Treatment of RuHCl(CO)(PPh3)(3) with CH2=C=CHCO2Me gives the allyl complex Ru(77 3 -CH2CHCHCO2Me)CI(CO)(PPh3)(2). The analogous allyl complexes Os(eta(3)-CH2CHCHR)Cl(CO)(PPh3)(2) (R = Ph, CH2Ph) are also produced from the reactions of OsHCI(CO)(PPh3)(3) with CH2=C=CHR. In contrast, MHCl(PPh3)(3) (M = Ru, Os) react with CH2=C=CHR to give the vinyl complexes MCl((C(CH3)=CHR)(CH2 C=CHR)(PPh3)(2) (M = Ru, R = CMe3, M = Os, R = CMe3, Ph, CO2Et). The difference in the reaction pathways for the reactions of allenes with MHCl(CO)(PPh3)(3) and MHCl(PPh3)(3) (M = Ru, Os) has been studied by computational chemistry. The theoretical study shows that the observed different reactivity can be related to the significant difference in the electron density increase at the central carbon of the allene ligand upon coordination to the metal fragments.