Ferrocenium tetrafluoroborate is an organometallic compound with the formula [Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>]BF4. This salt is composed of the cation [Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>]+ and the tetrafluoroborate anion . The related hexafluorophosphate is also a popular reagent with similar properties. The ferrocenium cation is often abbreviated Fc+ or Cp2Fe+. The salt is deep blue in color and paramagnetic.Ferrocenium salts are sometimes used as one-electron oxidizing agents, and the reduced product, ferrocene, is inert and readily separated from ionic products. The ferrocene–ferrocenium couple is often 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.[1] A variety of other oxidants work well also, such as nitrosyl tetrafluoroborate.[2] Many analogous ferrocenium salts are known.[3]
According to X-ray crystallography, the structures of the metallocene component of FcBF4 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.[4]