[Mn₂(AsS₄)₄]^8- and [Cd ₂(AsS₄)₂(AsS₅)₂] ^8-: Discrete clusters with high negative charge from alkali metal polythioarsenate fluxes

The reaction of Mn and Cd in alkali metal polythioarsenate fluxes afforded four new compounds featuring molecular anions. K₈[Mn ₂(AsS₄)₄] (I) crystallizes in the monoclinic space group P2/n with a = 9.1818(8) Å, b = 8.5867(8) Å, c = 20.3802(19) Å, and β = 95.095(2)°. Rb₈[Mn ₂(AsS₄)₄] (II) and Cs₈[Mn ₂(AsS₄)₄] (III) both crystallize in the triclinic space group P1̄ with a = 9.079(3) Å, b = 9.197(3) Å, c = 11.219(4) Å, α = 105.958(7)°, β = 103.950-(5)°, and γ = 92.612(6)° for II and a = 9.420(5) Å, b = 9.559(5) Å, c = 11.496(7) Å, α = 105.606(14)°, β = 102.999(12)°, and γ = 92.423(14)° for III. The discrete dimeric [Mn₂(AsS₄)₄]^8-clusters in these compounds are composed of two octahedral Mn²⁺ ions bridged by two [AsS₄]^3- units and chelated each by a [AsS ₄]^3- unit. Rb₈-[Cd₂(AsS ₄)₂(AsS₅)₂] (IV) crystallizes in P1̄ with a = 9.122(2) Å, b = 9.285(2) Å, c = 12.400(3) Å, α = 111.700-(6)°, β - 108.744°, and γ = 90.163(5)°. Owing to the greater size of Cd compared to Mn, the Cd centers in this compound are bridged by [AsS₅]^3- units. The [Cd₂(AsS₄)₄]^8- cluster is a minor component cocrystallized in the lattice. These compounds are yellow in color and soluble in water.