Chloroantimonic Acids

Complex compounds are known to which the formulae HSbCl₆, H₂SbCl₇ and H₃SbCl₈ have been ascribed; these may be regarded as meta-, pyro- and ortho-chlorantimonic acids respectively, analogous to the corresponding oxyacids, HSbO₃, H₄Sb₂O₇ and H₃SbO₄, respectively. 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 being allowed to exceed 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. Hydrolysis is prevented, however, by hydrochloric acid, and to some extent by nitric acid. The solutions in organic solvents are more stable. As a precipitate is not formed immediately on adding silver nitrate to a nitric acid solution, it is suggested that the chlorine and antimony combine to form a complex ion. On the grounds of the behaviour of the chromium salts of ortho- and meta-chlorantimonic acids towards hydrogen sulphide and towards silver nitrate, and also on account of the resemblance of these salts to the corresponding hydrated chlorides of chromium, the formulae suggested for the complex chlorides of chromium and antimony are

[Cr(H₂O)₆][SbCl₆]₃ + 7H₂O
and
[Cr(H₂O)₄Cl₂][SbCl₆] + 6H₂O

Salts of all these chlorantimonic acids are prepared in a similar manner; in general, a mixture of the metallic chloride and antimony trichloride (or pentachloride) in dilute hydrochloric acid is saturated with chlorine; crystallisation is effected over sulphuric acid. All the salts are hydrolysed by cold water, with the exception of those of the alkali and alkaline earth metals, and even these yield antimony pentoxide on warming. The presence of hydrochloric acid renders the solutions more stable.

Salts which have been obtained are given in the following table, together with their crystallographic characteristics.

(A) Salts of ortho-chlorantimonic acid.
CrSbCl8.10H2O	Grey, hygroscopic plates.
FeSbCl8.8H2O	Yellow, hygroscopic, four-sided tablets, tetragonal system (a:c = 1:1.0112).

(B) Salts of pyro-chlorantimonic acid.
MgSbCl7.9H2O	Greenish-yellow, hygroscopic tablets; triclinic system (a:b:c = 0.7144:1:2.595; α = 100°22', β = 88°3', γ = 91°16').

(C) Salts of meta-chlorantimonic acid.
LiSbCl6.4H2O	Square, hygroscopic tablets.
KSbCl6.H2O	Greenish-yellow, six-sided, hexagonal plates; rhombic system (a:b:c = 0.8889:1:0.7794).
RbSbCl6	Thin, six-sided, yellowish-green tablets, rhombic system (a:b:c = 0.6719:1:0.8136).
NH4SbCl6.H2O	Isomorphous with the potassium salt (a:b:c = 0.8909:1:0.8136).
Be(SbCl6)2.10H2O	Small, yellowish, hygroscopic needles
Ca(SbCl6)2.9H2O	Long, hygroscopic needles.
Zn(SbCl6)2.5H2O	Unstable
Al(SbCl6)3.15H2O	Greenish-yellow, hygroscopic needles
Cr(SbCl6)3.13H2O	Grey-violet, flat, hygroscopic needles.

The following complex compounds have also been prepared, their properties determined and their formulae discussed: NaSbCl₆.3H₂O.NaCl; Cu(SbCl₆)₂.3H₂O.HSbCl₆.5H₂O; Cd(SbCl₆)₂.12CdCl₂.2H₂O; SbCl₅.C₅H₅N.HCl; SbCl₅.C₉H₇N.HCl.

The following compounds with ammonia have also been obtained, in addition to a number of compounds with organic bases: Cu(SbCl₆)₂.5NH₃; AgSbCl₆.2NH₃; Zn(SbCl₆)₂.4NH₃; Cd(SbCl₆)₂.7NH₃; HSbCl₆.2NH₃.