The synthesis of a number of nickel(II) complexes containing the didentate phosphane ligand 1,2-bis(di(o-methoxyphenyl)phosphino)ethane (o-MeO-dppe) is reported. Two types of complexes have been synthesized, i.e., the mono(chelate! complex (I) of the general formula [Ni(o-MeO-dppe)X-2] (where X = Cl, Br or I) and the bis(chelate) complex (2) of the general formula [[Ni(o-MeO-dppe)(2)]Yr (where Y = PF6 or trifluoroacetate (TFA)). These complexes have been characterized using electronic absorption and NMR spectroscopy. The structures of the mono(chelate) complex [Ni(o-MeO-dppe)I-2] (1c) and of the bis(chelate) complex [Ni(o-MeO-dppe)(2)](PF6)(2) (2e) have been determined by X-ray crystallography. [Ni(o-MeO-dppe)I-2] crystallizes in the monoclinic space group P2(1)/c with Z = 4, a = 12.1309(1) Angstrom, b = 16.5759(3) Angstrom, c = 17.6374(2) Angstrom, beta = -119.3250(10)degrees. [Ni(oMeO-dppe)(2)](PF6): crystallizes in the monoclinic space group C2/c with Z = 4, a = 22.5326(3) Angstrom, b = 13.6794(2) A, c = 21.7134(3) Angstrom, beta = 107.1745(7)degrees; In both structures the nickel ion is in a square-planar geometry with a NiP2I2 and NiP4 chromophore, respectively. Using H-1 and P-31{1H} NMR spectroscopy the behavior-of the complexes in various solvents has been studied. It appears that in solution these nickel complexes are involved in an autoionization equilibrium: 2[Ni(o-MeO-dppe)X-2] reversible arrow [Ni(o-MeO-dppe)(2)](2+) + ["NiX4"](2-). The ionized complex (3) consists of a cationic unit in which a nickel atom is surrounded by two didentate phosphane ligands, and an anionic unit that stoichiometrically consists of a nickel atom and four anions. The position of the autoionization equilibrium is highly dependent on the anion and the solvent used. In a polar solvent in combination with weakly coordinating anions only the ionized complex is observed, whereas in;an apolar solvent in combination with coordinating anions only the mono(chelate) complex occurs. A comparison of the behavior of o-MeO-dppe with its unsubstituted analogue dppe in combination with nickel(II) acetate using P-31{H-1} NMR spectroscopy shows that the latter is more readily oxidized.