. 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 is between 00 and 00; X represents a halogen, selected from among chlorine, fluorine, bromine and iodine; wherein in formula Axe0x00x0, two ligand molecules Cp0 and Cp0, which may be identical or different, selected from a substituted or unsubstituted cyclopentadienyl and fluorenyl group are bonded to metal Ln; and in formula Bxe0x00x0, a ligand comprising Cp0 and Cp0 bonded to each other by a bridge P is bonded to metal Ln, wherein Cp0 and Cp0 comprise a substituted or unsubstituted cyclopentadienyl or fluorenyl group, bridge P corresponds to the formula MR0, in which M is silicon or another element of column IVA of Mendeleev""s periodic classification and R is an alkyl group having 0 to 00 atoms of carbon; and a co-catalyst selected from the group consisting of a magnesium alkyl, a lithium alkyl, an aluminum alkyl, a Grignard""s reagent and mixtures thereof. In a preferred embodiment