Structure and aromaticity of Sb5- and Sb5M (MÂ =Â Li, Na, and K) clusters

The equilibrium geometries, electronic structures, harmonic vibrational frequencies, and nucleus independent chemical shifts (NICS) of Sb5- and Sb5M (M = Li, Na, and K) clusters are calculated by the density functional theory (DFT) method. Our calculation show that the Sb5- species has the aromatic planar cyclic D5h structure and the Sb5M species have the pyramidal C5v structures as the ground states, and the Sb5- unit preserves its structural and electronic integrity in forming the Sb5M complexes. Molecular orbital analysis and NICS show that the planar Sb5- anion satisfies the Hükcel rule of 4n + 2π electrons and magnetic criteria for aromaticity with six delocalized π electrons and negative NICS values. The dissected NICS suggests that the aromaticity of Sb5-(D5h) and Sb5M (C5v) arise primarily from the contributions of Sb-Sb π bonds, Sb-Sb σ bonds, and the NICS distributions and contributions, and their changing tendencies of the Sb5M (C5v) species are similar to the isolated Sb5-(D5h) anion.