HSAB concept

Both the Edwards equation and Pearson s HSAB concept take as primary determinants of nucleophilicity the polarizability and basicity. A two-term equation of Bartoli and Todesco" uses these ideas also, but as a measure of polarizability the... 

With alkali cyanides, a reaction via a SN2-mechanism takes place the alkyl halide is attacked by cyanide with the more nucleophilic carbon center rather than the nitrogen center, and the alkylnitrile is formed. In contrast, with silver cyanide the reaction proceeds by a SnI-mechanism, and an isonitrile is formed, since the carbenium intermediate reacts preferentially with the more electronegative center of the cyanide—i.e. the nitrogen (Kornblum s rule, HSAB concept). ... 

It is obvious that the Lewis theory and the HSAB concept are very important for the description of the mechanisms of chemical reactions however, for electrochemistry the Br0nsted theory is quite adequate. 

The HSAB concept is the hard-soft acid-base concept. 

Next, we shall describe why the magnitudes of the E and C numbers are not just quantitative manifestations of the HSAB concept, but give insight into intermolecular interactions which are absent in the qualitative soft-soft and hard-hard labeling of interactions. As can be seen from the data in Tables 3 and 4, each acid and base has both a C and an E number which could be thought to correspond to possessing properties of softness and hardness. If this were the case, ammonia, which Pearson labels hard, has a larger Cb value than benzene, which is labeled soft. 

The question now remains, can a transformation matrix be found that converts these E and C numbers into a set that is appropriate The algebra involved is tedious, and we can use a comparable approach which shows that the HSAB concept is incomplete. That this is so and the extent to which the HSAB concept fails as a quantitative model can be shown by converting the HSAB concept as qualitatively applied to a quantitative equation and attempting to fit enthalpy data to it. If a transformation matrix exists, our least squares program will find it and fit the data as well as our E and C equation does. 

A literal translation of the HSAB concept would be given by ... 

As illustrated by the data in Table 8, we can conclude that the HSAB concept as it is most commonly employed is fundamentally incorrect. A hardness-softness-strength equation could be written as ... 

Eq. (1) with no obvious advantages for this additional complication since "strength of hardness and strength of softness would have to change in a way contrary to the accepted definition of the words. This equation doesn t correlate our quantitative data, and the HSAB concept as usually applied is not an adequate way to describe intermolecular... 

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,... 

The application of the HSAB concept to solutions leads to the rule that hard solutes dissolve in hard solvents and soft solutes dissolve in soft solvents (Pearson, 1987). For example, benzene is considered a very soft solvent since it contains only a basic function. Contrary to benzene, water is a very hard solvent, with respect to both its basic and acidic properties, ft is the ideal solvent for hard bases and hard acids. The hardness of water is reduced by the introduction of alkyl substituents in proportion to the size of the alkyl group. In alcohols, therefore, softer solutes become soluble. 

R. G. Pearson, Hard and soft acids and bases—the evolution of a concept. Coord. Chem. Rev. 100, 403-425 (1990) R. G. Pearson, Absolute electronegativity and hardness. Application to inorganic chemistry. Chem. Br. 31, 444-447 (1991) R. G. Pearson, Recent advances in the HSAB concept. J. Chem. Educ. 64, 561 (1987). 

Lewis acids are electron pair acceptors and Lewis bases are electron pair donors. However, according to the Hard and Soft Acids and Bases (HSAB) concept [17], Lewis acids are classified into hard and soft acids, while Lewis bases are classified into hard and soft bases. Hard acids interact strongly with hard bases, soft acids with soft bases. 

According to the hard and soft acids and bases (HSAB) concept, hard acids tend to interact strongly with hard bases, while soft acids tend to interact strongly with soft bases. The HSAB concept applies also to solute-solvent interactions. Figure 2.5 shows the polarographic half-wave potentials of metal ions in N-methyl-2-pyrrolidinone (NMP) and N-methyl-2-thiopyrrolidinone (NMTP) [13]. Here, we can compare the half-wave potentials in the two solvents, because they are referred to the half-wave potential of the bis(biphenyl)chromium(l)/(0) couple,... 

