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# Nitrosonium

The nitrosonium ion is NO+, the nitrogen atom is bonded to an oxygen atom with a bond order of 3, the overall diatomic species bearing a positive charge. This ion is usually obtained as the following salts: NOClO4, NOSO4H (nitrosyl sulfuric acid, more descriptively written ONSO2OH), and NOBF4. The ClO4 and BF4 salts are slightly soluble in CH3CN. NOBF4 can be purified by sublimation at 200–250 °C/0.01 mmHg.

For NOBF4: Selected data: density = 2.185 g cm–3.3 MW =116.82

NO+ is isoelectronic with CO and N2. It arises via protonation of nitrous acid:

HONO + H+ $\overrightarrow{\leftarrow}$ NO+ + H2O

## Chemical Properties

### Hydrolysis

NO+ reacts readily with water to form nitrous acid:

NOBF4 + H2O → HONO + HBF4

For this reason, NOBF4 must be protected from water or even moist air. With base, the reaction generates nitrite:

NOBF4 + 2 NaOH → NaNO2 + NaBF4 + H2O

### As a diazotizing agent

NO+ reacts with aryl amines, ArNH2, to give diazonium salt, ArN2+. This is useful because N2+ is a more readily leaving group than NH2.

### 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)

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. The example involves anisole:

CH3OC6H5 + NOBF4 → CH3OC6H4-4-NO + HBF4

Nitrosonium, NO+, is sometimes confused with nitronium, NO2+, 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 NO complexes

NOBF4 reacts with some metal carbonyl complexes to yield related metal nitrosyl complexes. One must be careful that [NO]+ is transferred vs. electron transfer (see above).

(C6Et6)Cr(CO)3 + NOBF4 → [(C6Et6)Cr(CO)2(NO)]BF4 + CO