Prior to Edwards developing his equation, other scientists were also working to define nucleophilicity quantitatively.
Brønsted and Pederson first discovered the relationship between basicity, with respect to protons, and nucleophilicity in 1924:[3] where kb is the rate constant for nitramide decomposition by a base (B) and βN is a parameter of the equation.
However, both the Swain–Scott equation and the Brønsted relationship make the rather inaccurate assumption that all nucleophiles have the same reactivity with respect to a specific reaction site.
[3] The Edwards equation attempts to account for this additional parameter by introducing a polarizability term.
[1] α and β are then parameters unique to specific nucleophiles that relate the sensitivity of the substrate to the basicity and polarizability factors.
To account for this, En was redefined in terms of basicity and polarizability (given as molar refractivity, RN): The values of a and b, obtained by the method of least squares, are 3.60 and 0.0624 respectively.
The alpha effect, basicity, and polarizability are still accepted as the main factors in determining nucleophilic reactivity.
This observation led Pearson to develop his hard-soft acid-base theory, which is arguably the most important contribution that the Edwards equation has made to current understanding of organic and inorganic chemistry.
The Edwards equation parameters have since been used to help categorize acids and bases as hard or soft, due to the approach's simplicity.