Phosphonium

The phosphonium (more obscurely: phosphinium) cation describes polyatomic cations with the chemical formula PR+
4.^[1] Salts of the parent PH+
4 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(CH
2OH)+
4Cl−

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

PR₃ + H⁺ → HPR+
3

Many organic quaternary phosphonium cations (PR+
4) 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 → CH
3PPh+
3I−

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

See also

• Ammonium (NH+
  4)
• Hydronium (H₃O⁺)
• Onium compounds

References

1. Corbridge, D. E. C. (1995). Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology (5th ed.). Amsterdam: Elsevier. ISBN 0-444-89307-5.