Ferrocenium tetrafluoroborate

Ferrocenium tetrafluoroborate is an organometallic compound with the formula BF₄. This salt is composed of the cation ⁺ 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 one-electron oxidizing agents, and the reduced product, ferrocene, is inert and often readily separated from ionic products. The ferrocene–ferrocenium couple is used as a reference in electrochemistry. The standard potential of ferrocene-ferrocenium is dependent on specific electrochemical conditions.

Preparation

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.

Structure

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.