Editing HSAB theory (section)

== 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 constant]]. An alternative quantitative measure is the heat ([[enthalpy]]) of formation of the Lewis acid-base adduct in a non-coordinating solvent. The [[ECW model]] is quantitative model that describes and predicts the strength of Lewis acid base interactions, -ΔH  . The model assigned E and C parameters to many Lewis acids and bases. Each acid is characterized by an E<sub>A</sub> and a C<sub>A</sub>. Each base is likewise characterized by its own E<sub>B</sub> and C<sub>B</sub>. The E and C parameters refer, respectively, to the electrostatic and covalent contributions to the strength of the bonds that the acid and base will form. The equation is

:-ΔH  =  E<sub>A</sub>E<sub>B</sub>  +  C<sub>A</sub>C<sub>B</sub>  +  W

The W term represents a constant energy contribution for acid–base reaction such as the cleavage of a dimeric acid or base.  The equation predicts reversal of acids and base strengths. The graphical presentations of the equation show that there is no single order of Lewis base strengths or Lewis acid strengths.<ref>{{cite journal |author=Vogel G. C. |author2=Drago, R. S.|year=1996|journal=Journal of Chemical Education|volume=73|issue=8|pages=701–707|title=The ECW Model|bibcode=1996JChEd..73..701V|doi=10.1021/ed073p701}}</ref> The ECW model accommodates the failure of single parameter descriptions of acid-base interactions.

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.<ref>{{cite journal | last = Hancock | first = R. D. |author2=Martell, A. E. | title = Ligand design for the selective complexation of metal ions in aqueous solution. | journal = Chemical Reviews | volume = 89| issue = 8 | pages = 1875–1914 | year = 1989|doi=10.1021/cr00098a011 }}</ref>

Another quantitative system has been proposed, in which Lewis acid strength toward Lewis base fluoride is based on gas-phase affinity for [[fluoride]].<ref>{{cite journal | last = Christe | first = K.O. |author2=Dixon, D.A. |author3=McLemore, D. |author4=Wilson, W.W. |author5=Sheehy, J.A. |author6= Boatz, J.A.  | title = On a quantitative scale for Lewis acidity and recent progress in polynitrogen chemistry | journal = Journal of Fluorine Chemistry | volume = 101| issue = 2 | pages = 151–153 | year = 2000 | issn = 0022-1139| doi=10.1016/S0022-1139(99)00151-7 | bibcode = 2000JFluC.101..151C }}</ref> Additional one-parameter base strength scales have been presented.<ref>Laurence, C. and Gal, J-F. Lewis Basicity and Affinity Scales, Data and Measurement, (Wiley 2010) p 51 {{ISBN|978-0-470-74957-9}}</ref> 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.<ref>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</ref>  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 .