Tetrahedral 4c-2e and Trigonal-Bipyramidal 5c-2e Bonds in Actinide-Containing Clusters with Quasi-Lewis Structures Identified via an Orbital Alignment and Localization Scheme

The study of metal-metal bonding, particularly in heterometallic clusters containing actinide elements, is a fascinating area of research due to its material and catalytic properties. However, understanding the complex bonding within these clusters poses challenges for traditional bonding analysis tools. The fragile bonding capability of actinide elements and the complicated orbital mixing of metal-metal and metal-ligand bonding interactions prevent the discovery of novel actinide-metal bonds and the precise bonding description of these clusters in terms of traditional Lewis structures. To address these issues, we introduced a novel orbital alignment and localization scheme, which helps separate the two above-mentioned bonding interactions. Herein, we employed the scheme to analyze two multinuclear actinide clusters containing U₂Ni₂ and U₂Ni₃ metal frameworks. Through the bonding analysis, we obtained a set of pure metallic localized molecular orbitals, which provide chemically intuitive bonding/nonbonding information. By observing these orbitals, we discovered a remarkable tetrahedral four-center two-electron σ bond and a unique trigonal-bipyramidal five-center two-electron σ bond for the U₂Ni₂ and U₂Ni₃ clusters, respectively. Furthermore, we were able to put forward quasi-Lewis structure descriptions for the two clusters, elucidating their metal-metal bonds, molecular structures, stability, and magnetic properties, successfully expanding the applicability of Lewis structures to the actinide clusters.