Phosphine (PH3)

Phosphine (PH3) is the most important hydride of phosphorus.

Preparation: it is typically prepared by treating white phosphorus with sodium hydroxide solution under an inert atmosphere of carbon dioxide or hydrogen (P4 + 3NaOH + 3H2O -> 3NaH2PO2 + PH3), a reaction that also generates sodium hypophosphite as a co-product. The crude gas is contaminated with a small amount of phosphine dihydride (diphosphane, P2H4), which is removed by passing the mixture through a freezing mixture — the dihydride condenses out while the phosphine (which has a much lower boiling/condensation point) passes through unaffected. Phosphine can also be obtained by the hydrolysis of metallic phosphides with water or dilute mineral acid (Ca3P2 + 6H2O -> 2PH3 + 3Ca(OH)2; AlP + 3HCl -> PH3 + AlCl3), by heating orthophosphorous acid, which disproportionates into orthophosphoric acid and phosphine (4H3PO3 --heat--> 3H3PO4 + PH3), or, to obtain it in a very pure form, by heating phosphonium iodide with caustic soda solution (PH4I + NaOH --heat--> PH3 + NaI + H2O).

Physical properties: phosphine is a colourless, poisonous gas with a characteristic rotten-fish smell, only slightly soluble in water and neutral to litmus (unlike ammonia, it shows negligible basicity in aqueous solution). It condenses to a colourless liquid at 188 K and freezes to a solid at 139.5 K.

Chemical properties: on heating in the absence of air to about 317 K, or when an electric spark is passed through it, phosphine decomposes thermally into its elements (4PH3 --317 K--> P4 + 6H2). On combustion in air or oxygen it burns to give metaphosphoric acid: 4PH3 + 8O2 -> P4O10 + 6H2O, followed by P4O10 + 6H2O -> 4H3PO4 (or, under some conditions, meta-phosphoric acid HPO3 forms directly). Phosphine is only weakly basic; it can nonetheless form phosphonium salts with the halogen acids (PH3 + HI -> PH4I), and these salts are readily hydrolysed back by water (PH4I + H2O -> PH3 + H3O+ + I-). It reacts with the halogens to give phosphorus pentahalides (PH3 + 4Cl2 -> PCl5 + 3HCl). As a reducing agent, phosphine precipitates certain metals as their phosphides from salt solutions, for example silver from silver nitrate (3AgNO3 + PH3 -> Ag3P + 3HNO3). It can also act as a Lewis base, forming coordination compounds with Lewis acids such as boron trichloride (BCl3 + PH3 -> Cl3B<-PH3, a coordinate/dative complex). …