A phosphite ion in inorganic chemistry usually refers to [HPO<sub>3</sub>]<sup>2−</sup> but includes [H<sub>2</sub>PO<sub>3</sub>]<sup>−</sup> ([HPO<sub>2</sub>(OH)]<sup>−</sup>). These anions are the conjugate bases of phosphorous acid (H<sub>3</sub>PO<sub>3</sub>). The corresponding salts, e.g. sodium phosphite (Na<sub>2</sub>HPO<sub>3</sub>) are reducing in character.

Nomenclature

The IUPAC recommended name for phosphorous acid is phosphonic acid. Correspondingly, the IUPAC-recommended name for the ion is phosphonate. In the US the IUPAC naming conventions for inorganic compounds are taught at high school, but not as a 'required' part of the curriculum. A well-known university-level textbook follows the IUPAC recommendations. In practice any reference to "phosphite" should be investigated to determine the naming convention being employed.

Salts containing HPO<sub>3</sub><sup>2−</sup>, called phosphonates or phosphites

:thumb|left|132px|Structural formula of [[disodium hydrogen phosphite|Na<sub>2</sub>HPO<sub>3</sub>. The anion has C<sub>3v</sub> symmetry.]]

From the commercial perspective, the most important phosphite salt is basic lead phosphite. Many salts containing the phosphite ion have been investigated structurally, these include sodium phosphite pentahydrate (Na<sub>2</sub>HPO<sub>3</sub>·5H<sub>2</sub>O). (NH<sub>4</sub>)<sub>2</sub>HPO<sub>3</sub>·H<sub>2</sub>O, CuHPO<sub>3</sub>·H<sub>2</sub>O, SnHPO<sub>3</sub> and Al<sub>2</sub>(HPO<sub>3</sub>)<sub>3</sub>·4H<sub>2</sub>O. The structure of is approximately tetrahedral.

has a number of canonical resonance forms making it isoelectronic with bisulfite ion, , which has a similar structure.

Organic esters of hydrogen phosphites are anions with the formula HP(O)<sub>2</sub>OR<sup>−</sup> (R = organic group). One commercial example is the fungicide fosetyl-Al with the formula [C<sub>2</sub>H<sub>5</sub>OP(H)O<sub>2</sub>]<sub>3</sub>Al.

Salts containing H<sub>2</sub>P<sub>2</sub>O<sub>5</sub><sup>2−</sup>, called diphosphites or pyrophosphites

Pyrophosphites (diphosphites) can be produced by gently heating acid phosphites under reduced pressure. They contain the ion , which can be formulated [HP(O)<sub>2</sub>O−P(O)<sub>2</sub>H]<sup>2−</sup>. and others have described this complicated situation and noted that calling phosphites fertilizers avoided the regulatory complication and negative public perceptions that might have been incurred by registering them as fungicides.

A major form of inorganic phosphite used in agriculture is monopotassium phosphite. This compound does not serve as a phosphorus fertilizer, but does serve as a potassium fertilizer.

See also

  • Hypophosphite –
  • Organophosphorus
  • Phosphine – PH<sub>3</sub> and the organic phosphines PR<sub>3</sub>
  • Phosphine oxide – OPR<sub>3</sub>
  • Phosphinite – P(OR)R<sub>2</sub>
  • Phosphonite – P(OR)<sub>2</sub>R
  • Phosphinate – OP(OR)R<sub>2</sub>
  • Phosphonate – organic phosphonates OP(OR)<sub>2</sub>R
  • Phosphate –
  • Organophosphate – OP(OR)<sub>3</sub>

Further reading

References