Na(9)Bi(5)Os(3)O(24): A Diamagnetic Oxide Featuring a Pronouncedly Jahn–Teller‐Compressed Octahedral Coordination of Osmium(VI)

The Jahn–Teller (JT) theorem constitutes one of the most fundamental concepts in chemistry. In transition‐element chemistry, the 3d(4) and 3d(9) configurations in octahedral complexes are particularly illustrative, where a distortion in local geometry is associated with a reduction of the electronic energy. However, there has been a lasting debate about the fact that the octahedra are found to exclusively elongate. In contrast, for Na(9)Bi(5)Os(3)O(24), the octahedron around Os(6+)(5d(2)) is heavily compressed, lifting the degeneracy of the t(2g) set of 5d orbitals such that in the sense of a JT compression a diamagnetic ground state results. This effect is not forced by structural constraints, the structure offers sufficient space for osmium to shift the apical oxygen atoms to a standard distance. The relevance of these findings is far reaching, since they provide new insights in the hierarchy of perturbations defining ground states of open shell electronic systems.