A series of nickel (II) complexes (L)NiCl2 (7–9) and (L)NiBr2 (10–12) were prepared by the reactions of the corresponding 2-carboxylate-6-iminopyridine ligands 1–6 with NiCl2·6H2O or (DME)NiBr2 (DME=1,2-dimethoxyethane), respectively. All the complexes were characterized by IR spectroscopy and elemental analysis. Solid-state structures of 7, 8, 10, 11 and 12 were determined by X-ray diffraction. In the cases of 7, 8 and 10, the ligands chelate with the nickel centers in tridentate fashion in which the carbonyl oxygen atoms coordinate with the metal centers, while the carbonyl oxygen atoms are free from coordinating with the nickel centers in 11 and 12. Upon activation with methylaluminoxane (MAO), these complexes are active for ethylene oligomerization (up to 7.97×105 g mol−1 (Ni) h−1 for 11 with 2 equivalents of PPh3 as auxiliary ligand) and/or polymerization (1.37×104 g mol−1 (Ni) h−1 for 9). The ethylene oligomerization activities of 7–12 were significantly improved in the presence of PPh3 as auxiliary ligands. The effects of the coordination environment and reaction conditions on the ethylene catalytic behaviors have been discussed.