A series of new nickel complexes NiCl2(L) based on tridentate NZN ligands (1, L=bis[2-(3,5-dimethyl-pyrazolyl)methyl]benzylamine; 2, L=bis[2-(3,5-dimethyl-l-pyrazolyl)methyl]butylamine; 3, L = bis[2-(3,5-dimethyl-l-pyrazolyl)methyl]sulfide; 4, L = bis[2-(3-phenyl-l-pyrazolyl)methyl]sulfide)) has been prepared and characterized by elemental analysis. Upon activation with methylaluminoxane (MAO) or diethylaluminum chloride (DEAC), these new pre-catalysts show high activity in ethylene oligomerization (TOF = 1.7-104.5 x 10(3) mol(ethylene)(mol(Ni))(-1) h(-1)), giving predominantly 1-butene (70-94% yield). The catalytic performance is substantially affected by the co-catalyst type and ligand environment, especially the bridge donor atom. Under optimized conditions ([Ni]=10 mu mol, 30 degrees C, 20bar ethylene, MAO-to-Ni = 250), pre-catalyst 3 leads to TOF = 104.5 x 10(3) h(-1) and 70% selectivity for 1-butene. (C) 2008 Elsevier B.V. All rights reserved.