Charge transfer halides

In this state, chromium compounds are usually coloured yellow or red (but due to charge transfer (p. 60) and not to the presence of d electrons on the chromium ion). The only halide known is the... 

As well as the cr-complexes discussed above, aromatic molecules combine with such compounds as quinones, polynitro-aromatics and tetra-cyanoethylene to give more loosely bound structures called charge-transfer complexes. Closely related to these, but usually known as Tt-complexes, are the associations formed by aromatic compounds and halogens, hydrogen halides, silver ions and other electrophiles. 

Opinions differ on the nature of the metal-adsorbed anion bond for specific adsorption. In all probability, a covalent bond similar to that formed in salts of the given ion with the cation of the electrode metal is not formed. The behaviour of sulphide ions on an ideal polarized mercury electrode provides evidence for this conclusion. Sulphide ions are adsorbed far more strongly than halide ions. The electrocapillary quantities (interfacial tension, differential capacity) change discontinuously at the potential at which HgS is formed. Thus, the bond of specifically adsorbed sulphide to mercury is different in nature from that in the HgS salt. Some authors have suggested that specific adsorption is a result of partial charge transfer between the adsorbed ions and the electrode. 

The adsorption strength of anions on Au electrodes follows the sequence of C104 < S042- < Cl < Br < I. The strong specific adsorption of halide ions leads to a partial charge transfer between the adsorbate and the metal electrode [234]. 

It is, of course, possible that the charge-transfer complex between metal halide and olefin, which is well known, is an intermediary in this reaction. There is here another variation on the theme of direct initiation. The thermochemical analysis, analogous to the previous ones, goes as follows ... 

In this state, chromium compounds are usually coloured yellow or red (but due to charge transfer (p. 60) and not to the presence of d electrons on the chromium ion). The only halide known is the unstable chromium(VI) fluoride CrF6, a yellow solid. However, oxide halides are known, for example Cr02Cl2 ( chromyl chloride ), formed as a red vapour when concentrated sulphuric acid is added to a chromate(VI) (or dichromate) mixed with a chloride ... 

Finally in this section, we note that the use of perturbation theory, particularly for quasi-resonant charge transfer, has been developed by Battaglia, George and Lanaro They show that first-order perturbation theory is satisfactory for high-velocity atoms, with Eq lying outside the solid band, and they have examined in detail protons scattered from alkali-halide... 

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