Tetrafluoroborate
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Tetrafluoroborate is the
anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
. This tetrahedral species is
isoelectronic Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in th ...
with tetrafluoroberyllate (),
tetrafluoromethane Tetrafluoromethane, also known as carbon tetrafluoride or R-14, is the simplest perfluorocarbon ( C F4). As its IUPAC name indicates, tetrafluoromethane is the perfluorinated counterpart to the hydrocarbon methane. It can also be classified as a ...
(CF4), and
tetrafluoroammonium The tetrafluoroammonium cation (also known as perfluoroammonium) 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 ...
() and is valence isoelectronic with many stable and important species including the
perchlorate A perchlorate is a chemical compound containing the perchlorate ion, . The majority of perchlorates are commercially produced salts. They are mainly used as oxidizers for pyrotechnic devices and to control static electricity in food packaging. Per ...
anion, , 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 Fluoroboric acid or tetrafluoroboric acid (archaically, fluoboric acid) is an inorganic compound with the chemical formula +BF4−], where H+ represents the solvated proton. The solvent can be any suitably Lewis-basic entity. For instance, in w ...
with base, or by treatment of
boric acid Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula . It may also be called hydrogen borate or boracic acid. It is usually encountered as colorless crystals or a white powder, that dissolve ...
with hydrofluoric acid.


As an anion in inorganic and organic chemistry

The popularization of has led to decreased use of in the laboratory as a weakly coordinating anion. With organic compounds, especially amine derivatives, forms potentially explosive derivatives. Disadvantages to include its slight sensitivity to
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolys ...
and decomposition via loss of a fluoride ligand, whereas does not suffer from these problems. Safety considerations, however, overshadow this inconvenience. With a formula weight of 86.8, BF is also conveniently the smallest weakly coordinating anion from the point of view of equivalent weight, often making it the anion of choice for preparing cationic reagents or catalysts for use in synthesis, in the absence of other substantial differences in chemical or physical factors. The anion is less nucleophilic and basic (and therefore more weakly coordinating) than nitrates, halides or even
triflates In organic chemistry, triflate (systematic name: trifluoromethanesulfonate), is a functional group with the formula and structure . The triflate group is often represented by , as opposed to −Tf, which is the triflyl group, . For example, ...
. Thus, when using salts of , one can usually assume that the cation is the reactive agent and this tetrahedral anion is inert. owes its inertness to two factors: (i) it is symmetrical so that the negative charge is distributed equally over four atoms, and (ii) it is composed of highly electronegative fluorine atoms, which diminish the basicity of the anion. In addition to the weakly coordinating nature of the anion, salts are often more soluble in organic solvents (lipophilic) than the related nitrate or halide salts. Related to are
hexafluorophosphate Hexafluorophosphate is an anion with chemical formula of . It is an octahedral species that imparts no color to its salts. is isoelectronic with sulfur hexafluoride, , and the hexafluorosilicate dianion, , and hexafluoroantimonate . In this an ...
, , and hexafluoroantimonate, , both of which are even more stable toward hydrolysis and other chemical reactions and whose salts tend to be more lipophilic. Illustrative of a fluoroborate salt is i(CH3CH2OH)6BF4)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 , so in such cases is not an "innocent" anion and less coordinating anions (e.g., SbF6–, BARF–, or l((CF3)3CO)4sup>–) must be employed. Moreover, in other cases of ostensibly "cationic" complexes, the fluorine atom in fact acts as a bridging ligand between boron and the cationic center. For instance, the gold complex ¼-(DTBM-SEGPHOS)(Au–BF4)2.html" ;"title="SEGPHOS.html" ;"title="¼-(DTBM-SEGPHOS">¼-(DTBM-SEGPHOS)(Au–BF4)2">SEGPHOS.html" ;"title="¼-(DTBM-SEGPHOS">¼-(DTBM-SEGPHOS)(Au–BF4)2was found crystallographically to contain two Au–F–B bridges. Despite the low reactivity of the tetrafluoroborate anion in general, serves as a fluorine source to deliver an equivalent of fluoride to highly electrophilic carbocationic species to generate carbon–fluorine bonds. The Balz–Schiemann reaction for the synthesis of aryl fluorides is the best known example of such a reaction. Ether and halopyridine adducts of HBF4 have been reported to be effective reagents for the Hydrohalogenation, hydrofluorination of alkynes. 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 is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...
,
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
,
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
, and
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
fluoroborates are prepared through electrolysis of these metals in a solution containing HBF4.


