Zintl Phase versus Covalent Metal: Chemical Bonding in Silicon Dumbbells of CaSi and CaSi.

Silicon dumbbells constitute identifiable anionic molecular species in Zintl phases and so-called covalent metals holding units with homopolar bonding inside a metallic framework. Based on electron-precise CaSi and metallic CaSi, the chemical bonding in Si units is investigated by computational quantum chemical methods considering the dual nature of the wave function. This concerted wave-vector and real space study substantiates that the Si dumbbells in CaSi can be referred to as molecular building units Si with additional metallic and ionic contributions in the solid. In the covalent metal CaSi, however, the bonding within the dumbbells falls short of fulfilling the octet rule. As a result, antibonding states of the Si building units are depopulated and attend metallic interactions, simultaneously giving rise to stronger covalent Si-Si bonds.