A density functional theory study of the competitive complexation of pyridine against H2O and Cl- to Cm3+ and Ce4+

Density functional theory was used to study the coordination of Cm3+ and Ce4+ with pridine (Py), water (H2O) and chloride anion (Cl-). The competitive coordination of Cl- and Py was investigated to simulate the ligand exchange between Cl(-)and Py at high concentration of hydrochloric acid (HCl), where Cm3+ and Ce4+ may exist in the form of [CmCl6](3-) and [CeCl6](2-), while that of water and Py to simulate the process at low concentration or without the presence of Cl-. The calculations show that Ce4+ has higher affinity to Py than Cm3+ in the absence of Cl-, while it binds much more weakly at high concentration of HCl. This is consistent with experimental data that at high concentration of HCl, Ce4+ has much shorter retention time than Cm3+ using tertiary pyridine resin (TPR). In view of the strengthening of M-Cl and the weakening of M-OW at bonds upon the coordination of Py, we conclude that the distinct coordination abilities of the three ligands to Ce4+ and Cm3+ are due to different strengths of the inner-shell electrostatic interaction between the ligands and the central metal ions.