Perfect Spherical Tetrahedral Metallo-Borospherene Ta₄B₁₈ as a Superatom Following the 18-Electron Rule

Cage-like metallo-borospherenes exhibit unique structures and bonding. Inspired by the newly reported smallest spherical trihedral metallo-borospherene D _3h Ta₃B₁₂ ^- (1), which contains two equivalent B₃ triangles interconnected by three B₂ units on the cage surface, we present herein a first-principles theory prediction of the perfect spherical tetrahedral metallo-borospherene T _d Ta₄B₁₈ (2), which possesses four equivalent B₃ triangles interconnected by six B atoms, with four equivalent nonacoordinate Ta centers in four η⁹-B₉ rings as integrated parts of the cage surface. As the well-defined global minimum of the neutral, Ta₄B₁₈ (2) possesses four 10c-2e B₉(π)-Ta(d_σ) and eight 10c-2e B₉(π)-Ta(d_δ) coordination bonds evenly distributed over four Ta-centered Ta@B₉ nonagons, with the remaining 18 valence electrons in nine 22c-2e totally delocalized bonds following the 18-electron principle (1S²1P⁶1D¹⁰) of a superatom. Such a bonding pattern renders spherical aromaticity to the tetrahedral complex, which can be used as building blocks to form the face-centered cubic crystal Ta₄B₁₅ (3). The IR, Raman, and UV-vis spectra of Ta₄B₁₈ (2) are theoretically simulated to facilitate its future experimental characterizations.