Synthesis, Structure, and Bonding of BaAuTl₃ and BaAuIn₃:  Stabilization of BaAl₄-Type Examples of the Heavier Triels through Gold Substitution

The title compounds have been synthesized by high temperature means and characterized by X-ray structural analysis, physical property measurements, and electronic structure calculations. The compounds crystallize in the three-dimensional tetragonal structure of BaAl(4), I4/mmm, Z = 2 (a = 4.8107(4), 4.8604(2) A, and c = 11.980(2), 12.180(2) A for BaAuIn(3) and BaAuTl(3), respectively). Gold randomly substitutes for 50% of the In or Tl in the apical (4e) positions in the network, generating apical-apical atom distances of 2.77 and 2.70 A, respectively, values that are comparable to the single bond metallic radii sum for Au plus In, and 0.08 A less than that for Au plus Tl. Relativistic effects appear to be important for both of the latter elements. The shrinkage in distances and increase in bond strengths evidently stabilize BaAuTl(3) relative to the distorted BaTl(4) with a presumably oversized triel lattice. EHTB band calculations indicate that the two compounds are electron-deficient relative to optimal Au-Tr and Au-Au bonding and metallic, the latter in agreement with measured properties of BaAuTl(3).