A cationic cyclopentadienyl nickel(II) complex of bis(mesityl-imino)acenaphthene, [Ni(η5-C5H5)(Mes-BIAN)]PF6 (Mes = mesityl = 2,4,6-Me3-C6H2; Mes-BIAN = bis(mesitylimino) acenaphthene), was synthesized via two alternative routes, being obtained in moderate to quantitative yields. Although electronically saturated, this new complex was tested in the polymerization of ethylene and propylene in the presence of the cocatalyst methylaluminoxane (MAO), and found to behave as an efficient catalyst precursor for the polymerization of ethylene under mild reaction conditions (low temperature and pressure), whereas it was inactive towards propylene. The polyethylenes produced show methyl branching and a low content of long branches.