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The interactions between Lewis acids (A) and bases (B) are fundamental in chemistry and biochemistry. The scope of these donor-acceptor interactions include classical Lewis acid-base reactions (e.g., charge-transfer complexes), all coordination chemistry, Bronsted acid-base interactions, hydrogen bonding, displacement reactions, nucleophilic and electrophilic attacks, and nonradical atom transfer reactions. The ubiquitous nature of donor-acceptor interactions in such diverse areas as synthesis, surface science, catalysis, and the collection and proper interpretation of thermodynamic data as well as their importance in the isolobal concept emphasizes that a fundamental understanding of Lewis acid-base interactions must be an essential component of the practicing chemist's knowledge. The ECW model discussed in most modern inorganic textbooks (1) is an empirical model of acid-base strength based on the Pauling (2) and Mulliken (3) bonding models and has been a powerful tool for developing an understanding of donor-acceptor interactions. A large database of E and C parameters now exists that allows the chemists and biochemists to predict and correlate quantitatively the enthalpies of Lewis acid-base interactions. These correlations provide insights on steric hindrance, pi-backbonding, spectral shifts, reactivity, etc. that are not easily obtained by other means. In textbooks too much attention has been placed on the model's ability to predicted enthalpies at the expense of its more far-reaching applications. The model can be used to determine if physicochemical measurements involving reactivity (equilibrium constants, rate constants, electrode potentials, etc.) or spectrochemical properties (NMR, EPR, UV-vis, IR, etc. spectral shifts) are dominated by donor-acceptor interactions. The purpose of this article is to emphasize the much broader scope of this model. Furthermore, a survey of the textbooks discussing this model indicates that the authors are using parameters of different vintages.l This article provides the teacher with the latest parameters as well as a discussion of some of the more important, often overlooked, applications of the model.
A consideration that must be noted when using the ECW model is that the parameters are based on enthalpies of donor-acceptor bond strengths and have minimal contributions from solvation or lattice energies. In polar solvents and in reactions involving solid phases, energy contributions from solvation and lattice energies can dominate the chemistry. For donor-acceptor enthalpies determined in poorly solvating media and ranging from 0 to -30 kcal mol^sup -1^, a molecular...