The first (η3-1-phosphaallyl)tricarbonylcobalt complex has been obtained by reaction of a (supermesityl)(vinyl)halophosphine with Co2(CO)8 at room temperature, and its structure established by X-ray diffraction study. The reaction of (mesityl)(propenyl)phosphine with nickelocene and allyl iodide gives the corresponding CpNi(I)(R2PH) complex, which, upon treatment with DABCO, gives a complex in which