Pivotal influence of ligand field stabilization energy on the extraction order of divalent metal ions by acidic extractants

Abstract

Even after decades of industrial use, the fundamental factors governing the selectivity of acidic extractants for various divalent metal ions in the fourth period of the periodic system have not yet been identified. In this paper, the case is made for ligand field stabilization energy as the principal quantity determining the extractability of a divalent metal ion by a particular acidic extractant. This stabilizing energetic contribution results from non-isotropic (covalent) interactions between the metal ion and the extractant, which acts as an inner-sphere ligand in the metal-extractant complex. The extractability of a metal ion appears to be largely determined by the difference in ligand field stabilization energy in the organic versus the aqueous phase. In order to investigate this correlation, the magnitude of the splitting of the d-orbitals in the metalextractant complexes was probed by ultraviolet-visible-near-infrared absorption spectroscopy.Crystal field theory was applied to estimate the anisotropic contribution to the stability of each metal-extractant combination (ligand field stabilization energy), and of the hydrated metal ion in the aqueous phase. It was found that the anisotropic contribution is diminished upon extraction by extractants that weakly split the d-orbitals. This leads to preferential extraction of metal ions in which the electron configuration results in zero covalent contribution. Conversely, the anisotropic contribution becomes more favorable upon extraction by extractants which induce a large splitting of the d-orbitals. Hence, these extractants preferentially extract metal ions with a large covalent contribution to the total energy balance. Extractants with an intermediate splitting of d-orbitals exhibit little difference in covalent stabilization between both phases, and hence the extraction sequence is primarily determined by electrostatic effects, governed by the charge density of the metal ion. The model was applied to phosphoric, phosphonic, phosphinic, dithiophosphinic and carboxylic acid extractants, as well as to βdiketone and β-hydroxyketoxime extractants