- published: 10 Jul 2015
- views: 670
Phosphorous acid is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is as an intermediate in the preparation of other phosphorus compounds.
H3PO3 is more clearly described with the structural formula HPO(OH)2. This species exists in equilibrium with a minor tautomer P(OH)3. IUPAC recommendations, 2005, are that the latter is called phosphorous acid, whereas the dihydroxy form is called phosphonic acid. Only the reduced phosphorus compounds are spelled with an "ous" ending. Other important oxyacids of phosphorus are phosphoric acid (H3PO4) and hypophosphorous acid (H3PO2). The reduced phosphorus acids are subject to similar tautomerism involving shifts of H between O and P.
The P(OH)3 tautomer has been observed as a ligand bonded to molybdenum.
In the solid state, HP(O)(OH)2 is tetrahedral with one shorter P=O bond of 148 pm and two longer P-O(H) bonds of 154 pm. Because the electronegativity of H and P are similar, the covalent P-H bond does not alter oxidation state of phosphorus, which is assigned the formal oxidation state P(IV).