Be₇Au₆: rare over-delocalization of valence electrons in a star-like planar hexacoordinate beryllium cluster

Although electron delocalization is essential for maintaining the stability of planar hypercoordinate carbon clusters, this delocalization is usually achieved through the formation of multicenter bonds between the central atom and the ligand atom and does not involve bridging atoms farther away from the central atom. Herein, we report a star-like Be₇Au₆ cluster with a planar hexacoordinate beryllium (phBe) atom. Unprecedentedly, its peripheral bridging Au atoms are involved in the bonding of the central phBe atom, which can be regarded as the first example of such bonding in planar hypercoordination chemistry. Unexpectedly, the increase in delocalization does not lead to improved stability. On the contrary, this over-delocalization of electrons brings relative instability, i.e., the Be₇Au₆ cluster can be identified as a global minimum or a competitive isomer due to different thermodynamic corrections (zero-point energy and Gibbs free energy).