Computational Investigation of the Chemical Bond between An(III) Ions and Soft-Donor Ligands

Author

Naomi E. Rehberg, University of South DakotaFollow

Date of Award

Spring 2025

Document Type

Honors Thesis

Department/Major

Chemistry

First Advisor

Bess Vlaisavljevich

Second Advisor

Pere Miro

Third Advisor

Rick Wang

Keywords

Actinide, Nuclear Energy, Organometallics, Chemical Bonding, Sofi-Donor Ligands, Transuranium, Complete Active Space Methods (CASSCF), Density Functional Theory (DFT), Second-Order Multireference Perturbation Theory (CASPT2), Molecular Orbital Analysis

Subject Categories

Chemistry

Abstract

The nature of chemical bonding of actinide metal ions with arene and borohydride ligands is investigated through quantum chemical methods to understand how the transuranium elements differ from uranium with respect to their interaction with soft-donor ligands. Specifically, this study examines [An(arene)(BH₄)₃] complexes (An^Me₆; An = Np, Pu, U, arene = C₆Me₆). Density functional theory (DFT) shows that, when the complexes are neutral, electrostatic interactions dominate the interaction between the metal ions and the soft-donor ligands. Molecular orbital analysis, with both the DFT and the complete active space (CASSCF) method, shows that as one goes from U to Pu, there is a gradual increase in the energy gap between the 5f-orbitals of the metal ion and the ligand π^∗ orbitals, leading to a weaker metal-ligand interaction. Upon reduction to [An(arene)(BH₄)₃]^– (An^Me₆–), the An–arene distances contract by 0.1 to 0.2 Å compared to the neutral complex, leading stronger metal–ligand interactions with varying degrees of δ-bonding depending on the actinide. In particular, orbital mixing decreases from U^Me₆– to Pu^Me₆–. In the high-spin state, U^Me₆– has two electrons in the two δ-bonding orbitals, while Np^Me₆– and Pu^Me₆– have only one electron in a single δ-bonding orbital. In the lower-spin states, these bonding orbitals become even more delocalized and the population of the δ orbital increases from U to Pu. This is in agreement with the increased An–arene distances, weaker interactions, and decreasing covalency across the series. In this study, both the neutral and reduced species contain an An(III)–borohydride bond so the An–B distance increasing by 0.06 Å does not affect the two as being perceived qualitatively similar. The Np complexes can be considered “uranium-like” although the complexes can be assigned to have slightly weaker bonding than the uranium analogs. Moreover, the Pu–arene interaction is predicted to be particularly weak, in part because the Pu complexes are predicted to have less covalent contributions to bonding in both the Pu–arene and Pu–borohydride interactions.

Recommended Citation

Rehberg, Naomi E., "Computational Investigation of the Chemical Bond between An(III) Ions and Soft-Donor Ligands" (2025). Honors Thesis. 360.
https://red.library.usd.edu/honors-thesis/360