Tetrafluoroborate
Encyclopedia
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
.
, 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.
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.
with boric acid and hydrofluoric acid.
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
Tetrafluoromethane
Tetrafluoromethane, also known as carbon tetrafluoride, is the simplest fluorocarbon . It has a very high bond strength due to the nature of the carbon–fluorine bond. It can also be classified as a haloalkane or halomethane...
, CF4 and tetrafluoroammonium
Tetrafluoroammonium
The tetrafluoroammonium cation is a positively-charged polyatomic ion with chemical formula . It is equivalent to the ammonium ion where the hydrogen atoms surrounding the central nitrogen atom have been replaced by fluorine....
NF4+, and is valence isoelectronic with many stable and important species including the closely related anion perchlorate
Perchlorate
Perchlorates are the salts derived from perchloric acid . They occur both naturally and through manufacturing. They have been used as a medicine for more than 50 years to treat thyroid gland disorders. They are used extensively within the pyrotechnics industry, and ammonium perchlorate is also a...
, ClO4−. It arises by the reaction of fluoride salts with the Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
BF3
Boron trifluoride
Boron trifluoride is the chemical compound with the formula BF3. This pungent colourless toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds.-Structure and bonding:...
, treatment of tetrafluoroboric acid with base, or by treatment of boric acid
Boric acid
Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...
with hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
.
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 hydrolysisHydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
, 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
Hexafluorophosphate
Hexafluorophosphate is an anion with chemical formula of . This octahedral species is isoelectronic with sulfur hexafluoride, SF6, and is valence isoelectronic with the highly stable superacid anion fluoroantimonate . As a non-coordinating anion, it is a poor nucleophile...
, 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
Non-coordinating anion
Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is 'weakly coordinating anion'. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found as counterions for cationic metal complexes...
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
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
, lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
fluoroborates are prepared through electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
of these metals in a solution containing HBF4.
Examples of BF4− salts
Potassium fluoroborate is obtained by treating potassium carbonatePotassium carbonate
Potassium carbonate is a white salt, soluble in water , which forms a strongly alkaline solution. It can be made as the product of potassium hydroxide's absorbent reaction with carbon dioxide. It is deliquescent, often appearing a damp or wet solid...
with boric acid and hydrofluoric acid.
-
- 2 HBF4 + K2CO3 → 2 KBF4 + H2CO3
Fluoroborates of alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
metals and ammonium ions crystallize as water-soluble hydrates with the exception of potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
, rubidium
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...
, and caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
.
Fluoroborate salts are often associated with highly reactive compounds. Some examples include
- Diazonium compoundDiazonium compoundDiazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group R-N2+ X- where R can be any organic residue such alkyl or aryl and X is an inorganic or organic anion such as a halogen...
of the formula ArN2+ are often isolated as their BF4− salts (Ar = aryl group). - Meerwein reagentsMeerwein saltTriethyloxonium tetrafluoroborate is the organic oxonium compound with the formula [3O]BF4. It is often called Meerwein's reagent after its discoverer Hans Meerwein. Also well known and commercially available is the related trimethyloxonium tetrafluoroborate. The compounds are exceptionally...
such as OEt3+, some of the strongest alkylating agents known, are famously obtained as BF4− salts. - NitrosoniumNitrosoniumThe nitrosonium ion is NO+, in which the nitrogen atom is bonded to an oxygen atom with a bond order of 3, and the overall diatomic species bears a positive charge. This ion is usually obtained as the following salts: NOClO4, NOSO4H , and NOBF4. The ClO and BF salts are slightly soluble in CH3CN...
tetrafluoroborate is a well known one-electron oxidizing agentOxidizing agentAn oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction... - Nitronium tetrafluoroborateNitronium tetrafluoroborateNitronium tetrafluoroborate is an inorganic compound with formula NO2BF4. It is a salt of nitronium cation and tetrafluoroborate anion. It is a colorless crystalline solid, which reacts heavily with water to form the corrosive acids HF and HNO3. As such, it must be handled under water-free conditions...
is a nitrationNitrationNitration is a general chemical process for the introduction of a nitro group into a chemical compound. The dominant application of nitration is for the production of nitrobenzene, the precursor to methylene diphenyl diisocyanate...
reagent. - Ferrocenium salts, [Fe(C5H5)2]+ are generally used as their tetrafluoroborates.
- Imidazolium and formamidiniumFormamidiniumFormamidinium cations are positively charged organic chemical compounds which can be represented by the chemical formula [R2N-CH=NR2]+.-Chemical applications:...
salts, precursors to stable carbenes. - An electrochemical cycle involving Ferrous/Ferric Tetraflouroborate is being used to replace thermal smeltingSmeltingSmelting is a form of extractive metallurgy; its main use is to produce a metal from its ore. This includes iron extraction from iron ore, and copper extraction and other base metals from their ores...
of lead sulfideLead sulfideLead sulfide is an ionic compound of lead and sulfur, having two possible proportions:*Lead sulfide, the ionic compound containing lead in the +2 oxidation state*Lead sulfide, the ionic compound containing lead in the +4 oxidation state...
ores by the Doe Run CompanyDoe Run CompanyDoe Run Resources Corporation is the largest integrated lead producer in North America and the largest primary lead producer in the western world...
.