Pandora's box of binary tungsten iodides.

The preparation of binary tungsten iodides by the reaction of elements is intrinsically difficult because most compounds are only stable at temperatures where the reaction rates between elemental tungsten and iodine are too low. Therefore, other preparative procedures have to be developed. Two synthetic methods have previously been reported for binary tungsten iodides: halide exchange reactions starting from tungsten chloride, and the reaction of tungsten hexacarbonyl with elemental iodine. These two methods have lead to the discovery of a number of binary tungsten iodide compounds. However, how many compounds can be expected in a binary sytem? A recently developed synthetic method has opened Pandora's box, leading to the discovery of a tremendous number of new binary tungsten iodide compounds with structures which have never been seen before in metal halide chemistry. The formation of binary tungsten iodides depends on a subtle interplay of pressure and temperature conditions as can be monitored by thermal scanning under variable I ₂ partial pressures. More than twenty binary tungsten iodide compounds with trigonal, tetrahedral, square pyramidal, and octahedral cluster cores are summarized in this review with their formation conditions, crystal structures, and transformation reactions. These preparative advances open new frontiers in metal halide chemistry and give an impact to the chemistry of octahedral [W ₆ I ₈ ] ⁴⁺ cluster compounds which can serve as powerful phosphorescent materials and singlet oxygen generators.