Four Polyanionic Compounds in the K–Au–Ga System: A Case Study in Exploratory Synthesis and of the Art of Structural Analysis

The K–Au–Ga system has been investigated
at 350 °C
for <50 at. % K. The potassium gold gallides K0.55Au2Ga2, KAu3Ga2, KAu2Ga4 and the solid solution KAuxGa3–x (x = 0–0.33)
were synthesized directly from the elements via typical high-temperature
reactions, and their crystal structures were determined by single
crystal X-ray diffraction: K0.55Au2Ga2 (I, I4/mcm, a = 8.860(3) Å, c = 4.834(2) Å, Z = 4), KAu3Ga2 (II, Cmcm, a = 11.078(2) Å, b = 8.486(2) Å, c = 5.569(1) Å, Z = 4), KAu2Ga4 (III, Immm, a = 4.4070(9) Å, b = 7.339(1) Å, c = 8.664(2) Å, Z = 2), KAu0.33Ga2.67 (IV, I-4m2, a = 6.0900(9)
Å, c = 15.450(3) Å, Z =
6). The first two compounds contain different kinds of tunnels built
of puckered six- (II) or eight-membered (I) ordered Au/Ga rings with completely different cation placements:
uniaxial in I and III but in novel 2D-zigzag
chains in II. III contains only infinite
chains of a potassium-centered 20-vertex polyhedron (K@Au8Ga12) built of ordered 6–8–6 planar Au/Ga
rings. The main structural feature of IV is dodecahedral
(Au/Ga)8 clusters. Tight-binding electronic structure calculations
by linear muffin-tin-orbital methods were performed for idealized
models of I, II, and III to
gain insights into their structure–bonding relationships. Density
of states curves reveal metallic character for all compounds, and
the overall crystal orbital Hamilton populations are dominated by
polar covalent Au–Ga bonds. The relativistic effects of gold
lead to formation of bonds of greater population with most post-transition
elements or to itself, and these appear to be responsible for a variety
of compounds, as in the K–Au–Ga system.