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+.
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. 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 [(P^P)(Au–BF4)2] (P^P = DTBM-SEGPHOS) was found crystallographically to contain two Au–F–B bridges.
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
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.
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