Aromaticity of the Square As42− Dianion in the MAs4− (M=Li, Na, K, Rb, and Cs) and MAs4 (M=Be, Mg, Ca, Sr, and Ba) Clusters

Geometrical structures, electronic structures, and vibrational frequencies of alkali metal MAs4− (M=Li, Na, K, Rb, and Cs) clusters and alkali earth metal MAs4 (M=Be, Mg, Ca, Sr, and Ba) clusters were investigated with density functional theory (DFT) methods. Calculation results show that the square As42− dianion can coordinate with metal atoms to form the pyramidal MAs4− and MAs4 complexes maintaining the square As42− dianion 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 MAs4− and MAs4 clusters confirmed that the square planar As42− dianion exhibits characteristics of π-aromaticity.