Chloroantimonic vs fluoroantimonic acid

Question

Does chloroantimonic acid exists? What do we get when we mix hydrochloric acid and antimony pentachloride and is the mixture stronger or weaker than fluoriantimonic acid. How about bromo and iodoantimonic acids?

Answer 1

Chlroantimonic acid has been synthesised by dissolving antimony trioxide in concentrated hydrochloric acid followed by saturation with chlorine. The relevant information can be found here[1]:

Complex compounds are known to which the formulae $\ce{HSbCl6}$, $\ce{H2SbCl7 }$ and $\ce{H3SbCl8}$ have been ascribed; these may be regarded as meta-, pyro- and ortho-chlorantimonic acids [...] Neither the ortho- nor the pyro-chlorantimonic acid appears to have been isolated, although salts are known. Meta-chlorantimonic acid has been obtained by dissolving antimony trioxide in concentrated hydrochloric acid and saturating the solution with chlorine. The solution darkens at first, afterwards changing to a bright greenish-yellow colour. Crystals can be obtained by concentrating slightly over a water-bath, adding hydrochloric acid and leaving over sulphuric acid, the temperature not exceeing 0° C. The crystals are very hygroscopic; they are soluble in cold water, alcohol, acetone and glacial acetic acid. The solution in water undergoes hydrolysis, especially on warming, hydrated antimony pentoxide separating out. It is prevented by hydrochloric acid and to some extent by nitric acid. The solutions in organic solvents are more stable. Various salts of chloroantimonic acid are known [...]

Bromoantimonic acid is prepared from crystallization from antimony bromide/bromine and sulfuric acid. From Here[2]:

Of the three possible acids only one has been obtained in the free state. By crystallisation from a solution containing antimony tri-bromide, bromine and either hydrobromic or sulphuric acid the compound meta-bromoantimonic acid, $\ce{HSbBr6.3H2O}$, has been prepared in the form of hygroscopic, irregular, six-sided, black tablets. It decomposes readily, with evolution of bromine, leaving a residue of antimony tribromide.

Pure iodoantimonic acid is not known although a mixed acid, $\ce{HSbBrI3}$ is known. Here[3]:

Salts of a complex antimony iodohydrobromic acid, $\ce{HSbBrI3}$, have been obtained by triturating equimolecular proportions of antimony triiodide and metallic bromide with a non-aqueous substance such as acetic acid or xylene.

Carefully note that in this acid, antimony is in the +3 rather than +5 oxidation state. Iodide ion, more easily oxidized than lighter halide ions, would accordingly be less stable in combination with antimony(V).

References

1. http://antimony.atomistry.com/chloroantimonic_acids.html

2. http://antimony.atomistry.com/antimony_pentabromide.html

3. http://antimony.atomistry.com/antimony_triiodide.html

4. $$\ce{Sb2O5 + 12HCl → 2H[SbCl6] + 5H2O}$$

   http://www.easychem.org/en/subst-ref/?id=3722