Roles of d- and f-orbital electrons in the complexation of Eu(III) and Am(III) ions with alkyldithiophosphinic acid and alkylphosphinic acid using scalar-relativistic DFT calculations

High-level radioactive liquid waste (HLLW) containing many types of metal ions is generated after the reprocessing of spent nuclear fuel. Minor-actinides (MA: Am and Cm) have long-term radiotoxicity due to their long half-lives and α-activities; thus, the partitioning and transmutation is a rational method for their disposal. In this method, MA ions are separated from HLLW, followed by transmutation of the MA ions to short-lived nuclides; this method was developed in order to reduce their environmental loads. However, the selective separation of MA ions from lanthanide (Ln) ions is difficult due to the similarity of their chemical properties, such as their oxidation states, geometric structures, and chemical stabilities in solution. To effectively separate MA ions, the molecular design of separation materials that have high selectivity for MA ions over Ln ions is desirable. Solvent extraction has been employed for the separation of MA ions from Ln ions. Selectivities for MA and Ln ions have been investigated using various extraction reagents. Previous studies indicated that Sand N-donor ligands such as alkyldithiophosphinic acid (R 2PS2H; Figure 1a) and N,N,N’,N’-tetrakis(4-pyridylmethyl)ethylenediamine (TPEN), respectively, preferably coordinate to MA ions over Ln ions, whereas O-donor ligands such as alkylphosphinic acid (R2PO2H; Figure 1b) selectively coordinate to Ln ions over MA ions. This dependency of MA/Ln selectivity on donor atoms has been explained using Pearson’s hard and soft acids and bases (HSAB) rule. Sand N-donor atoms are softer bases than O-donor atoms; they bond more strongly to MA ions than Ln ions because MA ions are softer acids than Ln ions due to the extended distribution of their electron orbitals in the valence region. The difference in covalency between MA and Ln complexes may determine the separation behaviors of MA ions from Ln ions; however, it is difficult to quantitatively discuss covalency using the HSAB rule. Density functional theory (DFT) is a powerful tool to understand the chemical stabilities and electronic states of f-block compounds, which has been successfully applied to evaluation of feasibility to separate MA ions from Ln ions. Discussion of the nature of the bonding between the metal ion and ligands in Ln and MA complexes has been controversial. Roles of dand f-orbital electrons in the complexation of Eu(III) and Am(III) ions with alkyldithiophosphinic acid and alkylphosphinic acid using scalar-relativistic DFT calculations