The invention relates to compounds, to the complexes they form with a lanthanide, and to the use of the complexes for fluorescence marking or NMR imaging.The complex consists of an Ln ion and a ligand R.sup.2--C(X--R.sup.1)(R.sup.3)--NR.sup.4R.sup.5. R.sup.1 is a functional group, X is a single bond or a hydrocarbon-based chain consisting of at least one alkylene or alkenylene group optionally comprising at least one hetero atom or an arylene. R.sup.2 is an anionic group A.sup.2 or a C.sub.1-C.sub.4 alkylene or alkenylene group bearing at least one such group A.sup.2 and optionally comprising at least one hetero atom. R.sup.3 is H or a C.sub.1-C.sub.5 alkylene or alkenylene group optionally containing at least one hetero atom, and optionally bearing at least one anionic group A.sup.3. R.sup.4 is a substituent with light-absorbing properties that forms chelate rings with Ln. R.sup.5 is a substituent that forms chelate rings with Ln.