Niobium and Chlorine, Nb₆Cl₁₄

Chloroniobium Chloride or niobium "dichloride", Nb₆Cl₁₄.7H₂O or [Nb₆Cl₁₂]Cl₂.7H₂O. - This compound is prepared by heating 1 part of niobium pentachloride with 7 parts of 3 per cent, sodium amalgam in a hard-glass combustion tube from which the air has previously been exhausted. The product is allowed to cool in a vacuum, is extracted with hot water, filtered from a brown oxide formed at the same time, and then evaporated with the addition of hydrochloric acid. It forms black, shining crystals, which give an olive-green powder, soluble in hot water. It is only slowly decomposed by ammonium hydroxide; concentrated boiling nitric acid precipitates niobic acid. Only two of the chlorine atoms are ionised in solution. On being treated with the equivalent quantity of caustic soda it yields chloro- niobium hydroxide, [Nb₆Cl₁₂](OH)₂.8H₂O, as a black, micro-crystalline precipitate, which in turn forms chloroniobium bromide, Nb₆Cl₁₂Br₂. 7H₂O, when evaporated with hydrobromic acid. Chloroniobium chloride and chloroniobium hydroxide are both soluble in excess of caustic soda, from which solution the addition of excess of concentrated hydrochloric acid precipitates a brown powder having the composition Nb₆Cl₁₄.9H₂O. This substance is less soluble than the olive-green compound into which it is converted by boiling water. The available evidence suggests that the brown compound differs constitutionally from the green compound; their behaviour recalls the different forms in which chromium trichloride is known to exist.