HSAB theory 1047111 224079654 2008-07-07T05:45:06Z Lightbot 7178666 Units/dates/other The '''HSAB concept''' is an [[acronym]] for 'hard and soft [[acid]]s and [[base (chemistry)|base]]s'. Also known as the '''Pearson acid base concept''', HSAB is widely used in [[chemistry]] for explaining stability of [[chemical compound|compounds]], [[chemical reaction|reaction]] mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical [[species]]. 'Hard' applies to species which are small, have high charge states (the charge criterion applies mainly to acids, to a lesser extent to bases), and are weakly polarizable. 'Soft' applies to species which are big, have low charge states and are strongly polarizable.<ref>{{cite book|title=Modern Inorganic Chemistry| author=Jolly, W. L.| isbn=0070327602}}</ref> The theory is used in contexts where a qualitative, rather than quantitative description would help in understanding the predominant factors which drive chemical properties and reactions. This is especially so in [[transition metal]] [[chemistry]], where numerous experiments have been done to determine the relative ordering of [[ligand]]s and transition metal ions in terms of their hardness and softness. HSAB theory is also useful in predicting the products of [[metathesis (chemistry)|metathesis]] reactions. Quite recently it has been shown that even the sensitivity and performance of explosive materials can be explained on basis of HSAB theory <ref>[http://www3.interscience.wiley.com/cgi-bin/abstract/109931709/ABSTRACT]E.-C. Koch, Acid-Base Interactions in Energetic Materials: I. The Hard and Soft Acids and Bases (HSAB) Principle-Insights to Reactivity and Sensitivity of Energetic Materials, ''Prop.,Expl.,Pyrotech. 30'' '''2005''', 5 </ref> [[Ralph Pearson]] introduced the HSAB principle in the early 1960s<ref>{{cite journal|title=Hard and Soft Acids and Bases|author=Pearson, Ralph G.|journal= [[J. Am. Chem. Soc.]] |date=1963| volume= 85 |issue=22|pages=3533–3539|doi=10.1021/ja00905a001}}</ref><ref>{{cite journal|title=Hard and soft acids and bases, HSAB|author= Pearson, Ralph G.|journal=[[J. Chem. Educ.]]| volume =1968|issue=45| pages= 581643| url=http://www.jce.divched.org/Journal/Issues/1968//jceSubscriber/JCE1968p1643.pdf| format=subscriber access}}</ref> as an attempt to unify inorganic and [[organic chemistry|organic]] reaction chemistry.<ref>[http://www3.interscience.wiley.com/cgi-bin/summary/112214582/SUMMARY]R. G. Pearson, Chemical Hardness - Applications From Molecules to Solids, Wiley-VCH, Weinheim, 1997, 198 pp</ref>. ==Theory== The gist of this theory is that ''soft'' acids react faster and form stronger bonds with ''soft'' bases, whereas ''hard'' acids react faster and form stronger bonds with ''hard'' bases, all other factors being equal.<ref name=IUPAC>[[IUPAC]], [http://www.iupac.org/reports/1999/7110minkin/h.html Glossary of terms used in theoretical organic chemistry], accessed 16 Dec 2006.</ref> The classification in the original work was mostly based on [[equilibrium constant]]s for reaction of two Lewis bases competing for a Lewis acid. '''Hard acids''' and '''hard bases''' tend to have: * small atomic/ionic radius * high [[oxidation state]] * low [[polarizability]] * high [[electronegativity]] * energy low-lying [[HOMO]] (bases) or energy high-lying [[LUMO]] (acids).<ref name=IUPAC/> Examples of hard acids are: H<sup>+</sup>, [[alkali metal|alkali]] ions, Ti<sup>4+</sup>, Cr<sup>3+</sup>, Cr<sup>6+</sup>, BF<sub>3</sub>. Examples of hard bases are: OH<sup>&ndash;</sup>, F<sup>&ndash;</sup>, Cl<sup>&ndash;</sup>, NH<sub>3</sub>, CH<sub>3</sub>COO<sup>&ndash;</sup>, CO<sub>3</sub><sup>2&ndash;</sup>. The affinity of hard acids and hard bases for each other is mainly [[ionic bond|ionic]] in nature. '''Soft acids''' and '''soft bases''' tend to have: * large atomic/ionic radius * low or zero oxidation state * high polarizability * low electronegativity * energy high-lying HOMO (bases) and energy-low lying LUMO (acids).