Novel metal complexes, particularly chromium complexes, which contain at least one tridentate ligand are disclosed and prepared. Olefins, particularly ethylene, can be reacted to form butene and/or other homo- or co-oligomers and/or polymers with high .alpha.-olefin concentrations by contacting a metal catalyst which contains a transition metal, particularly chromium, complexes having per metal atom at least one tridentate ligand with N, O, or N and O coordinating sites.