (6)), containing an alkenyl ligand beside the allenylidene, while the reaction with acetylene in dichloromethane at -20 degrees C gives the hydride-allenylidene-pi-alkyne [OsH(=C=C=CPh2)(eta(2)-HC CH)((PPr3)-Pr-i)(2)]BF4 (7), with the alkyne acting as a four-electron donor ligand. In acetonitrile under reflux, complexes 5 and 6 are transformed into the osmacyclopentapyrrole compounds [Os{C=C(CPh2CR=CH)CMe=NH}(CH3CN)(2)]BF4 (R = Ph (8), Cy (9)), as a result of the assembly of the allerrylidene ligand, the alkenyl group, and an acelonitrile molecule. The X-ray structures of 2, 5, and 8 are also reported.