Soft acids and bases

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. 

Ambident reactivity of A-4-thiazoline-2-thione has been discussed (101) in terms of the Hard and Soft Acids and Bases classification (199) and the Klopman-Hudson approach (200). 

T. L. Ho, Hard and Soft Acids and Bases Principle in Organic Chemisty, Academic Press, New York, 1977. 

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. 

Sometimes discussed in terms of hard and soft acids and bases. 

These concepts play an important role in the Hard and Soft Acid and Base (HSAB) principle, which states that hard acids prefer to react with hard bases, and vice versa. By means of Koopmann s theorem (Section 3.4) the hardness is related to the HOMO-LUMO energy difference, i.e. a small gap indicates a soft molecule. From second-order perturbation theory it also follows that a small gap between occupied and unoccupied orbitals will give a large contribution to the polarizability (Section 10.6), i.e. softness is a measure of how easily the electron density can be distorted by external fields, for example those generated by another molecule. In terms of the perturbation equation (15.1), a hard-hard interaction is primarily charge controlled, while a soft-soft interaction is orbital controlled. Both FMO and HSAB theories may be considered as being limiting cases of chemical reactivity described by the Fukui ftinction. 

The low stability of the complex 3.6 is consistent with the hard and soft acids and bases principle of Pearson (1963, 1968 Parr and Pearson, 1983 theoretical aspects Pearson, 1989 Chatteraj et al., 1991 monograph Ho, 1977). According to that principle hard acids will tend to complex with hard bases and soft acids with soft bases. Water is a hard base, whereas the nitrosyl ion is classified by Pearson as a borderline acid with a tendency to be soft. 

Nucleophilic catalysis is also observed with iodide ions. Fluoride ion does not form nitrosyl fluoride under diazotization conditions, as is to be expected from Pearson s hard and soft acids and bases principle which was discussed briefly in Section 3.2. More recently, nucleophilic catalysis has also been shown to occur with thiocyanate ion (SCN ), thiosulfate ion (HS2Of), dimethyl sulfide, and thiourea (H2NCSNH2) or its alkyl derivatives (see below). 

The concept of hard and soft acids and bases as applied to multicentre chemical reactions. B. Saville, Angew. Chem., Int. Ed. Engl., 1967, 6, 928-939 (84). 

The principle of hard and soft acids and bases and the problem of competitive coordination in complex compounds. A, D. Garnovskii, D. A. Osipov and S. B. Bulgarevich, Russ. Chem. Rev. (Engl. Transl), 1972, 41, 341-359 (441). 

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

The facility with which an acid-base reaction takes place depends of course on the strengths of the acid and the base. But it also depends on quite another quality, called the hardness or softness of the acid or base. Hard and soft acids and bases have these characteristics ... 

The concept of hard and soft acids and bases can be used to interpret many trends in chemical reactivity. These trends are summarized in the hard-soft acid-base principle (HSAB principle), an empirical summary of results collected from many chemical reactions studied through decades of research. 

C21-0023. State the hard-soft acid-base (HSAB) principle. Define and give examples of hard and soft acids and bases. 

In this review, CPOs constructed by covalent bonds are mainly focused on however, stable coordination bonds comparable to the stability of the covalent bonds have potential for future enhanced molecular design of novel CPOs. One representative is the bond between pyridine-type nitrogen and metal, which is widely used in supramolecular chemistry, that is, the cyclic supramolecular formation reaction between pyridine-substituted porphyrin and metal salts (Fig. 6d) [27,28]. Palladium salts are frequently used as the metal salts. From the viewpoint of the hard and soft acid and base theory (HSAB), this N-Pd coordination bond is a well-balanced combination, because the bonds between nitrogen and other group X metals, N-Ni and Ni-Pt coordination bonds, are too weak and too strong to obtain the desired CPOs, respectively. For the former, the supramolecular architectures tend to dissociate into pieces in the solution state, and for the latter. 

This concept of Chatt and his coworkers was developed further by Pearson (1963, 1966, 1968a,b) in his theory of hard and soft acids and bases. Hard acids correspond with class (a) acceptors and soft acids with class (b) acceptors. 

According to Yatsimirskii, group (2) and (3) species are equivalent to Pearson s hard acids and bases, and group (4), (5) and (6) species correspond to Pearson s soft acids and bases. This classification is of more value than HSAB theory to our subject. It can be seen that all cementforming anions come from group (3) and cations from groups (3), (4) and (5). Thus, the bonding in cement matrices formed from cation-anion combinations is not purely a but contains some n character. 

Pearson, R. G. (1963). Hard and soft acids and bases. Journal of the American Chemical Society, 85, 3533-9. 

Another feature of the metal ions that are typically involved in cementitious bonding in AB cements is that most of them fall into the category of hard in Pearson s Hard and Soft Acids and Bases scheme (Pearson, 1963). The underlying principle of this classification is that bases may be divided into two categories, namely those that are polarizable or soft, and those that are non-polarizable or hard. Lewis acids too may be essentially divided into hard and soft, depending on polarizability. From these classifications emerges the useful generalization that hard acids prefer to associate with hayd bases and soft acids prefer to associate with soft bases (see Section 2.3.7). 

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