Novel iron and cobalt complexes of certain novel tricyclic ligands containing a “pyridine” ring and substituted with two imino groups are polymerization and/or oligomerization catalysts for olefins, especially ethylene. Depending on the exact structure of the ligand, and polymerization process conditions, products ranging from α-olefins to high polymers may be produced. The polymers, especially polyethylenes, are useful for films and as molding resins.