Tetrafluoroborate: Difference between revisions

Tetrafluoroborate is the anion BF⁻
₄. This tetrahedral species is isoelectronic with tetrafluoromethane, CF₄ and tetrafluoroammonium NF⁺
₄, and is valence isoelectronic with many stable and important species including the perchlorate anion, ClO⁻
₄, which is used in similar ways in the laboratory. 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 BF⁻
₄ has led to decreased use of ClO⁻
₄ in the laboratory. With organic compounds, especially amine derivatives, ClO⁻
₄ forms potentially explosive derivatives. One disadvantage to BF⁻
₄ is its slight sensitivity to hydrolysis, whereas ClO⁻
₄ does not suffer from this problem. Safety considerations, however, overshadow this inconvenience.

The utility of BF⁻
₄ arises because its salts are often more soluble in organic solvents than the related nitrate or halide salts. Furthermore, BF⁻
₄ is less nucleophilic and basic than nitrates and halides. Thus, when using salts of BF⁻
₄, one can usually assume that the cation is the reactive agent and this tetrahedral anion is inert. BF⁻
₄ 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 BF⁻
₄ is hexafluorophosphate, PF⁻
₆, 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 BF⁻
₄, so in such cases BF⁻
₄ is not an "innocent" anion and less coordinating anions must be employed. Moreover, in other cases of ostensibly "cationic" complexes, the fluorine atom acts as a bridging ligand between boron and the cationic center. For instance, the gold complex [μ-(DTBM-SEGPHOS)(Au–BF₄)₂] was found crystallographically to contain two Au–F–B bridges.^[2]

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 is often used to isolate highly electrophilic cations. Some examples include:

• Diazonium compounds (ArN⁺
  ₂).
• Meerwein reagents such as OEt⁺
  ₃, some of the strongest alkylating agents known, are famously obtained as BF⁻
  ₄ salts.
• NO⁺ a one-electron oxidizing agent.
• NO₂⁺, a nitration reagent.
• Ferrocenium salts, Fe(C
  ₅H
  ₅)⁺
  ₂.
• Imidazolium and formamidinium salts, ionic liquids and precursors to stable carbenes.
• Selectfluor a fluorination agent
• Silver tetrafluoroborate and thallium tetrafluoroborate^[3] are convenient halide abstracting agents (although the thallium salt is highly toxic). Most transition metal tetrafluoroborates exist as solvates (e.g., [Cu(NCMe)₄][BF₄]).

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