Complexes of the type MX2(CO)2 diphos2 [M = Mo, W; X = Cl, Br; diphos = bis(diphenylphosphino)methane] have been prepared and characterized. The complexes are neutral, diamagnetic, seven-coordinate species with one of the diphosphine molecules acting as a monodentate ligand. Nuclear magnetic resonance, conductance, and molecular weight studies support this view. The monomeric complex MoBr2(CO)3 diphos has also been prepared.