Perfect Spherical Tetrahedral Metallo-Borospherene Ta(4)B(18) as a Superatom Following the 18-Electron Rule

[Image: see text] Cage-like metallo-borospherenes exhibit unique structures and bonding. Inspired by the newly reported smallest spherical trihedral metallo-borospherene D(3h) Ta(3)B(12)(–) (1), which contains two equivalent B(3) triangles interconnected by three B(2) units on the cage surface, we present herein a first-principles theory prediction of the perfect spherical tetrahedral metallo-borospherene T(d) Ta(4)B(18) (2), which possesses four equivalent B(3) triangles interconnected by six B atoms, with four equivalent nonacoordinate Ta centers in four η(9)-B(9) rings as integrated parts of the cage surface. As the well-defined global minimum of the neutral, Ta(4)B(18) (2) possesses four 10c-2e B(9)(π)–Ta(d(σ)) and eight 10c-2e B(9)(π)–Ta(d(δ)) coordination bonds evenly distributed over four Ta-centered Ta@B(9) nonagons, with the remaining 18 valence electrons in nine 22c-2e totally delocalized bonds following the 18-electron principle (1S(2)1P(6)1D(10)) 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 Ta(4)B(15) (3). The IR, Raman, and UV–vis spectra of Ta(4)B(18) (2) are theoretically simulated to facilitate its future experimental characterizations.