Oxyhalides donor numbers

Donor Numbers and Dielectric Constants of Oxyhalide Solvents... 

According to the different donor properties found in this group of solvents a distinction may be made between (a) oxyhalides with low donor numbers including carbonyl chloride, nitrosyl chloride, thionyl chloride, sulphuryl chloride, acetyl and benzoyl halides and (b) oxyhalides with medium donor numbers, namely phosphorus oxychloride, selenium oxychloride and phenyl phosphonic halides, the latter having donor properties approaching those of water or of the ethers. 

Oxyhalides solvents of low donor numbers are good media for the preparation of halide complexes because coordination of acceptor compounds by solvent molecules is not taking place to any significant degree. 

Oxyhalides with Low Donor Numbers Table 49. Formation Constants K of [(C6H5)3C]+[MCln+i]-in Benzoyl Chloride at 20° ... 

In the oxyhalides discussed so far the donor properties at the 0-atoms are too small to be considered in connection with the formation of anionic complexes and their capacities for solute-dissolution are poor. Solvents of higher donor number allow a wider range of solubilities, since more energy is provided by the interactions with acceptors, such as metal ions. Selenium oxychloride , phosphorus oxychloride and phenylphosphonic dichloride are much better donor-solvents than the oxyhalides discussed above. Coordination chemistry in their solutions may be explained in terms of competing reactions between the donor solvent molecules and the competitive ligands towards the acceptor molecules or acceptor ions ... 

Potentiometric and spectrophotometric methods have been used to obtain information about the extent of chloride ion transfer with the formation of chloronium ions and thus about the chloride ion donor strength of metal chlorides in different oxyhalides solvents. The chloride ion donor properties are increased by a solvent of high donor number and decreased by a weak donor solvent, if the chloronium ions are solvated. Thus the chloride ion donor strength of a particular chloride is higher in phenylphosphonic dichloride than in phosphorus oxychloride and is usually non-apparent in benzoyl chloride ... 

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