New Hexachalcogeno–Hypodiphosphates of the Alkali Metals: Synthesis, Crystal Structure and Vibrational Spectra of the Hexathiodiphosphate(IV) Hydrates K 4 [P 2 S 6 ] · 4 H 2 O, Rb 4 [P 2 S 6 ] · 6 H 2 O, and Cs 4 [P 2 S 6 ] · 6 H 2 O

The new hexathiodiphosphate(IV) hydrates K 4 [P 2 S 6 ] · 4 H 2 O ( 1 ), Rb 4 [P 2 S 6 ] · 6 H 2 O ( 2 ), and Cs 4 [P 2 S 6 ] · 6 H 2 O ( 3 ) were synthesized by soft chemistry reactions from aqueous solutions of Na 4 [P 2 S 6 ] · 6 H 2 O and the corresponding heavy alkali‐metal hydroxides. Their crystal structures were determined by single crystal X‐ray diffraction. K 4 [P 2 S 6 ] · 4 H 2 O ( 1 ) crystallizes in the monoclinic space group P 2 1 / n with a = 803.7(1), b = 1129.2(1), c = 896.6(1) pm, β = 94.09(1)°, Z = 2. Rb 4 [P 2 S 6 ] · 6 H 2 O ( 2 ) crystallizes in the monoclinic space group P 2 1 / c with a = 909.4(2), b = 1276.6(2), c = 914.9(2) pm, β = 114.34(2)°, Z = 2. Cs 4 [P 2 S 6 ] · 6 H 2 O ( 3 ) crystallizes in the triclinic space group $P {\bar 1}$ with a = 742.9(2), b = 929.8(2), c = 936.8(2) pm, α = 95.65(2), β = 112.87(2), γ = 112.77(2)°, Z = 1. The structures are built up by discrete [P 2 S 6 ] 4− anions in staggered conformation, the corresponding alkali‐metal cations and water molecules. O ··· S and O ··· O hydrogen bonds between the [P 2 S 6 ] 4− anions and the water molecules consolidate the structures into a three‐dimensional network. The different water‐content compositions result by the corresponding alkali‐metal coordination polyhedra and by the prefered number of water molecules in their coordination sphere, respectively . The FT‐Raman and FT‐IR/FIR spectra of the title compounds have been recorded and interpreted, especially with respect to the [P 2 S 6 ] 4− group. The thermogravimetric analysis showed that K 4 [P 2 S 6 ] · 4 H 2 O converted to K 4 [P 2 S 6 ] as it was heated at 100 °C.