Organometallic complexes of titanium or zirconium having only three ligands (namely a cyclopentadienyl ligand, a phosphinimine ligand and an activatable ligand) are catalyst components for olefin polymerization. Preferred polymerization systems are prepared by combining the organometallic complexes with an ionic activator and/or an alumoxane. Preferred catalyst components contain titanium (III) and are used in ethylene polymerization.