Thermal Iodine Loss Cascade of W 5 I 16 .

Tungsten iodide compounds feature a surprising diversity of binary compounds. Their formation conditions and phase equilibria depend very much on the surrounding iodine partial pressure and temperature. Herein we focus on square-pyramidal tungsten iodide cluster compounds with their iodine loss, structural rearrangements, cross-linking behavior, and final transformation into the octahedral cluster compound β-W ₆ I ₁₂ at elevated temperatures. Reactions depart from W ₅ I ₁₆ at different iodine pressures and temperatures. The thermal decomposition of W ₅ I ₁₆ at low iodine pressure passes through a number of cluster compounds (W ₅ I ₁₅ , W ₅ I ₁₃ ·xI ₂ , β-W ₅ I ₁₂ , and W ₅ I ₁₁ ) under release of iodine. At higher iodine pressures, only one intermediate compound (α-W ₅ I ₁₂ ) was found. Thermal decomposition of W ₅ I ₁₆ was examined by differential thermal/thermogravimetric analysis, differential scanning calorimetry, and X-ray diffraction (XRD) measurements. New compounds (W ₅ I ₁₅ , W ₅ I ₁₃ ·xI ₂ , and W ₅ I ₁₁ ) were structurally characterized by means of XRD techniques. The crystal structure of W ₅ I ₁₁ reveals a nice relationship to its transformation product W ₆ I ₁₂ .