Polyatomic Cations of Group VII

The existence of Is and Ig was deduced over 30 years ago by Masson (I) from his studies of aromatic iodination reactions, but it is only recently that his conclusions have been confirmed by physical measurements. The controversy over the nature of the blue solutions of iodine in various highly acidic media has now been resolved, and it has been shown conclusively that these solutions contain Ig (2-4) and not I+ as suggested earlier (5). There is, moreover, no convincing evidence for the existence of Cl or Br as stable species in solution or in the solid state. There is, however, evidence for polyatomic cations of chlorine and bromine analogous to the iodine cations, i.e., Clg , Brg, and Brg.  

The first evidence for the existence of a stable iodine cation was obtained by Masson (2) in 1938. He postulated the presence of Ig and Ig in solutions of iodine and iodic acid in sulfuric acid in order to explain the stoichiometry of the reaction of such solutions with chlorobenzene to form both iodo and iodoso derivatives. Later, Symons and co-workers (6) gave conductometric evidence for I3 formed from iodic acid and iodine in 100% sulfuric acid and suggested that Ig may be formed on the basis of changes in the UV and visible spectra when iodine is added to la solutions. Gillespie and co-workers (7) on the basis of detailed conductometric and cryoscopic measurements confirmed that Ig is formed from HlOg and Ig in 100% sulfuric acid according to Eq. (1). The Ig cation may also be prepared in fluoro- 

Solutions of Ig in 100% HgSO (7), or in fluorosulfuric acid (2), dissolve at least 1 mole of iodine per mole of Ig +, and a new absorption 

In 1906, Ruff (9) reported that excess iodine and SbFg react to form a brown solid which he formulated as SbFgl. Kemmitt et al. (4) have since shown from the absorption spectrum of the solid in liquid AsFg that it contains some I3 cation. However, it must be concluded from the method of preparation that this material is not a single compound, and almost certainly contains Sb(III). 

Gillespie and Milne (2) have shown, by conductometric, spectro-photometric, and magnetic susceptibility measurements in fluorosulfuric acid, that the blue iodine species observed in strong acids is la. When iodine was oxidized by peroxodisulfuryl difluoride in fluorosulfuric acid, the concentration of the blue iodine species reached a maximum at the 2 1 Ij/SgOeFa mole ratio [Eq. (4)] and not at the 1 1 mole ratio as would 

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