Position‐Space Bonding Indicators for Hexaborides of Alkali, Alkaline‐Earth, and Rare‐Earth Metals in Comparison to the Molecular Crystal K2[B6H6]

The crystal structure of the hexaborides MB6 of alkali, alkaline‐earth, and rare‐earth metals displays a network of interconnected B6 octahedra, while isolated B6H6 units occur in the molecular crystal K2[B6H6]. For the case of a total charge transfer of two electrons from the metal atoms, the B6 units serve as classical examples of electron deficient clusters. QTAIM and ELI‐D analyses of chemical bonding based on solid state DFT/APW+lo quantum chemical calculations were performed. Consistent with Wade's rules, a number of about seven endohedral bonds for the octahedral units is recovered from the delocalization index (DI). Detailed analyses of two‐ and three‐center delocalization indices yield a clear two‐center character of the exohedral B–B bonds and a mixed two‐ and three‐center character of the endohedral B–B bonds. The picture obtained by topological analysis of ELI‐D is in agreement with the DI results. With changing effective electron transfer from the metal atoms, electronic saturation of the intra‐cluster bonding is found, which leads to the notion of generally two‐valent rare‐earth atoms in MB6 hexaborides.