Phosphonium

The phosphonium (more obscurely: phosphinium) cation describes polyatomic cations with the chemical formula PR+4 (R = H, alkyl, aryl, halide). They are tetrahedral and generally colorless. The phosphonium (more obscurely: phosphinium) cation describes polyatomic cations with the chemical formula PR+4 (R = H, alkyl, aryl, halide). They are tetrahedral and generally colorless. The parent phosphonium is PH+4. One example is phosphonium iodide PH+4I−. 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: Many phosphonium salts are produced by protonation of primary, secondary, and tertiary phosphines: 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 (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: Solid phosphorus pentachloride is an ionic compound, formulated PCl+4PCl−6, i.e. a salt containing tetrachlorophosphonium cation. Dilute solutions dissociate according to the following equilibrium: 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. Tetrakis(hydroxymethyl)phosphonium chloride has industrial importance in the production of crease-resistant and flame-retardant finishes on cotton textiles and other cellulosic fabrics. A flame-retardant finish can be prepared from THPC by the Proban Process, in which THPC is treated with urea. The urea condenses with the hydroxymethyl groups on THPC. The phosphonium structure is converted to phosphine oxide as the result of this reaction.