A series of 1-(2,4-dibenzhydryl-6-chlorophenylimino)-2-aryliminoacenaphthylene derivatives (L1–L5) was synthesized and reacted with nickel halides to form the corresponding nickel complexes LNiX₂ (X = Br, Ni1–Ni5; X = Cl, Ni6–Ni10). The molecular structures of representative complexes Ni2 and Ni5 were determined by single crystal X-ray diffraction, indicating the distorted square planar geometry around the nickel atom of complex Ni2 and the distorted tetrahedral geometry around the nickel atom of complex Ni5, respectively. Upon activation with low amounts of ethylaluminium sesquichloride (Et₃Al₂Cl₃, EASC), all nickel complexes exhibited high activities up to 1.09 × 10⁷ g of PE per mol of Ni per h toward ethylene polymerization, producing branched polyethylenes. Most importantly, these systems showed good thermo-stability, even at 80 °C maintaining the activity with 3.76 × 10⁶ g of PE per mol of Ni per h.