HSAB theory


  • A related method adopting the E and C formalism of Drago and co-workers quantitatively predicts the formation constants for complexes of many metal ions plus the proton with
    a wide range of unidentate Lewis acids in aqueous solution, and also offered insights into factors governing HSAB behavior in solution.

  • An attempt to quantify the ‘softness’ of a base consists in determining the equilibrium constant for the following equilibrium: Where CH3Hg+ (methylmercury ion) is a very
    soft acid and H+ (proton) is a hard acid, which compete for B (the base to be classified).

  • Chemical hardness In 1983 Pearson together with Robert Parr extended the qualitative HSAB theory with a quantitative definition of the chemical hardness (η) as being proportional
    to the second derivative of the total energy of a chemical system with respect to changes in the number of electrons at a fixed nuclear environment:[11] .

  • [15] Another quantitative system has been proposed, in which Lewis acid strength toward Lewis base fluoride is based on gas-phase affinity for fluoride.

  • [20] Criticism[edit] Reanalysis of a large number of various most typical ambident organic system reveals that thermodynamic/kinetic control describes reactivity of organic
    compounds perfectly, whereas the HSAB principle fails and should be abandoned in the rationalization of ambident reactivity of organic compounds.

  • [1] 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.

  • Modifications If the interaction between acid and base in solution results in an equilibrium mixture the strength of the interaction can be quantified in terms of an equilibrium

  • [6] Theory Essentially, the theory states that soft acids prefer to form bonds with soft bases, whereas hard acids prefer to form bonds with hard bases, all other factors
    being equal.

  • This is especially so in transition metal chemistry, where numerous experiments have been done to determine the relative ordering of ligands and transition metal ions in terms
    of their hardness and softness.

  • The HASB classification in the original work was largely based on equilibrium constants of Lewis acid/base reactions with a reference base for comparison.

  • The first derivative of the energy with respect to the number of electrons is equal to the chemical potential, μ, of the system, from which an operational definition for the
    chemical potential is obtained from a finite difference approximation to the first order derivative as which is equal to the negative of the electronegativity (χ) definition on the Mulliken scale: μ = −χ.

  • HSAB is widely used in chemistry for explaining the stability of compounds, reaction mechanisms and pathways.

  • It is claimed that the knowledge of absolute rate constants and not of the hardness of the reaction partners is needed to predict the outcome of alkylations of the cyanide

  • [17] However, it has been shown that to define the order of Lewis base strength (or Lewis acid strength) at least two properties must be considered.

  • The equation is The W term represents a constant energy contribution for acid–base reaction such as the cleavage of a dimeric acid or base.

  • [18] For Pearson’s qualitative HSAB theory the two properties are hardness and strength while for Drago’s quantitative ECW model the two properties are electrostatic and covalent

  • This expression implies that the chemical hardness is proportional to the band gap of a chemical system, when a gap exists.


Works Cited

[‘Jolly, W. L. (1984). Modern Inorganic Chemistry. New York: McGraw-Hill. ISBN 978-0-07-032760-3.
2. ^ [1] 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
3. ^ Pearson, Ralph G. (1963). “Hard and Soft Acids and Bases”. J. Am. Chem. Soc. 85 (22): 3533–3539. doi:10.1021/ja00905a001.
4. ^ Pearson, Ralph G. (1968). “Hard and soft acids and bases,
HSAB, part 1: Fundamental principles”. J. Chem. Educ. 1968 (45): 581–586. Bibcode:1968JChEd..45..581P. doi:10.1021/ed045p581.
5. ^ Pearson, Ralph G. (1968). “Hard and soft acids and bases, HSAB, part II: Underlying theories”. J. Chem. Educ. 1968
(45): 643–648. Bibcode:1968JChEd..45..643P. doi:10.1021/ed045p643.
6. ^ [2] R. G. Pearson, Chemical Hardness – Applications From Molecules to Solids, Wiley-VCH, Weinheim, 1997, 198 pp
7. ^ Muller, P. (1994-01-01). “Glossary of terms used in physical
organic chemistry (IUPAC Recommendations 1994)”. Pure and Applied Chemistry. 66 (5): 1077–1184. doi:10.1351/pac199466051077. ISSN 1365-3075.
8. ^ Pearson, Ralph G. (1963). “Hard and Soft Acids and Bases”. Journal of the American Chemical Society.
85 (22): 3533–3539. doi:10.1021/ja00905a001. ISSN 0002-7863.
9. ^ Jump up to:a b IUPAC, Glossary of terms used in theoretical organic chemistry, accessed 16 Dec 2006.
10. ^ Jump up to:a b Miessler G.L. and Tarr D.A. “Inorganic Chemistry” 2nd ed.
Prentice-Hall 1999, p.181-5
11. ^ Jump up to:a b Robert G. Parr & Ralph G. Pearson (1983). “Absolute hardness: companion parameter to absolute electronegativity”. J. Am. Chem. Soc. 105 (26): 7512–7516. doi:10.1021/ja00364a005.
12. ^ Ralph G. Pearson
(2005). “Chemical hardness and density functional theory” (PDF). J. Chem. Sci. 117 (5): 369–377. CiteSeerX doi:10.1007/BF02708340. S2CID 96042488.
13. ^ Delchev, Ya. I.; A. I. Kuleff; J. Maruani; Tz. Mineva; F. Zahariev (2006).
Jean-Pierre Julien; Jean Maruani; Didier Mayou (eds.). Strutinsky’s shell-correction method in the extended Kohn-Sham scheme: application to the ionization potential, electron affinity, electronegativity and chemical hardness of atoms in Recent Advances
in the Theory of Chemical and Physical Systems. New York: Springer-Verlag. pp. 159–177. ISBN 978-1-4020-4527-1.
14. ^ Vogel G. C.; Drago, R. S. (1996). “The ECW Model”. Journal of Chemical Education. 73 (8): 701–707. Bibcode:1996JChEd..73..701V.
15. ^ Hancock, R. D.; Martell, A. E. (1989). “Ligand design for the selective complexation of metal ions in aqueous solution”. Chemical Reviews. 89 (8): 1875–1914. doi:10.1021/cr00098a011.
16. ^ Christe, K.O.; Dixon, D.A.;
McLemore, D.; Wilson, W.W.; Sheehy, J.A.; Boatz, J.A. (2000). “On a quantitative scale for Lewis acidity and recent progress in polynitrogen chemistry”. Journal of Fluorine Chemistry. 101 (2): 151–153. doi:10.1016/S0022-1139(99)00151-7. ISSN 0022-1139.
17. ^
Laurence, C. and Gal, J-F. Lewis Basicity and Affinity Scales, Data and Measurement, (Wiley 2010) p 51 ISBN 978-0-470-74957-9
18. ^ Cramer, R. E., and Bopp, T. T. (1977) Great E and C plot. Graphical display of the enthalpies of adduct formation
for Lewis acids and bases. Journal of Chemical Education 54 612-613
19. ^ 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
20. ^ Tishkov, Alexander A.; Mayr, Herbert (2004). “Ambident Reactivity of the Cyanide Ion: A Failure
of the HSAB Principle”. Angewandte Chemie International Edition. 44 (1): 142–145. doi:10.1002/anie.200461640. PMID 15599920.
21. ^ Mayr, Herbert (2011). “Farewell to the HSAB Treatment of Ambident Reactivity”. Angewandte Chemie International Edition.
50 (29): 6470–6505. doi:10.1002/anie.201007100. PMID 21726020.
Photo credit: https://www.flickr.com/photos/piecar/4673984698/’]