Halogens interhalogen cations

In this section, the polyatomic cations of group 17 elements (halogen and interhalogen cations and polycations), cations and polycations of group 16 elements (O, S, Se, and... 

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. 

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). 

There are numerous examples of cations of the interhalogens, and a great deal is known about the behavior of such species. The species that have been more fully studied involve only one type of halogen such as I3+, Br3+, and Cl3+. In general, the production of these species requires rather stringent conditions that may include nonaqueous solvent systems. For example, a reaction that takes place in anhydrous sulfuric acid can be used to produce I3 +. ... 

The triatomic cations are usually prepared from the corresponding interhalogen, by reaction with a halogen acceptor molecule. This is impossible for some of the more exotic species which have no interhalogen precursor. For example, the [Br3]+ cation has been characterized20 in [Br3]+[AsF6], formed from the interaction of arsenic pentafluoride and a bromine/bromine pentafluoride mixture and the [Cl FJ cation results from the interaction of antimony pentafluoride with chlorine monofluoride.21... 

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... 

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]. 

Pseudo-halogens may be used in a similar manner. The pseudo-interhalogens C1N3 or BrN3 react with iron pentacarbonyl to produce a remarkable bridged, binuclear, doubly charged cation, which is paramagnetic. 

The interhalogens, particularly the halogen fluorides, are chemically very reactive materials and hydrolyze easily. The handling and manipulation of these, their derivatives, and cationic or anionic species have to be performed using the safety precautions customary in fluorine chemistry (exclusion of moisture and air, and in vacuum lines constracted from materials that are resistant to fluorine). The properties of the interhalogens are intermediate between those of their parent halogens, and some physical properties are summarized in Table 4. 

Cation formation is not restricted, as normally observed, to association with metallic species. It can also occm among nonmetals and even the most typical ones such as halogens or interhalogens, which are usually more noted for halide and polyhalide anion formation. 

A solid polyhalide salt with the structure M+ tends to dissociate into a simple monohalide derivative of the cation M+ with the most electronegative halogen, and a neutral halogen or interhalogen. 

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