A series of nickel (II) complexes ligated by 2-imino-1,10-phenanthrolines were synthesized and characterized by elemental and spectroscopic analysis as well as by single-crystal X-ray crystallography. X-ray crystallographic analysis reveals complexes 3, 5, 7 and 11 as the five-coordinated distorted trigonal-bipyramidal geometry. Upon activation with Et2AlCl, these complexes exhibited considerably high activity for ethylene oligomerization (up to 3.76×107gmol−1(Ni)h−1 for 12 with 10equiv. of PPh3). The ligand environment and reaction conditions significantly affect the catalytic activity of their nickel complexes.