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 standard heat (enthalpy) of formation of the adduct in a non-coordinating solvent. Drago and Wayland proposed a two-parameter equation which predicts the formation of a very large number of adducts quite accurately.
- –ΔH
O(A—B) = EAEB + CACB
Value of the E and C parameters can be found in Drago et al. Hancock and Martell found that an E and C equation analogous to that of Drago gave an excellent quantitative prediction of formation constants for complexes of 34 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.
Another quantitative system has been proposed, in which Lewis acid strength is based on gas-phase affinity for fluoride.
Read more about this topic: HSAB Theory