Na9Bi5Os3O24: A Unique Diamagnetic Oxide Featuring a Pronouncedly Jahn-Teller Compressed Octahedral Coordination of Osmium(VI).

The Jahn-Teller theorem constitutes one of the most popular and stringent concepts, applicable to all fields of chemistry. In open shell transition elements chemistry and physics, 3d4, 3d9, and 3d7(low-spin) configurations in octahedral complexes serve as particular illustrative and firm examples, where a striking change (distortion) in local geometry is associated to a substantial reduction of electronic energy. However, there has been a lasting debate, about the fact that the octahedra are found to exclusively elongate, (at least for eg electrons). Against this background, the title compound displays two marked features, (1) the octahedron of oxygen atoms around Os6+ (d2) is drastically compressed, in contrast to the standard JT expectations, and (2) the splitting of the t2g set induced by this compression is extreme, such that a diamagnetic ground state results. What we see is obviously a Jahn-Teller distortion resulting in a compression of the respective octahedron and acting on the t2g set of orbitals. Both these issues are unprecedented. Noteworthy, the splitting into a lower dxy (hosting two d electrons with opposite spin) and two higher dxz and dyz orbitals is so large that for the first time ever the Hund's coupling for t2g electrons is overcome. We show that these effects are not forced by structural frustration, the structure offers sufficient space for Os to shift the apical oxygen atoms to a standard distance. Local electronic effects appear to be responsible, instead. 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. The system studied here, offers substantially more structural and compositional degrees of freedom, such that a configuration could form that enables Os6+ to adopt its apparently genuine diamagnetic ground state.