Nitrosonium

File:Nitrosonium-2D-dimensions.svg

The structure of the nitrosonium ion

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⁺ <math>\overrightarrow{\leftarrow}</math> 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.

File:Nitrosonium ion with amine reaction.png

Reaction of nitrosonium with aniline to form a diazonium salt.

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

See also

• nitronium

References

• N. G. Connelly, W. E. Geiger, "Chemical Redox Agents for Organometallic Chemistry" Chemical Reviews 1996, vol. 96, pp. 877-910.
• T. W. Hayton, P. Legzdins, W. B. Sharp "Coordination and Organometallic Chemistry of Metal-NO Complexes" Chemical Reviews 2002, volume 102, pp. 935-991
• E. Bosch and J. K. Kochi, "Direct Nitrosation of Aromatic Hydrocarbons and Ethers with the Electrophilic Nitrosonium Cation" Journal of Organic Chemistry, 1994, volume 59, pp. 5573–5586.
• G. A. Olah, G. K. S. Prakash, Q. Wang,X.-Y. Li ”Nitrosylsulfuric Acid” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.