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A series of new nickel (II) complexes of general formula [NiBr 2 (N,O) 2 ] (Ni1-Ni5) based on bidentate pyrazolyl-phosphinoyl ligands were prepared and characterized by IR spectroscopy, elemental analysis, high-resolution mass spectrometry (HRMS), and X-ray crystallography for Ni1. X-ray crystallographic analyses of Ni1 revealed a six-coordinate species, in which Ni (II) lies in an octaedral environment. The nickel center is chelated by two pyrazolyl-phosphinoyl ligands with the two bromide ligands occupying mutually trans positions. Upon activation with methylaluminoxane (MAO), all nickel complexes were able to oligomerize ethylene with turnover frequencies (TOFs) varying from 5.0 to 16.6 × 10 3 (mol ethylene) (mol Ni) −1 h −1 producing selectively 1-butene (91.0-95.5 wt%). The substitution on the pyrazolyl group and the length of the backbone chain play a significant role in the catalytic activity. The use of MAO/TMA (1:1) or MAO/TiBA (1:1) instead of MAO drastically reduced the activities. The TMA content has no significant influence on the selectivity. However, the use of TiBA leads to a reduction of 1-butene (from 94.9 to 71.0 wt%) and an improvement in the hexene production (from 1.5 to 23.3 wt%). Under optimized reaction conditions (10°C, 20 min, 20 bar ethylene, [Al/Ni] = 600), Ni1/MAO catalytic system yielded TOF = 24.7 × 10 3 (mol ethylene) (mol Ni) −1 h −1 with good selectivities for α-C4 (94.9 wt%).

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