Abstract Nickel(II) halides react under controlled conditions with the diphosphine bis(diphenylphosphino)methane, Ph 2PCH 2PPh 2 (dpm), forming the monomeric, four-coordinate, low-spin chelate complexes [NiX 2(dpm)], where X is Cl, Br, or I. The four-coordinate monomer [NiI 2(dpm)] can be transformed into a five-coordinate, low-spin iodo-bridged dimer [Ni 2I 4(dpm) 2], which in turn reverts slowly to the monomer when dissolved in dichloromethane. A similar five-coordinate dimer, stable only in the solid state, is formed by dpm with nickel(II) thiocyanate. The complexes [NiX 2(dpm)] are remarkably stable both in the solid state and in solution, although the diphosphine forms a highly strained four-atom chelate ring. Above 220°, however, the coordinated diphosphine slowly undergoes air-oxidation resulting chiefly in the formation of nickel(II) complexes of the related diphosphine, dioxide, Ph 2P(O)CH 2P(O)Ph 2 (dmpO 2). These oxidation products are identical with the complexes [Ni(dpmO 2) 3] [NiX 4] (octahedral tris-chelate cation, tetrahedral anion with X = Cl, Br, or I), obtained directly by reaction of nickel(III) halides with the O-donor chelating dpmO 2 ligand.