The hard-soft acid-base HSAB principle

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

Aquated metal ions tend to group into two categories in their reactions with halide ions.27a One class favors reactions with small, unpolarizable bases (e.g. fluoride ion) and the other favors reactions with large, polarizable bases (e.g. iodide ion). Swarzenbach referred to these classes of metal ions as 

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 

Pearson has described S how the concepts of hardness and softness can be understood through application of density functional theory (DFT) to chemical systems and, further, how the results correlate with molecular orbital theory. A firm theoretical basis for the HSAB principle is evolving from these studies that links chemical hardness to absolute electronegativity which, in this sense, refers to the electronic chemical potential of a system rather than the electronegativity of a single atom within a molecule. 

10 Factors affecting the acid/base properties of coordination compounds 

---

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. 

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. 

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

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... 

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... 

Metal-solvent interactions can be conveniently considered in terms of the hard-soft acid-base (HSAB) principle (Chapter 1). For example, palladium(II) is a soft metal center and so the hard oxygen donor solvent, diethyl ether, interacts only poorly with it. Simple valence bond models have been presented that adequately explain such soft metal-hard base interactions. In this chapter complexes containing coordinated halocarbons are treated in a separate section (3.7) from those that contain other "hard bases" since such species have only been recognized as well-defined complexes in recent years and their potential for exploitation in coordination chemistry merits special attention. 

---