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Bromotantalum Bromide, Ta6Br14.7H2O

Bromotantalum Bromide or tantalum dibromide. The anhydrous substance is unknown. A compound which has the composition (Ta3Br6)Br.3½H2O or Ta6Br14.7H2O is obtained by reducing tantalum pentabromide with sodium amalgam at a red heat in the absence of air; the product is extracted with water which has been acidified with hydrobromic acid, and the extract concentrated. Minute, black, hexagonal crystals result, which give a dark green powder. They yield an intensely green aqueous solution which is apparently stable in air. Addition of ammonium hydroxide precipitates brown flakes of an unstable lower hydroxide of tantalum. When the compound containing seven atoms of bromine is treated with silver nitrate solution, only one atom of bromine is removed as silver bromide, which indicates that six atoms of bromine form part of a complex anion and do not undergo ionisation as Br' ions. Similarly, when the aqueous solution of bromotantalum bromide is treated with equimolecular proportions of caustic soda in the cold, dark green, thin, hexagonal plates of bromotantalum hydroxide, (Ta3Br6)OH.5H2O, are obtained. Evaporation of the hydroxide with hydrochloric acid yields bromotantalum chloride, (Ta3Br6)Cl.3½H2O, which is very similar in appearance and general properties to bromotantalum bromide. Evaporation of the hydroxide with hydriodic acid gives rise to bromotantalum iodide, (Ta3Br6)I.3½H2O, in long, hexagonal prisms. All these substances contain the group (Ta3Br6), which, however, has not been separately isolated.

It is important to note that recent investigation into the composition of the analogous chlorine compound of tantalum, viz. chlorotantalum chloride, has shown that the formula H.Ta3Cl6.Cl.4H2O is to be preferred to the older formula Ta3Cl6.Cl.4H2O. By analogy, the composition of the bromine compound is H.Ta3Br6.Br.4H2O, and this assumption receives confirmation from the fact that the acid H[Ta3Br6.Br.H2O] has recently been prepared. Theforegoing formulae for the hydroxy-, chloro- and iodo-derivatives also presumably require correction.

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