Theoretical study on the aromaticity of the pyramidal MB6 (M = Be, Mg, Ca, and Sr) clusters

A series of metal-polyboron clusters with the general formula MB6 (M = Be, Mg, Ca, and Sr) were investigated using ab initio self-consistent field and density functional theory (DFT) methods. Calculation results show that the two clusters of BeB6 and MgB6 have C-6, Pyramidal structure with M2+ cation interacting with a planar hexagonal B-6(2-) dianion. Whereas the other two CaB6 (C-1) and SrB6 (Cs) clusters possess quasi-pyramidal structure with M2+ cation interacting with a chairlike B-6(2-) dianion. Molecular orbital (MO) analysis and nucleus-independent chemical shifts (NICS) further reveal that the four MB6 species all have three delocalized pi MOs and two delocalized sigma MOs and therefore exhibit the multiple-fold aromaticity.