Metal–metal bonding in molecular actinide compounds: electronic structure of [M2X8]2− (M = U, Np, Pu; X = Cl, Br, I) complexes and comparison with d-block analogues

Abstract

Density functional and multiconfigurational (ab initio) calculations have been performed on [M₂X₈]²⁻ (X = Cl, Br, I) complexes of 4d (Mo, Tc, Ru), 5d (W, Re, Os), and 5f (U, Np, Pu) metals in order to investigate general trends, similarities and differences in the electronic structure and metal–metal bonding between f-block and d-block elements. Multiple metal–metal bonds consisting of a combination of σ and π interactions have been found in all species investigated, with δ-like interactions also occurring in the complexes of Tc, Re, Np, Ru, Os, and Pu. The molecular orbital analysis indicates that these metal–metal interactions possess predominantly d_z² (σ), d_xz and d_yz (π), or d_xy and d_x²−y² (δ) character in the d-block species, and f_z³ (σ), f_z²x and f_z²y (π), or f_xyz and f_z (δ) character in the actinide systems. In the latter, all three (σ, π, δ) types of interaction exhibit bonding character, irrespective of whether the molecular symmetry is D_4h or D_4d. By contrast, although the nature and properties of the σ and π bonds are largely similar for the D_4h and D_4d forms of the d-block complexes, the two most relevant metal–metal δ-like orbitals occur as a bonding and antibonding combination in D_4h symmetry but as a nonbonding level in D_4d symmetry. Multiconfigurational calculations have been performed on a subset of the actinide complexes, and show that a single electronic configuration plays a dominant role and corresponds to the lowest-energy configuration obtained using density functional theory.