Arylnickel(II) phosphine complexes containing substituted N-O bidentate ligands, of the type [NiR(N-O)L] [N-O = 4-nitropyridine-2-carboxylate (4-NO2-pyca), R = o-tolyl, L = PPh3; N-O = 2-pyrazinecarboxylate (pyzca), R = o-tolyl, L = PPh3; and N-O = 4-methoxypyridine-2-carboxylate (4-MeO-pyca), R = o-tolyl, L = PPh3] have been prepared and characterised. Single crystal X-ray studies of the complexes [Ni(o-tolyl)(pyca)PPh3], 1, and the isomorphous analogue [Ni(o-tolyl)(4-NO2-pyca)PPh3], 3, show the expected square planar coordination about the nickel centres, with the pyridine nitrogens being trans to the phosphine ligand for both compounds. The coordination spheres of the two complexes are very similar, no elongation of the Ni-N bond for complex 3, which contains the 4-NO2-pyca ligand, being evident. In complex 3 the o-tolyl ligand is disordered over two sites indicating the presence, in the solid state, of two conformers in which the o-methyl groups of o-tolyl are located to either side of the coordination plane. The complexes with substituted pyca ligands form single component catalysts for the conversion of ethene to high molecular weight polyethene and for the copolymerisation of ethene and carbon monoxide to polyketone under mild conditions. The nature of the product, whether predominantly high molecular weight polymer or a mixture of polymer and lower oligomer, is dependent on the basicity of the N-O chelate ligand. From an NMR study of the effect of added ethene on the complex [Ni(o-tolyl)(4-NO2-pyca)PPh3], a mechanism involving alkene promoted ligand dissociation is suggested. (C) 1997 Elsevier Science S.A.