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The crystal structure of the hexaborides MB6 of alkali, alkaline-earth, and rare-earth metals displays a network of interconnected B-6 octahedra, while isolated B6H6 units occur in the molecular crystal K-2[B6H6]. For the case of a total charge transfer of two electrons from the metal atoms, the B-6 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.
