Mixed valent cold oxides: Syntheses, structures, and properties of Rb5Au3O2, Rb7Au5O2, and Cs7Au5O2

摘要

The title compounds Rb5Au3O2, Rb7Au5O2, and Cs7Au5O2 are the first examples of mixed valent phases containing gold in the oxidation states +1 and -1. Their crystal structures (Rb5Au3O2, Pbam, a=736.4(1)pm, b=1430.8(2)pm, c=567.9(1)pm, Z = 2, R(F)=0.053, 647 reflections; Rb7Au5O2, Immm, a = 567.1 (2) pm, b = 930.1(1) pm, C = 1659.4(3) pm, Z = 2, R (F) = 0.066, 409 reflections; Cs7Au5O2, Immm, a=599.4(1)pm, b = 960.6(3) pm, c = 1720.8(12) pm, Z = 2, R (F) = 0.039, 386 reflections) are characterized by the combination of distinctive structural features of gold(I) oxides and aurides: for Au(+1) a typical linear coordination by oxygen is found and the surroundings of Au(-1) bear a close resemblance to the binary 1:1 aurides, In consequence the overall structures of Rb5Au3O2 and M7Au5O2 can be described as intergrowths of M3AuO2 and MAu (M = Rb, Cs), constituting members of a homologous series [MAu](n)[M3AuO2] with n = 2 and 4, respectively. The crystal chemical evidence for the valence states assumed, also confirmed by Mossbauer spectroscopy, is supported by various band structure calculations (Hartee-Fock acid density functional) clearly indicating the coexistence of two different oxidation states. The compounds have been synthesized by reacting binary aurides MAu and alkali monoxides M2O (M = Rb, Cs) with elemental gold in the required stochiometric amounts. Hereby, a further astonishing parallel to the chemistry of halogens is revealed. Like these, gold disproportionates upon interaction with bases. (C) 2000 Academic Press. [References: 33]