Selenium Tetrachloride, SeCl₄

Selenium Tetrachloride, SeCl₄, may be obtained as follows:

1. By the action of excess of chlorine on selenium or on selenium monochloride in carbon tetrachloride solution; the tetrachloride is insoluble and precipitates as rapidly as it is formed as a white powder:
   
   Se₂Cl₂ + 3Cl₂ = 2SeCl₄.
   
   Baudrimont obtained the tetrachloride by the action of phosphorus pentachloride vapour on lead selenide.
2. By various methods from selenium dioxide; for example, by the action of hydrogen chloride, thionyl chloride or phosphorus pentachloride. The last-named is perhaps the most convenient chlorinating agent. A mixture of the pentachloride and the dioxide is heated until chemical change causes it to solidify. Phosphorus oxychloride can be distilled off with the aid of a current of carbon dioxide and the residual selenium tetrachloride purified by sublimation.
   
   SeO₂ + 2SOCl₂ = SeCl₄ + 2SO₂;
   SeO₂ + 2PCl₅ = 2POCl₃ + SeCl₄.
   
   The disappearance of selenium during the evaporation of a solution of selenium dioxide in concentrated hydrochloric acid is probably due to the formation of the tetrachloride.
3. Selenates, on distillation with sulphuric acid and sodium chloride, yield a mixture of selenium tetrachloride and chlorine; anhydrous selenic acid also gives an excellent yield of selenium tetrachloride on treatment with acetyl chloride at 0° C.

Selenium tetrachloride is a colourless solid which on heating vaporises without melting. Sublimation in an atmosphere of chlorine is therefore a convenient method of purification. The density of the vapour (which is yellow in colour) indicates that dissociation occurs to a considerable extent above 200° C., although below this temperature the results are in accordance with the formula SeCl₄. The products of dissociation are probably the lower chloride and chlorine. There are indications that at higher temperatures (500° C.) the dichloride, SeCl₂, is formed. When the tetrachloride is heated in a sealed tube with selenium, the monochloride is produced.

The tetrachloride may be purified by recrystallisation from phosphorus oxychloride, from which it separates as cubical crystals. It is practically insoluble in carbon disulphide, which can therefore be used to remove any monochloride present. The heat of formation of the tetrachloride from chlorine and amorphous selenium is 46.1 Calories. Water decomposes the tetrachloride, forming the dioxide,

SeCl₄ + 2H₂O = SeO₂ + 4HCl,

but if the quantity of water is very small, the oxychloride, SeOCl₂, is obtained.

The action of dry ammonia on a suspension of the tetrachloride in carbon disulphide gives ammonium chloride and the very explosive orange-yellow amorphous solid nitrogen selenide, N₄Se₄.

Excess of phosphorus displaces the selenium from the tetrachloride with the formation of phosphorus trichloride.

When copper is heated with the tetrachloride at 105° C. in an evacuated tube, the reaction proceeds according to the equations:

(1) 2SeCl₄ + 3Cu = 3CuCl₂ + Se₂Cl₂,
(2) Se₂Cl₂ + 5Cu = 2Cu₂Se + CuCl₂,
and, if the chloride is present in excess,
(3) 2Cu₂Se + 4Se₂Cl₂ = 4CuCl₂ + 10Se.