Synthesis, Structure, and Bonding in K12Au21Sn4. A Polar Intermetallic Compound with Dense Au-20 and Open AuSn4 Layers

The new phase K12Au21Sn4 has been synthesized by direct reaction of the elements at elevated temperatures. Single crystal X-ray diffraction established its orthorhombic structure, space group Pmmn (No. 59), a = 12.162(2); b = 18.058(4); c = 8.657(2) angstrom, V = 1901.3(7) angstrom(3), and Z = 2. The structure consists of infinite puckered sheets of vertex-sharing gold tetrahedra (Au-20) that are tied together by thin layers of alternating four-bonded-Sn and -Au atoms (AuSn4). Remarkably, the dense but electron-poorer blocks of Au tetrahedra coexist with more open and saturated Au-Sn layers, which are fragments of a zinc blende type structure that maximize tetrahedral heteroatomic bonding outside of the network of gold tetrahedra. LMTO band structure calculations reveal metallic properties and a pseudogap at 256 valence electrons per formula unit, only three electrons fewer than in the title compound and at a point at which strong Au-Sn bonding is optimized. Additionally, the tight coordination of the Au framework atoms by K plays an important bonding role: each Au tetrahedra has 10 K neighbors and each K atom has 8-12 Au contacts. The appreciably different role of the p element Sn in this structure from that in the triel members in K3Au5In and Rb2Au3TI appears to arise from its higher electron count which leads to better p-bonding (valence electron concentrations = 1.32 versus 1.22).