Two isostructural compounds Na(3)Mln(2) (M = Au, Ag) with a NiTi2-type structure (Fd-3m) were synthesized via typical high-temperature reactions. The anionic M-In structure consists of tetrahedral star building units [ln(4)M(4/2)] (TS) that are connected into a three-dimensional framework via shared TS vertexes, with the Na atoms filling the cages. On the basis of TB-LMTO-ASA calculations, the electronic structures of both compounds show substantial participation of sodium in the overall bonding of the structure. Compared with the Ag compound, relativistic effects in the Au phase appear to be significant, as shown in the M-M and M-Na bond length decreases of 0.03-0.04 angstrom.