High-symmetry tubular Ta@B183-, Ta2@B18, and Ta2@B27+ as embryos of α-boronanotubes with a transition-metal wire coordinated inside    

     Transition-metal doping leads to dramatic structural changes and results in novel bonding patterns in small boron clusters. Based on the experimentally derived mono-ring planar C_9v TaB₉^2- (1) and extensive first-principles theory calculations, we present herein the possibility of high-symmetry doublering tubular D9d Ta@B₁₈^3- (2) and C_9v Ta₂@B₁₈ (3) and triple-ring tubular D_9h Ta₂@B₂₇⁺ (4), which may serve as embryos of single-walled metalloboronanotube a-Ta₃@B_48(3,0) (5) wrapped up from the recently observed most stable free-standing boron a-sheet on a Ag(111) substrate with a transition-metal wire (–Ta–Ta–) coordinated inside. Detailed bonding analyses indicate that, with an effective dz²–dz² overlap on the Ta–Ta dimer along the C₉ molecular axis, both Ta₂@B₁₈ (3) and Ta₂@B₂₇⁺ (4) follow the universal bonding pattern of s + p double delocalization with each Ta center conforming to the 18-electron rule, providing tubular aromaticity to these Ta-doped boron complexes with magnetically induced ring currents. The IR, Raman, and UV-vis spectra of 3 and 4 are computationally simulated to facilitate their future experimental characterization.