A series of nickel(II) halide complexes [NiCl2(L)] (1a-12a) and [NiBr2(L)] (1b-12b) bearing 2-pyrazolyl substituted 1,10-phenanthroline derivatives (L-1-L-12) have been synthesized and characterized by elemental analysis, IR spectroscopy and X-ray diffraction analysis (for 1a', 10b and 12b). Upon activation with methylaluminoxane (MAO), these complexes display good catalytic activities in ethylene oligomerization (up to 3.01 x 10(5) g mol(Ni)(-1) h(-1) at 10 atm of ethylene for 12a with L-12 =2-(3,5-diphenylpyrazol-1-yl)-9-mesityl-1,10-phenanthroline). Complexes with bulkier aryl groups in the 9-position of the phenanthroline ring and the 3- and 5-position of the pyrazolyl ring give higher activities. This is explained with the formation of a more directional environment, a reaction channel around the nickel center which enhances the probability of insertion over dissociation after ethylene coordination to Ni. Also, bulkier aryl substituents can suppress chain-transfer reactions (typically beta-H elimination) and therefore increase the activity. (C) 2008 Elsevier B.V. All rights reserved.