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Tetrafluoridoborate is the anion BF4. This tetrahedral species is isoelectronic with many stable and important species including tetrafluoromethane, CF4, and the closely related anion perchlorate, ClO4. It arises by the reaction of fluoride salts with the Lewis acid BF3 or by treatment of tetrafluoridoboric acid with base.
BF4 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 hexafluoridophosphate, PF6, which is even more stable toward hydrolysis and whose salts tend to be more lipophilic.
Illustrative of a fluoridoborate salt is (BF4)2, a kinetically labile octahedral complex, which is used as a source of Ni.
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 fluoridoborates are produced in the same manner as other fluoridoborate salts; the respective metal salts are added to reacted boric and hydrofluoric acids. Tin, lead, copper, and nickel fluoridoborates are prepared through electrolysis of these metals in a solution containing HBF4.
Examples of BF4 salts
Potassium fluoridoborate is obtained by treating potassium carbonate with boric acid and hydrofluoric acid.
- 2 HBF4 + K2CO3 → 2 KBF4 + H2CO3
Fluoridoborates of alkali metals and ammonium ions crystallize as water-soluble hydrates with the exception of potassium, rubidium, and caesium.
Fluoridoborate salts are often associated with highly reactive compounds. Some examples:
- 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 tetrafluoridoborate is a well known one-electron oxidizing agent
- Nitronium tetrafluoridoborate is a nitration reagent.
- Ferrocenium salts, are generally used as their tetrafluoridoborates.
- Imidazolium and formamidinium salts, precursors to stable carbenes.
See also
References
- Willem L. Driessen, Jan Reedijk (1992). "Solid Solvates: The Use of Weak Ligands in Coordination Chemistry". Inorganic Syntheses. 29: 111–118. doi:10.1002/9780470132609.ch27.