A MOLECULAR MODELING STUDY OF ANIONIC METAL-COMPLEXES EXTRACTION BY MONOCATIONIC AND DICATIONIC EXTRACTANTS

Molecular modelling method is used as an aid to predict and to account for the results of solvent extraction of dianionic metal complexes by anion-exchange extractants such as mono- and diphosphonium compounds (butyltrioctyl-phosphonium and polymethylenebis(trioctylphosphonium)s abbreviated as BuMP and CnBPs respectively). The extraction ability of CnBPs extractants was surprisingly dependent on the length of bridging methylene chain connecting the two phosphonium cationic centers which offers an effective molecular recognition field. The studied dianionic species, noted A2-, are MCl-4(2-) M(CN)42- and M(C2O4)22- with M = Zn2+. The software used is the one meant to produce special molecular geometries for organic molecules. So, to model complexes with a metal cation, it is necessary to introduce characteristic parameters of the metal and its bonds. The extracted complexes considered are ion-pair complexes such as (CnBP2+, A2-), (2 CnBp2+, 2 A2-) with n = 2, 4, 6, 8, 10, 12 and (2 BuMP+, A2-) These complexes are modelled and minimized. We have obtained the total energy value of the complex which takes account of its steric and electrostatic components by using the energy minimization procedure. This quantification allows the comparison of complexes with the assumption that the complex with the smallest value of total energy should correspond to the most stable extractable complex. So, the results show that the extraction of ZnX-4(2-) (with X = CN-, Cl-) and Zn(C2O4)22-involve the formation of 2:2 ion-pair complex and 1:1 ion-pair complex respectively. Graphical relationships have been obtained between the total or electrostatic energy and the experimental extraction percentage taken from the literature.