Tungsten Dichloride

By careful reduction of tungsten hexa chloride in hydrogen, or of the tetrachloride in carbon dioxide, there is obtained a grey, amorphous, non-volatile, though unstable mass, which appears to contain tungsten dichloride. The hexachloride is more readily reduced by ignition in a stream of nitrogen with powdered aluminium and quartz; on extracting the product with hydrochloric acid, and saturating the concentrated extract with hydrogen chloride, slender yellow needles, of composition H[W₃Cl₇.H₂O]aq., separate. This compound is less stable than the corresponding molybdenum compound, and rapidly loses hydrogen chloride on exposure to air. In alcoholic solution or on gentle heating, 1 molecule of hydrogen chloride is lost and an insoluble yellow residue of composition [W₃Cl₆.H₂O] remains. With concentrated sulphuric acid the compound W₃Cl₄(SO₄) is formed. A little water hydrolyses it to the compound [W₃Cl₄. 2H₂O]Cl₂; much water or alkali hydroxides convert it completely to tungstic acid or tungstate. When the chloracid is dissolved in hot concentrated hydrobromic acid and the solution cooled, thin yellow leaflets, of composition H[W₃Br₄Cl₃.H₂O].9H₂O, separate.