From inverse sandwich Ta(2)B(7)(+) and Ta(2)B(8) to spherical trihedral Ta(3)B(12)(−): prediction of the smallest metallo-borospherene

Transition-metal-doped boron nanoclusters exhibit interesting structures and bonding. Inspired by the experimentally discovered inverse sandwich D(6h) Ta(2)B(6) and spherical trihedral D(3h) La(3)B(18)(−) and based on extensive first-principles theory calculations, we predict herein the structural transition from perfect di-metal-doped inverse sandwich D(7h) Ta(2)B(7)(+) (1) and D(8h) Ta(2)B(8) (2) to tri-metal-doped spherical trihedral D(3h) Ta(3)B(12)(−) (3). As the smallest metallo-borospherene reported to date, Ta(3)B(12)(−) (3) contains three octa-coordinate Ta atoms as integral parts of the cage surface coordinated in three equivalent η(8)-B(8) rings which share two eclipsed equilateral B(3) triangles on the top and bottom interconnected by three B(2) units on the waist. Detailed orbital and bonding analyses indicate that both Ta(2)B(7)(+) (1) and Ta(2)B(8) (2) possess σ + π dual aromaticity, while Ta(3)B(12)(−) (3) is σ + π + δ triply aromatic in nature. The IR, Raman, and UV-vis or photoelectron spectra of the concerned species are computationally simulated to facilitate their future spectroscopic characterizations.