Chromium Silicofluoride, Cr₂F₆.(SiF₄)₃

Chromium Silicofluoride, Cr₂F₆.(SiF₄)₃, is obtained in solution when chromium sesquioxide is dissolved in hydrofluosilicic acid.6 It has not yet been isolated, since in aqueous solution it undergoes decomposition; the violet solution spontaneously and gradually becomes bluish green, and silicon tetrafluoride is liberated, thus:

Cr₂F₆.(SiF₄)₃ = SiF₄ + Cr₂F₆.(SiF₄)₂,

the silicon tetrafluoride then being decomposed by water. On evaporation more silicon tetrafluoride is liberated, and a green hydrated fluoride can be isolated, to which Recoura gives the formula (CrF.5H₂O)SiF₆, considering it analogous to the two silicofluorides prepared by Jorgensen, of composition (CrCl.5NH₃)SiF₆ and (CrCl.4NH₃.H₂O)SiF₆.

The freshly prepared violet solution of chromium silicofluoride reacts with potassium chloride, thus:

Cr₂F₆.(SiF₄)₃ + 6KCl = 3K₂SiF₆ + 2CrCl₃;

but if the solution has been allowed to stand, the products are potassium silicofluoride and a compound, (Cr₂F₂)Cl₄, from which the fluorine cannot be precipitated by adding barium chloride.

The hydrated compound (CrF.5H₂O)SiF₆ is stable in dry air or in a vacuum; but in contact with moist air it slowly loses silicon tetra- fluoride, and ultimately yields hydrated chromic fluoride, Cr₂F₆.7H₂O.