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 ClO−4 and BF−4 salts are slightly soluble in acetonitrile CH3CN. NOBF4 can be purified by sublimation at 200–250 °C and 0.01 mmHg (1.3 Pa).[2]
NO+ is isoelectronic with CO, CN− and N2. It arises via protonation of nitrous acid:
In its infrared spectrum of its salts, νNO is a strong peak in the range 2150-2400 cm−1.[3]
NO+ reacts readily with water to form nitrous acid:
For this reason, nitrosonium compounds must be protected from water or even moist air. With base, the reaction generates nitrite:
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.
NO+, e.g. as NOBF4, is a strong oxidizing agent:[4]
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.
Electron-rich arenes are nitrosylated using NOBF4.[5] One example involves anisole:
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).
NOBF4 reacts with some metal carbonyl complexes to yield related metal nitrosyl complexes.[6] In some cases, [NO]+ does not bind the metal nucleophile but acts as an oxidant.