Diolefincobalt(O) complexes Co(L2)(PMe3)2 {L2 -[CH2 = CHSi(OMe)2]2O (1), [CH2 =CHSi(OEt)2]2O (2), (CH2 =CHCH2)2SiMe2 (3), [(CH2 =CHCH2)2N]2CH2 (4)} have been synthesized from Co(C5H8)(PMe3)3 and diolefins at ambient temperatures. Reactions with allyl ethers or with allyltin or allylphosphorus compounds involve transfer of allyl groups to the cobalt, while allylamine simply replaces cyclopentene (C5H8) to give Co(CH2 =CHCH2NH2)(PMe3)3 (5). Cationic cobalt(I) complexes containing olefin ligands as in 1-4 could not be obtained under conditions where norbornadiene readily gave [Co(C7H8)(PMe3)3 ]BF4 (6). Paramagnetic 4 shows a magnetic moment μeff = 2.0 μв (between 3.6 and 293 K), corresponding to one unpaired electron per cobalt atom. Compound 4 crystallizes in the space group P21/c with Z = 4, a = 14.986(4), b = 17.223(5), c = 15.436(3) Å,β = 117.98(2)°, V = 3518.4 Å3 . Each cobalt atom is η4-coordinated to a diolefin involving the smaller of two possible chelating rings. The Co-C distances range only from 2.029(6) to 2.058(6) Å for the olefin ligands, whereas Co-P bond lengths at each cobalt differ significantly: Co1-P1 2.188(2), Co1-P2 2.248(2); Co2-P3 2.185(2), Co2-P4 2.258(2) Å. This feature of two significantly different Co-L bond lengths is hitherto seen in all structurally characterized (bisolefin)CoL2 complexes (L = PMe3, CH3CN) and can be rationalized by molecular orbital calculations (Extended Hückel) on the 17 electron model system (PH3)2Co(C2H4)2.