Examples of salts

Potassium fluoroborate is obtained by treating
potassium carbonate Potassium carbonate is the inorganic compound with the formula K2 CO3. It is a white salt, which is soluble in water. It is deliquescent, often appearing as a damp or wet solid. Potassium carbonate is mainly used in the production of soap and gl ...
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 Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosph ...
, rubidium, and
cesium Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that ar ...
. Fluoroborate is often used to isolate highly electrophilic cations. Some examples include: * Solvated proton (H+ (solv.),
fluoroboric acid Fluoroboric acid or tetrafluoroboric acid (archaically, fluoboric acid) is an inorganic compound with the chemical formula +BF4−], where H+ represents the solvated proton. The solvent can be any suitably Lewis-basic entity. For instance, in w ...
), including H+·(H2O)''n'' ("
hydronium In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid i ...
"), H+·(Et2O)''n'' * Diazonium compounds (). * Meerwein reagents such as , the strongest commercial alkylating agents. * NO+ a one-electron
oxidizing agent An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxi ...
and
nitrosylation Nitrosylation is the general term for covalent incorporation of a nitric oxide "nitrosyl" moiety into another (usually organic) molecule. There are multiple chemical mechanisms by which this can be achieved; including biological enzymes and indust ...
reagent. * NO2+, a
nitration In organic chemistry, nitration is a general class of chemical processes for the introduction of a nitro group into an organic compound. The term also is applied incorrectly to the different process of forming nitrate esters between alcohols an ...
reagent. *
Ferrocenium Ferrocenium tetrafluoroborate is an organometallic compound with the formula e(C5H5)2F4. This salt is composed of the cation e(C5H5)2sup>+ and the tetrafluoroborate anion (). The related hexafluorophosphate is also a popular reagent with simi ...
, , and other cationic metallocenes. * Selectfluor, a fluorination agent, and other N–F electrophilic fluorine sources. *Bromonium and iodonium species, including py2X+ (X = Br; X = I: Barluenga's reagent) and Ar2I+ ( diaryliodonium salts) *
Silver tetrafluoroborate Silver tetrafluoroborate is an inorganic compound with the chemical formula AgBF4. It is a white solid that dissolves in polar organic solvents as well as water. In its solid state, the Ag+ centers are bound to fluoride. Preparation Silver tetraf ...
and thallium tetrafluoroborate are convenient halide abstracting agents (although the
thallium Thallium is a chemical element with the symbol Tl and atomic number 81. It is a gray post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists William Crookes an ...
salt is highly toxic). Most other transition metal tetrafluoroborates only exist as solvates of water, alcohols, ethers, or nitriles. *
Transition metal nitrile complexes Transition metal nitrile complexes are coordination compounds containing nitrile ligands. Because nitriles are weakly basic, the nitrile ligands in these complexes are often labile. Scope of nitriles Typical nitrile ligands are acetonitrile, prop ...
, e.g. u(NCMe)4F4 An electrochemical cycle involving ferrous/ferric tetrafluoroborate is being used to replace thermal
smelting Smelting is a process of applying heat to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a ...
of
lead sulfide Lead sulfide refers to two compounds containing lead and sulfur: * Lead(II) sulfide, PbS, containing lead in the +2 oxidation state, naturally occurring as the mineral galena *Lead(IV) sulfide Lead(IV) sulfide is a chemical compound with the formu ...
ores by the
Doe Run Company The Doe Run Resources Corporation, known by the trade name The Doe Run Company, is a privately held natural resources company and global producer of lead, copper, and zinc concentrates. It owns four mills, six mines and a lead battery recycling pl ...
.
Imidazolium Imidazole (ImH) is an organic compound with the formula C3N2H4. It is a white or colourless solid that is soluble in water, producing a mildly alkaline solution. In chemistry, it is an aromatic heterocycle, classified as a diazole, and has non-a ...
and formamidinium salts, ionic liquids and precursors to stable carbenes, are often isolated as tetrafluoroborates.


See also

*
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 ...
*
Fluoroboric acid Fluoroboric acid or tetrafluoroboric acid (archaically, fluoboric acid) is an inorganic compound with the chemical formula +BF4−], where H+ represents the solvated proton. The solvent can be any suitably Lewis-basic entity. For instance, in w ...


References

{{Tetrafluoroborates Tetrafluoroborates, Non-coordinating anions