Aromaticity of the planar pentagonal As5− anion in the MAs5 (M=Li, Na, K, Rb, and Cs) and MAs5+ (M=Be, Mg, Ca, Sr, and Ba) clusters

Geometrical structures, electronic structures, and vibrational frequencies of alkali metal MAs5 (M=Li, Na, K, Rb, and Cs) clusters and alkali earth metal MAs5+ (M=Be, Mg, Ca, Sr, and Ba) clusters were investigated with density functional theory (DFT) methods. Calculation results showed that the planar pentagonal As5− anion could coordinate with metal atoms to form the pyramidal MAs5 and MAs5+ complexes maintaining the planar pentagonal As5− anion structure. Structural and electronic criteria, the presence of six delocalized π electrons (satisfying the 4n+2 electron counting rule), and maintaining its structural and electronic integrity inside the MAs5 and MAs5+ clusters confirmed that the planar pentagonal As5− anion exhibited characteristics of π aromaticity.