Polyolefins represent about half the market of the synthesized polymeric materials. The catalytic oligomerization of ethylene is currently the primary source of linear [alpha]-olefins which are extensively used for the preparation of synthetic lubricants, detergent intermediates, surfactants, plasticizers and as comonomers to synthesize linear low-density polyethylene (LLDPE). This work deals with the synthesis of 2-imino-1,10-phenanthroline and 2-imino-9-phenyl-1,10-phenanthroline tridentate ligand and of their iron, cobalt and nickel complexes of which some are very active in the catalytic ethylene oligomerization. Stereoelectronic parameters influencing the catalytic properties have been examined in detail. Furthermore, mono- and dinuclear cobalt complexes with phosphino- and phosphinito-oxazoline ligands have been prepared in the course of this thesis. They have shown moderate catalytic activity in the presence of MAO or AlEtCl2 as cocatalyst.