<Li> typical soft bases: organophosphines, thioethers, carbon monoxide, iodide </Li> <P> For example, an amine will displace phosphine from the adduct with the acid BF . In the same way, bases could be classified . For example, bases donating a lone pair from an oxygen atom are harder than bases donating through a nitrogen atom . Although the classification was never quantified it proved to be very useful in predicting the strength of adduct formation, using the key concepts that hard acid--hard base and soft acid--soft base interactions are stronger than hard acid--soft base or soft acid--hard base interactions . Later investigation of the thermodynamics of the interaction suggested that hard--hard interactions are enthalpy favored, whereas soft--soft are entropy favored . </P> <P> 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 and a C. Each base is likewise characterized by its own E and C. 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 </P> <Dl> <Dd> − ΔH = E E + C C + W </Dd> </Dl>

Bf3 lewis base and nh3 is lewis base