The ethylene oligomerization was investigated by using new hydrazone nickel complexes Ni(N<^>O)(2)Cl-2 (2a, N<^>O = 4,5-diazafluorene-9-one-benzoylhydrazone; 2b, N<^>O = 4,5-diazafluorene-9-one-4-nitrobenzoylhydrazone; 2c, N<^>O = 4,5-diazafluorene-9-one-3-nitrobenzoylhydrazone) with methylaluminoxane (MAO) in toluene. Those catalytic systems mainly assisted the dimerization of ethylene with good catalytic activity (10(5) to 10(4) g mol(-1) h(-1)) at ambient pressure. The reaction conditions, such as the ratios of Al/Ni, reaction temperature and reaction time, were investigated. The best catalytic activity for the three complexes was observed for complex 2a without nitro-substituent on its aryl ring. (C) 2002 Elsevier Science B.V. All rights reserved.