Novel Group 4 metal complexes wherein the metal is in the +2, +3, or +4 formal oxidation state containing two ligand groups bound by n-electrons, at least one of which is a cyclic or noncyclic, non-aromatic, anionic, dienyl ligand group and having a bridged ligand structure, catalytic derivatives of such complexes; and the use thereof as catalysts for polymerizing addition polymerizable monomers are disclosed.