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.