Solubility, Complex Formation, and Redox Reactions in the Tl₂O₃−HCN/CN^-−H₂O System. Crystal Structures of the Cyano Compounds Tl(CN)₃·H₂O, Na[Tl(CN)₄]·3H₂O, K[Tl(CN)₄], and Tl^I[Tl^III(CN)₄] and of Tl^I₂C₂O₄^†

Thallium(III) oxide can be dissolved in water in the presence of strongly complexing cyanide ions. Tl^III is leached from its oxide both by aqueous solutions of hydrogen cyanide and by alkali-metal cyanides. The dominating cyano complex of thallium(III) obtained by dissolution of Tl₂O₃ in HCN is [Tl(CN)₃(aq)] as shown by ²⁰⁵Tl NMR. The Tl(CN)₃ species has been selectively extracted into diethyl ether from aqueous solution with the ratio CN^-/Tl^III = 3. When aqueous solutions of the MCN (M = Na⁺, K⁺) salts are used to dissolve thallium(III) oxide, the equilibrium in liquid phase is fully shifted to the [Tl(CN)₄]^- complex. The Tl(CN)₃ and Tl(CN)₄^- species have for the first time been synthesized in the solid state as Tl(CN)₃·H₂O (1), M[Tl(CN)₄] (M = Tl (2) and K (3)), and Na[Tl(CN)₄]·3H₂O (4) salts, and their structures have been determined by single-crystal X-ray diffraction. In the crystal structure of 1, the thallium(III) ion has a trigonal bipyramidal coordination with three cyanide ions in the equatorial plane, while an oxygen atom of the water molecule and a nitrogen atom from a cyanide ligand, attached to a neighboring thallium complex, form a linear O−Tl−N fragment. In the three compounds of the tetracyano−thallium(III) complex, 2−4, the [Tl(CN)₄]^- unit has a distorted tetrahedral geometry. Along with the acidic leaching (enhanced by Tl^III−CN^- complex formation), an effective reductive dissolution of the thallium(III) oxide can also take place in the Tl₂O₃−HCN−H₂O system yielding thallium(I), while hydrogen cyanide is oxidized to cyanogen. The latter is hydrolyzed in aqueous solution giving rise to a number of products including (CONH₂)₂, NCO^-, and NH₄⁺ detected by ¹⁴N NMR. The crystalline compounds, Tl^I[Tl^III(CN)₄], Tl^I₂C₂O₄, and (CONH₂)₂, have been obtained as products of the redox reactions in the system.