Sulphur Nitride, S₄N₄

The solid nitride was discovered by Gregory in 1835, and then completely investigated by a number of scientists.

The preparations follow three main lines:

The liquid medium in which the reaction takes place may be carbon disulphide, benzene, or chloroform. A solution containing 25 grams of SCl₂ in 200 c.c. of benzene is saturated with ammonia. The orange-coloured precipitate is washed with water to free it from ammonium chloride and recrystallised from carbon disulphide, or carbon tetrachloride, to free it from sulphur. It crystallises on cooling in golden crystals. A further quantity is obtained by evaporation of the benzene mother-liquor, and may be recrystallised from benzaldehyde.

It is probably formed by the reaction

12SCl₄ + 16NH₃ = 3S₄N₄ + 2N₂ + 48HCl.

The yield actually obtained exceeds that calculated from this equation by about 10 per cent. This is attributed to the reaction between S₂Cl₂ and NH₃, as stated below.

When sulphur monochloride, (S₂Cl₂), is used, there is an abundant separation of sulphur. By mixing ice-cold solutions of ammonia and of sulphur monochloride, both in chloroform, there is obtained a mixed precipitate, probably formed by the equation

6S₂Cl₂ + 16NH₃ = S₄N₄ + 12NH₄Cl + 8S.

Other sulphides are found in the mother-liquor.

Thionyl chloride gives a mixture of S₄N₄, ammonium chloride, and ammonium salts of thionic acids.

Sulphur dissolves freely in liquid ammonia, giving a red-brown solution, and the probability that a chemical reaction takes place was pointed out. It was likewise shown that, after the addition of AgI to remove the H₂S, and removal of the Ag₂S by filtration, the subsequent evaporation of the ammonia from the filtrate yields S₄N₄. The evaporation of a solution containing both H₂S and S₄N₄ in NH₃ leaves pure sulphur. These facts, taken together, point to the establishment of an equilibrium. Thus:

4NH₃ + 10S ⇔ S₄N₄ + 6H₂S.

This salt decomposes on heating, giving various products, among which an orange sublimate was found of empirical composition NS, formed probably according to the equation

3Ba(SO₃NH₂)₂ = 3BaSO₄ + HN(SO₃NH₄)₂ + NH₃ + SN + ½N₂.

The compound forms reddish crystals which belong to the rhombic, or monoclinic systems. The density is 2.2. Under atmospheric pressure it sublimes at 135° C., while under increased pressure it melts at 178° C. When sublimed in a vacuum over silver gauze it condenses as a blue substance, which may be a polymer. It decomposes slowly when heated to 185° C., and over 200° C. explodes, giving nitrogen, sulphur, and possibly another sulphide. It is an endothermic compound, the heat of formation being -0.7007 Cals. per gram, or -129 Cals. per mol., S₄N₄. It is insoluble in water, but soluble in the solvents mentioned above, in alcohol, ether, and many other solvents. The molar weight in these corresponds to the formula given, e.g. by the ebullioscopic method in CS₂.

The compound is slowly hydrolysed by hot water, more quickly by alkalies and acids, giving ammonia or its salts, and salts of sulphur oxy-acids or free sulphur. These reactions prove that it is a nitride of sulphur, not a nitrogen sulphide.

It is decomposed by dry hydrogen chloride, according to the equation

S₄N₄ + 12HCl = 4NH₃ + 4S + 6Cl₂.

With liquid hydrogen chloride it gives ammonium chloride and chlorides of sulphur. It forms many addition compounds: thus S₄N₄.2NH₃ can be obtained from the solution in liquid ammonia, S₄N₄Cl₄ and S₄N₄Br₄ are yellow and red substances respectively, and S₄N₄.SCl₂ is a yellow substance. These compounds are prepared by the addition of the respective elements and SCl₂ to S₄N₄ in solution.

Many compounds with acid radicals, such as NSO and S₄N₃Cl, have been prepared; the latter substance by the reaction between S₄N₄ and sulphur chloride or acetyl chloride. It behaves like a base, giving salts, e.g. a nitrate with nitric acid.

The preparation from SCl₂ and NH₃, and the decomposition with HCl into ammonia and sulphur, show that the sulphur takes the place of the hydrogen of NH₃, and that the nitrogen atoms are not linked together but are united to sulphur by 12 valencies. The valency of the sulphur is probably greater than 3 - that is, it may be 4.

Further, in the formation of thiodiamines from secondary amines by the equation

3S₄N₄ + 24HNR₂ = 12S(NR₂)₂ + 8NH₃ + 2N₂

the average valency of each sulphur atom exerted towards -NR₂ has fallen to 2, so that the hydrogen of the amine is insufficient to give ammonia, and nitrogen is set free. A solution in liquid ammonia reacts with lead iodide to give PbN₂S₂, with mercuric iodide to give HgN₂S; in which, as shown by their decomposition when acted on by HCl, the metal is united to nitrogen but not to sulphur. They are therefore considered to be the dithiodi-imide and the thiodi-imide respectively, and to contain the radicals and -N=S=N- The structure assigned to the compound S₄N₄ is:



On sublimation of nitrogen sulphide with sulphur, small quantities of a dark red liquid are obtained which solidifies at - 80° C., and on analysis gives data corresponding to the empirical formula NS₂.