On activation by MAO, tetrahedral Co-II dichloride complexes supported by 6-organyl-2-(imino)pyridine ligands generate active catalysts for the dimerisation of propylene to hexenes. The nature of the substituent on the 6-position of the pyridine ring influences both the selectivity and activity of the catalysts. The 6-thienyl complexes CoCl2N22Th and CoCl2N22BT dimerise propylene prevalently to linear hexenes through a propagation pathway involving 1,2-insertion into Co-H, followed by 2, 1 -insertion and P-hydride elimination. The 6-phenyl Complex CoCl2N2Ph is much less active and selective producing linear hexenes and methyl branched penteres in comparable yields. The same activity trend and selectivity is observed for the dimerisation of 1-hexene, which however is one order of magnitude slower as compared to the dimefisation of propylene. (c) 2007 Published by Elsevier B.V.