The stability of the polyhalides

Molecular halogens or inter halogens compounds have, for many years, been known to be capable of associating with halide ions either in the solid state or in solution to form stable univalent species. The classic example of this is the enhanced solubility of iodine in an aqueous potassium iodide solution to form the triiodide anion. This was crystallised by Johnson in 1877 and was the first inorganic triiodide identified. The earliest reference to a triiodide was however the strychnine triiodide of Pelletier and Caventore in 1819. Filhol made the first reference to an inorganic polyhalide in 1839. This was the mixed polyhalide KIC1\\(_4\\). Lowig and Roozeboom also investigated polyhalides during the nineteenth century. It is interesting that whilst Roozeboom was one of the leaders in the use of ternary phase rule investigation, his work on ammonium tribromide was not pursued in this manner. At the close of the nineteenth century Wells, Penfield and Wheeler began pioneering investigations into the polyhalide field. Much of the work was of a qualitative nature, dealing with the preparation of a particular compound or studying specific physical properties. This work was followed up by Abegg and Hamburger, Foote and Chalker, Parsons and Whittemore and Kremann and Schoulz over the period 1908 - 1912. They were mainly interested in the controversy over the formula of the potassium polyiodide. Most of the preparative methods at this stage involved crystallisation from an aqueous solution. It was not realized that solvation could occur and although analysis often suggested this, conflicting reports were given based on an anhydrous polyhalide. It was not until 1931 when Grace recognized the possibility of solvation that many of these earlier discrepancies were resolved. In the period 1910 - 1930 interest in the polyhalides lapsed to some extent apart from a review of structural considerations by Clark in 1923, the results of which however were inconclusive. The early 1930's saw a revival of interest in both the homogeneous and mixed polyhalides. In England, Grace, Cheesman, Duncan, Harris and coworkers and in America Briggs and coworkers began studying the former type of polyhalide by using phase rule techniques, in particular three component systems, which characterized solvates. At this time Cremer and Duncan reviewed the polyhalide work up to this stage and re examined the preparative methods, properties and reactions of this type of compound. Their main interest was in the mixed polyhalide of the type MIBr\\(_2\\), but they did deal with CsBr\\(_3\\) and CsI\\(_3\\).