Hard-soft acid base theory bases, Lewis

The Hard-Soft-Add-Base (HSAB) theory was developed by Pearson in 1963. According to this theory, Lewis acids and Lewis bases are divided into two groups on one hand hard acids and bases, which are usually small, weakly polarizable species with highly localised charges, and on the other hand soft acids and bases which are large, polarizable species with delocalised charges. A selection of Lewis acids, ordered according to their hardness in aqueous solution is presented in Table 1.3. 

The concept of hard and soft acids and bases ( HSAB ) should also be mentioned here. This is not a new theory of acids and bases but represents a useful classification of Lewis acids and bases from the point of view of their reactivity, as introduced by R. G. Pearson. 

Cation-selective ionophores are the most successful in polymeric ISEs and selectivi-ties exceeding ten orders of magnitude became quite common. The cation-ionophore binding occurs dominantly due to Lewis interactions and could be understood in terms of hard and soft acid and bases theory (HSAB). While hard base oxygen atoms originate from ester, ether or carbonyl functionalities, and interact with hard acid alkaline cations, the softer sulfur or nitrogen atoms better bind with transition metal ions. Cation... 

Pearson and Songstad have suggested that the nucleophilicity of a reagent can also be described in terms of hard and soft acid and base theory.51 In Lewis acid-base terms, the mechanism of an SN2 displacement can be written as in Equation 4.23. We already know that methyl cations (and, by analogy, other... 

Klopman has applied HOMO-LUMO arguments to hard-soft Lewis acid—base theory.10 Although his treatment is a quantitative one, we shall describe only its... 

Acidity and basicity are relative properties. Many compounds are amphoteric and behave as acids or as bases according to a partner. Metal oxides are classified as acidic, amphoteric or basic. Experimentally, this classification corresponds to the adsorption of probe molecules[7, 8]. NH3 is a base probe molecule that reacts with the electron deficient metal atoms (Lewis acid) or the protons adsorbed on the hydrated surface, CO2 is usually considered as acidic and thus it is expected to adsorb more strongly on basic sites. According to this classification, Ti02 belongs to an amphoteric species and MgO to a basic species. A general difficulty for such classifications is that the order can vary with the choice of the probe. The Hard and Soft Bases and Acids theory[9, 10] responds to the necessity to refine the model with a second scale it is better to couple... 

Particularly relevant to the present crmtext is the fact that the olefinic double bond is considered as a soft base in Pearson s theory, while many Lewis acids used in cationic polymerisation (BF3, BCI3, AICI3, etc.) are classed as hard acids. Obviously, n-acceptors like chloranil or tetracyanoethylene are considered as soft acids. Thus, the interactions between Lewis acids and olefins must be considered as very weak in the context of the HSAB theory. This prediction is well substantiated by the tenuous character of the complexes observed in experimental studies (see Chap. IV). On the other hand, carbenium ions are usually placed at the borderline between hard and soft acids and are definitely softer than the Lewis acids mentioned above. Consequently, their interactions with olefins must be rather strong, which suggests that that propagation in cationic polymerisations promoted by Lewis acids should be faster than initiation. 

W. B. Jensen, The Lewis Acid-Base Concepts An Overview, Wiley-Interscience, New York, 1980, and H. L Finston and Allen C. Rychtman, A New View of Current Acid-Base Theories, John Wiley Sons, New York, 1982, provide good overviews of the history of acid-base theories and critical discussions of the different theories. R. G. Pearson s Hard and Soft Acids and Bases, Dowden, Hutchinson, Ross, Stroudsburg, PA, 1973, is a review by one of the leading exponents of HSAB. For other viewpoints, the references provided in this chapter should be consulted. 

The HSAB theory of Pearson has been one of the key organizing concepts in the study of nucleophiles. This theory is applied and examined in Chapters 15 and 16. In Chapter 15, Fuji applies the HSAB principles to design nucleophilic reagents for cleaving C-X bonds. Fuji notes that all bonds are made of a combination of Lewis acid and Lewis base and have hard-soft dissymmetry for the typical C-X bond, the carbon is a soft acid and the X is a hard base. Thus, in accord with the HSAB principles, a soft base (the nucleophile) and a hard acid are required to cleave this bond selectively. Applying these ideas, Fuji then shows the utility of several soft base-hard acid reagents for cleaving various C-X bonds in complex molecules. 

The HSAB theory provides a useful precept for understanding Lewis acid-base interactions in that hard acids prefer hard bases and soft acids prefer soft bases. The principle can be applied to chemical equilibria in the form of the principle of maximum hardnesswhich states that molecules arrange themselves so as to... 

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. 

However, the strength of Lewis acid-base interaction can be expressed in energy terms, such as the exothermic molar heat, —for the equilibrium (III) of adduct formation. The enthalpy term is preferred because entropy effects accompanying the formation of coordinative bonds are difficult to determine. Various models have been proposed for the theoretical estimation of the enthalpy term based on molecular properties of reactants and are reviewed in Ref 5. The most significant developments have been the hard and soft acid-base principle of Pearson [6], the E C equation of Drago and Wayland [7], the donor and acceptor numbers of Gutmann [8], and the perturbation theory of Hudson and Klopman [9]. 

Why is such a trend observed Actually, the reason that hard acids and bases prefer to interact with each other is different than the reason that soft acids and bases prefer to interact. To see this, let s examine some mathematics that is meant to model the interaction between Lewis acids and bases in an early stage of their interaction. The analysis derives from pertur-bational molecular orbital theory (PMOT), which was briefly introduced in Chapter 1, and is explored in more depth in Chapter 14. In essence, three forces are considered to mediate the energy of interaction (Ej) between the acid and base as they approach each other in space (Eq. 5.28). One is the electrostatic repulsion between the electron clouds of the two entities, referred to as Ecore/ a positive destabilizing term. The second and third factors are both attractive and stabilizing. An electrostatic attraction between an acid and base occurs due to opposite charges on the acid and base this is called E s- Lastly, a term called Eoveriap/ which is related to the net overlap of the nucleophilic and electrophilic orbitals, is found to lower the energy of the system as the nucleophilic electrons delocalize into the empty electrophilic orbital. 

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