Phosphite anion

For phosphites of the type P(OR)₃, see Phosphite ester.

A phosphite anion or phosphite in inorganic chemistry usually refers to [HPO₃]²⁻ but includes [H₂PO₃]⁻ ([HPO₂(OH)]⁻). These anions are the conjugate bases of phosphorous acid (H₃PO₃). The corresponding salts, e.g. sodium phosphite (Na₂HPO₃) are reducing in character.

Phosphite anion

Names
IUPAC name Phosphonate
Systematic IUPAC name Phosphite[1]
Identifiers
CAS Number	15477-76-6 N
3D model (JSmol)	hypervalent form: Interactive image ionic form: Interactive image
ChEBI	CHEBI:16215 N
ChemSpider	10449627 N
Gmelin Reference	1618
MeSH	Phosphorite
PubChem CID	406
CompTox Dashboard (EPA)	DTXSID80164154
InChI InChI=1S/O3P/c1-4(2)3/q-3 N Key: AQSJGOWTSHOLKH-UHFFFAOYSA-N N
SMILES hypervalent form: P([O-])([O-])=O ionic form: [H][P+]([O-])([O-])[O-]
Properties
Chemical formula	HPO2− 3
Molar mass	79.9810 g mol−1
Related compounds
Other anions	Phosphinite Phosphonite
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Nomenclature

The IUPAC recommended name for phosphorous acid is phosphonic acid. Correspondingly, the IUPAC-recommended name for the HPO²⁻
₃ 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.^[2] A well-known university-level textbook follows the IUPAC recommendations.^[3] In practice any reference to "phosphite" should be investigated to determine the naming convention being employed.

Salts containing HPO32−, called phosphonates or phosphites

Structural formula of Na₂HPO₃. The anion has C_3v 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 (Na₂HPO₃·5H₂O). (NH₄)₂HPO₃·H₂O, CuHPO₃·H₂O, SnHPO₃ and Al₂(HPO₃)₃·4H₂O.^[4] The structure of HPO²⁻
₃ is approximately tetrahedral.^[5][6]

HPO²⁻
₃ has a number of canonical resonance forms making it isoelectronic with bisulfite ion, HSO⁻
₃, which has a similar structure.^[7]

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⁻. Aypical derivative is the salt [NH₄][HP(O)₂OH].^[7][6] 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.^[8]

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 [C₂H₅OP(H)O₂]₃Al.^[9]

Salts containing H2P2O52−, called diphosphites or pyrophosphites

Pyrophosphites (diphosphites) can be produced by gently heating acid phosphites under reduced pressure. They contain the ion H
₂P
₂O²⁻
₅, which can be formulated [HP(O)₂O−P(O)₂H]²⁻.^[7][6]

Parallels in arsenic chemistry

In contrast to the paucity of evidence for PO³⁻
₃, the corresponding arsenic ion, ortho-arsenite, AsO³⁻
₃ is known. An example is Ag₃AsO₃ as well as the polymeric meta-arsenite (AsO⁻
₂)
_n.^[7] The iso-electronic sulfite ion, SO²⁻
₃ is known from its salts.^[7]

Use as fungicides

Inorganic phosphites (containing HPO²⁻
₃) 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.^[10] Lemoynie^[11] 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.^[10]

A major form of inorganic phosphite used in agriculture is monopotassium phosphite. This compound does serve as a potassium fertilizer.

See also

• Hypophosphite – H
  ₂PO⁻
  ₂
• 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 – PO³⁻
  ₄
• Organophosphate – OP(OR)₃

Further reading

• A. Earnshaw; Norman Greenwood (1997). The Chemistry of the Elements (2nd ed.). pp. 513–514.

References

1. "Phosphorite - PubChem Public Chemical Database". The PubChem Project. USA: National Center for Biotechnology Information.
2. Physical setting/ chemistry core curriculum, The University of the State of New York, The State Education Department, http://www.p12.nysed.gov/ciai/mst/pub/chemist.pdf Archived 2018-03-29 at the Wayback Machine
3. Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ISBN 0-12-352651-5
4. "Synthesis and crystal structures of aluminum and iron phosphites", D.M. Poojary, Y. Zhang, D.E. Cox, P.R. Rudolf, S. Cheng & A. Clearfield, J. Chem. Crystallogr. 24 (1994) 155–163
5. L. E. Gordon, W. T. A. Harrison. "Bis(melaminium) hydrogen phosphite tetrahydrate". Acta Crystallogr. 59 (2): o195–o197. doi:10.1107/S1600536803001247
6. "Crystal chemistry of inorganic phosphites", J. Loub, Acta Crystallogr. (1991), B47, 468–473, doi:10.1107/S0108768191002380
7. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
8. Kosterina EV, Troyanov SI, Kemnitz E, Aslanov LA (2001). "Synthesis and Crystal Structure of Acid Phosphites RbH₂PO₃, CsH₂PO₃, and TlH₂PO₃". Russian Journal of Coordination Chemistry. 27 (7): 458–462. doi:10.1023/A:1011377229855. S2CID 91297300.
9. Franz Müller; Peter Ackermann; Paul Margot (2012). "Fungicides, Agricultural, 2. Individual Fungicides". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.o12_o06. ISBN 978-3-527-30673-2.
10. Thao; Yamakawa (2008). "Phosphite (phosphorous acid): Fungicide, fertilizer or bio-stimulator?". Soil Science and Plant Nutrition. 55 (2): 228–234. doi:10.1111/j.1747-0765.2009.00365.x. S2CID 95723306.
11. "Phosphites and Phosphates: When Distributors and Growers alike could get confused!" by Jean-Pierre Leymonie. Courtesy of New Ag International, September 2007 edition.