optionally be substituted a substituent, including a halogen atom; and X is an leaving group such as a halogen atom, etc.; the bond represented by formula below is a single bond or a double bond; M represents a metal belonging to the Group III to Group V or a lanthanide metal; L1 and L2, which may be the same or different, independently represents an anionic ligand, provided that L1 and L2 may be