Phosphite.html
 
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The phosphite ion (HPO32−) is a polyatomic ion with a phosphorus central atom. Its geometry is tetrahedral.

Ball-and-stick model of the phosphite ion

The bonding can be described in terms of a number of resonance canonicals,(there are three equivalent canonicals containing a single double bond) effectively delocalising the negative charges across the three equivalent oxygen atoms.

Four resonance structures of the phosphite ion

Many phosphite salts, such as ammonium phosphite, are highly water soluble. The term phosphite is sometimes used to mean phosphite ester, an organophosphorus compound with the formula P(OR)3.

The structure of a typical phosphite group.

Contents

Phosphorous acid

The conjugate acid of the phosphite anion is phosphorous acid (H3PO3 formulated HP(O)(OH)2). Other names for this acid are orthophosphorous acid and dihydroxyphosphine oxide. (H3PO3 is also sometimes referred to as phosphorus trihydroxide and trihydroxyphosphine, though these names are misleading as they imply a structure of P(OH)3). Phosphorous acid is a diprotic acid, since the hydrogen bonded directly to the central phosphorus atom is not ionizable. Thus, a more logical chemical formula for phosphorous acid is HPO(OH)2, since only two hydroxy groups are actually present on the acid. The acid can be synthesized by treatment of a carboxylic acid, alcohol, or water, with phosphorus tribromide or more commonly phosphorus trichloride.

PBr3 + 3 ROH → 3 RBr + HP(O)(OH)2

PBr3 + 3 RCOOH → 3 RCOBr + HP(O)(OH)2

PCl3 + 3 H2O → HP(O)(OH)2 + 3 HCl

Synthesis of phosphite compounds

Organophosphorus compounds called phosphite esters (or sometimes just phosphites) have the formula (RO)3P. They are prepared by reacting phosphorus trichloride (or phosphorus tribromide) with an alcohol and a tertiary amine.

PCl3 + 3 ROH + 3 R'3N → P(OR)3 + 3R'3NHCl

Acid phosphites are compounds which contain a metal cation and a possible dihydrogen phosphite anion. These compounds are formed by reacting phosphorous acid with a metal carbonate (typically Rb, Cs, or Tl). Acid phosphites have crystalline structures consisting of alternating layers of HPO3 tetrahedrons and metal cations.

See also

References

  • A. Earnshaw, Norman Greenwood (1997). The Chemistry of the Elements - Second Edition. 
  • Kosterina, E. V., Troyanov, S. I., Kemnitz, E. & Aslanov, L. A. (2001). "Synthesis and Crystal Structure of Acid Phosphites RbH2PO3, CsH2PO3, and TlH2PO3". Russian Journal of Coordination Chemistry 27: 458–462. doi:10.1023/A:1011377229855. 
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