What is "phosphonium"

Phosphonium \Phos*pho"ni*um\, n. [Phosphorus + ammonium.] (Chem.) The hypothetical radical PH4, analogous to ammonium, and regarded as the nucleus of certain derivatives of phosphine.

n. (context chemistry English) the tetravalent positively-charged phosphorus cation R₄P⁺

The phosphonium (more obscurely: phosphinium) cation describes polyatomic cations with the chemical formula . Salts of the parent are rarely encountered, but this ion is an intermediate in the preparation of the industrially useful tetrakis(hydroxymethyl)phosphonium chloride:

PH + HCl + 4 CHO →

Organic phosphonium salts are common reagents in the laboratory. Those with a P–H bond are produced through protonation of phosphines:

PR + H →

Many organic quaternary phosphonium cations are produced by alkylation of organophosphines. For example the reaction of triphenylphosphine with methyl iodide gives methyltriphenylphosphonium iodide, the precursor to a Wittig reagent:

PPh + CHI →

The cation tetraphenylphosphonium is a useful precipitating agent, analogous to quaternary ammonium salts used in phase transfer catalysis.