Non-coordinating anion

<h2 id="pre-barf-era">Pre-"BARF" era</h2>
Before the 1990s, <a href="/facts/Tetrafluoroborate/46EEToo6">tetrafluoroborate</a>, <a href="/facts/Hexafluorophosphate/mXYXC2Bf">hexafluorophosphate</a>, and <a href="/facts/Perchlorate/WCC53zS2">perchlorate</a> were considered weakly coordinating anions. Only by exclusion of conventional solvents were <a href="/facts/Transition_metal_perchlorate_complexes/LxdhUe9u">transition metal perchlorate complexes</a> found to exist, for example. It is now appreciated that BF−4, PF−6, and ClO−4 bind to strongly electrophilic metal centers of the type use in some catalytic reactions.<a class="footnote-ref" id="fnref:2" href="#fn:2">2</a><a class="footnote-ref" id="fnref:3" href="#fn:3">3</a> Tetrafluoroborate and hexafluorophosphate anions are coordinating toward highly electrophilic metal ions, such as cations containing Zr(IV) centers, which can abstract <a href="/facts/Fluoride/LpqNMzwa">fluoride</a> from these anions. Other anions, such as <a href="/facts/Triflate/6sbvDSyt">triflates</a> are considered to be low-coordinating with some cations.

<h2 id="era-of-barf">Era of BARF</h2>

A revolution in this area occurred in the 1990s with the introduction of the <a href="/facts/Tetrakis(3%2c5-bis(trifluoromethyl)phenyl)borate/I2htIrYD">tetrakis[3,5-bis(trifluoromethyl)phenyl]borate</a> ion, B[3,5-(CF3)2C6H3]−4, commonly abbreviated as B(ArF)4− and colloquially called "BARF".<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a> This anion is far less coordinating than tetrafluoroborate, hexafluorophosphate, and perchlorate, and consequently has enabled the study of still more electrophilic cations.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a> Related tetrahedral anions include <a href="/facts/Lithium_tetrakis(pentafluorophenyl)borate/mK9Chcvt">tetrakis(pentafluorophenyl)borate</a> B(C6F5)−4, and <a href="/facts/Polyfluoroalkoxyaluminates/Y1E1XWCm">Al[OC(CF3)3]−4.</a>

In the bulky borates and aluminates, the <a href="/facts/Electron_density/L5yDsk6j">negative charge</a> is symmetrically distributed over many <a href="/facts/Electronegative/H7Vq7Uah">electronegative</a> atoms. Related anions are derived from <a href="/facts/Tris(pentafluorophenyl)boron/DpzRgHAH">tris(pentafluorophenyl)boron</a> B(C6F5)3. Another advantage of these anions is that their salts are more <a href="/facts/Soluble/kLE9h3Jb">soluble</a> in non-polar organic solvents such as <a href="/facts/Dichloromethane/mHyeTD2D">dichloromethane</a>, <a href="/facts/Toluene/Pzdt8dwD">toluene</a>, and, in some cases, even <a href="/facts/Alkane/Qam7Hvmf">alkanes</a>. <a href="/facts/Polar_solvent/LoMPMz8S">Polar solvents</a>, such as <a href="/facts/Acetonitrile/fNpbvRp6">acetonitrile</a>, <a href="/facts/THF/P5t6SkMK">THF</a>, and <a href="/facts/Water/0lkXnJPK">water</a>, tend to bind to electrophilic centers, in which cases, the use of a non-coordinating anion is pointless.
Salts of the anion B[3,5-(CF3)2C6H3]−4 were first reported by Kobayashi and co-workers. For that reason, it is sometimes referred to as Kobayashi's anion.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a> Kobayashi's method of preparation has been superseded by a safer route.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a>

The neutral molecules that represent the parents to the non-coordinating anions are strong Lewis acids, e.g. <a href="/facts/Boron_trifluoride/2qa0NIH8">boron trifluoride</a>, BF3 and <a href="/facts/Phosphorus_pentafluoride/VbqstNnq">phosphorus pentafluoride</a>, PF5. A notable Lewis acid of this genre is <a href="/facts/Tris(pentafluorophenyl)borane/DpzRgHAH">tris(pentafluorophenyl)borane</a>, B(C6F5)3, which abstracts alkyl <a href="/facts/Ligand/6etB3qeW">ligands</a>:<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a>

