Theoretical Study on the Aromaticity of the Pyramidal MB6 (M = Be, Mg, Ca, and Sr) Clusters

Abstract

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 C6v pyramidal structure with M2+ cation interacting with a planar hexagonal B62- dianion. Whereas the other two CaB6 (C1) and SrB6 (CS) clusters possess quasi-pyramidal structure with M2+ cation interacting with a chairlike B62- dianion. Molecular orbital (MO) analysis and nucleus-independent chemical shifts (NICS) further reveal that the four MB6 species all have three delocalized π MOs and two delocalized σ MOs and therefore exhibit the multiple-fold aromaticity.