Detailed ethylene oligomerizations were performed over highly active nickel complexes ligated by N-((4-butyl-6-phenylpyridin-2-yl)methylene)-2,6-diisopropylaniline derivatives. The activity trend according to the structure of the complex was attempted to understand using the concept of ion-pair formation during activation process. The active species were characterized by comparing the oligomerization results with the UV-Vis spectroscopic and the cyclic voltammetry experiments.