Tantalum Pentafluoride, TaF₅

Tantalum Pentafluoride, TaF₅, is the only known fluoride of tantalum, and has been successfully isolated by methods that avoid hydrolysis: (1) Tantalum and fluorine are brought into reaction exactly as in the preparation of niobium pentafluoride. (2) Tantalum pentachloride is treated in the cold with dry hydrofluoric acid; the hydrochloric acid liberated and excess of hydrofluoric acid are evaporated off, and the resulting tantalum pentafluoride is purified by redistillation in a platinum crucible between 300° and 400° C. (3) The double barium tantalum fluoride, 3BaF₂.2TaF₅, is very strongly heated in a platinum tube, one end of which is kept cold.

Tantalum pentafluoride forms hygroscopic, colourless, doubly refracting, tetragonal prisms which melt at 96.8° C. and boil between 229.2° and 229.5° C. at 760 mm. pressure. Its density varies between 4.981 at 15° C. and 4.744 at 19.5° C. It is soluble in water; solution is followed by hydrolysis which, however, does not proceed so readily as with tantalum pentachloride, niobium pentafluoride, and vanadium pentafluoride. The aqueous solution evolves hydrogen fluoride on being evaporated, and leaves an insoluble tantalum oxyfluoride which is converted into tantalum pentoxide on being ignited. Caustic alkalis and alkali carbonates in concentrated solution attack the pentafluoride vigorously, and yield an alkali tantalum oxyfluoride of composition 4RF. Ta₂O₅.2TaF₅. Dilute alkalis yield tantalic acid, while fusion with potassium fluoride yields the double fluoride 2KF.TaF₅. Tantalum pentafluoride is also soluble in cold sulphur monochloride, sulphuryl chloride, stannic chloride, arsenious chloride, antimony pentachloride, alcohol, chloroform, glacial acetic acid, and acetic anhydride.

When solutions of tantalum pentoxide in hydrofluoric acid are treated with solutions of the fluorides of the alkali (and other) metals, double fluorides are obtained which possess the general formula 4RF.TaF₅, where n usually varies between 1 and 3; in the most important series n = 2. These double fluorides are much more stable than tantalum pentafluoride. They were among the first tantalum compounds to receive examination, and still form an important class of tantalum compounds. A study of their isomorphism with the corresponding compounds of niobium enabled the formula of " tantalic acid " (and hence of other tantalum compounds) to be correctly established; precipitation of potassium tantalum fluoride, 2KF.TaF₅, still constitutes the classical method for the separation of tantalum from niobium, and analyses of this salt provided the first reliable data for the atomic weight of tantalum. Balke repeated and confirmed the previous investigations of Marignac, and showed that tantalum pentafluoride forms several double salts with almost every one of the alkali fluorides, from which it appears that the preparation of any particular double fluoride in the pure state is not an easy matter; the purest to be prepared hitherto are the potassium salt, 2KF.TaF₅, and the sodium salt, 3NaF. TaF₅. The greater stability of these double fluorides as compared with tantalum pentafluoride has led to the assumption that their solutions contain complex anions into which the tantalum enters. The two salts just mentioned ionise, for instance, thus:

K₂TaF₇ = 2K^• + [TaF₇]'',
Na₃TaF₈ = 3Na^• + [TaF₈]'''.

When viewed from the point of view of the Werner co-ordination theory, it is observed that in these complexes the common co-ordination number is 7, thus [TaF₇]K₂ and [TaF₇]Cu.4H₂O; the co-ordination number 8 also occurs, thus [TaF₈]Na₃ and , and less frequently the co-ordination number 6, thus [TaF₆](NH₄).

The following double fluorides with tantalum pentafluoride are known:

Acid Tantalum Fluoride or tantalum hydrogen fluoride, HTaF₆.6H₂O

Acid Tantalum Fluoride or tantalum hydrogen fluoride, HF.TaF₅.6H₂O or HTaF₆.6H₂O, has recently been prepared in clusters of feathery crystals which melt at about 15° C. by dissolving tantalum pentoxide in hydrofluoric acid and crystallising at about -10° C. It can be looked upon as the acid which gives rise to the series RF.TaF₅.

Ammonium Tantalum Fluorides

The compound NH₄F.TaF₅ or NH₄TaF₆ results from the action of ammonium fluoride on acid tantalum fluoride. When a solution of tantalum pentoxide in hydrofluoric acid is treated with gaseous ammonia and evaporated, the compound 2NH₄F.TaF₅ or (NH₄)₂TaF₇ is obtained in thin, right-angled plates, or in flattened needles belonging to the tetragonal system. It can be dried at 100° C. without undergoing decomposition, is readily soluble in water, from which it can be recrystallised unchanged, but the aqueous solution on being boiled throws down a white precipitate. Its preparation in the perfectly pure state is difficult. A compound which has the approximate composition 3NH₄F.TaF₅ or (NH₄)₃TaF₈ has also been obtained.

Barium Tantalum Fluoride, Ba₃Ta₂F₁₆

Barium Tantalum Fluoride, 3BaF₂.2TaF₅ or Ba₃Ta₂F₁₆, separates as a white, micro-crystalline precipitate from the action of barium chloride on a solution of tantalum pentoxide in slight excess of hydrofluoric acid.

