SmB6– Cluster Anion: Covalency Involving f Orbitals

While boride clusters of alkali and transition metals have been observed and extensively characterized, so far little is known about lanthanide–boron clusters. Lanthanide–boride solids are intriguing, however, and therefore it is of interest to understand the fundamental electronic properties of such systems, also on the subnano scale. We report a joint experimental photoelectron spectroscopic and theoretical study of the SmB6– anion, iso-stoichiometric to the SmB6 solid—a topological Kondo insulator. The cluster is found to feature strong static and dynamic electron correlations and relativistic components, calling for treatment with CASPT2 and up sixth-order Douglass–Kroll–Hess (DKH) relativistic correction. The cluster has a C2v structure and covalent Sm–B bonds facilitated by f atomic orbitals on Sm, which are typically thought to be contracted and inert. Additionally, the cluster retains the double antiaromaticity of the B62– cluster.