Hard-Soft Acid-Base HSAB principle

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

Ho, T.-L. The hard soft acids bases (HSAB) principle and organic chemistry. Chem. Rev. 1975, 75, 1-20. 

To predict which of the two alkyne carbons, C1 or C2, HNC will preferentially attack, one now invokes the local hard-soft acid-base (HSAB) principle (cf. [157]), which says that interaction is favored between electrophile/nucleophile (or radical/radical) of most nearly equal softness. The HNC carbon softness of 1.215 is closer to the softness of C1 (1.102) than that of C2 (0.453) of the alkyne, so this method predicts that in the reaction scheme above the HNC attacks C1 in preference to C2, i.e. that reaction should occur mainly by the zwitterion A. This kind of analysis worked for -CH3 and -NH2 substituents on the alkyne, but not for -F. 

A very important finding is that hard cations (e.g., Be2+, Al3+, Ti4+) prefer ligands with hard donor atoms (e.g., F, O, Cl), and soft cations (e.g., Ag+, Cd2+, Tl+) prefer ligands with soft donor atoms (e.g., I, Te, As). This fact has a myriad of applications, because it can be advantageously used for analysis, selective reactions, metal ion separation, removal, recovery, etc. It is an application of the Hard Soft Acid Base (HSAB) principle enunciated by Pearson at the end of the 1960s. 

The solubilities of salts in water (clearly of importance in aquatic, analytical, and geochemistry) can be fairly well predicted and explained using two principles later we shall return to the more familiar of these, the hard soft acid base (HSAB) principle (see Hard Soft Acids and Bases). To control this principle, we must first consider oifly salts of anions that are hard bases, that is, salts of 0x0 and fluoro anions, and oxides, hydroxides, and fluorides. The solubilities of salts of these anions can be fairly well predicted and explained on the basis of the acidity classification of the cation and the basicity classification of the anion, that is, on a principle of acid base strength. The numerous solubility rules taught in General Chemistry could be replaced with four solubility principles (Table 4), two of which are quite reliable and two of which are less reliable, for known reasons. 

Finally, it must be mentioned that acid-base interactions can also be analyzed in terms of Pearson s hard-soft acid-base (HSAB) principle [56,57]. At present, the application of this concept to solid-solid interactions and thus to adhesion is under investigation. 

Now, the Hard-Soft Acid-Base (HSAB) principle suggests that if the reaction follows the sequence ... 

The concept of hardness has been introduced by Pearson " through his famous hard soft acid base (HSAB) principle which states that, Hard acids prefer to react with hard bases and soft acids prefer to react with soft bases for both their kinetic and thermodynamic considerations . The amount of electron transfer (AFV) and the associated change in energy (A ) during an acid (A) base (B) reaction have been shown to be . 

In chemistry, the American chemist Ralph G. Pearson (1963) introduced the concept of hardness. Originally, the definition referred to hard and soft acids and bases. The hard-soft acid-base (HSAB) principle states that hard Lewis acids prefer hard Lewis bases and conversely that soft Lewis acids prefer soft Lewis bases. We will not dig deeply into the chemical background here. What is important is that Pearson (-1980) later found that hardness may simply be defined as... 

Extension of the Hard-Soft Acid-Base(HSAB) Principle to Solid Adhesion... 

APPLICATIONS OF HARD-SOFT ACID-BASE (HSAB) PRINCIPLE TO SOLID ADHESION AND SURFACE INTERACTIONS BETWEEN METALS AND POLYMERS... 

We attempt to extend the Hard-Soft Acid-Base (HSAB) principle for the reactions in solutions to interactions in solids. First we point out the important link between the absolute hardness of acid-and-base and the average energy gap. Then we discuss the electronic band structures of various solids, e.g., metals, semimetals, semiconductors and insulators. On the basis of energy gaps, we elaborate various consequences of the acid-base interactions in solids. The applications of HSAB principle and the frontier orbital concept to the solid adhesion and surface interactions between metals and polymers will be verified by experimental results reported in the literature. The new findings reported in this paper should be beneficial to those who are carrying out research in or processing thin-film microelectronic devices or thick-film multilayer structures. 

For the acid-base interaction in solutions, in 1963, Pearson proposed the hard-soft acid-base (HSAB) principle to describe some basic rules about the kinetics and equilibrium of the reaction. In this paper, we attempt to apply the HSAB principle to solid interactions with the aid of the frontier orbital method. We shall first describe the HSAB principle as it has been evolved in recent years " and then the band structures of solids. After we demonstrate the compatibility between the HSAB principle and the band structures in the solid state, we then illustrate with several examples of adhesion and tribointeractions between metals and... 

The hard-soft acid-base (HSAB) principle... 

The hard-soft acid-base (HSAB) principle states that hard acids prefer to associate with, and react readily with, hard bases while soft acids prefer to associate with, and react readily with, soft bases. The HSAB principle embodies both kinetic and thermodynamic meaning. Thus, interaction between a Lewis acid and a Lewis base of comparable hardness or softness is predicted to proceed readily and result in the formation of a thermodynamically stable product. Applications of the HSAB principle to coordination chemistry abound.29 For example, DMSO is an ambidentate ligand with both hard (oxygen) and soft (sulfur) donor sites. When complexes are formed with platinum(II), a soft acid, DMSO will typically coordinate via sulfur, while, with the harder acid nickel(II), coordination via oxygen is favored. O... 

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