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 C_2v 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 B₆^2- cluster.