The electronically saturated and air-stable complexes [Ni(η5-C5H5)(Mes-BIAN)][PF6] (1) and [Ni(η5-C5H5)(Mes-DAD)][PF6] (2) were found to behave as efficient catalyst precursors for the polymerisation of ethylene under mild reaction conditions (temperature and pressure), when activated with small excesses of AlEt2Cl (DEAC), and exhibiting activities in the range 4×104 to 22×104 g PE/(mol Ni·h·bar). Compound 2 was synthesised and fully characterised as reported hereby for the first time, and exhibits a catalytic activity of ca. 30×104 g PE/(mol Ni·h·bar) in the polymerisation of ethylene. The reactivity of these complexes towards DEAC was found to lead to the formation of diamagnetic cationic Ni(II) complexes (4 and 3, respectively) containing the same initial cation [Ni(η5-C5H5)(α-diimine)]+ and a [AlEtCl3]- anion, whereby α-diimine = BIAN (1 and 4) and DAD (2 and 3). In the corresponding reaction of complex [Ni(η5-C5H5)(Mes-BIAN)][PF6] (1), an unusual paramagnetic complex incorporating a cationic trinuclear α-diimine Ni cluster (5), containing five bridging Cl ligands and a [AlCl4]- anion, was also isolated and fully characterised, in which the cyclopentadienyl ligand was absent. The intermediates 3 and 5 also catalysed efficiently the polymerisation of ethylene when the DEAC co-catalyst was used in slight excess. The polyethylene produced (ca. 10×104 to 30×104 g PE/(mol Ni·h·bar), respectively) showed branching numbers ranging from 7 to 75 branches/1000 carbon atoms according to 1H NMR spectroscopy analysis. These findings are surprising given that the new 18-electron nickel precursors show remarkable properties towards ethylene polymerisation catalysis when activated with a small excess of aluminium activator under mild conditions, and also that some unusual intermediates responsible for the observed activity were isolated and characterised by X-ray crystallography.