Aromaticity of Planar B₅⁻Anion in the MB₅(M = Li, Na, K, Rb, and Cs) and MB₅⁺(M = Be, Mg, Ca, and Sr) Clusters

The ground-state geometries, electronic structures, and vibrational frequencies of alkali metal−B 5 - MB 5 (M = Li, Na, K, Rb, and Cs) and alkaline earth metal−B 5 - MB 5 + (M = Be, Mg, Ca, and Sr) clusters were investigated using ab initio self-consistent field and density functional theory (DFT) methods. Calculation results show that planar B 5 - anion can coordinate with the metal atom to form metal−polyboron MB 5 and MB 5 + species. On the basis of the molecular orbital (MO) analysis and nucleus-independent chemical shifts (NICS), we revealed that the planar B 5 - anion exhibits characteristic of π-aromaticity with two delocalized π electrons and maintains its structural and electronic integrity inside the five MB 5 and four MB 5 + clusters.