Tantalum Dichloride, TaCl₂

Tantalum Dichloride, TaCl₂, is stated to be obtained by reducing the pentachloride with aluminium in the presence of aluminium chloride; the product is heated to 600° C. so as to volatilise the aluminium salt, and the residue is extracted with cold water. The trichloride dissolves readily and leaves the dichloride as a dark olive-green powder, which oxidises on exposure to air and is converted into the pentoxide on being heated. The dichloride dissolves in warm water with evolution of hydrogen to form an oxychloride of trivalent tantalum. Cold dilute caustic soda solution dissolves it without evolution of hydrogen or alteration in valency, but on being warmed, the solution oxidises readily, evolves hydrogen, and precipitates an unstable brown lower oxide which is also thrown down by the addition of ammonia to the aqueous solution of the dichloride, or by the action of nitric acid or other strong oxidising agent on the acid solution. The composition of the green powder and the existence of tantalum dichloride have been rendered doubtful by the investigations of Lindner and Feit, who reduced tantalum pentachloride with lead powder at 600° C. in an atmosphere of nitrogen, and extracted the product with dilute hydrochloric acid, from which, after removal of lead with hydrogen sulphide, six-sided, dark green crystals of a chloro-acid having the composition HTa₃Cl₇.4H₂O were obtained. This compound loses only three of its water molecules at 205° C., and at slightly higher temperatures undergoes decomposition with evolution of hydrochloric acid. It contains a complex ion the chlorine atoms of which scarcely dissociate in alcohol solution; in water, dissociation is followed by hydrolysis, with the splitting off of two molecules of hydrochloric acid. The co-ordinated formula is H[Ta₃Cl₇.H₂O].3H₂O, analogous to the formulae for the corresponding molybdenum and tungsten chloro-acids. The formula was previously written both as TaCl₂.2H₂O and as (Ta₆Cl₁₂)Cl₂.7H₂O.

A study of the reactions of this chloro-acid of tantalum shows that in acid solution only one of the seven chlorine atoms is dissociated, and a number of compounds have been prepared which are derived from the chloro-acid by substitution either of this differently held chlorine atom or of the co-ordinated water:

1. A monobromochloro-acid, H[Ta₃Cl₆.Br.H₂O].3H₂O, is obtained as green crystals by the action of hydrobromic acid on tantalum pentachloride.
2. A sulphatochloro-acid, H₂[Ta₃Cl₆.SO₄].H₂O, is obtained by treating tantalum pentachloride with dilute sulphuric acid.
3. Evaporation of the alcohol solution of the chloro-acid yields a brown resinous mass of composition H[Ta₃Cl₆.Cl.C₂H₅OH].C₂H₅OH.
4. With pyridine and hydrochloric acid in aqueous or alcoholic solutions, deep green, crystalline compounds are produced: C₅H₅N. H[Ta₃Cl₆.Cl.C₅H₅N]; C₅H₅N.H[Ta₃Cl₆.Br.C₅H₅N]. All these compounds have the co-ordination number 8, and all of them contain the group (Ta₃Cl₆) as part of the complex anion, which is probably a derivative of the complex anion [Ta₃Cl₈]''. The acid of composition H₂Ta₃Cl₈ is unknown, but a green, crystalline pyridinium salt, (C₅H₅N₂)H₂[Ta₃Cl₈).3H₂O, has been prepared. Other pyridinium derivatives in which the co-ordination number is 7 or 9 are known. On being treated with aqueous caustic potash, the chloro-acid loses two of its chlorine atoms and yields a dark brown, amorphous precipitate of composition [Ta₃Cl₅(H₂O)₅].OH aq., in which the co-ordination number is 10.