Enthalpies of formation of alkali-metal hexabromo-zirconate, -hafnate, and -stannate

Abstract

Enthalpies of reaction have been determined by solution calorimetry for the process (i) where M′₂M = K₂Zr, Cs₂Zr, Cs₂Hf, or K₂Sn. The data lead to the following standard enthalpies of formation, ΔH_f^⊖(298 K): K₂[ZrBr₆], 2M′Br(c)+ MBr₄(c)→ M′₂[MBr₆](c)(i)–1 603; Cs₂[ZrBr₆], –1 665; Cs₂[HfBr₆], –1 793; and K₂[SnBr₆], –1 214 kJ mol^–1. The halide-ion affinities of the gaseous MBr₄ and MCl₄ molecules have been calculated by assuming semi-empirical values for the lattice energies of the complexes. Tin halides show markedly less affinity for halide than zirconium and hafnium halides. Hafnium tetrabromide has an unusually high affinity for bromide. In this respect it differs considerably from the zirconium halides.