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 p pi 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.