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Concept
Phosphonium
Summary
In chemistry, the term phosphonium (more obscurely: phosphinium) describes polyatomic cations with the chemical formula PR4+ (where R is a hydrogen or an alkyl, aryl, or halide group). These cations have tetrahedral structures. The salts are generally colorless or take the color of the anions.
The parent phosphonium is PH4+ as found in the iodide salt, phosphonium iodide. 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−
Many organophosphonium salts are produced by protonation of primary, secondary, and tertiary phosphines:
PR3 + H+ → HPR3+
The basicity of phosphines follows the usual trends, with R = alkyl being more basic than R = aryl.
The most common phosphonium compounds have four organic substituents attached to phosphorus. The quaternary phosphonium cations include tetraphenylphosphonium, (C6H5)4P+ and tetramethylphosphonium P(CH3)4+.
Quaternary phosphonium cations (PR4+) are produced by alkylation of organophosphines. For example, the reaction of triphenylphosphine with methyl bromide gives methyltriphenylphosphonium bromide:
PPh3 + CH3Br → [CH3PPh3]+Br−
The methyl group in such phosphonium salts is mildly acidic, with a pKa estimated to be near 15:
[CH3PPh3]+ + base → CH2=PPh3 + [Hbase]+
This deprotonation reaction gives Wittig reagents.
Solid phosphorus pentachloride is an ionic compound, formulated PCl4+PCl6-, that is, a salt containing the tetrachlorophosphonium cation. Dilute solutions dissociate according to the following equilibrium:
PCl5 PCl4+ + Cl−
Triphenylphosphine dichloride (Ph3PCl2) exists both as the pentacoordinate phosphorane and as the chlorotriphenylphosphonium chloride, depending on the medium. The situation is similar to that of PCl5. It is an ionic compound (PPh3Cl)+Cl− in polar solutions and a molecular species with trigonal bipyramidal molecular geometry in apolar solution.
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