 preparing copolymers of ethylene and a conjugated diene. The present invention provides an improved catalytic system for preparing copolymers of ethylene and a conjugated diene which do not have the aforementioned disadvantages of the art. The present invention provides a catalytic system for copolymerizing ethylene and a conjugated diene comprising an organometallic complex compound represented by one of the following generic formula Axe0x00x0 or Bxe0x00x0:  in which Ln represents a metal of a lanthanide, the atomic number of which 