Hard soft acid-base principles

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. 

The hard-soft acid-base principle is not restricted to the usual types of acid-base reactions. It is a guiding principle that for all types of interactions species of similar electronic character interact best. We have already seen some applications (such as the relative strength of HF and HI) of this principle, which we will continue to call HSAB, but we now consider a number of other types of applications. 

The hard-soft acid-base principle just illustrated is one of the most useful principles in all of chemistry for predicting how many types of interactions occur, ft is not restricted to acid-base interactions, so it is better called the hard-soft interaction principle, ft predicts that hard acids (high charge, small size, low... 

Ho, Tse-Lok (1977). Hard and Soft Acid and Base Principle in Organic Chemistry. Academic Press, New York. The applications of the hard-soft acid base principle to many organic reactions. 

In Chapter 9, the hard-soft acid-base principle was discussed, and numerous applications of the principle were presented. This principle is also of enormous importance in coordination chemistry. First-row transition metals in high oxidation states have the characteristics of hard Lewis acids (small size and high charge). Consequently, ions such as Cr3+, Fe3+, and Co3+ are hard Lewis acids that bond best to hard Lewis bases. When presented with the opportunity to bond to NH3 or PR3, these metal ions bond better to NH3, which is the harder base. On the other hand, Cd2+ bonds better to PR3 because of the more favorable soft acid-soft base interaction. 

Ayers, P. W., Parr, R. G., and Pearson, R. G. 2006. Elucidating the hard/soft acid/base principle A perspective based on half-reactions. J. Chem. Phys. 124 194107. 

More recently, the interaction energy between different molecular systems has been expressed in terms of the chemical potential and molecular hardness (cf. Section IV.B) based on the local hard-soft acid-base principle (Chandrakumar and Pal, 2003), and respective applications include the calculation of acidities and bond dissociation energies of 4-substituted phenols (Romero and Mendez, 2003a, 2003b). 

Chandrakumar, K.R.S. and Pal, S., Study of local hard-soft acid-base principle to multiple-site interactions,. /. Phys. Chem. A, 106 5737-5744, 2002b. 

Regarding the primary coordination sphere of iron, the identity of the axial ligand is critical to determining the chemical reactivity of heme. In general, the interaction between the axial ligand and heme iron follow the hard-soft acid-base principle... 

The products of acid-base interactions such as those shown in Equations (5.36) through (5.39) are not properly considered as salts because they are not ionic compounds. Because in many cases these products are formed from two neutral molecules, they are more properly considered as addition compounds or adducts held together by the formation of coordinate covalent bonds. In that connection, they are similar to coordination compounds except that the latter ordinarily involve the formation of coordinate bonds to metal ions by the electron donors ligands). There are some useful generalizations that correlate to the stability of bonds during this type of acid-base interaction, and these are largely summarized by the hard-soft acid-base principle. 

Hard-Soft Acid-Base Principle (HSAB)... 

Both oxygen and nitrogen nucleophiles react more rapidly with fluoroaromatics than corresponding sulphur and carbon nucleophiles, in accordance with Hard-Soft Acid-Base principles [51]. It should be remembered, however, that the situation can become more complex with, for example, base catalysis, where the rate of the second stage becomes important under these rather unusual conditions, an order of replacement Br > Cl > F has been observed [52]. 

The separation factor relies on the design of the active sites on the resins and the solution chemistry. Mechanisms contributing to chemical selectivity reportedly include the hard-soft acid-base principle (HSAB), hydrophilicity/hydrophobicity selectivity, and pore size recognition. These mechanisms have common characteristics and their effects are thus not easy to distinguish. 

Inert electrolytes show ion specificity, as discussed in Chapter 2 and illustrated in Figures 2.4, 2.5, 2.9, and 2.10. The anions affect the positive branches of the charging and electrokinetic curves, and the cations affect the negative branches. The affinity to particular monovalent anions and cations depends on the character of the surface, and follows the hard- soft acid-base principle that is, hard surfaces prefer to adsorb hard ions, and soft surfaces prefer to adsorb soft ions. The effects of ion specihcity on the charging and electrokinetic curves are usually minor. The affinity series observed in coagulation behavior are termed Hoffmeister series. 

Popular qualitative chemical concepts such as electronegativity [1] and hardness [2] have been widely used in understanding various aspects of chemical reactivity. A rigorous theoretical basis for these concepts has been provided by density functional theory (DFT). These reactivity indices are better appreciated in terms of the associated electronic structure principles such as electronegativity equalization principle (EEP), hard-soft acid-base principle, maximum hardness principle, minimum polarizability principle (MPP), etc. Local reactivity descriptors such as density, Fukui function, local softness, etc., have been used successfully in the studies of site selectivity in a molecule. Local variants of the structure principles have also been proposed. The importance of these structure principles in the study of different facets of medicinal chemistry has been highlighted. Because chemical reactions are actually dynamic processes, time-dependent profiles of these reactivity descriptors and the dynamic counterparts of the structure principles have been made use of in order to follow a chemical reaction from start to finish. 

The qualitative concepts of hardness and softness were first introduced by Pearson [30-32,34], which later culminated in enunciation of the famous hard-soft acid-base principle. Quantification of these concepts had been in order and was accomplished within density functional theory by Parr and Pearson [52]. The energy stabilization due to soft-soft interaction can be expressed by rearranging Eq. (61) as [52] ... 

In terms of Pearson s hard-soft acid-base principle, the soft Lewis acid copper ion is not compatible with the hard base H20 molecule, but a recent article provided the first example of a copper(I)-water bond (240). The novel copper complex with 2,3-diphenylquinoxaline (L 1), [Cu(L91)(H20)]C104, is diamagnetic and indefinitely stable in air in the solid state. Its structure consists of an infinite polymeric cationic chain in which adjacent metal centers are bridged by the aro-... 

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