Trihalide anions

Fig. 12. 3 Center four electron p —c bonds in symmetrical linear trihalide anions...

These efforts were extended to the CI3 and Brs anions, as well as to the mixed trihalide anions. The reaction of alkali chloride salts with CI2 in argon gave rise to distinctive bands between 200 and 400 cm in both the infrared and Raman spectra (48). These have been assigned to CI3, which showed similar band positions in solution with bulky tetraalkylammonium cations (49). A shift in band position was observed with a change in cation. 

Trihalide anions (I3,1-I-Br, Br-I-Br, Br-I-Cl, etc.) were identified in BEDT-TTF salts by Raman spectroscopy and confirmed by a good agreement with ab OTr/rb-calculated bond lengths and vibrational frequencies <2000JCP7634>. 

Pyridines react easily with halogens and inter-halogens to give crystalline compounds, largely undissociated when dissolved in solvents such as carbon tetrachloride. Structurally they are best formulated as resonance hybrids related to trihalide anions. 1-Fluoropyridinium triflate is also crystalline and serves as an electrophilic fluorinating agent (31.1). ... 

These salts must be distinguished from pyridinium tribromide, obtained by treating pyridine hydrobromide with bromine, which does not contain an iV-halogen bond, but does have a trihalide anion. The stable, crystalline, commercially available salt can be used as a source of molecular bromine, especially where small accurately known quantities are required. 

This laek of diseussion does not mean that hypervalent bonding is uneommon. It oeeurs for nearly all of the p-block elements. " Noble gas compounds, of course, have 3e-4e bonds. The central atom in trihalide anions sueh as li" is hypervalent. Similar 3c-4e interactions occur for halides of group 13-17 elements examples include anions such as AIF, ... 

Figure 19. Free energies of solvation of halide and trihalide anions as a function of inverse ionic radius, in accordance with the Born Model. The arrow indicates an upper limit on the trifluoride solvation energy.

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. 

Table 2 Thermodynamic characteristics of the unsymmetrical trihalide anions a 7 = 0.63—0.70 mol 1-1...

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