Treatment of 1-(2′-azidophenyl)-3,5-dimethylpyrazole (1) with Ph2PR (R = Ph, Me) and (Ph2P)2CH2, respectively, affords the pyrazolyliminophosphoranes 2, 3 and 4. Reaction of 2 or 3 with [NiCl2(dme)] or NiBr2 yields the N,N-chelate nickel complexes 5–8, and with CoCl2 complexes 9 and 10. Reaction of 4 with [NiCl2(dme)], NiBr2 and CoCl2, respectively, affords the N,N,P-chelate complexes 11–13. Compounds 2–4 were characterised by 1H, 13C, 31P NMR and IR spectroscopy and elemental analysis, while complexes 5–13 were characterised by IR spectroscopy and elemental analysis. The structures of complexes 5, 9 and 12 were further characterised by single-crystal X-ray diffraction techniques. Complexes 5–13 are active catalysts for ethylene oligomerisation upon activation with alkylaluminium derivatives(Et2AlCl, MAO or MMAO). These complexes exhibit good to high catalytic activities (up to 3.54 × 106 g mol–1 h atm for the nickel complexes and 5.48 × 105 g mol–1 h atm for the cobalt complexes). The effects of varying ethylene pressure, temperature and aluminium co-catalyst/Ni or Co ratios with complexes 5, 9, 11 and 12 are reported. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)