Thermodynamic and spectroscopic studies of cadmium(II)–N-(phosphonomethyl)glycine (PMG) complexes

• Published in: Inorganica Chimica Acta Vol. 357; no. 4; pp. 1185 - 1192
• Main Authors: Ramstedt, Madeleine, Norgren, Caroline, Sheals, Julia, Boström, Dan, Sjöberg, Staffan, Persson, Per
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 10.03.2004
• ISSN: 0020-1693; 1873-3255; 1873-3255
• DOI: 10.1016/j.ica.2003.10.029

Speciation and equilibria in the H+–Cd2+–N-(phosphonomethyl)glycine (PMG, H3L) system have been studied using potentiometry, and formation constants for CdHL, CdL−, CdL24− and CdL(OH)2− were determined. The structures of CdL−, and CdL24− in solution were investigated using EXAFS and IR spectroscopic techniques. The results indicated chelating PMG molecules forming two 5-membered chelate rings. Speciation and equilibria in the H+–Cd2+–N-(phosphonomethyl)glycine (PMG, H3L) system have been studied in 0.1 M Na(Cl) medium at 25.0 °C. Formation constants for a series of mononuclear complexes, CdHL, CdL−, CdL24− and CdL(OH)2− were determined from potentiometric titrations. The structures of the predominating species CdL−, and CdL24− in solution were investigated using EXAFS and IR spectroscopic techniques. In the 1:1 complex bonds are formed between the Cd(II) ion and all three donor groups (amino, carboxylate, phosphonate) of the PMG molecule resulting in two 5-membered chelate rings. At the remaining three of the corners of the distorted Cd(II) octahedra oxygens were found which are replaced by donor groups of a second PMG molecule in the 1:2 complex. Furthermore, a solid phase consisting of Cd9(PMG)6(H2O)12·6H2O crystals was synthesized and the crystal structure was determined. The structure consists of six CdL− octahedra connected through a seventh Cd–O octahedron in the centre of the entity, with two additional Cd–O octahedra located at the apices of the unit formed.