K(23)Au(12)Sn(9)--an intermetallic compound containing a large gold-tin cluster: synthesis, structure, and bonding.

A polyanionic unit {Au(12)Sn(9)} with a novel "corrugated sheet" shape occurs in K(23)Au(12)Sn(9). 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) A, Z = 2. The large heteroatomic cluster has D(2d) 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 approximately 40 % of the total -ICOHP. Extended-Huckel 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.