The novel metallic compound Na₆TlSb₄ consists of four-membered TlSb₃ rings joined by pairs of Sb atoms into Tl₂Sb₈ units, the last of which is further interconnected into 1D anionic chains via Tl−Tl bonds. The contrast of its metallic conductivity with that of the 2 − e^- poorer, electron precise, and semiconducting Zintl phase K₆Tl₂Sb₃, which has virtually the same anionic network, has been investigated by ab initio LMTO-DFT methods. Sodium ion participation is found to be appreciable in the (largely) Sb p valence band and especially significant in an additional low-lying conduction band generated by antimony pπ and sodium orbitals. The one pyramidal 3-bonded Sb atom appears to be largely responsible for the interchain conduction process. The substitution of one Tl by Sb, which occurs when the countercation is changed from potassium in K₆Tl₂Sb₃ to sodium, yielding only Na₆TlSb₄, is driven by a distinctly tighter packing, a corresponding increase in the Madelung energy, and binding of the excess pair of electrons in the new conduction band.