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Abstract:
Thiny black air-sensitive crystals of Bi65Cu13Ge41O6Br122 =
(Bi-5)(2)(Bi-9)(6)[O(GeBr2)(3)](6)[Cu6Ge5Br20][BiCu7Ge18Br66] were
obtained by very slow cooling of a melt of bismuth, bismuth tribromide,
copper, and partly oxidized germanium. The crystal structure was
described in the hexagonal space group P6(3) (no. 173) with a =
2584.2(4) pm and c = 1277.4(3) pm at 296(1) K. The polarity of the
structure is subtle; large parts of the structure show a pseudo mirror
plane. The highly diversified structure comprises five distinct
structural fragments. For the first time, two polyhedral bismuth
polycations, Bi-5(3+) (trigonal bipyramid, D-3h) and Bi-9(5+) (tricapped
trigonal prism, D-3h), occur in one and the same chemical compound. A
hitherto unknown group is the C-3h-symmetric
(3)-oxido-tris[dibromidogermanate(II)] anion [O(GeBr2)(3)](2-), in which
the germanium atoms of three (GeBr2)-Br-II units bind to a central oxide
ion with sp(2) hybridization. Density-functional-theory-based
calculations on an isolated [O(GeBr2)(3)](2-) anion confirm the
stability of the group and its shape. Real-space chemical bonding
analysis supports the polar nature of the Ge-O and Ge-Br bonds as well
as the assigned oxidation states. The [O(GeBr2)(3)](2-) anion and the
bismuth polycations are embedded between two kinds of one-dimensional
networks, which are running parallel to the c axis, a
bromido-cuprate(I)-germanate(II) network (1)[Cu6Ge5Br20](4-) and a
bromido-bismuthate(III)-cuprate(I)-germanate(II) network
(1)[BiCu7Ge18Br66](20-). The latter features pentacoordinate
germanium(II) atoms with bond angles close to 90 degrees. The solution
of the crystal structure from X-ray diffraction data was hampered not
only by severe pseudo-symmetry and the similar scattering power of
copper, germanium, and bromine atoms, but also by substantial atomic
disorder and orientational disorder of the Bi-5(3+) polycation.
Bi65Cu13Ge41O6Br122 may be seen as a link between low-valent germanium
compounds and low-valent bismuth compounds.
