Polyethylene was synthesized using iron(II) and cobalt(II) complexes stabilized by tris(2-pyridyl)X (X=N, P) ligands in the presence of methylaluminoxane (MAO) as cocatalyst. The catalysts showed activities ranging from moderate to high. The iron complexes showed higher activities than the cobalt analogous and it was found that the bridgehead atom of the ligand tpX (X = P, N) did not affect significantly the activity of the active species as well as the molecular weight of the polymers. The polyethylenes obtained were classified as HDPE with linear structure and broad monornodal distribution. (c) 2006 Elsevier B.V All rights reserved.