Halogens 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. 

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. 

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. 

In the reaction of halogen or interhalogen with a solid polyhalide, the reaction product is usually the most stable of the possible polyhalide ions. For example, chlorine will react with the dibromoiodate(I) ion to give dichloroio-date(I) ion,... 

The halogens form compounds that are binary and ternary combinations of themselves. With the exception of BrCl, IC1, IC13 and IBr, the compounds are all halogen fluorides (Table 16-6) (below) such as C1F, BrF3, IF5 and IF7. Ternary compounds occur only as polyhalide ions, the principal types of which are listed in Tables 16-7 and 16-8 (page 485). 

A few ternary compounds, such as IFCI2 and IF2CI, are also known. As can be seen from the formulas, the neutral interhalogen molecules always contain an even number of halogen atoms. A variety of polyhalide ions have also been identified. Many contain an odd number of halogen atoms. 

In aqueous solution, astatine is usually found in the free (0) state. However, unlike the other halogens, astatine(O) is not diatomic. It can be volatilized from solution and extracted (like iodine) into organic solvents such as benzene and carbon tetrachloride. Coextraction with halogens, X2, into chloroform produces interhalogen compounds, AtX (X = Cl, Br, I). Reaction with halide ion, X , yields polyhalide ions, AtX2 (X = Br, I) and AtIBr , which can be extracted into isopropylether. 

The triiodide ion. Is , is one of a group of species called polyhalide ions that are produced by the reaction of a halide ion with a halogen molecule. In the reaction below, the F ion acts as a Lewis base (an electron-pair donor) and the I2 molecule as a Lewis acid (an electron-pair acceptor). 

In a polyhalide ion two or more halogen atoms are covalently bonded into a polyatomic anion, e.g., 13 . 

Compounds, such as the polyhalides and polysulphides, formally also the azides, could be regarded as produced by the attachment of one or more neutral non-polar halogen, sulphur or nitrogen molecules to a halogen, sulphide or nitride ion. With the last one this way of representation is definitely incorrect the azide-ion N3 is linear in contrast to the triangular structure to be expected for ionic bonding. 

The types of ion fragments produced by the decomposition of organic halogen compounds in the mass spectrometer have been summarised by McLafferty - . Brief reviews have also been given by Beynon and by Budzikiewicz et al . These authors have also summarised the results of other workers, such as the early studies on the monohalides by Stevenson and Hippie and by Dibeler and Reese (ref. 146) and on the polyhalides by Bernstein et by McDowell et al and by Dibeler et Electron impact studies on aromatic halogen compounds have been reported by Majer and Patrick . The interpretations of the mass spectra of halogen compounds have been provided in some detail by McLaffierty , and the main features only of these spectra are discussed very briefly here. 

Structure of I5- Ion The structure of pentaiodide in (CH3)4NI5 is quite different from those of other polyhalides containing five halogens. It is planar and V-shaped. 

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