<ref name=IUPAC/> Examples of soft acids are: CH<sub>3</sub>Hg<sup>+</sup>, Pt<sup>4+</sup>, Pd<sup>2+</sup>, Ag<sup>+</sup>, Au<sup>+</sup>, Hg<sup>2+</sup>, Hg<sub>2</sub><sup>2+</sup>, Cd<sup>2+</sup>, BH<sub>3</sub>. Examples of soft bases are: H<sup>&ndash;</sup>, R<sub>3</sub>P, SCN<sup>&ndash;</sup>, I<sup>&ndash;</sup>. The affinity of soft acids and bases for each other is mainly [[covalent bond|covalent]] in nature. {|align="center" class="wikitable" |colspan=4 align="center"|'''Acids'''||colspan=4 align="center"|'''Bases''' |- |colspan=2 align="center"|'''hard'''||colspan=2 align="center"|'''soft'''||colspan=2 align="center"|'''hard'''||colspan=2 align="center"|'''soft''' |- |[[Hydronium]]||H<sup>+</sup>||[[Mercury (element)|Mercury]]||[[methylmercury|CH<sub>3</sub>Hg<sup>+</sup>]], Hg<sup>2+</sup>, [[Calomel|Hg<sub>2</sub><sup>2+</sup>]]||[[Hydroxyl]]||OH<sup>-</sup>||[[Hydride]]||H<sup>-</sup> |- |[[Alkali metals]]||Li<sup>+</sup>,Na<sup>+</sup>,K<sup>+</sup> ||[[Platinum]]||Pt<sup>4+</sup>||[[Alkoxide]]||RO<sup>-</sup>||[[Thiolate]]||RS<sup>-</sup> |- |[[Titanium]]||Ti<sup>4+</sup>||[[Palladium]]||Pd<sup>2+</sup>||[[Halogens]]||F<sup>-</sup>,Cl<sup>-</sup>||[[Halogens]]||I<sup>-</sup> |- |[[Chromium]]||Cr<sup>3+</sup>,Cr<sup>6+</sup>||[[Silver]]||Ag<sup>+</sup>||[[Ammonia]]||NH<sub>3</sub>||[[Phosphine]]||PR<sub>3</sub> |- |[[Boron trifluoride]]||BF<sub>3</sub> ||[[borane]]||BH<sub>3</sub>||[[Carboxylate]]||CH<sub>3</sub>COO<sup>-</sup>||[[Thiocyanate]]||SCN<sup>-</sup> |- |[[Carbocation]]||R<sub>3</sub>C<sup>+</sup>||[[P-chloranil]]||||[[Carbonate]]||CO<sub>3</sub><sup>2-</sup>||[[carbon monoxide]]||CO |- |||||bulk [[Metals]]||M<sup>0</sup>||[[Hydrazine]]||N<sub>2</sub>H<sub>4</sub>||[[Benzene]]||C<sub>6</sub>H<sub>6</sub> |- |||||[[Gold]]||Au<sup>+</sup>|||||||| |- | colspan=8 align=left style="background: #ccccff;"| <center>''Table 1. Hard and soft acids and bases''</center> |- |} Borderline cases are also identified: '''borderline acids''' are [[trimethylborane]], [[sulfur dioxide]] and [[ferrous]] Fe<sup>2+</sup>, [[cobalt]] Co<sup>2+</sup> and [[lead]] Pb<sup>2+</sup> cations. '''Borderline bases''' are: [[aniline]], [[pyridine]], [[nitrogen]] N<sub>2</sub> and the [[azide]], [[bromine]], [[nitrate]] and [[sulfate]] anions. Generally speaking, acids and bases interact and the most stable interactions are hard-hard ([[ionic bond|ionogenic]] character) and soft-soft ([[covalent bond|covalent]] character). An attempt to quantify the 'softness' of a base consists in determining the [[equilibrium constant]] for the following equilibrium: :BH + CH<sub>3</sub>Hg<sup>+</sup> ↔ H<sup>+</sup> + CH<sub>3</sub>HgB Where CH<sub>3</sub>Hg<sup>+</sup> ([[methylmercury]] ion) is a very soft acid and H<sup>+</sup> (proton) is a hard acid, which compete for B (the base to be classified). Some examples illustrating the effectiveness of the theory: * Bulk metals are soft acids and are [[Catalyst poisoning|poisoned]] by soft bases such as phosphines and sulfides. * Hard [[solvent]]s such as [[hydrogen fluoride]], [[water]] and the [[protic solvent]]s tend to [[solvatation|solvatate]] strong solute bases such as the fluorine anion and the oxygen anions. On the other hand dipolar aprotic solvents such as [[dimethyl sulfoxide]] and [[acetone]] are soft solvents with a preference for solvatating large anions and soft bases. * In [[coordination chemistry]] soft-soft and hard-hard interactions exist between ligands and metal centers. ==Chemical hardness== {|align="right" class="wikitable" |colspan=6 align="center"|Chemical hardness in [[electron volt]] |- |colspan=3 align="center"|'''Acids'''||colspan=3 align="center"|'''Bases''' |- ||[[Hydrogen]]|| H<sup>+</sup>||[[infinite]]||[[Fluoride]]|| F<sup>-</sup>||7 |- ||[[Aluminum]]|| Al<sup>3+</sup>||45.8||[[Ammonia]]|| NH<sub>3</sub>||6.8 |- ||[[Lithium]]|| Li<sup>+</sup>||35.1||[[hydride]]|| H<sup>-</sup>||6.8 |- ||[[Scandium]]|| Sc<sup>3+</sup>||24.6||[[carbon monoxide]]|| CO ||6.0 |- ||[[Sodium]]|| Na<sup>+</sup>||21.1||[[hydroxyl]]|| OH<sup>-</sup>||5.6 |- ||[[Lanthanum]]|| La<sup>3+</sup>||15.4||[[cyanide]]|| CN<sup>-</sup>||5.3 |- ||[[Zinc]]|| Zn<sup>2+</sup>||10.8||[[phosphane]]|| PH<sub>3</sub>||5.0 |- ||[[Carbon dioxide]]|| CO<sub>2</sub>||10.8||[[nitrite]]|| NO<sub>2</sub><sup>-</sup>||4.5 |- ||[[Sulfur dioxide]]|| SO<sub>2</sub>||5.6||[[Hydrosulfide]]|| SH<sup>-</sup>||4.1 |- ||[[Iodine]]|| I<sub>2</sub>||3.4||[[Methane]]|| CH<sub>3</sub><sup>-</sup>||4.0 |- | colspan=6 align=left style="background: #ccccff;"| <center>''Table 2. Chemical hardness data'' <ref name=abshardess/></center> |- |} In 1983 Pearson together with [[Robert Parr]] extended the qualitative HSAB theory with quantitative '''chemical hardness''' ([[Eta (letter)|η]]) defined as <ref name=abshardess>{{cite journal | author = Robert G. Parr and Ralph G. Pearson | title = Absolute hardness: companion parameter to absolute electronegativity | journal = [[J. Am. Chem. Soc.]] | year = 1983 | volume = 105 |issue = 26 | pages = 7512–7516 | doi = 10.1021/ja00364a005}}</ref>: :<math>\eta = 0.5(I - A) \,</math> with <math>I\,</math> the [[ionization potential]] and <math>A\,</math> the [[electron affinity]]. (More recently Pearson recommends following modern usage and dropping the factor of 0.5<ref>Pearson, "Chemical hardness and density functional theory"; J. Chem. Soc; Indian Academy of Sciences; Vol. 117, No. 5, September 2005, pp. 369–377.</ref>.) When the [[electronegativity]] ([[Chi (letter)|χ]]) as the [[Mulliken scale]]: :<math>\chi = 0.5(I + A) \,</math> is the [[first derivative]] in a plot of energy <math>E\,</math> versus the amount of [[electrons]] <math>N\,</math> with fixed [[nuclear charge]] <math>Z\,</math> in an atom or molecule: :<math>\chi = \left(\frac{\partial E}{\partial N}\right)_Z \,</math> then the chemical hardness is simply the [[second derivative]]: :<math>\eta = 0.5\left(\frac{\partial^2 E}{\partial N^2}\right)_Z \,</math> Hardness and electronegativity are related as: :<math>2\eta = -\left(\frac{\partial \chi}{\partial N}\right)_Z \,</math> and in this sense hardness is a measure for resistance to deformation or change. Likewise a value of zero denotes maximum '''softness'''. (Softness is the reciprocal of hardness.) In a compilation of hardness values only that of the [[hydride]] anion deviates. Another discrepancy noted in the original 1983 article are the apparent higher hardness of [[Thallium|Tl<sup>3+</sup>]] compared to Tl<sup>+</sup>. ==Kornblum's rule== An application of HSAB theory is the so-called '''Kornblum's rule''' which states that in reactions with [[ambident nucleophile]]s, the more [[electronegative]] atom reacts when the [[reaction mechanism]] is [[SN1 reaction|S<sub>N</sub>1]] and the less electronegative one in a [[SN2 reaction|S<sub>N</sub>2]] reaction. This rule (established in 1954) <ref>''The Mechanism of the Reaction of Silver Nitrite with Alkyl Halides. The Contrasting Reactions of Silver and Alkali Metal Salts with Alkyl Halides. The Alkylation of Ambident Anions'' Nathan Kornblum, Robert A. Smiley, Robert K. Blackwood, Don C. Iffland [[J. Am. Chem. Soc.]]; '''1955'''; 77(23); 6269-6280. {{DOI|10.1021/ja01628a064}}</ref> actually predates HSAB theory but in HSAB terms its explanation is that in a S<sub>N</sub>1 reaction the [[carbocation]] (a hard acid) reacts with a hard base (high electronegativity) and that in a S<sub>N</sub>2 reaction tetravalent carbon (a soft acid) reacts with soft bases. ==References== {{reflist}} ==See also== *[[Acid-base reaction]] [[Category:Acid-base chemistry]] [[Category:Inorganic chemistry]] [[de:HSAB-Konzept]] [[fr:Principe HSAB]] [[it:Teoria HSAB]] [[ja:HSAB則]] [[pt:Teoria HSAB]] [[zh:软硬酸碱理论]]