Tetrafluoroborate
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[[Image:Tetrafluoroborate-ion-3D-balls.png|thumb|right|The structure of the '''tetrafluoroborate''' anion, BF<sub>4</sub><sup>−</sup>]]
'''Tetrafluoroborate''' is the [[anion]] BF<sub>4</sub><sup>−</sup>. This tetrahedral species is [[isoelectronic]] with many stable and important species including tetrafluomethane, CF<sub>4</sub>, and the closely related anion [[perchlorate]], ClO<sub>4</sub><sup>−</sup>. It arises by the reaction of fluoride salts with the [[Lewis acid]] [[boron trifluoride|BF<sub>3</sub>]] or by treatment of tetrafluoroboric acid with base.
==BF<sub>4</sub><sup>−</sup> as an anion in inorganic and organic chemistry==
The popularization of BF<sub>4</sub><sup>−</sup> has led to decreased use of ClO<sub>4</sub><sup>−</sup> in the laboratory. With organic compounds, especially amine derivatives, ClO<sub>4</sub><sup>−</sup> forms potentially explosive derivatives. One disadvantage to BF<sub>4</sub><sup>−</sup> is its slight sensitivity to hydrolysis, whereas ClO<sub>4</sub><sup>−</sup> does not suffer from this problem. Safety considerations, however, overshadow this inconvenience.
The utility of BF<sub>4</sub><sup>−</sup> arises because its salts are often more soluble in organic solvents than the related nitrate or halide salts. Furthermore, BF<sub>4</sub><sup>−</sup> is less nucleophilic and basic than nitrates and halides. Thus, when using salts of BF<sub>4</sub><sup>−</sup>, one can usually assume that the cation is the reactive agent and this tetrahedral anion is inert. BF<sub>4</sub><sup>−</sup> 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<sub>4</sub><sup>−</sup> is [[hexafluorophosphate]], PF<sub>6</sub><sup>−</sup>, which is even more stable toward hydrolysis and whose salts tend to be more lipophilic.
Illustrative of a fluoroborate salt is [Ni(CH<sub>3</sub>CH<sub>2</sub>OH)<sub>6</sub>](BF<sub>4</sub>)<sub>2</sub>, a kinetically labile octahedral complex, which is used as a source of Ni<sup>2+</sup>.<ref>{{cite journal | author = Willem L. Driessen, Jan Reedijk | title = Solid Solvates: The Use of Weak Ligands in Coordination Chemistry | journal = [[Inorganic Syntheses]] | year = 1992 | volume = 29 | pages = 111–118 | doi = 10.1002/9780470132609.ch27}}</ref>
Extremely reactive cations such as those derived from Ti, Zr, Hf, and Si do in fact abstract fluoride from BF<sub>4</sub><sup>−</sup>, so in such cases BF<sub>4</sub><sup>−</sup> is not an "innocent" anion and [[non-coordinating anion|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 HBF<sub>4</sub>.
==Examples of BF<sub>4</sub><sup>−</sup> salts==
Potassium fluoroborate is obtained by treating [[potassium carbonate]] with boric acid and hydrofluoric acid.
:: 2 HBF<sub>4</sub> + K<sub>2</sub>CO<sub>3</sub> → 2 KBF<sub>4</sub> + H<sub>2</sub>CO<sub>3</sub>
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:
* [[diazonium compound]] of the formula ArN<sub>2</sub><sup>+</sup> are often isolated as their BF<sub>4</sub><sup>−</sup> salts (Ar = aryl group).
* [[Meerwein salt|Meerwein reagents]] such as OEt<sub>3</sub><sup>+</sup>, some of the strongest alkylating agents known, are famously obtained as BF<sub>4</sub><sup>−</sup> salts.
* [[Nitrosonium]] tetrafluoroborate is a well known one-electron [[oxidizing agent]]
* [[Nitronium]] tetrafluoroborate is a [[nitration]] reagent.
* [[Ferrocenium]] salts, [Fe(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>]<sup>+</sup> are generally used as their tetrafluoroborates.
* [[Imidazolium]] and [[formamidinium]] salts, precursors to [[stable carbene]]s.
==See also==
* [[Non-coordinating anion]]
==References==
<references/>
[[Category:Fluorides]]
[[Category:Non-coordinating anions]]