text: A Lewis acid, A, and a Lewis base, B, can be considered to form a complex AB.
  There are three major theories relating to the strength of Lewis acids and bases and the interactions between them.
  #Hard and soft acid–base theory (HSAB). This is used mainly for qualitative purposes.
  #Drago and Wayland proposed a two-parameter equation which predicts the standard enthalpy of formation of a very large number of adducts quite accurately. −ΔH (A − B) = EE + CC. Values of the E and C parameters are available.
  #Guttmann donor numbers: for bases the number is derived from the enthalpy of reaction of the base with antimony pentachloride in 1,2-Dichloroethane as solvent. For acids, an acceptor number is derived from the enthalpy of reaction of the acid with triphenylphosphine oxide.
  For more details see: acid–base reaction, acid catalysis, Extraction (chemistry)