Question

Which of the following contain $P - P$ bond?
(A) \[{P_2}O_7^{4 - }\]
(B) \[{P_2}O_6^{3 - }\]
(C) \[{P_2}O_9^{3 - }\]
(D) \[{P_2}O_6^{4 - }\]

Answer 1

Hint: P-P bond means two phosphorus atoms are joined with each other and form a bridge between the molecules. P-P bond is a covalent bond, formed by the sharing of electrons by different atoms present in a molecule.

Complete step by step answer:
The prefix 'hypo-' means beneath or less than. Hypophosphoric acid has an oxidation state as \[ + 4\] that is in between phosphoric acid and phosphorus acid. Thus, "hypophosphoric" refers to less than phosphoric but above the phosphorous acid.
Among the given options, hypophosphoric acid i.e. \[{H_4}{P_2}O_6^{4 - }\] is a mineral acid. It has a \[ + 4\] oxidation state. The two phosphorus atoms of the molecule form a covalent P-P bond as shown in the figure.

The structure of hypophosphorous acid is containing two P-O bonds with \[151{\text{ }}pm\] length, \[P - OH\] bond with \[151{\text{ }}pm\] length and P-P bond of \[219{\text{ }}pm\] length. Hypophosphoric acid also has oxonium ions with formula ${[{H_3}{O^ + }]_2}{[{H_2}{P_2}{O_6}]^{2 - }}$ . The hypophosphoric acid is isostructural with the diammonium salt which containing ${[HOP{O_2}P{O_2}OH]^{2 - }}$ anion with a P−P bond length of \[219{\text{ }}pm.\]
Dihydrate ${H_4}{O_2}{P_6}.2{H_2}O$ is the solid state of hypophosphoric acid, formed by the reaction of red phosphorus and sodium chlorite at room temperature.
$2P + 2NaCl{O_2} + 2{H_2}O \to N{a_2}{H_2}{P_2}{O_6} + 2HCl$
Hence, the correct option is (A) \[{P_2}O_7^{4 - }\].

Additional information:
Hypophosphoric acid is used as a bleaching agent, reducing agent, stimulant in pharmaceutical agents and as a wetting agent. Hypophosphoric acid is triprotic and have dissociation constants \[p{K_{a1}}\; = {\text{ }}2.2,{\text{ }}p{K_{a2}}\; = {\text{ }}2.8,{\text{ }}p{K_{a3}}\; = {\text{ }}7.3{\text{ }}and{\text{ }}p{K_{a4}}\; = {\text{ }}10.0\].

Note:
In air the hypophosphites oxidise and form pyrophosphates containing the \[{P_2}O_7^{4 - }\] ion where P has an oxidation state of \[ + 5\]. They are stable to alkali hydroxides. It converts to the orthophosphate when fused with sodium hydroxide.