Niobium Monoxide, NbO

Niobium Monoxide, NbO, is of considerable interest as a compound of divalent niobium, but its existence is doubtful. Attempts to prepare it by reduction of the pentoxide with hydrogen at high temperatures and in the carbon electric vacuum furnace have not been successful. It is stated to be formed together with the dioxide, NbO₂, by treating the pentoxide with hydrogen at about 150 atmospheres pressure and 2500° C. Rose obtained it by reducing potassium niobium oxyfluoride, K₂NbOF₅, with sodium in an iron crucible and then removing the fluorides of sodium and potassium in the product with water. Both Rose and Hermann overlooked the presence of the oxygen atom, and looked upon this substance as being metallic niobium. A study of the amount of oxygen absorbed in its conversion into the pentoxide showed, however, that it was more probably the oxide NbO. This conclusion is supported by the following facts: (1) Niobium monoxide is stated to be prepared also by reducing niobium oxytrichloride, NbOCl₃, or niobium oxytrifluoride, NbOF₃, with magnesium; (2) it is converted into niobium oxytrichloride, NbOCl₃, on being heated in a stream of chlorine,

2NbO + 3Cl₂ = 2NbOCl₃;

(3) it is converted into niobium oxytrichloride on being dissolved in hydrochloric acid,

2NbO + 6HCl = 2NbOCl₃ + 3H₂.

The monoxide has been variously described, according to the method of preparation, as a black powder, or glistening crystals, or beautiful black cubes. Its density varies from 6.3 to 6.7, and it is a good conductor of electricity. On being heated in air it oxidises readily to the pentoxide with considerable evolution of heat. It dissolves in hydrofluoric acid and hydrochloric acid with evolution of hydrogen, and a pentavalent niobium salt is formed in solution. It is also dissolved by boiling potassium hydroxide to form potassium niobate. This behaviour of niobium monoxide towards acids and alkali indicates that divalent salts of niobium are too unstable to exist. A divalent chloride of tantalum has, however, recently been isolated.