Studies of Oxygen Exchange Between Trivalent Lanthanide – EDTA Complexes and Solvent Water: Evidence for a Structural Change Within the Series

Abstract

Oxygen-18 labelled EDTA, dissolved in normal water, has been used to examine the kinetics of oxygen transfer between the solvent and a variety of metal–EDTA complexes, including most of the lanthanides, Ca and Th. The lability of the oxygens is profoundly affected by the ionic charge of the metal in the chelate. Thus oxygen exchange in the Th–EDTA–H2O system shows a half-life of 10 min at 100 °C, while for comparable conditions, Ca–EDTA–H2O shows a half-life in excess of 100 days. The lanthanides as a group show behavior intermediate between these extremes. Within the latter group the lability varies systematically with atomic number, Pr–EDTA (element 59) being the most reactive and Yb–EDTA (element 70) the least reactive. The maximum variation across the group is about 10-fold. Interpretation of these results is presented in terms of the polarizing effect of the metal ion on the electron density at the acetate groups. The variation in lability within the lanthanide group can be accounted for quantitatively by changes in coordination number of the central metal ion.