The original Lewis definitions

The Lewis definitions of acid-base interactions are now over a half a century old. Nevertheless they are always useful and have broadened their meaning and applications, covering concepts such as bond-formation, central atom-ligand interactions, electrophilic-nucleophilic reagents, cationic-anionic reagents, charge transfer complex formation, donor-acceptor reactions, etc. In 1923 Lewis reviewed and extensively elaborated the theory of the electron-pair bond, which he had first proposed in 1916. In this small volume which had since become a classic, Lewis independently proposed both the proton and generalized solvent-system definitions of acids and bases. He wrote  

The important point that Lewis revealed is that though the acid-base properties of species are obviously modified by the presence or absence of a given solvent, their ultimate cause should reside in the molecular structure of the acid or base itself, and in light of the electronic theory of matter, not in a common constituent such as or OH, but in an analogous 

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Acid-Base Interactions 3.2.2.1 The Original Lewis Definition... 

In the original Lewis definition (1923 [29], 1938 [30]), aeids are electron-pair acceptors and bases are electron-pair donors. The fundamental reaction between a Lewis acid A and a Lewis base B is the formation of a complex (or adduct or coordination compound or addition compound) A-B (reaction 1.17) ... 

With the extension of the original Lewis definition, and its application to many fields of chemistry, many terms specific to those fields have been substituted for the very general Lewis acid and Lewis base terms. These synonyms are collected in Table 1.6. 

Following the use of boron acids by Brown et al. (1953-1955) [91] in his classical work on steric effects in the complexation of amines with trimethylboron, an extensive set of calorimetric measurements was provided by the studies of BF3 complexes by Gal, Maria et al. (1970-1992) [80]. Boron trifluoride is the archetype of Lewis acids in the original Lewis definition. The promising BF3 affinity scale is presented in Chapter 3. 

The original Arrhenius definition of a base has been extended by the Lowry-Bronsted theory and by the Lewis theory. See acid. 

The discussion above has indicated some of the limitations of the original Lewis/ Kossel descriptiOTi of chemical bonding and the manner in which it has been adapted to assimilate the multitude of new compounds being reported from chemical laboratories during the last century. Central to the model is the definition of the chemical bond as a pair of electrons and the adherence to the octet rule. 

If hypervalent literally means exceeding the lowest chemical valence, then even NH4+ is hypervalent, despite the fact that the Lewis-Langmuir theory easily accounts for its stability. Furthermore, if the definition of hypervalency is restricted to Groups 15-18, then this artificially excludes species such as SiFs and SiFe, which are isoelectronic and isostructural with PF5 and SFe, respectively. The ambiguities in the original definition have led to several changes. Today, no one would regard NH4+ as hypervalent, while those that consider PF5 and SFe hypervalent would classify SiFs" and SiFe in the same way. 

The Usanovich definition of acids and bases has not been widely used, probably because of (I) the relative inaccessibility of the original to non-Russian-reading chemists and (2) the awkwardness and circularity of Usanovich s original definition. The Usanovich definition includes all reactions of Lewis acids and bases and extends the latter concept by removing the restriction that the donation or acceptance of electrons be as shared pairs. The complete definition is as follows An add is any chemical species which reacts with bases, gives up cations, or accepts anions or electrons, and, conversely, a base is any chemical species which reacts with acids, gives up anions or electrons, or combines with cations. Although perhaps unnecessarily complicated, this definition simply includes all Lewis acid-base reactions plus redox reactions, which may consist of complete transfer of one or more electrons. Usanovich also stressed unsaturation involved in certain acid-base reactions ... 

This is the equation given by Tunell in his classic paper on fugacity (1931), in which he points out some difficulties with the definition as we have presented it, and shows that equation (11.10) is preferable as a definition of fugacity from a mathematical point of view. Our presentation, which follov s the original one by Lewis and Randell (1923), seems preferable from a pedagogical point of view. 

A brief account is given of the origin and early development of the idea of hardness, introduced to chemistry as hard and soft Lewis acids and bases. There is also a discussion of the merging of this early view of hardness with the modern definition, based on density functional theory. 

Bronsted and Lewis acids and bases are frequently encountered in the chemistry of solid surfaces, but in some instances these terms are used in a slightly different context from their original proposals. From the point of view of the Bronsted definition, a solid is acidic if it is able to donate (or at least transfer partially) a proton to a basic probe molecule to which the proton bonds (or with which it, at least, becomes partially associated). According to the Lewis definition, a solid is acidic if it is able to accept an electron pear from a basic molecule and also form a coor dinative bond with it. The transfer of an electron pair in the meaning of the Lewis definition should be clearly distinguished from surface redox processes in which electron transfer (ofone or two electrons) occurs without coordination. 

Two important points should be noted (1) In the discussion of solid bases, the H. scale is treated as a parameter to describe the nature of individual basic sites. It is often assumed that there is a certain number of basic sites on solid surfaces and each of the basic sites has its own basic strength. In the original definition, H scale is used to describe basic property of solution, not that of individual basic molecules (or ions) in the solution. (2) In principle, the idea of the H. scale is only applicable to Bronsted bases. It can not be, at least directly, related to the capability of the sites as Lewis bases. 

Presentations of Jablohski s contribution to spectroscopy in the frame of the evolution of science and political situation in the 1930s and 1940s have been published [14, 15]. The contribution of Jacques and Francis Perrin, Jablonski, Vavilov, Levshin, Lewis, and Kasha to the definition of state diagram has been critically evaluated and the originality of each one commented by Nickel [16, 17]. The topic was discussed at the first congress on photoluminescence in Warsaw, May 20-25, 1936 (Pringsheim president, Jablonski secretary). The lively discussion that took place on that occasion has been retraced in detail by Nickel [18]. 

Figure 19 gives an example of a not-so-successful correlation for adsorption of hydrocarbons on silica gel. The adsorption isotherms of four hydrocarbons at 298 K were correlated with the experimental data by Lewis et al. [102]. Solid lines in Fig. 19 correspond to the Dubinin potential (53) with jS = 1 and dotted lines correspond to fS = 2. For the three substances propylene, ethylene, and ethane, predictions on the basis of the potential with jS = 1 are definitely better. The agreement with experimental data of predictions with jS = 2 is also reasonable if the small number of fitted parameters is taken into account The relative error does not exceed 5% (as for ethylene). However, in this case, the three best-fit curves exhibit similar systematic deviations from experimental data. Such deviations are not observed if the potential with jS = 1 is used. The adsorption isotherms of the fourth substance, propane, show an opposite tendency Predictions on the basis of the original DR potential are significantly better than those on the basis of the potential with /] = 1 (in the first case, the relative error is only 1.3%). This example shows that the potential theory has limitations when adsorption on silica gels is considered. 

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