Methods for making .alpha.-diimine metal complexes are described. The methods comprise forming an .alpha.-diimine metal complex imine bond in the presence of a metal salt or an .alpha.-acylimine metal complex. The method is particularly using for the production of .alpha.-diimine metal complexes having two different .alpha.-diimine nitrogen groups. The .alpha.-diimine metal complexes are useful for polymerizing or oligomerizing olefins.