Theoretical study on the aromaticity of dianions X32- (XÂ =Â Zn, Cd, Hg)

The equilibrium geometries, total electronic energies, and vibrational frequencies of the low-lying states of dianions X32- (X = Zn, Cd, Hg), anions NaX3- (X = Zn, Cd, Hg), and neutral clusters Na2X3 (X = Zn, Cd, Hg) are investigated with three density functional theory (DFT) and correlated ab initio methods B3LYP, B3PW91, and CCSD(T). We find that the most stable regular triangular D3h structure for X32-, pyramidal C3v structure for NaX3- and bi-pyramidal C3v structure for Na2X3 can be held together by a single π-bond. In both pyramidal NaX3- and bi-pyramidal Na2X3 species, the basic structural unit is an unusual X32- trigonal planar cluster, which exhibits properties of aromaticity. The detailed molecular orbital analysis for these isomers further reveals that one highly delocalized π-type MO play an important role in rendering them pure π-aromaticity without σ-framework.