The Polyhalides

From the trend in acidities of the hydrogen halides in water, it follows that fluoride is the most basic or nucleophilic of the halides and iodide the least basic if the hydrogen ion is considered the reference acid. It should be recalled (p. 169) that this order of halide basicities is the same as that toward small, multicharged ions with rare-gas structures (for example, Be2+, A 3, and Si4+). A different, and sometimes reversed, order of basicities or nucleophilicities is observed toward certain ions of the post-transition metals (for example, Cu+, Hg +). For a number of ions (for example, Be+2, B+3 and Ta+6), fluoride complexes may exist in aqueous solution, whereas the other halo-complexes do not. Only a few of the elements having positive valence states form no halo-complexes the most important of these are carbon, the rare earths, the alkali metals, and the heavier alkaline-earth metals. 

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Cl-I-Br] the I-Cl distance is greater than the I-Br distance, and in [Br-I-I] I-Br is greater than I-I. On dissociation, the polyhalide yields the solid monohalide corresponding to the smaller of the halogens present, e.g. CsIClj gives CsCl and ICl rather than Csl + Clj. Likewise for CsIBrCl the favoured products are CsCl(s) + IBr(g) rather than CsBr(s) + ICl(g) or Csl(s) + BrCl(g). Thermochemical cycles have been developed to interpret these results. 

The propensity for iodine to catenate is well illustrated by the numerous polyiodides which crystallize from solutions containing iodide ions and iodine. The symmetrical and unsymmetrical 13 ions (Table 17.15) have already been mentioned as have the I5- and anions and the extended networks of stoichiometry (Fig. 17.12). The stoichiometry of the crystals and the detailed geometry of the polyhalide depend sensitively on the relative concentrations of the components and the nature of the cation. For example, the linear ion may have the following dimensions ... 

Several other molecules have been studied in this way but none of them have the simplicity of the polyhalides just discussed and, even worse, n. q. r. measurements at all the sites are not available. Examples are ClFs (59), PClg 8) SbClg (8), PCl F5-n 60. 61), SeCU 62), TeCl2 63), ClSFs 64), BrSFa 65) but the incompleteness of the data at the present stage makes discussion rather pointless. 

Two points of view are applicable to these species, as they also are to the isoelectronic noble gas fluorides (1) a valence bond approach with promotion of electrons to d 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 die alternative approaches via VB or MO theory, all are agreed that Madelung energy ( ionic character ) is very important in stabilizing both the polyhalide tons and the polyhalogens.27... 

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... 

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 X3 and X X2 species are linear. We shall explore the characteristics and behavior of the polyhalide species in greater detail in the next section. 

Ion-non-polar molecule ( 11). The polyhalides such as KI3 belong to this group, at least formally. The bonding can here also be based on electrostatic polarization and on atomic bonding. 

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. 

Van Arkel and De Boer originally considered the polyhalides, suchasl 3, ICl 2etc., as complexes consisting of an ion and a molecule. These complexes are stable, especially in... 

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. 

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. 

The gaseous halogens or interhalogens can add directly to a solid halide to form a corresponding complex salt provided that the temperature of the reaction is below the decomposition temperature of the polyhalide. The reaction usually yields a trihalide, but in the case of the addition... 

One of the most comprehensive studies on the preparation of polyhalogen complexes in solutions has been made by Chattaway and Hoyle.1 These authors have prepared principally the tetraalkylammonium salts of the polyhalide ions. 

The choice of an appropriate solvent as the reaction medium is of primary importance and usually involves a compromise among various unfavorable factors. Because of the high chemical reactivity of the halogens and of the polyhalides, solvents which are easily susceptible to halo-genation or which effect solvolysis reactions are to be avoided. This, of course, precludes the use of aqueous solutions for many syntheses, with the exception of those of some polyiodides since iodine is fairly unreactive with water. 

The stability of the solid polyhalide depends on numerous factors, among them the size of the cation, the size and the nature of the polyhalide ion, and the chemical resistance of the compound to atmospheric moisture. The most stable salts are formed when the ionic sizes of the cation and the anion are similar. In the alkali metal series the stability of the polyhalides decreases in the order Cs > Rb > NH4 > K > Na, which corresponds to the order of decrease in cationic size. Not enough work has, as yet, been done on the substituted onium poly halides to allow any convincing generalizations. 

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