Ferrocenium tetrafluoroborate is an organometallic compound with the formula [Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>]BF<sub>4</sub>. This salt is composed of the cation [Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>]<sup>+</sup> and the tetrafluoroborate anion (). The related hexafluorophosphate is also a popular reagent with similar properties. The ferrocenium cation is often abbreviated Fc<sup>+</sup> or Cp<sub>2</sub>Fe<sup>+</sup>. The salt is deep blue in color and paramagnetic.
Ferrocenium salts are one-electron oxidizing agents, and the reduced product, ferrocene, is inert and often readily separated from ionic products. The ferroceneâÂÂferrocenium couple is used as a reference in electrochemistry. The standard potential of ferrocene-ferrocenium is dependent on specific electrochemical conditions.
Commercially available, this compound may be prepared by oxidizing ferrocene typically with ferric salts followed by addition of fluoroboric acid. A variety of other oxidants work well also, such as nitrosyl tetrafluoroborate. Many analogous ferrocenium salts are known.
According to X-ray crystallography, the structures of the metallocene component of FcBF<sub>4</sub> and the parent ferrocene are very similar. The Fe-C distances in the cation are 209.5 pm, about 2% longer than the Fe-C distances in ferrocene.