Interhalogen compounds cations

C21-0093. Some pure liquid interhalogen compounds are good electrical conductors, indicating that they contain cations and anions. Show a Lewis acid-base reaction between two bromine trifluoride molecules that would generate ionic species. 

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 ... 

In general, addition of more polar interhalogen compounds (ICI, IF, BrF, ClF) to fluoroolefins proceeds as an ionic process, starting with attack of halogen bearing positive charge on the C=C bond and formation of a cyclic halonium cation (Eq. 2) [10,18]. 

The halogens form many compounds and ions that are binary or ternary combinations of halogen atoms. There are three basic types (a) neutral interhalogen compounds, (b) interhalogen cations, and (c) interhalogen anions. 

One of the focal points of this work was the influence of traces of moisture on the cationic polymerization of epoxides. Using the previously described iodonium salt as an example, it was shown that water not only played an important role as co-catalyst in the photochemical formation of the superacid, but it also caused the hydrolysis of the iodonium salt via a nucleophihc attack at the T center. This reaction was mainly investigated by X-ray photoelectron spectroscopy (XPS). Interestingly, this hydrolysis only occurs with the hexafluoroantimonate salt and not with the corresponding chloride. Here it should be mentioned that the chloride salt resembles an interhalogen compound and the hexafluoroantimonate salt forms a real ion pair. Hydrolysis of the hexafluoroantimonate anion of the iodonium salt was not observed, but slow hydrolysis was observed for sodium hexafluoroantimonate. The hydrolysis is so slow overall that the influence of this effect can be excluded from the discussions into the influences of water and alcohol on the cationic polymerization of epoxides. 

Some interhalogen compounds form as well, and they form an extensive array of both neutral and cationic compounds with each other. These can be diatomic (CIF, BrF, IF, BrCl, ICl, and so on), tetratomic (CIF3, BrFj, IF3, and so on), hexatomic (CIF5, BrF, IF5, and so on), and even octatomic in the case of iodine heptaflouride (IF ). 

Numerous interhalogen compounds, as well as cations and anions, are known. Several engage in halide/fluoride ion transfer reactions. 

The interhalogen compound BrFsfZ) reacts with antimo-ny(V) fluoride to form the salt (BrF2 (SbF6 ). Write the Lewis structure for both the cation and anion in this substance, and describe the likely structure of each. 

The halogen pentafluorides and the [XeF5]+ cation have a square pyramidal configuration and any weak secondary bonds are found below the base of the pyramid and situated to avoid the axial, lone pair position. These contacts are much more significant for the xenon compounds than for the interhalogens, where they are so weak as to be virtually indistinguishable from normal intermolecular contacts, as seen in the structure of IF5.40... 

On the basis of self-ionic dissociation, these compounds can be prepared by acid-base reactions. Heteropolyhalogen cations are usually prepared by reacting the parent compound with a Lewis acid (equation 51) in which XY = interhalogen and MYm = Lewis acid, for example, hahdes of B, Al, P, As, and Sb, and so on (equations 52 and 53). Such reactions can be performed by direct interaction of the reactants with an excess of the more volatile reactant, which can then be pumped off, after completion of the reaction, leaving behind the pure product. Sometimes it is preferable to perform such reactions in solution, such as in anhydrous hydrogen fluoride (AHF), and pump off the solvent at the end of the reaction. 

Boron trifluoride forms addition compounds that incorporate an sp hybridized boron into a tetravalent structure. Salts of BF4 are readily formed with BF3 and a suitable fluoride donor. Halogen fluorides such as chlorine trifluoride react with BF3 to generate interhalogen cations such as [C1F2]+[BF4]. Some further examples are shown in equations (43) and (44). In an organic application, the Schiemann reaction provides an entry into fluorinated aromatics by thermal decomposition of a diazonimn tetrafluoroborate (equation 45). 

The XY2 cations can be considered as derivatives of the XY3 interhalogen parent compounds. No IQ3, however, is known but only its dimer, I2CI6, forming the ICI2 cation. Hie supposed parent compound of IBr2, namely IBrs, has not been isolated or identified. 

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