LANTHANIDE IONS transmetallation

Although Ca " selectivity over Mg + in the transmetalation reaction is extremely high, a lanthanide ion is bound more strongly in the central site to give helical complexes. X-ray crystallographic analysis revealed that the helical structures are more squeezed as the size of the lanthanide ion decreases. In the absence of Zn ions, the lanthanide ions do not interact efficiently with the ligand 29. This indicates that... 

When considering the kinetic stability of a lanthanide chelate, dissociation and trans-metallation reactions are the main processes that must be taken into account. For complexes formed with EDTA, the proton-assisted dissociation dominates over direct attack by another metal ion. Indeed, the structure does not leave space for direct attack, because none of the acetate arms remains exposed [7]. This can be contrasted with the DTPA case where the two residual negative charges on Ln(DTPA) make the direct attack reaction more likely. A study by Briicher et al. [8] demonstrated that at physiological pH the transmetallation mechanism dominates and that the kinetics of the process largely depends on the attacking... 

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