Cesium Tantalum Fluorides

Substances having the compositions CsF.TaF₅ and 2CsF.TaF₅ are prepared by crystallising a mixture of caesium fluoride and tantalum fluoride from dilute hydrofluoric acid, using excess of tantalum fluoride or of caesium fluoride respectively. The former yields glistening, rhombohedral crystals, and the latter thin needles. A stable, crystalline salt of the composition 15CsF.TaF₅ has also been reported.

Copper Tantalum Fluoride, CuTaF₇.4H₂O

Copper Tantalum Fluoride, CuF₂.TaF₅.4H₂O or CuTaF₇.4H₂O, can be obtained in blue, transparent, rhombic crystals by the action of copper oxide on a solution of tantalum pentoxide in excess of hydrofluoric acid. It is very readily soluble in water.

Lithium Tantalum Fluoride, LiTaF₆.2H₂O

Lithium Tantalum Fluoride, LiF.TaF₅.2H₂O or LiTaF₆.2H₂O, is obtained in colourless, prismatic crystals when a solution of tantalum pentoxide in excess of hydrofluoric acid is treated with lithium carbonate.

Potassium Tantalum Fluoride, K₂TaF₇

Potassium Tantalum Fluoride, 2KF.TaF₅ or K₂TaF₇ or [TaF₇]K₂, is the commonest of the double fluorides, and is in fact one of the commonest of tantalum compounds. It is prepared by adding potassium fluoride to a warm solution of tantalum pentoxide in hydrofluoric acid, or by boiling tantalic acid with potassium hydrogen fluoride, KHF₂, and cooling the product. The preparation of potassium tantalum fluoride by this method has already been referred to in describing the separation and estimation of niobium and tantalum. It can also be obtained by the action of caustic potash or potassium carbonate on a solution of tantalum pentoxide in hydrofluoric acid.

Potassium tantalum fluoride crystallises in small, thin, rhombic needles, which are isomorphous with the corresponding niobium compound K₂NbF₇. The density is variously reported as being 4.56 and 5.24. The salt is stable in dry air at ordinary temperatures. On being heated the pure substance decrepitates, melts to a clear liquid, and leaves a blue, infusible mass. It dissolves in water, but hydrolysis takes place, the extent of which depends on the conditions. In water containing very little hydrofluoric acid, 1 part of the salt dissolves in 200 parts of water at 15° C.; in the presence of rather more hydrofluoric acid 1 part dissolves in 150 to 160 parts of water, at 15° C. Ruff and Schiller have shown that the solubility increases (1) with rise in temperature, (2) with increase in the concentration of hydrofluoric acid, (3) with decrease in the concentration of potassium fluoride. The aqueous solution reacts acid to litmus, and on being boiled precipitates a white, insoluble, potassium tantalum oxyfluoride, Ta₂O₅. 4KF.2TaF₅ or Ta₂O₅.2(2KF.TaF₅); this reaction is used to detect the presence of tantalum in a niobium compound, since a solution of potassium niobium fluoride, K₂NbF₇, does not throw down a precipitate on being boiled. Addition of caustic soda precipitates tantalic acid.

Rubidium Tantalum Fluoride, Rb₂TaF₇

Rubidium Tantalum Fluoride, 2RbF.TaF₅ or Rb₂TaF₇, is obtained in white needles similarly to the analogous potassium compound. It dissolves in 40 parts of water.

Sodium Tantalum Fluorides

Slow evaporation of a solution of sodium tantalate in hydrofluoric acid precipitates the two compounds, 3NaF.TaF₅ or Na₃TaF₈, and 2NaF.TaF₅.H₂O or Na₂TaF₇-H₂O, the former first. Na₃TaF₈ yields rhombic prisms, a:b:c = 0.6017:1:0.2799, and dissolves in from 20.5 to 20.9 parts of water at 25° C. Na₂TaF₇.H₂O gives rise to six-sided plates belonging to the rhombic system, a:b:c = 0.838:1:1.274; it loses its water of crystallisation at 100° C., and can be heated to 150° C. without undergoing decomposition. Evaporation of its mother-liquors yields the salt NaF.TaF₅ or NaTaF₆ in regular, cubic crystals.

Analogous sodium niobium fluorides have not been isolated. Thallium Tantalum Fluoride, 2TlF.TaF₅ or Tl₂TaF₇, is obtained in glistening crystals by the action of thallium fluoride on tantalum pentoxide in hydrofluoric acid.

Zinc Tantalum Fluoride, ZnF₂.TaF₅

Zinc Tantalum Fluoride, ZnF₂.TaF₅, is prepared by the action of zinc oxide on a solution of tantalum pentoxide in excess of hydrofluoric acid; concentration yields a hygroscopic, crystalline mass.

Tantalum pentafluoride also forms crystalline double salts with pyridine, methylamine, and other organic bases. The following compounds with pyridine have been prepared: 3(C₅H₅N.HF).2TaF₅.2H₂O; C₅H₅N.HF.TaF₅.