Polyatomic halide ions

In addition to the common monatomic halide ions, numerous polyatomic species, both cationic and anionic, have been prepared. Many readers will be familiar with the brown triiodide ion, l3, formed from I2 and I ... 

As indicated in Table 5.3d, significant negative enthalpies and entropies of exchange for the exchange of halide ions on quaternary ammonium resins could be interpreted in terms of pure electrostatic interactions. However such a coulombic model invoking solvated ion size or distance of closest approach fails to conform to the observed affinity order I > Br > CI > F . If one includes a larger polyatomic anion such as perchlorate, Cl 04 the experimental sequence is Cl 04 > I > Br"> Cr > F" which clearly contradicts any simple coulombic attraction theory. 

In summary, the empirical approach to ionic solvation based on the MSA is quite successful for monoatomic ions of the main group elements. It helps one to understand the important differences between the way cations and anions are solvated in water. It can also be applied to other ions, including polyatomic ions, provided the solvation is essentially electrostatic in character. Thus, one may estimate effective radii for anions such as nitrate and perchlorate from the Gibbs solvation energy using the value of 8s calculated for the halide ions. Considering the simplicity of the model, it provides an useful means of understanding the thermodynamics of solvation. 

Ligands are most conveniently classified according to the number of potential donor atoms which they contain and are known as uni-, bi-, ter-, quadri-, quinqi- and sexi-dentate accordingly as the number is 1,2, 3, 4,5 or 6. Unidentate ligands may be simple monatomic ions such as halide ions, or polyatomic ions or molecules which contain a donor atom from Groups 16, 15 or even... 

As was mentioned in the preceding section, the infra-red stretching vibration bands of chalcogenoanions appear usually as more or less smeared-out bands. In contrcist, the infra-red spectra of small amounts of polyatomic anions isolated in eilkah halide host lattices have very sharp bands [see results in Refs. 204—209)]. Thus, one can clearly see that the bands are better defined when the anion can be effectively isolated and thereby ehminate the coupling between neighboring identical ions as well as reducing the other effects discussed earlier. More will be said about this in the next section. 

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