The nitronium ion, or sometimes the nitryl ion (incorrect because it is not a radical), NO+
2, is a generally unstable cation created by the removal of an electron from the paramagnetic nitrogen dioxide molecule, or the protonation of nitric acid.
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:
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 stable nitronium salts with anions of weak nucleophilicity can be isolated. These include nitronium perchlorate (NO+
4), nitronium tetrafluoroborate (NO+
4), nitronium hexafluorophosphate (NO+
6), nitronium hexafluoroarsenate (NO+
6), and nitronium hexafluoroantimonate (NO+
6). These are all very hygroscopic compounds.
The nitronium ion is isoelectronic with carbon dioxide and like that molecule has a linear structure with an ONO bond angle of 180°. For this reason it has a similar vibrational spectrum to carbon dioxide: the Raman active symmetrical stretch was first use to identify the ion in nitrating mixtures.
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).
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