Nitronium ion

    The nitronium ion (improperly called nitryl ion, because it is not a radical), NO₂⁺ is a generally unstable cation created by the removal of an electron from the paramagnetic nitrogen dioxide molecule, or the protonation of nitric acid.

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It is not stable enough to exist in normal conditions, but it is used extensively as an electrophile in the nitration of other substances. The ion is generated in situ for this purpose by mixing sulfuric acid and nitric acid according to the equilibrium:

H₂SO₄ + HNO₃ ⇌ HSO₄⁻ + NO₂⁺ + H₂O

The nitronium ion also exists in the solid form of dinitrogen pentoxide, N₂O₅, which is an ionic solid formed from nitronium and nitrate ions. Its liquid and gaseous forms, however, are molecular and do not contain nitronium ions. A few nitronium salts with anions of low nucleophilic power, such as nitronium perchlorate, NO₂⁺ClO₄⁻ can be isolated but are extremely reactive.

The nitronium ion is isoelectronic with carbon dioxide and like that compound has a linear structure with a ONO bond angle of 180°.

Related species

The compounds nitryl fluoride, NO₂F, and nitryl chloride, NO₂Cl, are not nitronium salts but rather molecular compounds, as shown by their low boiling points (−72 °C and −6 °C respectively) and short N-X bond lengths (N-F 135 pm, N-Cl 184 pm).^[1]

Addition of one electron forms the neutral nitryl radical, ·NO₂; in fact, this is fairly stable and known as the compound nitrogen dioxide.

The related negatively charged species is NO₂⁻, the nitrite ion.

Notes and references

1. ^ F.A.Cotton and G.Wilkinson, Advanced Inorganic Chemistry, 5th edition (1988), Wiley, p.333