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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
[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.
CrSbCl8.10H2OGrey, hygroscopic plates.
FeSbCl8.8H2OYellow, hygroscopic, four-sided tablets, tetragonal system (a:c = 1:1.0112).

(B) Salts of pyro-chlorantimonic acid.
MgSbCl7.9H2OGreenish-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.
LiSbCl6.4H2OSquare, hygroscopic tablets.
KSbCl6.H2OGreenish-yellow, six-sided, hexagonal plates; rhombic system (a:b:c = 0.8889:1:0.7794).
RbSbCl6Thin, six-sided, yellowish-green tablets, rhombic system (a:b:c = 0.6719:1:0.8136).
NH4SbCl6.H2OIsomorphous with the potassium salt (a:b:c = 0.8909:1:0.8136).
Be(SbCl6)2.10H2OSmall, yellowish, hygroscopic needles
Ca(SbCl6)2.9H2OLong, hygroscopic needles.
Al(SbCl6)3.15H2OGreenish-yellow, hygroscopic needles
Cr(SbCl6)3.13H2OGrey-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.

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