Pyrophosphite

A phosphite ion in inorganic chemistry usually refers to PO₃sup>2− but includes ₂PO₃sup>− ( PO₂(OH)sup>−). These anions are the conjugate bases of phosphorous acid (H₃PO₃). The corresponding salts, e.g. sodium phosphite (Na₂HPO₃) 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.Egon Wiberg, Arnold Frederick Holleman (2001) ''Inorganic Chemistry'', Elsevier In practice any reference to "phosphite" should be investigated to determine the naming convention being employed.

Salts containing HPO₃²⁻, called phosphonates or phosphites

:
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 (Na₂HPO₃·5H₂O). (NH₄)₂HPO₃·H₂O, CuHPO₃·H₂O, SnHPO₃ and Al₂(HPO₃)₃·4H₂O. The structure of is approximately tetrahedral.L. E. Gordon, W. T. A. Harrison. "Bis(melaminium) hydrogen phosphite tetrahydrate". ''Acta Crystallogr.'' 59 (2): o195–o197. "Crystal chemistry of inorganic phosphites", J. Loub, ''Acta Crystallogr.'' (1991), B47, 468–473,

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

Salts containing HP(O)₂OH⁻

Acid or hydrogen phosphites are called hydrogenphosphonates or acid phosphites. IUPAC recommends the name hydrogenphosphonates). They are anions HP(O)₂OH⁻. A typical derivative is the salt H₄HP(O)₂OH]. Many related salts are known, e.g., RbHPHO₃, CsHPHO₃, TlHPHO₃. These salts are prepared by treating phosphorous acid with the metal carbonate. These compounds contain a layer polymeric anion consisting of HPO₃ tetrahedra linked by hydrogen bonds. These layers are interleaved by layers of metal cations.

Organic esters of hydrogen phosphites are anions with the formula HP(O)₂OR⁻ (R = organic group). One commercial example is the fungicide fosetyl-Al with the formula ₂H₅OP(H)O₂sub>3Al.

Salts containing H₂P₂O₅²⁻, 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 P(O)₂O−P(O)₂Hsup>2−.

Parallels in arsenic chemistry

In contrast to the paucity of evidence for , the corresponding arsenic ion, ortho-arsenite, is known. An example is Ag₃AsO₃ as well as the polymeric meta-arsenite . The iso-electronic sulfite ion, is known from its salts.

Use as fungicides

Inorganic phosphites (containing ) have been applied to crops to combat fungus-like pathogens of the order oomycetes (water molds). The situation is confusing because of the similarity in name between phosphite and phosphate (a major plant nutrient and fertilizer ingredient), and controversial because phosphites have sometimes been advertised as fertilizers, even though they are converted to phosphate too slowly to serve as a plant's main phosphorus source. In fact, phosphites may cause phytotoxicity when a plant is starved of phosphates. Lemoynie"Phosphites and Phosphates: When Distributors and Growers alike could get confused!" by Jean-Pierre Leymonie. Courtesy of ''New Ag International'', September 2007 edition.
/ref> 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 serve as a potassium fertilizer.

See also

* Hypophosphite –
* Organophosphorus
* Phosphine – PH₃ and the organic phosphines PR₃
* Phosphine oxide – OPR₃
* Phosphinite – P(OR)R₂
* Phosphonite – P(OR)₂R
* Phosphinate – OP(OR)R₂
* Phosphonate – organic phosphonates OP(OR)₂R
* Phosphate –
* Organophosphate – OP(OR)₃

Further reading

*

References

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Phosphorus oxyanions
Functional groups
Phosphorus(III) compounds