Niobium Oxytrichloride, NbOCl₃

Niobium Oxytrichloride, NbOCl₃, is of historical interest in that it was one of the oxy-halogen compounds prepared originally by Rose in which the oxygen atom was overlooked. Rose called this substance "hyponiobium chloride"; its correct formula, and hence the true valency of niobium, were first established by Blomstrand.

Niobium oxytrichloride is prepared by the action of chlorine on a mixture of niobium pentoxide and carbon, or on the lower oxide, NbO₂, or by the action of the vapours of carbon tetrachloride on the pentoxide. Some niobium pentachloride is produced in the same reaction, and this is removed either by distilling it away at the lowest temperature possible in an atmosphere of carbon dioxide, or by subliming the product in a current of chlorine over the ignited oxide. The oxychloride has also been prepared by passing the vapours of niobium pentachloride over niobium pentoxide at a red heat.

Niobium oxytrichloride forms white, silky crystals, which sublime to silky needles on being heated in a sealed tube. It is very stable. It can be volatilised at 400° C. without melting; its vapour density at 440° C. is 7.89, and at 860° C. 7.87 (air = 1. NbOCl₃ requires 7.47). Its vapour is colourless. On being gently heated in hydrogen, carbon dioxide, or chlorine, it sublimes unchanged; with very strong heating in hydrogen or carbon dioxide, however, it decomposes and forms niobium pentoxide and niobium pentachloride (hydrogen also produces some of the lower oxides). It is soluble in alcohol and ether. Like the pentachloride it undergoes ready hydrolysis with water, producing niobic acid and hydrochloric acid; it is attacked by damp air. It dissolves in alkalis, also in concentrated sulphuric acid, to form a clear liquid with evolution of hydrochloric acid gaS> but it is only very sparingly soluble in concentrated hydrochloric acid.

Niobium oxytrichloride gives rise to two series of double salts with alkali chlorides and with the organic bases pyridine and quinoline. These have the general formulae NbOCl₃.RCl and NbOCl₃.2RCl. They are prepared either by addition of the alkali chloride or organic base to a solution of niobic acid in concentrated hydrochloric acid, or by the action of the alkali chloride or organic base on niobium oxytrichloride in alcohol solution. They crystallise well, are stable in dry air but are decomposed by water. The following are known:

NbOCl₃.2NH₄Cl Very unstable.
NbOCl₃.2RbCl Yellow, regular octahedra.
NbOCl₃.2CsCl Yellow, regular octahedra.
NbOCl₃.C₅H₅N.HCl Pale greenish-yellow crystals.
NbOCl₃.C₅H₅N.HCl.H₂O. Colourless crystals.
NbOCl₃.2C₅H₅N.HCl.H₂O. Large, colourless, transparent prisms.
NbOCl₃.C₉H₇N.HCl Almost colourless needles or greenish-yellow truncated prisms.
NbOCl₃.2C₉H₇N.HCl Almost colourless needles.

The corresponding bromine compounds are very comparable, but attempts to prepare corresponding iodo-derivatives were unsuccessful owing to the insolubility of niobic acid in hydriodic acid. Double compounds of vanadium oxytrichloride with pyridine, and tantalum oxytrichloride with pyridine and quinoline, have also been prepared.