Homoatomic Stella Quadrangula [Tl₈]⁶⁻ in Cs₁₈Tl₈O₆, Interplay of Spin−Orbit Coupling, and Jahn−Teller Distortion

Cs18Tl8O6 was synthesized reacting the binary compounds CsTl and Cs2O. According to single crystal X-ray analysis, the title compound crystallizes as a novel structure type in the cubic space group I23 and is diamagnetic. The electronic structure of the extended solid and of excised Cs6Tl8 clusters has been examined by relativistic density functional calculations including spin−orbit coupling. Cs18Tl8O6 comprises a clusteranion [Tl8]6− in the shape of a tetrahedral star. An isoelectronic cluster was found previously in Cs8Tl8O, however, with the shape of a parallelepiped. Both clusteranions can be derived from a homocubane unit by displacive distortions. It has been shown by quantum mechanical analyses that the closed-shell electronic structure of the parallelepiped is the result of a Jahn−Teller distortion, while in contrast the tetrahedral star in Cs18Tl8O6 would still exhibit an open-shell degenerate HOMO within a scalar relativistic approximation. Only if spin−orbit coupling is considered, a closed-shell electronic system is obtained in accordance with the diamagnetic behavior of Cs18Tl8O6.