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

The chemical bonding of actinide metal ions with both arene and borohydride ligands is explored via quantum chemical methods to understand how the transuranic elements differ from uranium with respect to their interaction with soft donor ligands. Specifically, the [An(arene)(BH4)3] complexes (AnMe6 , An = Np, Pu, U, arene C6Me6) are studied. Density functional theory (DFT) shows that, when the complexes are neutral, the interaction between the metal ions and the soft-donor ligands is governed by electrostatic interactions. Molecular orbital analysis, with both the DFT and the complete active space (CASSCF) method, shows that as one moves from U to Pu, the energy gap between the 5f orbitals of the metal ion and the ligand π∗ orbitals gradually increases, leading to a weaker metal-ligand interaction. Upon reduction to AnMe6−, the An–arene distances contract by 0.1-0.2 ̊A compared to the neutral complex, leading stronger metal-ligand interactions with varying degrees of δ-bonding depending on the actinide. Specifically, orbital mixing decreases from UMe6− to PuMe6−. In the high-spin state, UMe6− has two electrons in the two δ-bonding orbitals, while NpMe6− and PuMe6− 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 consistent with the increased An–arene distances, weaker interactions, and decreasing covalency across the series.