The E-C model accommodates the failure of single parameter descriptions of acids and bases.  In 1965 Russell S. Drago and Bradford Wayland published the two term equation such that each acid and each base is described by two parameters.  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. These parameters have been empirically obtained by using enthalpies for adducts that form only σ bonds between the acid and base as well as adducts that have no steric repulsion between the acid and base.
This equation reproduces and predicts the enthalpy, ΔH, of a reaction between many acids and bases. ΔH is a measure of strength of the bond between the acid and the base, both in the gas phase and in weakly solvating media.  Entropy effects are ignored. A matrix presentation of the equation enhances its utility.
Four values, two E and two C were assigned as references.  E and C of I were chosen as standards.  Since I has little tendency to undergo electrostatic bonding, the E parameter was assigned a small value, 0.5, while the value of C for the covalent property was set at 2.0. For the two base parameters, E for CHC(O)N(CH) (DMA) was set at 2.35 and C for (CH)S, diethyl sulfide, was set at 3.92.  Fixing the parameters in this way imposed the covalent-electrostatic model on the data set by fixing the EE and CC products of the DMA and (CH)S adducts with iodine, and these four values ensured that none of the parameters had negative values. Due to increasing enthalpy data that became available since the EC equation was first proposed the parameters have been improved.  Mixing E and C numbers from the improved set of parameters with older parameters will result in incorrect calculations and is to be avoided. A select set of the improved E and C numbers is found in this article and the complete set is available in the literature.
 E and C parameters for phosphines that can be used in combination with the improved parameters for oxygen, nitrogen, and sulfur donors to measure σ-basicity have been reported.