MB82- (M = Be, Mg, Ca, Sr, and Ba): Planar octacoordinate alkaline earth metal atoms enclosed by boron rings

Complexes involving planar octacoordinate alkaline earth metal atoms in the centers of eight-membered boron rings have been investigated by two density functional theory (DFT) methods. BeB82- with D-8h symmetry is predicted to be stable, both geometrically and electronically, since a good match is achieved between the size of the central beryllium atom and the eight-membered boron ring. By contrast, the other alkaline earth metal atoms cannot be stabilized in the center of a planar eight-membered boron ring because of their large radii. By following the out-of-plane imaginary vibrational frequency, pyramidal C-8v MgB82-, CaB82-, SrB82-, and BaB82- structures are obtained. The presence of delocalized pi and sigma valence molecular orbitals in D-8h BeB82- gives rise to aromaticity, which is reflected by the value of the nucleus-independent chemical shift. The D-8h BeB82- structure is confirmed to be the global minimum on the potential energy surface.