Hexacarbonyltungsten reacts with allyl halides in cyclohexane under u.v. radiation at room temperature to yield the derivatives W2(CO)6(π-allyl)Cl3, W(CO)4(π-allyl)Br, and W(CO)4(π-allyl)l. The bromo- and iodo-complexes readily yield substituted derivatives on reaction with one or more of the three ligands 2,2′-bipyridyl, pyridine, or triphenylphosphine by displacement of two molecules of carbon monoxide. Structural information about these complexes has been obtained by n.m.r. spectroscopy and other physical measurements.