New Sulfido Antimonates of the Heavy Alkali Metals: Synthesis, Crystal Structure and Chemical Bonding of (K/Rb/Cs)3SbS3 and Cs3SbS4 ·H2O

The new sulfido antimonates(III) (Rb/Cs)₃SbS₃ were prepared from the alkali metal sulfides Rb₂S/Cs₂S₂ and elemental antimony and sulfur or Sb₂S₃ at reaction temperatures of about 700 °C. The known isotypic potassium compound was similarily synthesized from the elements. The structures of the light-yellow crystals were refined using single-crystal X-ray data. Both compounds are isotypic to the respective Na salt forming the Na₃AsS₃ structure type (cubic, space group P2₁3, K/Rb/Cs: a = 947.21(7)/982.28(5)/1025.92(5) pm, Z = 4, R1 = 0.0159/0.0560/0.0582). The ψ-tetrahedral SbS₃^3– anions with Sb−S bond lengths of 242 pm are arranged in a cubic face centered packing, in which the three crystallographically different A⁺ cations occupy the tetrahedral and octahedral voids, overall exhibiting a distorted octahedral sulfur coordination. The chemical bonding and the characteristics of the stereochemically active lone electron pair have been investigated by means of FP-LAPW band structure calculations. Needle-shaped crystals of the monohydrate of the antimony(V) salt Cs₃SbS₄ · H₂O were obtained from a suspension of Sb₂O₃, CsOH and elemental sulfur. Cs₃SbS₄ · H₂O crystallizes in a new structure type (monoclinic, space group P2₁/c, a = 987.17(10), b = 994.83(7), c = 1600.46(14) pm, β = 126.895(8)^○, Z = 4, R1 = 0.0234). As expected, the Sb−S distances (233.1–234.7 pm) in the nearly ideally tetrahedral anion SbS₄^3– are considerably shorter than in the antimonates(III) but match the bond lengths in the anhydrous sulfido antimonate(V) Cs₃SbS₄. Due to their similar fcc-like anion packing and the stereochemically active lone electron pair of Sb in the antimonates(III), the whole series of compounds A₃Sb^III,VS_3/4 shows a uniform structure relation, which is elucidated using crystallographic group-subgroup relations.