Tantalum Pentachloride, TaCl₅

Tantalum Pentachloride, TaCl₅, is the best known of the chlorides of tantalum. It has been prepared by several methods:

1. By the action of chlorine or carbonyl chloride on heated metallic tantalum. The pentachloride can be sublimed away.
2. By the action of carbon tetrachloride, carbon tetrachloride and chlorine, sulphur monochloride and chlorine, or phosphorus pentachloride on tantalum pentoxide.
3. By the action of chlorine on a heated mixture of tantalum pentoxide and sugar charcoal.

Tantalum pentachloride is usually obtained as a yellow powder which forms a mass of white crystals on being melted or sublimed. Its melting-point is variously reported as 211.3° C., 221° C., and between 230° and 240° C., and its boiling-point as 241.6° and 233° C. Its density at 17° C. is 3.68, and its vapour density at 360° C. is 12.8 (air = 1); TaCl₅ requires 12-5, so that decomposition does not take place at this temperature. The electrical conductivity in the fused state is 0.30×10^-6 reciprocal ohms (the corresponding figure for copper at ordinary temperatures is 64×10⁴), so that molten tantalum pentachloride is an insulator of the order of the best conductivity water. It is quite stable in dry air, and sublimes unchanged in chlorine or in carbon dioxide. When brought into contact with water it emits a hissing noise, and decomposes into tantalic acid and hydrochloric acid, with evolution of heat. It is only sparingly soluble in hot concentrated hydrochloric acid, but with addition of water it dissolves completely to an opalescent solution which does not throw down a precipitate even on being boiled. Concentrated hydrochloric acid attacks it, hydrogen chloride fumes being evolved, but the tantalic acid formed at the same time is dissolved; the solution becomes cloudy on being boiled, and throws down a gelatinous precipitate of tantalic acid on being cooled. Tantalum pentachloride is soluble in cold alcohol, chloroform, carbon tetrachloride and carbon bisulphide; solutions in these solvents yield crystalline addition compounds when treated with organic compounds. The following addition compounds with pyridine and piperidine have been isolated: TaCl₅.2C₅H₅N; TaCl₅.6C₅H₅N.2C₂H₅OH. Rose was unable to obtain double chlorides of tantalum pentachloride and the alkali chlorides which would correspond to the double fluorides.