2-Chloro/pheny1-7-arylimino-6,6-dimethylcyclopenta[b]pyridylnickel chlorides (Nil-Ni8) were synthesized from the respective ligands L1-L8 and characterized. Upon activation with either methylaluminoxane (MAO) or ethylaluminium sesquichloride (EASC), they show high catalytic activity of up to 10.84 x 10⁶ g(oligomer) mol^-1(Ni) h^-1 in ethylene oligomerization. The products range froth butenes to dodecenes for Ni1-Ni4, but are limited to butenes and hexenes in the case of Ni5-Ni8. DFT calculations indicate that the Ni-C bond length in the model alkyl complexes depends both on the nature of the substituents at the heterocycles and the kind of the alkyl group, shedding some light on the preferences of the precatalysts toward oligomers or polyethylene.