Aromaticity of the planar quadrilateral CN2O2− dianion in the MCN2O− (M = Li, Na, K, Rb, and Cs) and MCN2O (M = Be, Mg, Ca, Sr, and Ba) clusters

Investigation of equilibrium geometries, total energies, harmonic vibrational frequencies, and nucleus-independent chemical shifts (NICS) of the low-lying states for the planar quadrilateral CN2O2− dianion, alkali metal MCN2O− (M = Li, Na, K, Rb, and Cs) clusters, and alkali earth metal MCN2O (M = Be, Mg, Ca, Sr, and Ba) clusters were done with two density functional theory (DFT) methods. The calculations reveal that the planar quadrilateral CN2O2− dianion can coordinate with metal atoms to form the pyramidal MCN2O− and MCN2O complexes maintaining the planar quadrilateral CN2O2− dianion structure. From 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 MCN2O− and MCN2O clusters confirm that the planar quadrilateral CN2O2− dianion exhibits characteristics of π-aromaticity.