Theoretical reasons for metallic behavior among diverse Zintl phases have generally not been pursued at an advanced level. Here, the electronic structure of Ca₅Ge₃ (Cr₅B₃ type), which can be formulated (Ca⁺²)₅(Ge₂^-6)Ge^-4 in oxidation states, has been explored comparatively by means of semiempirical and first-principles density functional methods. The FP-APW calculations show that alkaline-earth-metal and germanium orbitals, particularly the d orbitals on the cations and the p-π* orbitals of the halogen-like dimeric Ge₂^-6, mix considerably to form a conduction band. This covalency perfectly explains the unusual metallic properties of the nominally electron-precise Zintl phase Ca₅Ge₃ and its numerous relatives. Similar calculational results are obtained for Sr₅Ge₃, Ba₅Ge₃, and Ca₅Sn₃. Cation d orbitals appear to be a common theme among Zintl phases that are also metallic.