An oxo-nitrogenated iron complex having general formula (I) or (II) wherein: R1 and R2 identical or different, represent a hydrogen atom; or are selected from linear or branched, optionally halogenated C1-C20, preferably C1-C15, alkyl groups, optionally substituted cycloalkyl groups, optionally substituted aryl groups; R3, identical or different, represent a hydrogen atom; or are selected from linear or branched, optionally halogenated C1-C20, preferably C1-C15, alkyl groups, optionally substituted cycloalkyl groups, optionally substituted aryl groups; X1 and X2, identical or different, represent a halogen atom such as, for example, chlorine, bromine, iodine; or are selected from linear or branched C1-C20, preferably C1-C15, alkyl groups, —OCOR4 groups or —OR4 groups wherein R4 is selected from linear or branched C1-C20, preferably C1-C15, alkyl groups. Said oxo-nitrogenated iron complex having general formula (I) or (II) can be advantageously used in a catalytic system for the (co)polymerization of conjugated dienes.