Rhodium Tribromide, RhBr₃

Bromine begins to act on finely divided rhodium at about 250° С , but the product has an uncertain composition attributable to partial dissociation at the temperature of formation. Thus 3:

2Rh + 3Br₂ ⇔ 2RhBr₃.

The dihydrate, RhBr₃.2H₂O, may be prepared by heating to 80° to 100° C. in a sealed tube spongy rhodium with 40 per cent, hydrobromic acid and a little bromine. The product is distilled under diminished pressure to remove excess of hydrobromic acid, the residue diluted with water and concentrated in vacuo over sulphuric acid, and finally over fused potash. The crystals obtained are blackish red in colour, and very soluble in water. At 100° to 140° C. they lose water and hydrogen bromide. With potassium hydroxide solution a basic salt is precipitated, namely, Rh(OH)₂Br.2H₂O, the solution containing potassium pentabromrhodite, K₂RhBr₅. By adding a slight excess of rhodium bromide solutions to those of alkali bromides, double salts are obtained. These have the general formula RhBr₃.2MBr or M₂RhBr₅, and are known as: Pentabromrhodites or Rhodopentabromides.

Ammonium Pentabromrhodite, (NH₄)₂RhBr₅, is conveniently prepared by precipitating potassium bromide from a solution of potassium pentabromrhodite by passing in hydrogen bromide. To the filtrate dilute ammonium bromide solution is added. On standing for several days the salt separates out in blackish green crystals.

Caesium Pentabromrhodite, Cs₂RhBr₅, results when a solution of the potassium salt is decomposed by addition of a concentrated solution of caesium bromide. The salt separates out as a green precipitate.

Potassium Pentabromrhodite, K₂RhBr₅, is obtained by heating finely divided rhodium with potassium bromide in a stream of bromine vapour. On cooling, the mass is extracted with water, and the filtered solution worked for crystals. These are obtained as small irregular leaflets, dark green in colour.

Rubidium Pentabromrhodite, Rb₂RhBr₅, has been prepared in a similar manner to the caesium salt. It has a darker green colour than the last named.