Nitrosonium
Diatomic cation
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Through Nitrogen, Oxygen and Carbon monoxide
Synthesis and spectroscopy
NO+ is isoelectronic with CO, CN− and N2. It arises via protonation of nitrous acid:
- HONO + H+ ⇌ NO+ + H2O
In its infrared spectrum of its salts, νNO is a strong peak in the range 2150–2400 cm−1, indicating it is a loose cation rather than coordinated to another species.
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Chemical properties
Hydrolysis
NO+ reacts readily with water to form nitrous acid:
- NO+ + H2O → HONO + H+
For this reason, nitrosonium compounds must be protected from water or even moist air. With base, the reaction generates nitrite:
- NO+ + 2 NaOH → NaNO2 + Na+ + H2O
As a diazotizing agent
NO+ reacts with aryl amines, ArNH2, to give diazonium salts, ArN+2. The resulting diazonium group is easily displaced (unlike the amino group) by a variety of nucleophiles.
As an oxidizing agent
NO+, e.g. as NOBF4, is a strong oxidizing agent:
- vs. ferrocene/ferrocenium, [NO]+ in CH2Cl2 solution has a redox potential of 1.00 V (or 1.46–1.48 V vs SCE),
- vs. ferrocene/ferrocenium, [NO]+ in CH3CN solution has a redox potential of 0.87 V vs. (or 1.27–1.25 V vs SCE).
In organic chemistry, it selectively cleaves ethers and oximes, and couples diarylamines.
NOBF4 is a convenient oxidant because the byproduct NO is a gas, which can be swept from the reaction using a stream of N2. Upon contact with air, NO forms NO2, which can cause secondary reactions if it is not removed. NO2 is readily detectable by its characteristic orange color.
Nitrosylation of arenes
Electron-rich arenes are nitrosylated using NOBF4. One example involves anisole:
- CH3OC6H5 + NOBF4 → CH3OC6H4NO + HBF4
Nitrosonium, NO+, is sometimes confused with nitronium, NO+
2, the active agent in nitrations. These species are quite different, however. Nitronium is a more potent electrophile than is nitrosonium, as anticipated by the fact that the former is derived from a strong acid (nitric acid) and the latter from a weak acid (nitrous acid).
As a source of nitrosyl complexes
NOBF4 reacts with some metal carbonyl complexes to yield related metal nitrosyl complexes. In some cases, [NO]+ does not bind the metal nucleophile but acts as an oxidant.
- (C6Et6)Cr(CO)3 + NOBF4 → [(C6Et6)Cr(CO)2(NO)]BF4 + CO