(C5H5)2Zr(CH3)2 + B(C6F5)3 → [(C5H5)2Zr(CH3)]+[(CH3)B(C6F5)3]−
<h2 id="other-types-of-non-coordinating-anions">Other types of non-coordinating anions</h2>
Another large class of non-coordinating anions are derived from <a href="/facts/Carborane/rBGVnA2R">carborane</a> anion CB11H−12. Using this anion, the first example of a three-coordinate silicon compound, the salt [(<a href="/facts/Mesityl/4JCRbj1Y">mesityl</a>)3Si][HCB11Me5Br6] contains a non-coordinating anion derived from a carborane.<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">I. Krossing & I. Raabe (2004). "Noncoordinating Anions - Fact or Fiction? A Survey of Likely Candidates". Angewandte Chemie International Edition. 43 (16): 2066–2090. doi:10.1002/anie.200300620. PMID 15083452. <a href="/wiki/Angewandte_Chemie_International_Edition" target="_blank">/wiki/Angewandte_Chemie_International_Edition</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Honeychuck, R. V.; Hersh, W. H. (1989). "Coordination of "Noncoordinating" Anions: Synthesis, Characterization, and X-ray Crystal Structures of Fluorine-Bridged [SbF6]−, [BF4]−, and [PF6]− Adducts of [R3P(CO)3(NO)W]+. An Unconventional Order of Anion Donor Strength". Inorganic Chemistry. 28 (14): 2869–2886. doi:10.1021/ic00313a034. <a href="/wiki/Inorganic_Chemistry_(journal)" target="_blank">/wiki/Inorganic_Chemistry_(journal)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Mayfield, H. G.; Bull, W. E. (1971). "Co-ordinating Tendencies of the Hexafluorophosphate Ion". J. Chem. Soc. A (14): 2279–2281. doi:10.1039/J19710002279. <a href="/wiki/Journal_of_the_Chemical_Society" target="_blank">/wiki/Journal_of_the_Chemical_Society</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">N. A. Yakelis; R. G. Bergman (2005). "Sodium Tetrakis(3,5-trifluoromethyl)phenylborate (NaBArF24): Safe Preparation, Standardized Purification, and Analysis of Hydration". Organometallics. 24 (14): 3579–3581. doi:10.1021/om0501428. PMC 2600718. PMID 19079785. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2600718" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2600718</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">M. Brookhart; B. Grant; A. F. Volpe, Jr. (1992). "[(3,5-(CF3)2C6H3)4B]-[H(OEt2)2]+: a convenient reagent for generation and stabilization of cationic, highly electrophilic organometallic complexes". Organometallics. 11 (11): 3920–3922. doi:10.1021/om00059a071. <a href="/wiki/Maurice_Brookhart" target="_blank">/wiki/Maurice_Brookhart</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">H. Nishida; N. Takada; M. Yoshimura; T. Sonods; H. Kobayashi (1984). "Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate. Highly lipophilic stable anionic agent for solvent-extraction of cations". Bulletin of the Chemical Society of Japan. 57 (9): 2600. doi:10.1246/bcsj.57.2600. <a href="https://doi.org/10.1246%2Fbcsj.57.2600" target="_blank">https://doi.org/10.1246%2Fbcsj.57.2600</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">N. A. Yakelis; R. G. Bergman (2005). "Sodium Tetrakis(3,5-trifluoromethyl)phenylborate (NaBArF24): Safe Preparation, Standardized Purification, and Analysis of Hydration". Organometallics. 24 (14): 3579–3581. doi:10.1021/om0501428. PMC 2600718. PMID 19079785. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2600718" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2600718</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">G.Erker (2005). "Tris(pentafluorophenyl)borane: a Special Boron Lewis Acid for Special Reactions". Dalton Transactions (11): 1883–1890. doi:10.1039/b503688g. PMID 15909033. <a href="/wiki/Dalton_Transactions" target="_blank">/wiki/Dalton_Transactions</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">Kim, K.-C.; Reed, C. A.; Elliott, D. W.; Mueller, L. J.; Tham, F.; Lin, L.; Lambert, J. B. (2002). "Crystallographic Evidence for a Free Silylium Ion". Science. 297 (5582): 825–827. Bibcode:2002Sci...297..825K. doi:10.1126/science.1073540. PMID 12161650. S2CID 9072540. <a href="http://www.escholarship.org/uc/item/88j06088" target="_blank">http://www.escholarship.org/uc/item/88j06088</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
</ol>

Non-coordinating anion open-in-new

Non-coordinating anion