K₂₃Au₁₂Sn₉; An intermetallic compound containing a large gold-tin cluster: Synthesis, structure, and bonding

A polyanionic unit {Au₁₂Sn₉} with a novel corrugated sheet shape occurs in K₂₃Au₁₂Sn₉. The compound was obtained by fusion of the pure elements in tantalum ampules at high temperatures followed by programmed cooling, and the structure was determined by X-ray diffraction: I42m (No. 121), a = 20.834(3), c = 6.818(1) Å, Z = 2. The large heteroatomic cluster has D2d point symmetry and features a central four bonded (4b-) Sn, eight 3b-or 2b-Sn on the perimeter, and 24 linking nearly linear Sn-Au bonds at 12 Au atoms. Formula splitting according to the Zintl concept suggests that the compound is one electron deficient, and linear muffin-tin-orbital (LMTO) electronic structure calculations show that the Fermi level (E_F) lies near a band gap at around 0.5 eV, that is, an incompletely filled valence band in concert with favorable atom packing. Large relative-ICOHP values for Au-Sn are consistent with the observed maximization of the number of heteroatomic bonds, whereas the numerous K-Sn and K-Au contacts contribute ̃ 40 % of the total-ICOHP. Extended-Ḧuckel population and molecular orbital analyses indicate that the open band feature originates from 5p states that are associated with the 2b-corner Sn atoms. In accord with the electronic structure calculations, magnetic susceptibility measurements show a nearly temperature-independent paramagnetic property.