Uncovering Heavy Actinide Covalency: Implications for Minor Actinide Partitioning

Abstract

Across the actinide period, the stability of the trivalent oxidation state predominates in the heavy actinides, making their chemical nature close to that of rare earth elements. The resemblance in their chemistry poses difficulties in separating heavy actinides from lanthanides, which is a vital separation in the minor actinide partitioning process. Actinide contraction has conventionally implied electrostatic actinide-ligand interactions among the heavy actinides. The present study challenges this conventional understanding and reveals increasing covalency in the actinide-ligand bond across Am to Cf. Complexes of Am, Cm, Bk, and Cf have been examined for their electronic structure with a focus on the nature of their interactions with different ligands within the framework of density functional theory, where the relativistic effects have been incorporated by using zero-order regular approximation and spin–orbit coupling. The choice of ligands selected for this study facilitates the effect of the donor atom as well as denticity to be accounted for. Hence, heavy actinide complexes of the N- and O-donor ligand dipicolinic acid, S and O mixed donor ligands of the Cyanex type, and an octadentate ligand N,N,N′N′-tetrakis­[(6-carboxypyridin-2-yl)­methyl]­ethylenediamine have been optimized and evaluated. In each case energy decomposition analysis has been used to explicitly decompose the metal–ligand interaction energy into components which have then been analyzed. Irrespective of the hard–soft characteristics of donor atoms or the denticity of the ligands, steadily increased covalency has been observed across Am to Cf. Inspection of the ligand highest energy occupied molecular orbitals and metal orbitals sheds light on the origin of the unexpected covalency. An overall increase in bonding and also the orbital contribution along the Am–Cf series is clearly due to the enhancement in covalency, which is complementary to the orbital degeneracy induced covalency proposed very recently by Batista and co-workers.

Covalency of Trivalent Actinide Ions with Different Donor Ligands: Do Density Functional and Multiconfigurational Wavefunction Calculations Corroborate the Observed “Breaks”?

“…Optimized An–ligand distances for [An­(S 2 PMe 2 ) 3 ] (model An–Cyanex), [An­(DPA) 3 ] 3– , and [An­(HOPO)] − are displayed in Figure (bottom panels). Concerning the model An–Cyanex complex, the optimized An–S bond distances decrease monotonically across the actinide row and follow closely the DFT/PBE bond lengths reported in ref . Our optimized Am–S bond distance (2.826 Å) agrees well with the optimized distance (2.830 Å) by Dolg, Cao, and Ciupka with an f-in-valence PP and the Becke–Perdew , nonhybrid functional.…”

“…Present calculations (except for HOPO complexes) indicated by data points connected with lines. Additional data shown for the [AnCl 6 ] 3– , [An­(S 2 PMe 2 ) 3 ] 0 , and [An­(DPA) 3 ] 3– complexes are from refs − . Calculated and experimental An–O bond distances for the [An­(HOPO)] − complexes were collected from refs and .…”

Ligand-Induced U Mobilization from Chemogenic Uraninite and Biogenic Noncrystalline U(IV) under Anoxic Conditions

“…While the existence of 1:2 and 1:3 U(IV) DPA complexes has been established in the literature, stability constants for U(IV)-DPA complexes that would be needed to quantitatively predict the effect of DPA on UO 2 solubility are not available. 57 − 59 …”

Theoretical examination of covalency in berkelium(IV) carbonate complexes

“…Recently, Chandrasekar reported energy decomposition analyses for AmCf complexes using solvent corrections considering a Van der Waals radius of 2.223 Å for all actinide atoms. [ 26 ] We conducted a couple of tests considering VdW radii of 2.100 and 2.223 Å for berkelium, which yielded significantly different results. Therefore, we decided to show only gas‐phase results to keep the discussion as simple as possible, bearing in mind that they lack solvation effects.…”

“…Studies devoted to the understanding of covalency in heavy actinides are scarce. [22,[24][25][26] The virtue of QTAIM is that the partition of the 3D space is said to be natural since no chemical intuition is required.…”