Thermodynamic stability and structure of Tl2Br+ in molten alkali-metal nitrate

Abstract

The complexation between thallium(l) and bromide has been studied in molten 57:43 (K,Li)NO₃ between 150 and 250°C, and in 50 : 50 (K,Na)NO₃ between 250 and 300°C in systems TlBr–TlNO₃–AlkNO₃(Alk = K,Li or K,Na). Solubility data and e.m.f. measurements with Ag/AgBr electrodes give consistent evidence for TlBr and Tl₂Br⁺ complexes. ΔH^°_mn and ΔS^°_mn have been evaluated for the stepwise associations in (K,Li)NO₃. The results are: ΔH^°₁₁=–7.4 kJ mol^–1, ΔS^°₁₁= 14.2 J K^–1 mol^–1 for Tl⁺+ Br^–→ TlBr and ΔH^°₂₁=–31.1 kJ mol^–1, ΔS^°₂₁=–47 J K^–1 mol^–1 for Tl⁺+ TlBr → Tl₂Br⁺. A qualitatively similar pattern is observed for the reactions in (K,Na)NO₃. Liquid X-ray scattering measurements on TlNO₃–(K,Na)NO₃ and TlBr–TlNO₃–(K,Na)NO₃ melts at 280°C reveal that Tl^l is solvated by four nitrate ions, which coordinate to Tl in an asymmetrical bidentate manner, the Tl–O distances being 2.66 and 3.23 Å. The Tl–Br–Tl angle in Tl₂Br⁺ is 92° and the Tl–Tl separation in the complex, 4.54 Å, does not imply any direct Tl–Tl interaction.