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.

A set of chromium(III) complexes CrCl 3 (L) based on tridentate ligands (1, L ) bis [2-(3,5dimethylpyrazolyl)have been prepared and characterized by elemental analysis, ESI-HRMS, and an X-ray diffraction study for 3. Upon activation with methylaluminoxane (MAO), these precatalysts show high activity in ethylene oligomerization (TOF ) (3.4-131.0) × 10 3 (mol ethylene) (mol Cr) -1 h -1 at 80 °C), producing R-olefins in the range C 4 -C 14+ with high selectively. The catalytic performances are substantially affected by the ligand environment, especially the bridge donor atom, and reaction conditions. Under optimized conditions ([Cr] ) 10 µmol, 80 °C, 40 bar ethylene, MAOto-Cr ) 300), precatalyst 5 leads to TOF ) 131.0 × 10 3 h -1 and selectivity for R-olefins in the range 89-97%.