Oxoacids of Phosphorus

Phosphorus forms a rich family of oxoacids, summarised in Table 7.5 along with their formulas, oxidation states, characteristic bonds, and how each is prepared.

Reading down the table, the acids are related to one another largely through loss or gain of a water molecule, or of an oxygen atom — moving, for instance, from an ortho-acid to its pyro- or meta-acid by condensation. The structural diagram (Fig. 7.4) lays out several of these acids side by side: orthophosphoric acid $H_3PO_4$ (a tetrahedral P bearing one $=O$ and three $-OH$ groups); pyrophosphoric acid $H_4P_2O_7$ (two such tetrahedra bridged through a single $P-O-P$ oxygen); orthophosphorous acid $H_3PO_3$ and hypophosphorous acid $H_3PO_2$ (each retaining one $P=O$ but replacing one or two of the $-OH$ groups with a direct $P-H$ bond); pyrophosphorous acid $H_4P_2O_5$; and the ring and chain forms of metaphosphoric acid — cyclotrimetaphosphoric acid as a six-membered $P_3O_3$ ring with pendant $=O$ and $-OH$ groups, and polymetaphosphoric acid as an open, indefinitely repeating $P-O-P-O-\ldots$ chain.

In every one of these structures, phosphorus sits at the centre of a tetrahedron, and every oxoacid contains at least one $P=O$ bond and one $P-OH$ bond. Acids in which phosphorus carries a lower oxidation state (below +5) additionally contain either a $P-P$ bond (as in $H_4P_2O_6$) or a $P-H$ bond (as in $H_3PO_2$) — but never both in the same acid.

Disproportionation

Acids where phosphorus is in the +3 state are unstable with respect to disproportionation into the +5 and −3 states. On heating, orthophosphorous acid (phosphorous acid) disproportionates into orthophosphoric acid and phosphine:

Reducing Character

Any oxoacid that retains a direct $P-H$ bond is a strong reducing agent, because that hydrogen is available to be transferred. Hypophosphorous acid, with two $P-H$ bonds, is a particularly effective reductant — it will reduce silver nitrate to metallic silver:

Basicity …