Zintl Phase versus Covalent Metal: Chemical Bonding in Silicon Dumbbells of Ca₅Si₃ and CaSi₃.

Abstract

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 Ca₅Si₃ and metallic CaSi₃, the chemical bonding in Si₂ 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 Si₂ dumbbells in Ca₅Si₃ can be referred to as molecular building units Si₂^6- with additional metallic and ionic contributions in the solid. In the covalent metal CaSi₃, however, the bonding within the dumbbells falls short of fulfilling the octet rule. As a result, antibonding states of the Si₂ building units are depopulated and attend metallic interactions, simultaneously giving rise to stronger covalent Si-Si bonds.