Recently the increasing use of the lanthanide(III)-aminopolycarboxylates (LnAPC) as shift reagents and relaxation probes for the study of the structure of biologically important macromolecules has renewed the interest in these complexes. In this respect the formation of ternary complexes is particularly important, because the macromolecules investigated are bound to the LnAPC. By the use of the LnAPC relaxation probes the slow rate of dissociation of complexes may have also some importance (e. g. in NMR tomography). The results of the kinetic study of the exchange reactions occurring between the complexes CeEDTA, CeDPTA, CemDBTA, CerDBTA and Pb**2** plus , Cu**2** plus , Co**2** plus as well as Ni**2** plus ions has shown that with the introduction of methyl groups into the ethylenediamine moiety the rate of the dissociation of complexes slows down.
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