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, ΔHf⊖(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.