Rubidium chalcogenido diferrates(III) containing dimers [Fe₂Q₆]⁶⁻ of edge-sharing tetrahedra (Q=O, S, Se)

The two isotypic rubidium chalcogenido diferrates Rb₁₂[Fe₂Q₆](Q₂)₃ (Q=S/Se), which both form needles with green-metallic lustre, were synthesized from Rb₂S, elemental iron, rubidium and sulfur (Q=S) or from the pure elements (Q=Se) at maximum temperatures of 500–800°C. Their triclinic crystal structures were determined by means of X-ray single crystal data (space group P1̅, a=863.960(10)/903.2(3), b=942.790(10)/982.1(3), c=1182.70(2)/1227.4(4) pm, α=77.4740(10)/77.262(6), β=71.5250(10)/71.462(6), γ=63.7560(10)/63.462(5)°, Z=1, R1=0.0308/0.0658 for Q=S/Se). The structures contain isolated dinuclear anions [Feᴵᴵᴵ₂Q₆]⁶⁻ composed of two edge-sharing [FeQ₄] tetrahedra (dFₑ₋Q =223.4–232.3/236.2–244.8 pm), which are also found in the two polymorphs of the pure alkali diferrates Rb₆[Fe₂Q₆]. The diferrate ions are arranged in layers running in the a/b plane around z=0. Inbetween (around z≈12$z \approx {1 \over 2}$), two crystallographically different disulfide/diselenide ions Q22−$Q_2^{2 - }$ (dQ₋Q =211.1–213.4/237.9–241.1 pm), which are arranged in slightly puckered 3⁶ nets, are intercalated. The intra-anionic distances and angles, the Rb coordination numbers and the molar volumes of these two ‘double-salts’ are in accordance with their corresponding reference compounds, Rb₆[Fe₂Q₆] and Rb₂Q₂. In addition, the two polymorphs of Rb₆[Fe₂Se₆], which are both isotypic with the sulfido analogous (Cs₆[Ga₂Se₆]-type, monoclinic, space group P2₁/c, a=827.84(5), b=1329.51(7), c=1074.10(6) pm, β=127.130(5)°, R1=0.0443 and Ba₆[Al₂Sb₆]-type, orthorhombic, space group Cmce, a=1963.70(3), b=718.98(3), c=1348.40(7) pm, R1=0.0264) were prepared and characterized to complete the series of alkali diferrates(III) with oxido, sulfido and selenido ligands. The electronic band structures of the three Rb salts Rb₆[Fe₂Q₆], which have been calculated within the GGA+U approach applying an AFM spin ordering in the dimers and appropriate Hubbard parameters, allow a comparison of the chemical bonding characteristics (e.g. covalency) and the magnetic properties (magnetic moments) within the series of chalcogenido ligands. An analysis of the spin densities enables a comparative consideration of the mechanisms crucial for the magnetic ordering in chalcogenido ferrates. Ultimately, the electronic structure of the new compound Rb₁₂[Fe₂S₆](S₂)₃ nicely compares with those of the S₂-free reference compound Rb₆[Fe₂S₆].