  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.
