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.