Hypophosphoric acid H4P2O6 and hypophosphates

There has been much confusion over the structure of these compounds but their diamagnetism has long ruled out a monomeric formulation, H2PO3. In fact, as shown in Table 12.7, isomeric forms are known (a) hypophosphoric acid and hypophosphates in which both P atoms are identical and there is a direct P-P bond (h) isohypophosphoric acid and isohypophos-phates in which 1 P has a direct P-H bond 

Hypophosphoric acid, (H0)2P(0)-P(0)(0H)2, is usually prepared by the controlled oxidation of red P with sodium chlorite solution at room temperature the tetrasodium salt, Na4P20(i, lOH O, crystallizes at pH 10 and the disodium salt at pH 5.2  

Ion exchange on an acid column yields the crystalline dihydrate H4P2O6.2H2O which is actually the hydroxonium salt of the dihydrogen hypophosphate anion [H Olj [(H0)P(0)2 P(0)2(0H)] it is isostructural with the corresponding ammonium salt for which X-ray diffraction studies establish the staggered structure shown. 

The anhydrous acid is obtained cither by the vacuum dehydration of the dihydratc over P4O10 

Both H4P2O6 and its dihydrate are stable at 0° in the absence of moisture. The acid begins to melt (with decomposition) at 73 but even at room temperature it undergoes rearrangement and disproportionation to give a mixture of isohypophosphoric, pyrophosphoric, and pyrophosphorous acids as represented schematically on the previous page. 

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