π-Cyclo-octenyl-π-cyclo-octa-1,5-dienecobalt, Co(C8H13)(C-H12), was treated with CO in pentane at –60° to –80° to give an orange monocarbonyl complex, Co(CO)(C8H13)(C8H12). From this were derived phosphine or phosphite complexes, Co(CO)(C8H13) L2[L = PPh3, PMePh2, P(OPh)3] and hydridocarbonylphosphine complexes, Co(H)(CO)L3. All these new complexes are diamagnetic, soluble in organic solvents, and show well defined proton n.m.r. and i.r. spectra on which structural assignments are based.