DISSOCIATION KINETICS OF CERIUM(III) AND GADOLINIUM(III) COMPLEXES OF MACROCYCLIC DTPA BIS(AMIDE) LIGANDS

The dissociation kinetics of Ce(III) and Gd(III) complexes of macrocyclic DTPA bis(amide) have been studied in aqueous solution using Cu(II) ions as the scavenger. The measurements were made at 25-degrees-C in a solution of 0. 100 M (NaClO4) ionic strength. The rates of dissociation of these complexes have been found to be independent of [acetate]. Dissociation of the complexes takes place via both dissociative and associative pathways. The rates of both processes decrease in the order Gd(DTPA)-EAM > Gd(DTPA)-PAM congruent-to Gd(DTPA)-XAM > Gd(DTPA)-BAM > Gd(DTPA)-PenAM. An increase in ring size of the DTPA bis(amide) ligands leads to an increase in kinetic inertness, reflecting the enhanced participation of the amide carbonyl oxygen in lanthanide(III) ion coordination. The substitution of an aromatic ring between the two amide nitrogens, however, results in a significant decrease in kinetic inertness of the resulting DTPA-XAM complex.