Polyhalide ions

At(0) reacts with halogens X2 to produce interhalogen species AtX, which can be extracted into CCI4, whereas halide ions X yield polyhalide ions AtX2 which are not extracted by CCLt but can be extracted into Pr O. The equilibrium formation constants of the various trihalide ions are intercompared in Table 17.28. 

This type of treatment may be readily extended to the bonding in the hexacoordinated species such as SiFg", PFJ and SFe or to the simple polyhalide ions. 

The polyhalide ions may conveniently be classified into two groups (Xf-type ions belong to both groups) (I) those I hat are isoelectron ic with noble gas compounds and... 

Polyelectronic atom, 20-43 Polyhalide ions, 839-843 Polyhalogen cations, 848 Polymers, and phosphazene polymers, 773-775 Polynuclear carbonyl complexes, 633-639... 

In addition to the polyhalide ions discussed previously, which were all anionic, there are comparable cationic species known, although they have been studied considerably less. Many pure interhalogen compounds arc thought to undergo autoionization (see Chapter 10) with the formation of appropriate cationic species ... 

The coordination number of the halogens in the -1 state is normally only 1, but exceptions are found in HXH+ cations, in polyhalide ions such as FC1F and I3, and when X ions occur as bridging ligands where the coordination number is 2. There are also triply bridging X ions in some metal atom cluster compounds. 

The pseudohalides form ions analogous to polyhalide ions ... 

Two points of view are applicable to these species, as they also are to the isoelectronic noble gas fluorides (I) a valence bond approach with promotion of electrons to orbitals and (2) three-center, four-electron bonds. The same arguments, pro and con. apply as given previously, so they will not be repeated here. Independent of the alternative approaches via VB or MO theory, all are agreed that Madelung energy ( ionic character ) is very important in stabilizing both the polyhalide ions and the polyhalogens.- ... 

Halide ions often react with molecules of halogens or interhalogens and form polyhalide ions. Iodine is only slightly soluble in water. Its solubility is greatly increased if same iodide ions are present in the solution. The increase in solubility is due to the formation of polyhalide ion, in this case triiodide ion I3 This is stable both in aqueous solution and in ionic crystals. 

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. 

Table I. Known Polyhalide Ions in Crystalline Solids...

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