The reactions of the bis(solvento) complexes [OsCp(MeCN) 2 (P i Pr 3 )]PF 6 (1; Cp = cyclopentadienyl) and [OsTp(κ 1 -OCMe 2 ) 2 (P i Pr 3 )]BF 4 (1a; Tp = hydridotris(pyrazolyl)borate) with allenes have been investigated. Complex 1 reacts with 1-methyl-1-(trimethylsilyl)allene and 1,1-dimethylallene to give the π-allene derivatives [OsCp(η 2 -CH 2 dCdCRMe)(MeCN)(P i Pr 3 )]PF 6 (R = SiMe 3 (2), Me (3)). In fluorobenzene at 80 °C, complexes 2 and 3 are moderately stable and evolve into the isopropenyldiisopropylphosphine derivative [OsCp{κ 3 P,C,C-P i Pr 2 [C(Me)dCH 2 ]}(MeCN)]PF 6 (4) by hydrogen transfer from an isopropyl substituent of the phosphine to the coordinated double bond of the allene. Under an ethylene atmosphere the acetonitrile ligand of 4 is displaced by the olefin. The resulting π-ethylene derivative [OsCp(η 2 -CH 2 dCH 2 ){κ 3 P,C,C-P i Pr 2 [C(Me)dCH 2 ]}]PF 6 ( 5) is obtained through a one-pot synthesis procedure by the stirring of 3 in fluorobenzene at 80 °C under 2 atm of ethylene. Treatment of 2 and 3 with dimethyl acetylenedicarboxylate gives [OsCp{η 2 -CH(CO 2 Me)dCH(CO 2 Me)}{κ 3 P,C,C-P i Pr 2 [C(Me)dCH 2 ]PF 6 (6). The reaction of 1a with 1,1dimethylallene leads to [OsTp(η 2 -CH 2 dCdCMe 2 )(κ 1 -OCMe 2 )(P i Pr 3 )]BF 4 (7). In contrast to its Cp counterpart, complex 7 evolves into the hydride-alkenylcarbyne derivative [OsHTp(tCCHdCMe 2 )-(P i Pr 3 )]BF 4 (8), by means of a double migration of the hydrogen atoms of the terminal CH 2 group of the allene. One of them migrates to the central carbon atom of the allene, and the other one goes to the metal center. The alkenylcarbyne group of 8 is selectively deprotonated in the presence of the hydride ligand to afford the hydride-alkenylvinylidene OsHTp{dCdCHC(Me)dCH 2 }(P i Pr 3 ) (9). The X-ray structures of 2, 5, 8, and 9 are also reported.