A cationic Group 3 or Lanthanide metal complex for coordination polymerization of olefins is disclosed. The precursor metal complex is stabilized by an anionic multidentate ancillary ligand and two monoanionic ligands. The ancillary ligand and the transition metal form a metallocycle having at least five primary atoms, counting any .pi.-bound cyclopentadienyl group in the metallocycle as two primary atoms. Olefin polymerization is exemplified.