Electronic and structural relations between solid CaB6 and the molecular dianion B6H6 (2-): A computational study

Abstract

By means of the isolobality concept between a hydrogen atom and the radical dianion [B6H5 • ]2− derived from an extraction of a hydrogen atom in the dianion [B6H6]2−, forming part of the K2(B6H6) structure, we rationalize the 3D boron skeleton architecture in the solids CaB6 and KB6. The above statement is supported by electronic structure computations based on the Electron Localisation Function (ELF) in the dianion [B6H6]2− and radical anion [B6H5 •]2−. Valence-Bond (VB) theory is also used in the study of the B–B sigma bond connecting the octahedra in the CaB6 structure, with the model dimer [(H5B5)B–B(B5H5)]4−. Quantum-chemical geometry optimizations of clusters extracted from the CaB6 solid structure show the analogies between molecular chemistry and solid-state chemistry: In the same way as benzene is the building block of graphene and graphite, the anion [B6H6]2− is the building block of CaB6 provided every hydrogen atom in [B6H6]2− is connected to a further [(B•)6]2− octahedron in the three directions of the 3D space, and every void at the Ca positions is embedded with two electrons.