XBe6Au62+ (X = Si, Ge): Electron-Sharing Covalently Bonded Planar Hexacoordinate Silicon and Germanium Atoms

Abstract Examples of planar hexacoordinate molecular systems are scarce in the literature, particularly for the covalently bonded planar hexacoordinate group 14 elements. In the previously reported such systems, three bonds were covalent in nature, and the remaining three bonds were predominantly supported by electrostatic interactions. Here, we report that the global minimum of the 20-valence electronic XBe6Au62+ (X = Si, Ge) clusters has a D6h-symmetric star-like structure featuring planar hexacoordinate silicon and germanium (phX) atoms, which are strongly covalently bonded to the Be6 ring through three σ and one π-electron-sharing bonds. Additionally, electrostatic interactions also contribute considerably to the stabilization of the phX clusters. These clusters show high stability against dissociation and kinetic stability against distortion from planarity. The stability of phX species can be attributed to the electronic stabilization (2π + 6σ double aromaticity) and the optimal geometrical match between the planar X center and the peripheral Be6Au6 ring, making them promising candidates for experimental realization. We have further explored the possibility of blocking the reactivity of phX by [10]cycloparaphenylenes ([10]CPP), forming a viable host–guest complex while maintaining the novel electronic and structural integrity of phX motifs.