Polyhalides trihalides

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. 

All these extended spoke adducts structurally resemble Z -shaped X82-polyhalides (X = I, Br) in which two asymmetric X3 trihalides are bridged by an X2 molecule.15 This further supports the above-mentioned similarity in the chemical nature of asymmetric X3 trihalides and linear E-X-Y systems in C.-T. adducts. 

Rubidium and caesium also form tribromides, and mixed trihalides, particularly of the MIClg type, are known in these the (Cl—I—Cl) ion is linear. The compounds KIBrCl and CsIBrF have also been isolated. All the alkali metals form stable orange-red polyhalides of the type MICI4, containing a square complex ion (p. 127). 

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. 

Not all metals are capable of forming polyhalide salts. Although considerable research remains to be done in this field, it seems at the present time that in order to form a polyhalide salt the cation must have a large radius and a small charge. The best known and the most stable of the metal polyhalogen salts are those of cesium and rubidium. The potassium salts are more difficult to form and are somewhat more unstable in the solid state. The sodium and lithium polyhalides have not been prepared in anhydrous form. Polyvalent cations apparently do not yield salts with the trihalides, f However, salts of magnesium, zinc,... 

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... 

Ion solvation in general has been extensively studied, and polyhalide ions in solution have also been investigated. In fact, the gas-phase bond strengths in the trihalide systems could have been theoretically predicted with reasonable accuracy in the early part of this century. The difference between D(X2-X ) in the gas phase and solution is equal to the difference in the solvation enthalpies of X and [X2 + X ]. The solvation... 

Although polyhalide anions are most frequently studied in aqueous solution, they are in fact more stable in less polar solutions. For example, D(l2-I )=17 kJ/mol in aqueous solution, and 47 kJ/mol in acetone. While halide anions are spherical, the trihalide anions are oblate. The Born model is still reasonably accurate for these systems if a spherical ion approximation is used with either an experimental value of the radius or with the assumption that the volume of X is simply three times the volume of X . However, the model does not work for the pentahalide ions IJ and Br. Thus, application of the Born model to larger anions is an oversimplification. As more data become available, it may be possible to use more sophisticated models of solvation to correlate gas- and solution-phase data. 

Ferric chloride Addition of polyhalides to carbon-carbon double bonds 1,1,3-Trihalides... 

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