Phosphite (ion)
The phosphite ion is a polyatomic anion with the formula HPO32-. It is also occasionally referred to as hydrogen phosphite or phosphonate. This species is the conjugate base of the hydrogen phosphite ion, H2PO3-, and the dihydrogen phosphite ion, H3PO3. Note that the term "phosphite" is also used (and sometimes misused) to refer to esters of phosphorous acid, P(OR)3, which are trialkyl or triaryl derivatives of phosphorous acid.
In terms of structure, the phosphite ion features a central phosphorus atom bonded to one hydrogen atom, and three oxygen atoms, two of which carry a negative charge. The phosphorus atom has a tetrahedral geometry. A key point to note is that the phosphite ion is isomeric with the phosphonate ion, PO3H2-, where the hydrogen is directly bonded to the phosphorus atom in phosphite, but bonded to an oxygen atom in the (less common) phosphonate form. The hydrogen atom in the phosphite ion is directly bonded to the phosphorus and is not readily lost under normal conditions.
Salts containing the phosphite ion are known as phosphites. They can be formed by neutralizing phosphorous acid (H3PO3) with a base. Phosphites have various applications, including as reducing agents, ligands in coordination chemistry, and in some cases, as fungicides in agriculture. The use as fungicides is controversial due to concerns about resistance development in fungal populations. In environmental contexts, phosphites can be involved in phosphorus cycling.