The first example of a mixed valence ternary compound of silver with random distribution of Ag(i) and Ag(ii) cations

Abstract

The reaction between colourless AgSbF₆ and sky-blue Ag(SbF₆)₂ (molar ratio 2 : 1) in gaseous HF at 323 K yields green Ag₃(SbF₆)₄, a new mixed-valence ternary fluoride of silver. Unlike in all other Ag(I)/Ag(II) systems known to date, the Ag⁺ and Ag²⁺ cations are randomly distributed on a single 12b Wyckoff position at the axis of the I3d cell. Each silver forms four short (4 × 2.316(7) Å) and four long (4 × 2.764(6) Å) contacts with the neighbouring fluorine atoms. The valence bond sum analysis suggests that such coordination would correspond to a severely overbonded Ag(I) and strongly underbonded Ag(II). Thorough inspection of thermal ellipsoids of the fluorine atoms closest to Ag centres reveals their unusual shape, indicating that silver atoms must in fact have different local coordination spheres; this is not immediately apparent from the crystal structure due to static disorder of fluorine atoms. The Ag K-edge XANES analysis confirmed that the average oxidation state of silver is indeed close to +1⅓. The optical absorption spectra lack features typical of a metal thus pointing out to the semiconducting nature of Ag₃(SbF₆)₄. Ag₃(SbF₆)₄ is magnetically diluted and paramagnetic (μ_eff = 1.9 μ_B) down to 20 K with a very weak temperature independent paramagnetism. Below 20 K weak antiferromagnetism is observed (Θ = −4.1 K). Replacement of Ag(I) with potassium gives K(I)₂Ag(II)(SbF₆)₄ which is isostructural to Ag(I)₂Ag(II)(SbF₆)₄. Ag₃(SbF₆)₄ is a genuine mixed-valence Ag(I)/Ag(II) compound, i.e. Robin and Day Class I system (localized valences), despite Ag(I) and Ag(II) adopting the same crystallographic position.