The combination of the neutral compound (eta(3) : eta(0)-Ind(CH2)(2)NMe2)Ni(PPh3)Cl (1) and methylaluminoxane (MAO) produces catalysts for the dimerization and the polymerization of ethylene. On the other hand, activation of the cationic complex [(eta(3):eta(1)-Ind(CH2)(2)NMe2)Ni(PPh3)][BPh4] (2) by MAO or trimethylaluminum leads to a system which dimerizes ethylene with high turnover frequencies (2 x 10(3) s(-1)), but does not promote its polymerization. The effects of parameters such as ethylene pressure, reaction temperature and time, solvent type, and the type and amount of activator used have been studied in order to optimize the conditions for the formation of polyethylene. In addition, a number of reactions have been studied by NMR and GC-MS analysis in an effort to identify the catalytically active species. The results of these studies point to the involvement of cationic species in the dimerization of ethylene, whereas, the active catalyst for the polymerization of ethylene appears to be a non-cationic species. (C) 2002 Elsevier Science B.V. All rights reserved.