Phosphite

The phosphite ion (PO33−) is a polyatomic ion with a phosphorus central atom. Its geometry is trigonal pyramidal. Many phosphite salts, such as ammonium phosphite, are highly water soluble. The term phosphite is sometimes used to mean phosphite ester, an organophosphorus compound with the formula P(OR)3.

Phosphorous acid
The conjugate acid of the phosphite anion is phosphorous acid (H3PO3). Other names for this acid are orthophosphorous acid and dihydroxyphosphine oxide. H3PO3 is also sometimes referred to as phosphorus trihydroxide and trihydroxyphosphine, though these names are misleading. Phosphorous acid is a diprotic acid, since the hydrogen bonded directly to the central phosphorus atom is not ionizable. Thus, a more logical chemical formula for phosphorous acid is HPO(OH)2, since three hydroxy groups are not actually present on the acid. The acid can be synthesized hy treatment of a carboxylic acid, alcohol, or most practically water, with phosphorus tribromide or more commonly phosphorus trichloride.

PBr3 +  3 ROH  →  3 RBr  +  HP(O)(OH)2

PBr3 +  3 RCOOH  →  3 RCOBr  +  HP(O)(OH)2

PCl3 +  3 H2O  →  HP(O)(OH)2  + 3 HCl

Synthesis of phosphite compounds
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

Acid phosphites are compounds which contain a metal cation and a possible dihydrogen phosphite anion. These compounds are formed by reacting phosphorous acid with a metal carbonate (typically Rb, Cs, or Tl). Acid phosphites have crystalline structures consisting of alternating layers of HPO3 tetrahedrons and metal cations.