Several new nickel(II) and nickel(0) complexes have been prepared which are active and selective for the oligomerization of ethylene to higher alpha-olefins. Nickel(II) complexes of the type NiCl(PR3)S2PR2' (R = Ph, Bu; R' = Me, Ph) contain one bound halide ion, a monodentate phosphine, and a bidentate chelating dithiophosphinate ligand. The single-crystal X-ray structure of NiCl(PPh3)S2PMe2 is reported. Crystal data : triclinic, space group P (1) over bar (No. 2), a = 10.288 (2) Angstrom, b = 13.205 (3) Angstrom, c = 16.157 (2) Angstrom, alpha = 82.63(1)degrees, beta = 88.29(1)degrees, gamma = 78.87(2)degrees, Z = 4. Final R and R-W values were 0.042 and 0.048, respectively. There are two independent molecules A and B in the unit cell. In each case the molecular structure is based on a distorted square planar Ni(II) center subtended with a slightly asymmetrically bound chelated dithiophosphinate ligand with a relatively tight binding angle (S-Ni-S 87.4 degrees). Molecule A is slightly more distorted from the square planar geometry (in that the P(V) of the dithiophosphinate chelate deviates more significantly from the coordination plane) than B. The bond distances (Ni-S (av) (trans to Cl) 2.205(2) Angstrom, Ni-S (av) (trans to the phosphine ligand) 2.254(5) Angstrom, Ni-Cl (av) 2.179(5) Angstrom and Ni-P (ave) 2.021(3) Angstrom) do not differ significantly in the two molecules. Also prepared were the nickel(0) complexes Ni(Ph2P-N=C(H)Ph)(4) and Ni(Ph2PCH2P(Ph-2)NC6F4CN)(2). Each of the complexes is more active for the oligomerization of ethylene than are comparable conventional complexes of nickel with typical phosphine ligands. Batch reactions of the nickel complexes in combination with a 150 times molar excess of EtnAlCl3-n in toluene oligomerized ethylene at temperatures below 25 degrees C under 0.1-2.8 MPa of ethylene and a lesser amount of inert gas. The catalyst mixtures have high activity (for example, a rate of 7.64 x 10(5) mol of C-2(mol of cat)(-1) h(-1) was observed for Ni(Ph(H)NCPPh2)(4) with excess diethylaluminum chloride cocatalyst) and high selectivity to alpha-olefins, in particular 1-butene. The selectivity to 1-butene increases with increasing olefin pressure.