Hard Lewis acids and bases

Analogously, water is extremely efficient in weakening hard Lewis add - hard Lewis base interactions. Consequently, when aiming at catalysis by hard Lewis adds, the inefficiency of the interaction between the catalyst and the substrate is a serious problem. Strangely enough, this characteristic of water is not recognised by many researchers working with hard Lewis acids in 

Turning the argument around reactions that do not involve proton transfer steps will only experience a significant effect of the Lewis acids if a direct interaction exists between catalyst and reactant. The conventional Diels-Alder reaction is a representative of this class of reactions. As long as monodentate reactants are used, the effects of Lewis acids on this reaction do not exceed the magnitude expected for simple salt effects, i.e. there are no indications for a direct interaction between Lewis-acid and substrate. 

We conclude that, when employirg hard Lewis-acids in aqueous solution, the term Lewis-acid catalysis should be used with caution, and only after evidence for a direct interaction between Lewis-acid and substrate has been obtained. 

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In Chapter 6 we survey what has been accomplished and indicate directions for future research. Furthermore, we critically review the influence of water on Lewis acid - Lewis base interactions. This influence has severe implications for catalysis, in particular when hard Lewis acids and bases are involved. We conclude that claims of Lewis-acid catalysis should be accompanied by evidence for a direct interaction between catalyst and substrate. 

The first symposium on soft and hard (Lewis) acids and bases was organized by Prof. R. F. Hudson (now at the University of Kent, Canterbury) at our institute in May 1965, and the proceedings partly published in Structure and Bonding 1, pp. 207—248. A second symposium was organized by Professor Malcolm J. Frazer, Northern Polytechnic, Holloway, London, 29.—31. March 1967. The proceedings of this discussion were not published, but the present paper is based on the invited lecture, partly modified after further helpful comments by Professors /. Bjerrum, K. Fajans and G. Klopman. 

Hard Lewis acids and bases have inflexible electron orbitals that form ionic bonds. The electron orbitals of soft Lewis acids and bases are more polarizable and more likely to form covalent bonds. Soft Lewis acids and bases are also called covalent-bonding ions and are siderophile (sulfur-loving) ions in the geology literature. Organic ligands and soil organic matter range from hard to soft Lewis bases. ... 

Analogous to the classification of Lewis acids and bases in hard and soft species, Ahrland et al. have su ested a division of donors and acceptors into classes a and 6. See Ahrland, S. Chatt, J. Davies, N.R. Quart. Rev. 1958, 77, 265... 

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. 

Continuum effects indicated by hard and soft acid-base (Lewis acids) and bases. C. K. Jorgensen, Top. Curr. Chem., 1975,56,1-66 (210). 

Lewis Acids and Bases Hard and Soft Acids and Bases... 

Lewis acids and bases can be organized according to their polarizability. If polarizability is low, the species is categorized as hard. If polarizability is high, the species is soft. ... 

The terms hard and soft are relative, so there is no sharp dividing line between the two, and many Lewis acids and bases are intermediate between hard and soft. Example shows how to categorize Lewis acids and bases according to their hard-soft properties. 

Yatsimirskii considered that the hard and soft classification was too general and proposed instead a more detailed approach. He classified Lewis acids and bases into six groups, based on the nature of the adduct bonding. 

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. 

Jolly, W. L. Inorganic Applications of X-Ray Photoelectron Spectroscopy. 71, 149-182 (1977). Jorgensen, C. K. Continuum Effects Indicated by Hard and Soft Antibases (Lewis Acids) and Bases. 56, 1-66 (1975). 

This concept was introduced qualitatively in the late 1950s and early 1960s by Pearson, in the framework of his classification of Lewis acids and bases, leading to the introduction of the hard and soft acids and bases (HSAB) principle [19-21]. This principle states that hard acids prefer to bond to hard bases and soft acids to soft bases. In many contributions, the factor of 1/2 is omitted. The inverse of the hardness was introduced as the softness S=l/rj [22]. A third quantity, which can be expressed as a derivative with respect to the number of electrons is the Fukui function, was introduced by Parr and Yang [23,24] ... 

