Phosphine

Phosphine is the hydride of phosphorus as ammonia is the hydride of nitrogen, NH₃. The hydrides of phosphorus are referred to as phosphine and there are two hydrides of phosphorus namely, phosphorus(III) hydride, PH₃, and diphosphane,P₂H₄. Phosphorus(III) hydride is like ammonia escept that the nitrogen atom is substituted by phosphorus atom. It is the main phosphine as diphosphane is of lesser importance. Small wonder, we are discussing phosphorus(III) hydride as phosphine.

Phosphorus(III) hydride is a poisonous colourless gas with characteristic rotting fish smell. Whereas ammonia dissolves in water forming alkaline solution with hydrogen bond, Phosphorus(III) hydride only dissolves slightly to form a neutral solution as it does not form hydrogen bond.

Phosphorus(III) hydride is prepared in an inert environment where the air in the flask used for the preparation is displaced by natural gas. White phosphorus is heated with concentrated sodium hydroxide in the flask to form the phosphorus(III) hydride gas.

P₄(s) + 3NaOH(aq) + 3H₂O(l) → 3NaH₂PO₂(aq) + PH₃(g)

Phosphorus(III) hydride is also prepared from calcium phosphide, Ca₃P₂, where water (or acid) is reacted with the calcium phosphide to form the phosphorus(III) hydride gas.

Ca₃P₂(s) + 6H₂O → 3Ca(OH)₂(s) + 2PH₃(g)

Pure phosphorus(III) hydride burns explosively in air at about 150°C to form phosphorus(V) oxide,P₄O₁₀, and steam. The reaction is in two phase as the products will react further to form tetraoxo-phosphate(V) acid, H₃PO₄.

4PH₃(g) + 8O₂(g) → P₄O₁₀(s) + 6H₂O(g) → 4H₃PO₄(aq)

When heated to a temperature of about 440°C, phosphorus(III) hydride will decompose in an exothermic reaction to phosphorus and hydrogen.

4PH₃(g) → P₄(g) + 6H₂(g), ΔH=-ve

The decomposition of phosphorus(III) hydride is similar to ammonia escept that it takes place at a lower temperature and that of ammonia is endothermic.

Phosphorus(III) hydride is also a strong reducing agent. It reduces solutions of silver or copper(II) salts to metals or phosphides. It forms silver metal by reducing silver trioxonitrate(V) salt.

6AgNO₃(aq) + PH₃(g) + 3H₂O(l) → 6Ag(s) + H₃PO₄(aq) + 6HNO₃(aq)

Another important reduction reaction is how it ignites spontaneously in chlorine to form phosphorus(III) chloride.

PH₃(s) + 3Cl₂(g) → PCl₃(l) + 3HCl(g)

Whereas ammonia forms so many stable complexes, phosphorus(III) hydride does not. As the salts formed by ammonia are called ammonium salts, so the phosphorus salts are regarded as phosphonium salts. An example of a typical phosphonium salt is phosphonium chloride, PH₄Cl, that differs from ammonium chloride, NH₄Cl, by a substitution of the ammonium nitrogen with a phosphorus atom.

Phosphorus(III) hydride is also known for its weakly basic properties and the inability to form many stable complexes unlike ammonia, NH₃.