Phosphonium

The phosphonium (more obscurely: phosphinium) cation describes positively charged polyatomic cations with the chemical formula PR+
4. Salts of the parent PH₄+ are rarely encountered, but this ion is an intermediate in the preparation of the industrially useful tetrakis(hydroxymethyl)phosphonium chloride:

PH₃ + HCl + 4 CH₂O → P(CH₂OH)₄+Cl-

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

PR₃ + H+ → HPR₃+

Many organic quaternary phosphonium cations (P+R₄) are produced by alkylation of organophosphines. For example the reaction of triphenylphosphine with methyl iodide gives methyltriphenylphosphonium iodide, the precursor to a Wittig reagent:

PPh₃ + CH₃I → CH₃PPh₃+I-

The cation tetraphenylphosphonium (PPh₄+) is a useful precipitating agent, analogous to quaternary ammonium salts used in phase transfer catalysis.