Phosphonium
The phosphonium (more obscurely: phosphinium) cation describes polyatomic cations with the chemical formula PR+
4. A phosphonium salt is a salt containing either the phosphonium (PH4+) ion, such as phosphonium iodide (PH4+I−) or, more commonly, an organic derivative such as the quaternary phosphonium salts tetraphenylphosphonium chloride, (C6H5)4P+ Cl− and tetramethylphosphonium iodide, [P(CH3)4]+I−. 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:
Organic phosphonium salts are common reagents in the laboratory. Those with a P–H bond are produced through protonation of phosphines:
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:
The cation tetraphenylphosphonium (PPh+
4) is a useful precipitating agent, analogous to quaternary ammonium salts used in phase transfer catalysis.
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