Soft Lewis acids and bases

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. 

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

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. 

One of the most useful tools for predicting the outcome of chemical reactions is the principle of hard and soft acids and bases (HSAB), formulated by Pearson in 1963 [13-15]. This prindple states that hard acids will react preferentially with hard bases, and soft acids with soft bases, hard and soft referring to sparsely or highly polarizable reactants. A selection of hard and soft Lewis acids and bases is given in Table 1.1. 

Table 1.1. Hard and soft Lewis acids and bases [13,15,16] (Z = electron-withdrawing group, M = metal). The acidic or basic centers in molecules are in italics.

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. ... 

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. 

A selection of important metal ions and counterions, all of importance in atmospheric corrosion, have elsewhere been classified into hard, intermediate, and soft Lewis acids and bases [9]. The Lewis acid-base concept explains why hard acids, such as Ah+ and Ti" + commonly form oxides or sulfates as corrosion products, whereas soft acids, such as Cu" " and Ag+, commonly form sulfides. The concept also explains why intermediate acids, such as Zn +, Cu +, and Ni +, possess the ability to form corrosion products with a broader range of counterions [7]. 

Kurmach and coworkers reported the synthesis of A/-Ar -4-Ar -6-R -1 -R -2-oxo(thioxo)-l,2,3,4-tetrahydropyrimidine-5-carbothioamides 39 via a [2 + 1 + 3]-cyclocondensation reaction, using Biginelli reaction conditions (Scheme 26) (14CHC1770).It was determined that the regioselectivity of this reaction followed the hard/soft Lewis acids and bases principle. 

The chemical potential, chemical hardness and sofmess, and reactivity indices have been nsed by a number of workers to assess a priori the reactivity of chemical species from their intrinsic electronic properties. Perhaps one of the most successful and best known methods is the frontier orbital theory of Fukui [1,2]. Developed further by Parr and Yang [3], the method relates the reactivity of a molecule with respect to electrophilic or nucleophilic attack to the charge density arising from the highest occupied molecular orbital or lowest unoccupied molecular orbital, respectively. Parr and coworkers [4,5] were able to use these Fukui indices to deduce the hard and soft (Lewis) acids and bases principle from theoretical principles, providing one of the first applications of electronic structure theory to explain chemical reactivity. In essentially the same form, the Fukui functions (FFs) were used to predict the molecular chemical reactivity of a number of systems including Diels-Alder condensations [6,7], monosubstituted benzenes [8], as well as a number of model compounds [9,10]. Recent applications are too numerous to catalog here but include silylenes [11], pyridinium ions [12], and indoles [13]. 

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