# Phosphite (ion)

A phosphite ion in inorganic chemistry usually refers to [HPO$_3$]$^{2−}$ but includes [H$_2$PO$_3$]$^−$ ([HPO$_2$(OH)]$^−$). These anions are the conjugate bases of phosphorous acid (H$_3$PO$_3$). The corresponding salts, e.g. sodium phosphite (Na$_2$HPO$_3$) are reducing in character.

## Nomenclature

The IUPAC recommended name for phosphorous acid is phosphonic acid. Correspondingly, the IUPAC-recommended name for the HPO$^{2−}$ $_3$ 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. 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$_3$$^{2−}$, called phosphonates or phosphites

    Structural formula of Na$_2$HPO$_3$. 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 (Na$_2$HPO$_3$·5H$_2$O). (NH$_4$)$_2$HPO$_3$·H$_2$O, CuHPO$_3$·H$_2$O, SnHPO$_3$ and Al$_2$(HPO$_3$)$_3$·4H$_2$O. The structure of HPO$^{2−}$ $_3$ is approximately tetrahedral.
HPO$^{2−}$ $_3$ has a number of canonical resonance forms making it isoelectronic with bisulfite ion, HSO$^−$ $_3$, which has a similar structure.

## Salts containing HP(O)$_2$OH$^−$

Acid or hydrogen phosphites are called hydrogenphosphonates or acid phosphites. IUPAC recommends the name hydrogenphosphonates). They are anions HP(O)$_2$OH$^−$. A typical derivative is the salt [NH$_4$][HP(O)$_2$OH]. Many related salts are known, e.g., RbHPHO$_3$, CsHPHO$_3$, TlHPHO$_3$. These salts are prepared by treating phosphorous acid with the metal carbonate. These compounds contain a layer polymeric anion consisting of HPO$_3$ 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)$_2$OR$^−$ (R = organic group). One commercial example is the fungicide fosetyl-Al with the formula [C$_2$H$_5$OP(H)O$_2$]$_3$Al.

## Salts containing H$_2$P$_2$O$_5$$^{2−}$, called diphosphites or pyrophosphites

Pyrophosphites (diphosphites) can be produced by gently heating acid phosphites under reduced pressure. They contain the ion H $_2$P $_2$O$^{2−}$ $_5$, which can be formulated [HP(O)$_2$O−P(O)$_2$H]$^{2−}$.

## Parallels in arsenic chemistry

In contrast to the paucity of evidence for PO$^{3−}$ $_3$, the corresponding arsenic ion, ortho-arsenite, AsO$^{3−}$ $_3$ is known. An example is Ag$_3$AsO$_3$ as well as the polymeric meta-arsenite (AsO$^−$ $_2$) $_n$. The iso-electronic sulfite ion, SO$^{2−}$ $_3$ is known from its salts.

## Use as fungicides

Inorganic phosphites (containing HPO$^{2−}$ $_3$) 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. 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.
A major form of inorganic phosphite used in agriculture is monopotassium phosphite. This compound does serve as a potassium fertilizer.