Phosphonium

Phosphonium is the molecular ion analogous to the ammonium ion, in which the nitrogen atom has been replaced by a phosphorus atom. Analogous to the ammonium ion, the phosphonium cation is single positively charged, the phosphorus atom is present in the oxidation number -3. The simplest phosphonium ion is [H4P]⁺. Its salts decompose on contact with water into hydrogen phosphide and the acid of the anion, thus phosphonium chloride decomposes into hydrogen phosphide (monophosphane, PH3) and hydrochloric acid (HCl). Because of the splitting off of the hydrogen phosphide, phosphonium salts are very toxic. They are used in pest control to produce hydrogen phosphide.

Organic phosphonium ions have the form [_R4P]⁺, where R stands for an organic radical (e.g. alkyl or aryl groups). They are very stable, similar to the homologous nitrogen compounds. They can be prepared from phosphanes by reaction with alkyl and aryl halides, whereby a catalyst is necessary when using aryl halides (e.g. NiCl2).

Phosphonium salts have a great importance in the Wittig reaction.