Lewis acid-base concept

R. G. Pearson, J. Am. Chem. Soc. 85 3533 (1963) T. L. Ho, Hard and Soft Acids and Bases in Organic Chemistry, Academic Press, New York, 1977 W. B. Jensen, The Lewis Acid-Base Concept, Wiley-Interscience, New York, 1980, Chapter 8. 

The strength of the complexation is a function of both the donor atom and the metal ion. The solvent medium is also an important factor because solvent molecules that are potential electron donors can compete for the Lewis acid. Qualitative predictions about the strength of donor-acceptor complexation can be made on the basis of the hard-soft-acid-base concept (see Section 1.2.3). The better matched the donor and acceptor, the stronger is the complexation. Scheme 4.3 gives an ordering of hardness and softness for some neutral and ionic Lewis acids and bases. 

For a monograph on Lewis acid-base theory, see Jensen, W.B. The Lewis Acid-Base Concept Wiley NY, 1980. For a discussion of the definitions of Lewis acid and base, see Jensen, W.B. Chem. Rev, 1978, 78, 1. 

W. B. Jensen, The Lewis Acid-Base Concepts, An Overview, John Wiley Sons, Hoboken, NJ, 1980. 

C21-0089. The fluorides BF3, AIF3, SiFq, and PF5 are Lewis acids. They all form very stable fluoroanions when treated with lithium fluoride. In contrast, three other fluorides, CF4, NF3, and SFg, do not react with lithium fluoride. Use Lewis acid-base concepts to explain this behavior. 

Resonance such as (5.28a)-(5.28c) is inherently a quantal phenomenon, with no classical counterpart. In NBO language, each of the resonance interactions (5.28a)-(5.28c) corresponds to a donor-acceptor interaction between a nominally filled (donor Lewis-type) and unfilled (acceptor non-Lewis-type) orbital, the orbital counterpart of G. N. Lewis s general acid-base concept. As mentioned above, Lewis and Werner (among others) had well recognized the presence of such valence-like forces in the dative or coordinative binding of free molecular species. Thus, the advent of quantum mechanics and Pauling s resonance theory served to secure and justify chemical concepts that had previously been established on the basis of compelling chemical evidence. 

I. Definition of Basicity A. General Comments on the Acid-base Concept The current general definition of the acid-base concept is based on the definitions by Bronsted and Lewis. According to Bronsted s theory (1923) an acid-base interaction can be described by the general relation (1) ... 

These and similar solvents are distinguished by the fact that they themselves cannot eliminate any protons. On the other hand one is still dealing with water-like solvents in the case of alcohols, since the selfdissociation of the alcohols resembles that of water in order of magnitude (see Gurney, 1953). The ability of free electron pairs to function prompted Lewis (1923) to a more general definition of the acid-base concept. 

Jensen WB (1980) The Lewis acid-base concept. Wiley, New York... 

According to the acid-base concept of Pearson, A -phosphorins can be viewed as soft bases the lone electron pair at phosphoms is much more delocalized than the lone pair at nitrogen in pyridine. Thus, such soft Lewis acids as Hg ions are more likely to react with A -phosphorins (see p. 84). 

G.N. Lewis extended and generalized the acid-base concept to nonprotonic systems.5,6 He defined an acid as a substance that can accept electrons and defined a base as a substance that can donate electrons. Lewis acids are electron-deficient molecules or ions such as BF3 or carbocations, whereas Lewis bases are molecules that contain readily available nonbonded electron pairs (as in ethers, amines, etc.) [Eq. (1.6)]. 

Empirically measured parameters are additional solvent properties, which have been developed through the efforts of physical chemists and physical organic chemists in somewhat different, but to some extent related, directions. They have been based largely on the Lewis acid base concept, which was defined by G. N. Lewis. The concept originally involved the theory of chemical bonding which stated that a chemical bond must involve a shared electron pair. Thus, an atom in a molecule or ion which had an incomplete octet in the early theory, or a vacant orbital in quantum mechanical terms, would act as an electron pair acceptor (an acid) from an atom in a molecule or ion which had a complete octet or a lone pair of electrons (a base). Further developments have included the concepts of partial electron transfer and a continuum of bonding from the purely electrostatic bonds of ion-ion interactions to the purely covalent bonds of atoms and molecules. The development of the concept has been extensively described (see Ref. 11 for details). 

Lewis acid-base theory— To describe reactions where no proton transfer occurs, - Lewis has proposed a more general acid-base concept where an acid is a molecule or an ion with an incomplete outer electron sphere, i.e., an acceptor of electron pairs. A base is a molecule or an ion having a free electron pair, i.e., an electron pair donor. A neutralization reaction is characterized by the formation of a coordination or covalent bond. The following reactions illustrate the Lewis-definition ... 

Refs. [i] Finston H, Rychtman AC (1982) A new view of current acid-base theories. Wiley, New York [ii] Hand CW, BlewittHL (1986) Acid-base chemistry. Macmillan, New York [iii] )ensen WB (1980) The Lewis acid-base concepts. Wiley, New York [iv] Tanabe K (1989) New solid acids and bases. Elsevier, Amsterdam... 

Jensen, W. B. The Lewis Acid-Base Concept — an Overview. New York Wiley 1980... 

The concept of acids and bases has been important since ancient times. It has been used to correlate large amounts of data and to predict trends. Jensen has described a useful approach in the preface to his book on the Lewis acid-base concept ... 

The changing definitions described in this ehapter have generally led to a more inclusive and useful approach to acid-base concepts. Most of this chapter is concerned with the Lewis definition, its more recent explanation in terms of molecular orbitals, and its application to inorganic chemistry. 

FIGURE 6-12 HOMO-LUMO Diagrams for Hard-Hard and Soft-Soft Interactions. (Adapted with permission from W. B. Jensen, The Lewis Acid-Base Concepts, Wiley-fnterscience, New York, 1980, pp. 262-263. Copyright 1980, John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc.)... 

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