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 B6 2− cluster.