text: Compounds with the formula [PR]X comprise the phosphonium salts.  These species are tetrahedral phosphorus(V) compounds.  From the commercial perspective, the most important member is tetrakis(hydroxymethyl)phosphonium chloride, [P(CHOH)]Cl, which is used as a fire retardant in textiles.  Approximately 2M kg are produced annually of the chloride and the related sulfate.  They are generated by the reaction of phosphine with formaldehyde in the presence of the mineral acid:
  :PH  +  HX  +  4 CHO   →  [P(CHOH)]X
  A variety of phosphonium salts can be prepared by alkylation and arylation of organophosphines:
  :PR  +  RX  →  [PRR]X
  The methylation of triphenylphosphine is the first step in the preparation of the Wittig reagent.
  The parent phosphorane (σλ) is PH, which is unknown. Related compounds containing both halide and organic substituents on phosphorus are fairly common.  Those with five organic substituents are rare, although P(CH) is known, being derived from P(CH) by reaction with phenyllithium.
  Phosphorus ylides are unsaturated phosphoranes, known as Wittig reagents, e.g. CHP(CH).  These compounds feature tetrahedral phosphorus(V) and are considered relatives of phosphine oxides.  They also are derived from phosphonium salts, but by deprotonation not alkylation.