Compounds of general Formula I ##STR1## wherein R.sup.3 is a saturated, unsaturated, straight- or branched-chain or cyclic aliphatic hydrocarbon residue of up to 16 carbon atoms or, if R.sup.4 is a hydrogen atom, a cycloalkyl group or an aryl or aralkyl group optionally substituted by one or several C.sub.1 -C.sub.6 -dialkylamino groups or by one or several C.sub.1 -C.sub.6 -alkoxy groups, R.sup.4 is a hydrogen atom, a saturated, unsaturated, straight- or branched-chain or cyclic hydrocarbon residue of up to 16 carbon atoms, or R.sup.3 and R.sup.4 jointly mean a saturated or unsaturated 5- or 6-membered ring optionally substituted or containing an O, S, N atom or oxo substituent, and Y is COOX or CONR.sub.3 R.sub.4, are valuable complexing agents, complexes or complex salts, e.g., for use as NMR or X-ray diagnostic image-enhancement agents or radioactive diagnostic agents.