Structures of complexes formed by organophosphorus acid extractants and divalent metal ions

Abstract

The structure of the metal-extractant complexes was investigated for Mg(II), Ca(II), Mn(II), Ni(II), Cu(II) and Zn(II) after solvent extraction by bis(2-ethylhexyl)phosphoric acid (D2EHPA) and bis(2,4,4-trimethylpentylphosphinic acid) (Ionquest® 290). A combination of slope analysis, Karl Fischer titration and differential scanning calorimetry (DSC) allowed the determination of the stoichiometry of the complex (both inner and outer sphere). UV-VIS-NIR absorption spectroscopy was used to establish the coordination geometry and symmetry around the central metal ion, while infrared spectroscopy and 31P NMR spectroscopy revealed structural characteristics of the extractant molecules and their coordination to the metal ion. Co(II), for which the structure is well-established in the literature, was used as a reference for infrared spectroscopy. The results indicate that all metals form oligomeric or polymeric structures at high loading, either as inner-sphere coordination polymers, or as outer-sphere inverse micelles in the case of Ni(II). At lower loading, structures are more diverse, with the eventual structure of the complex being influenced by factors such as ionic radius, crystal field effects, steric demands of the extractant and the Jahn–Teller effect.