A series of N-(2-alkyl-5,6,7-trihydroquinolin-8-ylidene)arylaminonickel(II) dichloride complexes were synthesized in a one-pot reaction with nickel dichloride. All nickel complexes were characterized by elemental and spectroscopic analysis. The molecular structures of representative nickel complexes, as determined by the single crystal X-ray diffraction, are reported. All nickel complexes, when treated with ethylaluminium sesquichloride (Et3Al2Cl3), showed high activities (up to 1.1 × 106 g mol−1 h−1) for ethylene oligomerization, with good thermal stability at 80 °C at 10 atm ethylene. The influence of the reaction parameters on the catalytic behavior was investigated for these nickel-based systems, including variation of Al/Ni molar ratio and reaction temperature.