Four iron(II) and cobalt(II) complexes ligated by 2,6-bis(4-nitro-2,6-R 2 -phenylimino)pyridines, LMCl 2 (1: R=Me, M=Fe; 2: R=iPr, M=Fe; 3: R=Me, M=Co; 4: R=iPr, M=Co) have been synthesized and fully characterized, and their catalytic ethylene polymerization properties have been investigated. Among these complexes, the iron(II) pre-catalyst bearing the ortho-isopropyl groups (complex 2) exhibited higher activities and produced higher molecular weight polymers than the other complexes in the presence of methylaluminoxane (MAO). A comparison of 2 with the reference non-nitro-substituted catalyst (2,6-bis(2,6-diisopropylphenylimino)pyridyl)FeCl 2 (FeCat 5) revealed a modest increase of the catalytic activity and longer lifetime upon substitution of the para-positions with nitro groups (activity up to 6.0×10 3 kgmol −1 h −1 bar −1 for 2 and 4.8×10 3 kgmol −1 h −1 bar −1 for 5), converting ethylene to highly linear polyethylenes with a unimodal molecular weight distribution around 456.4kgmol −1 . However, the iron(II) pre-catalyst 1 on changing from ortho-isopropyl to methyl groups displayed much lower activities (over an order of magnitude) than 2 under mild conditions. As expected, the cobalt analogues showed relatively low polymerization activities.