OXIDATION OF FIRST ROW TRANSITION ELEMENTS WITH AsF5 IN AsF3 AT AMBIENT TEMPERATURE: SYNTHESES AND STRUCTURES OF M(AsF6)2·2AsF3, M = Fe, Co, Ni
Karel Lutar, Horst Borrmann, Adolf Jesih and Boris Žemva
Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
A novel synthetic approach via reactions between pure metals (Cr, Mn, Fe, Co, Ni, Cu) and AsF5 in AsF3 as a solvent has been investigated. AsF5 proved to oxidize these metals to their (+2) oxidation state. In the case of Cr, Mn or Cu the same compounds as those in the superacidic systems MF2-AsF5-aHF (anhydrous hydrogen fluoride) were obtained: CrFAsF6, Mn(AsF6)2, and CuFAsF6, respectively. In the case of Fe, Co and Ni a new type of fluoroarsenates has been isolated: M(AsF6)2.2AsF3. The latter compounds are isostructural and crystallize in the monoclinic space group P21/n (no. 14) with Z=2. Unit cell dimensions of Fe(AsF6)2.2AsF3 are: a = 733.8(4) pm, b = 1303.0(8) pm, c = 735.4(5) pm, $ = 91.43(7)° at 292 K, and for Co(AsF6)2.2AsF3: a = 725.9(4) pm, b = 1292.9(7) pm, c = 727.6(5) pm, $ = 91.64(5)° at 288 K. The structure is characterized by layers formed through transcoordinating AsF6 units in the basal plane of the octahedron about the transition metal. Both apical positions of this octahedron are occupied by AsF3 molecules. A main feature of the layers are characteristic rings resulting from the apex-sharing connectivity of AsF6 and MF6 octahedra. As AsF3 molecules of neighbouring layers are localized above and below the center of a particular ring, a stacking sequence ABAB... results.