Polyhalogen complex salts

Polyhalogen complex salts can be defined as the addition products of halides with one or more molecules of a halogen or of an interhalogen compound. They contain, therefore, 

The list of existing polyhalogen anions is rather impressive since, except for fluorine, nearly every combination of halogens is possible. Fluorine does not readily form polyhalogen complexes, and when it does, both the methods of preparation and the properties of the resulting complexes lie somewhat apart from the other poly halides. This discussion is limited primarily to the complexes of chlorine, bromine, and iodine. 

Since the polyhalide ions are formed by the addition of diatomic halogen molecules or of interhalogen molecules to a halide ion, the total number of halogen atoms in such a complex is always odd. The most common case is that of the trihalides. Some penta-, hepta-, and enneahalides are likewise known. The highest known polyhalogen complex is PCUBr 1 the structure of this has not been determined, but by analogy to other similar complexes it can probably be expressed as [PClsBr]+[Bri7]. Table I lists the known polyhalide ions. 

Not all metals are capable of forming polyhalide salts. Although considerable research remains to be done in this field, it seems at the present time that in order to form a polyhalide salt the cation must have a large radius and a small charge. The best known and the most stable of the metal polyhalogen salts are those of cesium and rubidium. The potassium salts are more difficult to form and are somewhat more unstable in the solid state. The sodium and lithium polyhalides have not been prepared in anhydrous form. Polyvalent cations apparently do not yield salts with the trihalides, f However, salts of magnesium, zinc, 

I5- i2ci- IC12- IjBr- IBrr BrClr Br2Cl- IBrCl- ICU- BrI7-( ) 

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Popov AI, Buckles RE. Polyhalogen complex salts. In Inorganic Syntheses. Vol.5. New York, NY. McGraw-Hill Book Co., Inc., 1957. p 167-175. 

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