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