A series of new pentacoordinated nickel complexes NiCl 2 (NZN) based on nitrogen-, oxygen-, or sulfurbridged bis(pyrazolyl) ligands (NZN) were synthesized and characterized by elemental analysis and an X-ray diffraction study for NiCl 2 {bis[2-(3,5-dimethylpyrazolyl)ethyl)]amine} (1). Upon activation with methylaluminoxane (MAO), these complexes show high activity in ethylene oligomerization (TOF ) 7-80 × 10 3 h -1 ), which varies according to the ligand environment. Up to 92% selectivity for 1-butene, combined with a TOF of 60 × 10 3 h -1 , has been obtained under moderate conditions (30 °C, 40 bar, MAO-to-Ni ) 250) using NiCl 2 {bis[2-(5-phenylpyrazolyl)ethyl)]ether} (3) as the catalyst precursor.