Novel .alpha.-diimine metal complexes, particularly iron complexes, having a phenyl sulfidyl or substituted phenyl sulfidyl metal complexing group are disclosed. The .alpha.-diimine metal complexes having a phenyl sulfidyl or substituted phenyl sulfidyl metal complexing group are produced by forming one of the .alpha.-diimine metal complex imine bonds in the presence of a metal salt or an .alpha.-acylimine metal complex. The .alpha.-diimine metal complexes having phenyl sulfidyl or substituted phenyl sulfidyl metal complexing group are useful for polymerizing or oligomerizing olefins.