Nitrosonium tetrafluoroborate, also called nitrosyl tetrafluoroborate, is a chemical compound with the chemical formula NOBF4. This colourless solid is used in organic synthesis as a nitrosating agent, diazotizing agent and a mild oxidant. NOBF4 is the nitrosonium salt of fluoroboric acid, and is composed of a nitrosonium cation, [NO]+, and a tetrafluoroborate anion, [BF4]−.

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The dominant property of NOBF4 is the oxidizing power and electrophilic character of the nitrosonium cation. It forms colored charge transfer complexes with hexamethylbenzene and with 18-crown-6. The latter, a deep yellow species, provides a means to dissolve NOBF4 in dichloromethane. Nitrosonium tetrafluoroborate may be used to prepare metal salts of the type [MII(CH3CN)x][BF4]2 (M = Cr, Mn, Fe, Co, Ni, Cu). The nitrosonium cation acts as the oxidizer, itself being reduced to nitric oxide gas: With ferrocene the ferrocenium tetrafluoroborate is formed. In its infrared spectrum of this salt, νNO is a strong peak at 2387 cm−1.