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%).

and the synthesis and ethylene reactivity of two new nickel complexes [NiCl (py)(E,SO)] (Ni1, E = Se; Ni2, E = S) are reported. The minimum energy structures of the complexes were determined by density functional theory (DFT) calculations. For both compounds, the triplet and singlet 1 structures (with Cl trans to chalcogen) are higher in energy compared with singlet 2 (with Cl cis to chalcogen). Under initial catalytic condition (MAO, [Al/Ni] = 300, time = 20 min, T = 30 C) complex Ni1 oligomerizes ethylene with turnover frequencies (TOF) of 32.6 Â 10 3 (mol ethylene) (mol Ni) À1 (h) À1 with 81.3% of 1-butene production. The use of different cocatalyst systems (MAO/TMA or MAO/TiBA in an equimolar ratio) led to lower activities and poor selectivity for 1-C 4 . Under optimized reaction conditions (MAO, [Al/Ni] = 600, time-= 20 min, T = 30 C), the phenylthionyl complex Ni2 was 1.7 times more active than the phenylselenyl analog, with TOF of 72.3 Â 10 3 (mol ethylene) (mol Ni) À1 (h) À1 with 84.3% of 1-butene selectivity. DFT calculations suggest that the lowest lowest unoccupied molecular orbital (LUMO) energy and highest natural charge at the Ni center could be associated to the higher activity of Ni2 compared with the phenylselenyl analog Ni1.

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