Na9Bi5Os3O24: 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 and stringent concepts in chemistry. In transition elements chemistry, the 3 d 4 , 3 d 9 , and 3 d 7 configurations in octahedral complexes are particularly illustrative, where a distortion in local geometry is associated to a reduction of 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+ (5 d 2 ) is heavily compressed, and the splitting of the t 2g set of 5 d orbitals induced by this compression is extreme, such that a diamagnetic ground state results. Both these issues are extremely rare. The splitting into a lower d xy (hosting two d electrons with opposite spin) and two higher d xz and d yz orbitals is so large that the Hund´s coupling for t 2g electrons is overcome. These effects are 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 insights in the hierarchy of perturbations defining ground states of open shell electronic systems.