The structure and chemical bonding in inverse sandwich B6Ca2 and B8Ca2 clusters: conflicting aromaticity vs. double aromaticity

The typical electron-deficiency of the boron element renders fascinating architectures and chemical bonding to boron-based nanoclusters. We theoretically predict two di-Ca-doped boron clusters, B6Ca2(D-2h,(1)A(g)) and B8Ca2(D-8h,(1)A(1g)), and both adopt interesting inverse sandwich geometries, showing an elongatedD(2h)B(6)or perfectly planarD(8h)B(8)ring being sandwiched by two Ca atoms only, respectively. Natural atomic charge analyses indicate that the Ca atoms donate nearly all the 4s electrons to the B-6(or B-8) ring, forming [Ca](2+)[B-6](4-)[Ca](2+)and [Ca](2+)[B-8](4-)[Ca](2+)charge transfer complexes. The interaction between the two Ca atoms and the boron rings is governed by robust electrostatics albeit by weaker B-Ca covalent interaction. Chemical bonding analyses show that B(6)Ca(2)has 4 sigma and 6 pi delocalized electrons on the elongated B(6)ring, leading to a conflicting aromatic system. B8Ca2, possessing 6 sigma and 6 pi delocalized electrons on the B(8)ring, is doubly aromatic. Additionally, the B(6)Ca(2)and B(8)Ca(2)clusters show noticeable structural and electronic transmutation relative to their equivalent electronic B(6)Be(2)and B(8)Mg(2)clusters, respectively. The intrinsic reasons behind the transmutations are elucidatedviain-depth bonding analyses.