Zintl Phase versus Covalent Metal: Chemical Bonding in Silicon Dumbbells of Ca 5 Si 3 and CaSi 3 .

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 Ca 5 Si 3 and metallic CaSi 3 , the chemical bonding in Si 2 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 2 dumbbells in Ca 5 Si 3 can be referred to as molecular building units Si 2 6- with additional metallic and ionic contributions in the solid. In the covalent metal CaSi 3 , however, the bonding within the dumbbells falls short of fulfilling the octet rule. As a result, antibonding states of the Si 2 building units are depopulated and attend metallic interactions, simultaneously giving rise to stronger covalent Si-Si bonds.