Atomic Orbital Energy Matching vs. Overlap in Actinide-Ligand Dative Bonding

Pre-bonding conditions and dative bond formation in actinide(IV)hexachloride com- plexes with U, Np, and Pu are studied by quantum chemical calculations, with a focus on the interplay of atomic orbital (AO) overlap and AO energy matching in the formation of molecular orbitals (MOs) and the subsequent identification of dative bonds. The extent of ligand-to-metal donation is tracked via population analysis, MO localization, and var- ious bond-order criteria. Fock matrices from density functional calculations are used to setup models in which varying numbers of suitably chosen valence AOs interact to form dative bonds in the complexes. The results confirm, among other effects, a contribution to the metal-ligand covalency from the actinide (An) 6p shell. Better matching of An(5f) and Cl(3p) AO energies occurs when going from U to Np to Pu because of the lowering of the An(5f) levels due to the incomplete nuclear charge screening. However, a near- degeneracy only occurs in the case of the 𝛼-spin orbitals of the Pu system, and it is shown to be inconsequential. Overall, a better AO energy matching for An(5f) and Cl(3p) along the series U, Np, Pu is counter-balanced by decreasing AO overlap and decreasing avail- ability of 5f acceptor orbitals, leading to similar bond orders and extents of donation in the three systems.