The effect of solvent, anion, and amount of ligand on the catalytic hydrogenation of 1-octene by nickel(II) complexes containing the ligands 1,2-bis(di(ortho-methoxyphenyl)phosphanyl)ethane (o-MeO-dppe) and 1,3-bis(di(ortho-methoxyphenyl)phosphanyl)propane (o-MeO-dppp) is reported. Catalysts containing the ligand o-MeO-dppe are more sensitive towards variations in solvents and anions than the catalysts containing the ligand o-MeO-dppp. The active catalyst precursor complex [Ni(o-MeO-dppp)(tfa)(2)] (tfa = trifluoroacetate) was characterized with X-ray crystallography. The complex has a crystallographic C-2 symmetry; the nickel ion is in a square-planar geometry with two phosphorus donors and two tfa oxygen donors in a cis configuration (Ni-P distance 2.1527(4) Angstrom, Ni-O distance 1.9186(10) Angstrom). In reaction mixtures containing o-MeO-dppe ligand redistribution takes place, leading to the formation of an inactive bis(ligand) complex. To prevent the ligand redistribution reaction the bulkier ligand 1,2-bis(di(ortho-ethoxyphenyl)phosphanyl)ethane (o-EtO-dppe) has been synthesized. Even though NMR analysis showed that its nickel complex also is involved in a ligand-redistribution equilibrium, the hydrogenation activity is considerably higher than that of a catalyst containing the ligand o-MeO-dppe. (C) 2002 Elsevier Science B.V. All rights reserved.