Equation (4.5) is also valid in this case. Reactions of this type are realized in polarography at a dropping mercury electrode, and the standard potentials can be obtained from the polarographic half-wave potentials ( 1/2)- Polarographic studies of metal ion solvation are dealt with in Section 8.2.1. Here, only the results obtained by Gritzner [3] are outlined. He was interested in the role of the HSAB concept in metal ion solvation (Section 2.2.2) and measured, in 22 different solvents, half-wave potentials for the reductions of alkali and alkaline earth metal ions, Tl+, Cu+, Ag+, Zn2+, Cd2, Cu2+ and Pb2+. He used the half-wave potential of the BCr+/BCr couple as a solvent-independent potential reference. As typical examples of the hard and soft acids, he chose K+ and Ag+, respectively, and plotted the half-wave potentials of metal ions against the half-wave potentials of K+ or against the potentials of the 0.01 M Ag+/Ag electrode. The results were as follows ... 

Another property that characterizes solvents is their softness, in terms of the HSAB concept (Pearson 1963), according to which the interactions of soft solvents are strongest with soft solutes, of hard solvents with hard solutes, but are weaker for hard solvents with soft solutes and vice versa. The applicability of the softness property takes into account that it is superimposed on the more general electron pair donation property discussed above. In fact, it can replace (Marcus 1987) the notion of the family dependence of the P scale, expressed by the , parameter (Kamlet etal. 1985). A few quantitative scales have been... 

Two proofs for the HSAB principle were provided under the restriction of a common chemical potential of the reaction partners [83, 84]. Later on, a local HSAB principle was provided by Gazqu z and Mendez [85], They showed that the interaction between two chemical species will not necessarily occur through their softest atoms, but through those whose softnesses are approximately equal. In Section 4.2, an intuitive application of the HSAB concept is provided, followed by an application of the local HSAB principle in the interpretation of regioselectivity in Diels Alder reactions. 

Table I summarizes compounds that can be expected to be poisons on oxide surfaces according to the HSAB concept. Column 1 denotes the surface site to be detected or poisoned by a compound given in column 2. Column 3 specifies the most probable adsorption processes for the various systems according to...

The adsorption of potential poisons listed in Table I according to the HSAB concept is discussed in this section. From the information accumulated regarding the modes of interaction of any one of these adsorbates with oxide surfaces, it will be concluded whether a particular molecule may be suitable as an effective specific poison or as a probe molecule for the characterization of certain surface properties. 

The aim of specific poisoning is the determination of the chemical nature of catalytically active sites and of their number. The application of the HSAB concept together with eight criteria that a suitable poison should fulfill have been recommended in the present context. On this basis, the chemisorptive behavior of a series of hard poisoning compounds on oxide surfaces has been discussed. Molecules that are usually classified as soft have not been dealt with since hard species should be bound more strongly on oxide surfaces. This selection is due to the very nature of the HSAB concept that allows only qualitative conclusions to be drawn, and it is by no means implied that compounds that have not been considered here may not be used successfully as specific poisons in certain cases. Thus, CO (145, 380-384), NO (242, 381, 385-392, 398), and sulfur-containing molecules (393-398) have been used as probe molecules and as specific poisons in reactions involving only soft reactants and products (32, 364, 368). 

HSAB concept — In 1963, Pearson published the concept of hard and soft acids and bases (HSAB). Based... 

In the gas phase, the reaction of ethyl cations, C2H , with the ambident 2,4-pentanedione (which is 92% enolized at 25 °C in the gas phase) leads predominantly (>95%) to alkylation at the hard oxygen site and not at the soft carbon atom, as predicted by the HSAB concept [662]. Accordingly, the gas-phase alkylation of the enolate ion of cyclohexanone gives only the O- and no C-alkylation product [848], and the gas-phase acylation of acetophenone enolate with trifluoroacetylchloride leads predominantly to the 0-acylation product (0/C ratio = 6.0) [849]. 

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