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

Abstract

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, ¹A_g) and B₈Ca₂ (D_8h, ¹A_1g), and both adopt interesting inverse sandwich geometries, showing an elongated D_2h B₆ or perfectly planar D_8h B₈ 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 B₆ (or B₈) ring, forming [Ca]²⁺[B₆]⁴⁻[Ca]²⁺ and [Ca]²⁺[B₈]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 B₆Ca₂ has 4σ and 6π delocalized electrons on the elongated B₆ ring, leading to a conflicting aromatic system. B₈Ca₂, possessing 6σ and 6π delocalized electrons on the B₈ ring, is doubly aromatic. Additionally, the B₆Ca₂ and B₈Ca₂ clusters show noticeable structural and electronic transmutation relative to their equivalent electronic B₆Be₂ and B₈Mg₂ clusters, respectively. The intrinsic reasons behind the transmutations are elucidated via in-depth bonding analyses.