Series of new arylnickel(II)–dimethylphenylphosphine complexes of types trans-[NiXR(PMe2Ph)2] and trans-[NiR2(PMe2Ph)2](X = anionic ligand; R =o-tolyl, mesityl, 1-naphthyl, pentachlorophenyl, pentafluorophenyl) are described. The methyl resonance spectra of the dimethylphenylphosphine ligands confirm the stereochemistry. The mesityl pentachloro- and pentafluoro-phenyl complexes show one 1 : 2 : 1 triplet but the o-tolyl and 1-naphthyl complexes show two 1 : 2 : 1 triplets because there is no plane of symmetry through the phosphorus–nickel bond. The ortho-methyl groups in the complex trans-[Ni(o-tolyl)2(PMe2Ph)2] are shown to adopt a mutual anti-configuration by n.m.r.