Nomenclature and Tautomerism
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.
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