Tetrafluoroborate

The structure of the tetrafluoroborate anion, BF₄⁻

Tetrafluoroborate is the anion BF₄⁻. This tetrahedral species is isoelectronic with tetrafluoromethane, CF₄ and tetrafluoroammonium NF₄⁺, and is valence isoelectronic with many stable and important species including the closely related anion perchlorate, ClO₄⁻. It arises by the reaction of fluoride salts with the Lewis acid BF₃, treatment of tetrafluoroboric acid with base, or by treatment of boric acid with hydrofluoric acid.

As an anion in inorganic and organic chemistry

The popularization of BF₄⁻ has led to decreased use of ClO₄⁻ in the laboratory. With organic compounds, especially amine derivatives, ClO₄⁻ forms potentially explosive derivatives. One disadvantage to BF₄⁻ is its slight sensitivity to hydrolysis, whereas ClO₄⁻ does not suffer from this problem. Safety considerations, however, overshadow this inconvenience.

The utility of BF₄⁻ arises because its salts are often more soluble in organic solvents than the related nitrate or halide salts. Furthermore, BF₄⁻ is less nucleophilic and basic than nitrates and halides. Thus, when using salts of BF₄⁻, one can usually assume that the cation is the reactive agent and this tetrahedral anion is inert. BF₄⁻ owes its inertness to two factors: (i) it is symmetrical so that the negative charge is distributed equally over several (four) atoms, and (ii) it is composed of highly electronegative fluorine atoms, which diminish the basicity of the anion. Related to BF₄⁻ is hexafluorophosphate, PF₆⁻, which is even more stable toward hydrolysis and whose salts tend to be more lipophilic.

Illustrative of a fluoroborate salt is [Ni(CH₃CH₂OH)₆](BF₄)₂, a kinetically labile octahedral complex, which is used as a source of Ni²⁺.^[1]

Extremely reactive cations such as those derived from Ti, Zr, Hf, and Si do in fact abstract fluoride from BF₄⁻, so in such cases BF₄⁻ is not an "innocent" anion and less coordinating anions must be employed.

Transition and heavy metal fluoroborates are produced in the same manner as other fluoroborate salts; the respective metal salts are added to reacted boric and hydrofluoric acids. Tin, lead, copper, and nickel fluoroborates are prepared through electrolysis of these metals in a solution containing HBF₄.

Examples of salts

Potassium fluoroborate is obtained by treating potassium carbonate with boric acid and hydrofluoric acid.

B(OH)₃ + 4 HF → HBF₄ + 3 H₂O

2 HBF₄ + K₂CO₃ → 2 KBF₄ + H₂CO₃

Fluoroborates of alkali metals and ammonium ions crystallize as water-soluble hydrates with the exception of potassium, rubidium, and caesium.

Fluoroborate salts are often associated with highly reactive compounds. Some examples include

• Diazonium compound of the formula ArN₂⁺ are often isolated as their BF₄⁻ salts (Ar = aryl group).
• Meerwein reagents such as OEt₃⁺, some of the strongest alkylating agents known, are famously obtained as BF₄⁻ salts.
• Nitrosonium tetrafluoroborate is a well known one-electron oxidizing agent
• Nitronium tetrafluoroborate is a nitration reagent.
• Ferrocenium salts, [Fe(C₅H₅)₂]⁺ are generally used as their tetrafluoroborates.
• Imidazolium and formamidinium salts, ionic liquids and precursors to stable carbenes.
• An electrochemical cycle involving Ferrous/Ferric Tetrafluoroborate is being used to replace thermal smelting of lead sulfide ores by the Doe Run Company.

See also

• Non-coordinating anion

References

1. Willem L. Driessen, Jan Reedijk (1992). "Solid Solvates: The Use of Weak Ligands in Coordination Chemistry". Inorganic Syntheses. Inorganic Syntheses 29: 111–118. doi:10.1002/9780470132609.ch27. ISBN 978-0-470-13260-9.