According to the hard and soft acids and bases (HSAB) principle, developed by Pearson in 1963232,233, Lewis acids and Lewis bases are divided into two groups hard and soft. Pearson correlated the hardness of acids and bases with their polarizability, whereby soft acids and bases are large and easily polarizable, and vice versa. A selected list of Lewis acids ordered according to their hardness in aqueous solution is presented in Table 18. The HSAB principle predicts strong association of like partners. Hard acid-soft base complexes mainly result from electrostatic interactions, while soft acid-soft base complexes are dominated by covalent interactions. 

Contents C. K. Jorgensen Continuum Effects Indicated by Hard and Soft Antibases (Lewis Acids) and Bases. 

Drago and co-workers Introduced an empirical correlation to calculate the enthalpy of adduct formation of Lewis acids and bases ( 5). In 1971, he and his co-workers expanded the concept to a computer-fitted set of parameters that accurately correlated over 200 enthalpies of adduct formation ( ). These parameters were then used to predict over 1200 enthalpies of interaction. The parameters E and C are loosely Interpreted to relate to the degree of electrostatic and covalent nature of the Interaction between the acids and bases. This model was used to generalize the observations involved in the Pearson hard-soft acid-base model and render it more quantitatively accurate. 

The principle of hard and soft Lewis acids and bases, proposed by Pearson (1963), is useful to describe these reactions. A Lewis acid is any chemical species that employs an empty electronic orbital available for reaction, while a Lewis base is any chemical species that employs a doubly occupied electronic orbital in a reaction. Lewis acids and bases can be neutral molecules, simple or complex ions, or neutral or charged macromolecules. The proton and all metal cations of interest in subsurface aqueous solutions are Lewis acids. Lewis bases include H, O, oxyanions, and organic N, S, and P electron donors. A list of selected hard and soft Lewis acids and bases found in soil solutions is presented in Table 6.1. 

An inner-sphere complex is formed between Lewis acids and bases, while an outer-sphere complex involves a water molecule interposed between the acid and the base. A hard Lewis acid is a molecular unit of small size, high oxidation state, high electronegativity, and low polarizability whereas a soft Lewis acid is a molecular unit of relatively large size, characterized by low oxidation state, low electronegativity, and high polarizability. Based on this characterization, hard bases prefer to complex hard acids, and soft bases prefer to complex soft acids, under similar conditions of acid-base strength. 

Table 6.1 Hard and soft Lewis acids and bases in solution (Sposito 1981)...

It has to be pointed out that Lewis acid and base sites produced during the regular dehydroxylation process can hardly be all involved in catalytic reactions as active sites. It has already been indicated [44] that only defect sites can be considered as active sites because of their low site density. The configuration of such defect sites can hardly be predicted from idealizing model considerations. 

The Lewis acid/base complex is formed via an overlap between a doubly occupied orbital of the donor D and vacant orbital of the acceptor A. This acid/base approach was extended by Pearson who divided Lewis acids and bases into two groups, hard and soft, according to their electronegativity and polarizability (principle of hard and soft acids and bases (HSAB concept). Hard acids (e.g., H, Lf, Na, BF3, AICI3, hydrogen-bond donors HX) and hard bases (e.g., F", CL, HO, RO, H2O, ROH, R2O,... 

Nucleophilic Displacement of Halogens at Saturated Carbon Atoms Box 13.1 The Concept of Hard and Soft Lewis Acids and Bases (HSAB) Illustrative Example 13.2 Some More Reactions Involving Methyl Bromide Illustrative Example 13.3 1,2-Dibromoethane in the Hypolimnion of the Lower Mystic Lake, Massachusetts Polyhalogenated Alkanes — Elimination Mechanisms... 

Box 13.1 The Concept of Hard and Soft Lewis Acids and Bases (HSAB)... 

Explain the terms hard and soft Lewis acids and bases. 

In Klopman s quantitative calculations, the presence of other orbitals close in energy to the frontier orbitals is taken into account in the determination of the reactivity, as is the change in solvation energy on complex formation. On these theoretical grounds, Klopman has been able to classify Lewis acids and bases into hard, soft, and borderline categories his results closely parallel Pearson s assignments (Section 3.5) made from consideration of chemical properties. 

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