Heteromultimetallic complexes consisting of three Co(II) ions and one lanthanide ion were synthesized and applied to the alternating copolymerization of CO2 and cyclohexene oxide. Unlike the conventional cobalt(III) salen complexes, the high thermal stability of the present catalyst allowed us to reach a turnover number of 13000, one of the highest values ever reported for multimetallic systems. The chain propagation was first-order to the catalyst, suggesting a cooperative behavior of the metal centers.