Complexation of trivalent lanthanide cations by different chelation sites of malic and tartric acid (composition, stability and probable structure)

The formation of colorless gadolinium complexes (x,y,z) between x gadolinium ions, y ligands and z protons of some organic acids has been studied in aqueous solution. In this work we shall present the results of investigations on the interaction of the gadolinium ion (Gd3+) with different chelation sites of malic and tartric acid formed in dilute solution for pH values between 5.50 and 7.50. The structures of these new organometallic complexes are Gd3(C4H4O5)2·(NO3)3·nH2O and Gd3(C4H4O6)2·(NO3)3·nH2O (C4H4O52-: malate ions and C4H4O62-: tartrate ions). These colorless gadolinium complexes of malate and tartrate ions have no absorption band UV-visible, the indirect photometry detection (IPD) study; have identified major tri-nuclear complexes of these dicarboxylic acids, giving for these colorless complexes a (3,2,2) and (3,2,3), respectively. Composition and apparent stability constant depends on the acidity of the medium. To complement previous results and to propose probable structures for these new complexes detected in solution FT-IR and FT-Raman spectroscopy have been conducted to identify the different chelation sites for both ligands.