Niobium Pentachloride, NbCl5

Niobium Pentachloride, NbCl₅, is the most important of the chlorides of niobium, and is the material from which the other chlorides are prepared. It can be obtained by several methods:

1. By heating metallic niobium in chlorine at a red heat.
2. A mixture of niobium pentoxide and carbon is gently heated in a stream of chlorine, air being excluded. The pentachloride is sublimed away from any oxychloride simultaneously produced by raising the temperature at the conclusion of the reaction.
3. By the action of the vapour of carbon tetrachloride on niobium pentoxide heated to 230° to 250° C. Some of the oxychloride is formed at the same time, but the change to the pentachloride is complete if the heating is carried out in a sealed tube at 200° to 225° C. The same change takes place if the niobium pentoxide is subjected to the action of a mixture of chlorine and carbon tetrachloride vapour or chlorine and sulphur dichloride, or phosphorus pentachloride, or sulphur monochloride. In the last case the pentachloride is removed from excess of sulphur monochloride by distillation.
4. By carefully heating niobium sulphide in chlorine free from oxygen.

Niobium pentachloride crystallises in long, yellow, transparent needles. Density 2.73 to 2.77. The specific electrical conductivity of the fused substance between 220° and 235° C. is 0.22×10^-6 reciprocal ohms (the corresponding figure for copper at ordinary temperatures is 64×10⁴), so that fused niobium pentachloride is an insulator of the order of the best conductivity water. It melts at 212° C. to a red liquid, which volatilises readily and boils at 240.5° C. Its vapour is yellow. Its density between 280° and 300° C. = 9.45 (air = 1. NbCl₅ requires 9.35). The chloride sublimes unchanged in the vapour of carbon bisulphide, and dissolves readily in sulphur monochloride to a red solution and in carbon tetrachloride to a yellow solution; both these solvents yield crystals on being concentrated. It is also soluble in chloroform, alcohol, and ether; a nitride of niobium is precipitated when ammonia is passed into the ether solution. Niobium pentachloride undergoes ready hydrolysis; on being exposed to damp air it evolves hydrochloric acid fumes and becomes coated with a skin of niobic acid. It is completely decomposed by water to hydrochloric acid and niobic acid, which latter can be obtained pure by repeated washing with water. It dissolves in concentrated hydrochloric acid to produce a clear solution, and in concentrated sulphuric acid with evolution of hydrochloric acid gas.

Double chlorides of niobium pentachloride

Double chlorides of niobium pentachloride with chlorides of other metals to correspond with the double fluorides given by niobium pentafluoride are not known. A double compound with piperidine, NbCl₅.6C₅H₁₁N, has been prepared, however, by addition of the organic base to a concentrated alcoholic solution of the pentachloride. It forms white needles which undergo decomposition on attempts to recrystallise. Similar compounds are formed with aniline, pyridine, etc.