Abstract: The geometries and chemical bonding of B₆X^- (X=N, P, As, Sb, Bi) clusters have been investigated using ab initio and density functional theory computations with MP2, B3LYP and B3PW91 methods. Analytical gradients with the polarized split-valence basis sets (6-311+G*) were used for B, N, P, and As. The relativistic effective core potential with the LANL2DZ basis set was chosen for Sb and Bi. It is seems that the very reasonable looking pyramidal minimum structure (C_2v symmetry) is the most stable. The analyses of molecular orbital, natural bond orbital, and nucleus-independent chemical shift for these quasi-pyramidal B₆X^- systems revealed a double-aromatic character(π and σ), which had been reported to play an important role in the chemical bonding, structure, and stability of the pure boron clusters.

Key words: Hetero-polyboron cluster, Ab initio calculation, Density fauctional theory, Aromaticity