Phosphite Identifiers CAS number PubChem ChemSpider MeSH ChEBI ChEMBL Gmelin Reference 68617 Jmol-3D images Image 1
Properties Molecular formula O3P3- Molar mass 78.9720 g mol-1 Exact mass 78.958505378 g mol-1 Related compounds Other anions Phosphinite
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
A phosphite is a salt of phosphorous acid. The phosphite ion (PO33−) is a polyatomic ion with a phosphorus central atom where phosphorus has an oxidation state of +3. Its molecular geometry is approximately tetrahedral like ammonia.
Because phosphorous acid exists as an equilibrium tautomeric mixture of P(OH)3 and HP(O)(OH)2, predominantly the latter, there is some confusion in nomenclature. The IUPAC recommends that the trihydroxy form be called phosphorous acid and its salts phosphites, with the dihydroxy form being called phosphonic acid and its salts phosphonates, but despite this, salts of HP(O)(OH)2 are often called phosphites rather than phosphonates.
Acid or hydrogen phosphites (which the IUPAC recommends be called acid or hydrogen phosphonates), such as NH4HP(O)2OH, can be prepared from phosphorous acid, HP(O)(OH)2. Hydrogen bonding between anions leads to polymeric anionic structures.  Recently some others, RbHPHO3, CsHPHO3, TlHPHO3 have been prepared by reacting 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.
Naming of phosphite ions
The traditional name for HPO32− is phosphite, and for HPO2(OH)− is hydrogenphosphite or acid phosphite. However IUPAC recommendations are that HPO32− is to be named hydrogenphosphite or phosphonate, HPO2(OH)− is to be named dihydrogenphosphite or hydrogenphosphonate and phosphite is reserved for the hypothetical PO33− ion. 
Synthesis of phosphite esters
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
Use in plants
Inorganic phosphites have been applied to crops to combat fungus-like pathogens of the order Oomycetes. 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. Lemoynie 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.
- Phosphine - PR3
- Phosphine oxide - OPR3
- Phosphinite - P(OR)R2
- Phosphonite - P(OR)2R
- Phosphinate - OP(OR)R2
- Phosphonate - OP(OR)2R
- Phosphate - OP(OR)3
- A. Earnshaw, Norman Greenwood (1997). The Chemistry of the Elements - Second Edition. pp. 513–514.
- ^ "Phosphorite - PubChem Public Chemical Database". The PubChem Project. USA: National Center for Biotechnology Information. http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=107908.
- ^ a b c Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0080379419.
- ^ a b c Crystal chemistry of inorganic phosphites, J. Loub, Acta Cryst. (1991), B47, 468-473, doi:10.1107/S0108768191002380
- ^ a b International Union of Pure and Applied Chemistry (2005). Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005). Cambridge (UK): RSC–IUPAC. ISBN 0-85404-438-8. Electronic version..
- ^ 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 (7): 458–462. doi:10.1023/A:1011377229855.
- ^ Phosphites and Phosphates: When Distributors and Growers alike could get confused! by Jean-Pierre Leymonie. Courtesy of New Ag International, September 2007 edition.
- ^ Thao; Yamakawa (2008). Soil Science and Plant Nutrition 55: 228–234.
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