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

The chemical bonding of actinide ions with arene and
borohydride
ligands is explored via quantum chemical methods to understand how
the transuranium elements interact with soft-donor ligands. Specifically,
the [An(C6Me6)(BH4)3] complexes (An = U, Np, and Pu) and their
reduced congeners are studied. Density functional theory (DFT) shows
that the metal–ligand interactions in the neutral complexes
are governed by electrostatic interactions. Both DFT and complete
active space (CASSCF) results show that as one moves from U to Pu,
the 5f-orbitals are stabilized leading to a poorer energy match with
the ligand orbitals. This contributes to progressively weaker metal-arene
and metal-borohydride interactions across the series due to a decrease
in energy-driven covalency. A reduction in orbital contributions to
bonding is obtained for the transuranium-arene interactions as well.
Upon reduction, the arene is reduced, forming a δ-bond. This
causes the An–arene distances to contract by 0.1–0.2
Å compared to the neutral complexes. The ground state is assigned
as the intermediate-spin state where the arene radical is antiferromagnetically
coupled to the metal-centered f-electrons in Np and Pu. On the other
hand, the ferromagnetically and antiferromagnetically coupled states
are close in energy in the uranium complex, but do not mix when spin–orbit
coupling is included using a state-interaction approach (SO-CASPT2).
The population of the CASSCF δ*-antibonding natural orbital
increases from U to Pu consistent with the increased An−arene
distances, weaker interactions, and decreasing covalency across the
series. Although the An–B distance increases by ca. 0.06 Å
upon reduction, both the neutral and reduced species involve an An(III)–borohydride
bond and as such are qualitatively similar. The Np complexes can be
assigned to have slightly weaker bonding than the uranium analogs
but are overall “uranium-like”. The Pu complexes are
predicted to have less covalent contributions to bonding in both the
Pu–arene and Pu–borohydride interactions; however, the
Pu–arene interaction is predicted to be particularly weak.