Nitrosonium

The nitrosonium ion is NO+, in which the nitrogen atom is bonded to an oxygen atom with a bond order of 3, and the overall diatomic species bears a positive charge. It can be viewed as nitric oxide with one electron removed. This ion is usually obtained as the following salts: NOClO4, NOSO4H (nitrosylsulfuric acid, more descriptively written ONSO3OH) and NOBF4. The ClO4(−) and BF4(−) salts are slightly soluble in acetonitrile CH3CN. NOBF4 can be purified by sublimation at 200–250 °C and 0.01 mmHg.

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.

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, ArN2(+). 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).

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