Recent progress on nickel-based complex pre-catalysts has been reviewed. Variation of the complex models by employing different types of heteroatom-containing ligands and the new features associated with the resulting highly branched polyethylenes is discussed. The discussions focus on the influence that fine tuning of the substituents has on the catalytic properties of their nickel complexes. The resultant polyethylene exhibits high branching and can be classified as an elastomeric material. Thus, such nickel complex pre-catalysts have a bright future in both the academic and industrial arenas.