A half-metallocene-type complex, La(C5Me5)[CH(SiMe3)(2)](2)tetrahydrofuran (THF) 1, showed the dual function of performing the controlled polymerizations of nonpolar monomers such as ethylene and styrene as well as polar monomers like methyl methacrylate, hexyl isocyanate, and acrylonitrile in high yields. On the other hand, the metallocene-type rare-earth metal complexes, [(C5H4SiMe3)(2)Y(mu -Me)](2) 2 and (C5Me5)(2)YMe(THF) 3, showed relatively low catalytic activity. The structure of complex 2 was characterized by X-ray analysis.