Synthesis and crystal structure of K₃AsSe₄ and K ₄As₂Se₅ Containing [AsSe₄] ^3- Anions and a Novel [As₂Se₅]^4- Isomer Featuring an As-As

Two novel selenoarsenates, K₃AsSe₄ and K ₄As₂Se₅, were synthesized by the mild solvothermal reaction of potassium carbonate and arsenic and selenium powder in the presence of superheated ethylenediamine and structurally characterized by single-crystal X-ray diffraction. K₃AsSe₄ crystallizes in the orthorhombic space group Pnma (no. 62) with a = 9:4169(4), b = 10:9695(5), c = 9:7568(8) Å, Z = 4 and V = 1007:86(10) ų and contains [AsSe₄]^3- anions with pentavalent arsenic. K ₄As₂Se₅ crystallizes in the monoclinic space group P21=n (no. 14) with a = 7:6167(2), b = 22:8733(4), c = 8:4687(2) Å, b = 107:241(2)°, Z = 4 and V = 1409:14(5) Å3, and shows isolated [As₂Se₅]^4- anions. The most prominent feature of the latter anions is the presence of arsenic in two different formal oxidation states (+4/+2) due to the presence of an As-As bond.