The subject of the invention is the compounds of the formula (I) their isomers, their physiologically acceptable salts and/or their Mn(II) complexes wherein the formula (I) R 1 and R 2 are jointly unsaturated or aromatic rings of 5 to 7 carbon atoms together with the enclosed ethylene group and R 3 and R 4 are jointly saturated, unsaturated or aromatic rings of 5 to 7 members together with the connecting N atom, in given case optionally substituted with alkyl group of 1 to 4 carbon atoms, carboxyl, nitro, amino groups or phenyl-alkyl group of 1 to 4 carbon atoms, which can be optionally in given case substituted with carboxyl, nitro or amino group in its phenyl part, or R 1 and R 2 are jointly saturated ring of 5 to 7 carbon atoms together with the enclosed ethylene group R 3 and R 4 are jointly saturated, unsaturated or aromatic ring of 5 to 7 members together with the connecting atom N, substituted with alkyl group of 1 to 4 carbon atoms, carboxyl, nitro or amino group or phenyl-alkyl group of 1 to 4 carbon atoms, which can be optionally substituted with carboxyl, nitro or amino group in its phenyl part, or R 1 and R 2 are jointly saturated, unsaturated or aromatic ring of 5 to 7 carbon atoms together with the enclosed ethylene group, R 3 is hydrogen atom and R 4 is alkyl group of 2 to 6 carbon atoms with straight or branching open carbon chain. The subject of the invention is the MRI diagnostic agents as well, which are containing the above compounds.