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 Tll 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 A. The Tl–Br–Tl angle in Tl2Br+ is 92° and the Tl–Tl separation in the complex, 4.54 A, does not imply any direct Tl–Tl interaction.