Borohydride metallocene complex of lanthanide, preparation process, catalytic system incorporating it, copolymerization of olefins employing catalytic system and ethylene/butadiene copolymer, the butadiene units comprise 1,2-cyclohexane or 1,2- and 1,4-cyclohexane links. The complex corresponds to of formulae A and/or B: where, in A two ligands Cp1 and Cp2, each of a fluorenyl group, are connected to the lanthanide Ln, where, in B, a ligand molecule, composed of two fluorenyl groups Cp1 and Cp2 are connected via bridge P of formula MR1R2, is an element from group IVa, R1 and R2, which are identical or different, represent an alkyl group comprising from 1 to 20 carbon atoms, connected to lanthanide Ln, L is alkali metal, N is molecule of a complexing solvent, x is integral or non-integral number ≧0 and p is integer ≧1.