An oxo-nitrogenated complex of lanthanides having general formula or: wherein: - Ln represents a metal of the series of lanthanides; - R 1 and R 2, equal to or different from each other, represent a hydrogen atom; or they are selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups optionally substituted, aryl groups optionally substituted; - R 3 represents a hydrogen atom; or it is selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups optionally substituted, aryl groups optionally substituted; or R 3 represents a ketoimine group having the formula: - wherein R' and R", equal to or different from each other, represent a hydrogen atom, or they are selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups optionally substituted, aryl groups, optionally substituted; - Y represents an oxygen atom; or a -N-R 4 group wherein R 4 represents a hydrogen atom, or it is selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups optionally substituted, - or, when Y represents a -N-R 4 group, R 2 and R 4 can be optionally bound to each other so to form, together with the other atoms to which they are bound, a saturated, unsaturated or aromatic cycle containing from 3 to 6 carbon atoms, optionally substituted with linear or branched C 1 -C 20 alkyl groups, said cycle optionally containing other heteroatoms such as oxygen, sulfur, nitrogen, silicon, phosphorous, selenium; - X 1, X 2 and X 3, equal to or different from each other, represent a halogen atom such as chlorine, bromine, iodine; or they are selected from linear or branched C 1 -C 20 alkyl groups, -OCOR 5 or -OR 5 groups wherein R 5 is selected from linear or branched C 1 -C 20 alkyl groups. - Ln represents a metal of the series of lanthanides; - R 1 and R 2, equal to or different from each other, represent a hydrogen atom; or they are selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups optionally substituted, aryl groups optionally substituted; - R 3 represents a hydrogen atom; or it is selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups optionally substituted, aryl groups optionally substituted; or R 3 represents a ketoimine group having the formula: - wherein R' and R", equal to or different from each other, represent a hydrogen atom, or they are selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups optionally substituted, aryl groups, optionally substituted; - Y represents an oxygen atom; or a -N-R 4 group wherein R 4 represents a hydrogen atom, or it is selected from linear or branched C 1 -C 20 alkyl groups, cycloalkyl groups optionally substituted, - or, when Y represents a -N-R 4 group, R 2 and R 4 can be optionally bound to each other so to form, together with the other atoms to which they are bound, a saturated, unsaturated or aromatic cycle containing from 3 to 6 carbon atoms, optionally substituted with linear or branched C 1 -C 20 alkyl groups, said cycle optionally containing other heteroatoms such as oxygen, sulfur, nitrogen, silicon, phosphorous, selenium; - X 1, X 2 and X 3, equal to or different from each other, represent a halogen atom such as chlorine, bromine, iodine; or they are selected from linear or branched C 1 -C 20 alkyl groups, -OCOR 5 or -OR 5 groups wherein R 5 is selected from linear or branched C 1 -C 20 alkyl groups.