Formation of Tetraselenodiarsenate(II) Anions in Ultra‐alkaline Medium. Issue 10 (29th November 2021) - Albrecht, Ralf
Abstract: SeO2 and As2 O3 were reacted at 200 °C in in a potassium hydroxide hydroflux with a molar water‐base ratio n (H2 O) : n (KOH)=1.8. Arsenate(III) disproportionates under these conditions into arsenate(V) and elemental arsenic. The reduction also yielded tetraselenodiarsenate(II) anions, which turned the solution yellow and precipitated at room temperature as plate‐shaped crystals of K4 (As2 Se4 ) ⋅ 2H2 O. The unexpected redox reaction as well as the formation of a hygroscopic compound in an aqueous solution are attributed to a strongly reduced activity of water in ultra‐alkaline media. K4 (As2 Se4 ) ⋅ 2H2 O crystallizes in the orthorhombic space group Cmce with lattice parameters a =784.78(3), b =1027.76(4) and c =1750.63(7) pm at 100 K. The (As2 Se4 ) 4− anion has C 2h symmetry and an As−As bond length of 245.3(1) pm. The water molecules are part of infinite zig‐zag chains ∞ 1 [K(H2 O)2/2 ] + and form hydrogen bonds to neighboring selenium atoms. Abstract :