The nitronium ion (also called the nitryl ion), ₂⁺ is a generally unstable cation created by the removal of an electron from the paramagnetic nitrogen dioxide molecule.

It is not stable enough to exist in normal conditions, but it is used extensively 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 the nitrogen oxide ₂₅, 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 O�N�O bond angle of 180°.

Cations

Nitronium | Nitroniumjon