Pearson concept hard-soft

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. 

Drago [Dr 73] considered how his concept could be correlated with Pearson s hard-soft acid-base theory [Pe 63b, Kl 68]. Setting out from the assumption that the terms E Eg and C Cg may serve as measures of the electrostatic and covalent interactions, respectively, and identifying the hard and soft interactions of the Pearson concept with the electrostatic and the covalent interactions, respectively, Drago wished to employ the C/E ratios for the acids or bases examined as a measure of the soft nature of the molecules. He considered that the higher the value of C/E, the softer is the given acid or base, whereas low value of C/E is indicative of a hard character. 

However, the sequence of C/E values is not in accordance with practical experience with the solvents. Drago s parameters and his equation also could not be used to describe Pearson s hard-soft concept. 

Drago and co-workers Introduced an empirical correlation to calculate the enthalpy of adduct formation of Lewis acids and bases ( 5). In 1971, he and his co-workers expanded the concept to a computer-fitted set of parameters that accurately correlated over 200 enthalpies of adduct formation ( ). These parameters were then used to predict over 1200 enthalpies of interaction. The parameters E and C are loosely Interpreted to relate to the degree of electrostatic and covalent nature of the Interaction between the acids and bases. This model was used to generalize the observations involved in the Pearson hard-soft acid-base model and render it more quantitatively accurate. 

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. 

It is possible to be consistent with our E and C equation and view intermolecular interactions in terms of concepts we could call hardness, softness and strength. However, in doing this, we will have to modify the qualitative ideas presented by Pearson (2) about what hardness and softness mean, vide infra. The approach involves converting the E and C equation to polar coordinates. Our acids and bases are represented as vectors in E and C space in Fig. 7. The dot product of these two vectors is given as... 

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,... 

Hardness and softness as chemical concepts were presaged in the literature as early as 1952, in a paper by Mulliken [138], but did not become widely used till they were popularized by Pearson in 1963 [139]. In the simplest terms, the hardness of a species, atom, ion or molecule, is a qualitative indication of how polarizable it is, i.e. how much its electron cloud is distorted in an electric field. The adjectives hard and soft were said to have been suggested by D.H. Busch [140], but they appear in Mulliken s paper [138], p. 819, where they characterize the response to spatial separation of the energy of acid-base complexes. The analogy with the conventional use of these words to denote resistance to deformation by mechanical force is clear, and independent extension, by more than one chemist, to the concept of electronic resistance, is no surprise. The hard/soft concept proved useful, particularly in rationalizing acid-base chemistry [141]. Thus a proton, which cannot be distorted in an electric field since it has no electron cloud (we ignore the possibility of nuclear distortion) is a very hard acid, and tends to react with hard bases. Examples of soft bases are those in which sulfur electron pairs provide the basicity, since sulfur is a big fluffy atom, and such bases tend to react with soft acids. Perhaps because it was originally qualitative, the hard-soft acid-base (HSAB) idea met with skepticism from at least one quarter Dewar (of semiempirical fame) dismissed it as a mystical distinction between different kinds of acids and bases [142]. For a brief review of Pearson s contributions to the concept, which has been extended beyond strict conventional acid-base reactions, see [143],... 

The concept of soft and hard acids and bases (7), which is in effect an extension of the Chatt-Ahrland classification (2) of A and B metals, is applied in the 1963 paper by Pearson (7) particularly to equilibria involving mainly inorganic systems. This paper follows an earlier discussion by Edwards and Pearson (3) of the Swain-Edwards equation (4) (1) for nucleophilic reactivity. 

The Pearson concept of hard and soft acids and bases considers the number of electrons in the outer shell. Elements with a saturated outer shell and low tendency for polarization (noble gas configuration) are called hard acids, while elements with only partially filled outer shell, low electronegativity, and high tendency for polarization are soft acids. 

Table 10 Classification of metal ions into A and B- type and after the Pearson concept into hard and soft acids with preferred ligands (after Stumm and Morgan 1996)...

W. B. Jensen, The Lewis Acid-Base Concepts An Overview, Wiley-Interscience, New York, 1980, and H. L Finston and Allen C. Rychtman, A New View of Current Acid-Base Theories, John Wiley Sons, New York, 1982, provide good overviews of the history of acid-base theories and critical discussions of the different theories. R. G. Pearson s Hard and Soft Acids and Bases, Dowden, Hutchinson, Ross, Stroudsburg, PA, 1973, is a review by one of the leading exponents of HSAB. For other viewpoints, the references provided in this chapter should be consulted. 

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] ... 

The HSAB theory of Pearson has been one of the key organizing concepts in the study of nucleophiles. This theory is applied and examined in Chapters 15 and 16. In Chapter 15, Fuji applies the HSAB principles to design nucleophilic reagents for cleaving C-X bonds. Fuji notes that all bonds are made of a combination of Lewis acid and Lewis base and have hard-soft dissymmetry for the typical C-X bond, the carbon is a soft acid and the X is a hard base. Thus, in accord with the HSAB principles, a soft base (the nucleophile) and a hard acid are required to cleave this bond selectively. Applying these ideas, Fuji then shows the utility of several soft base-hard acid reagents for cleaving various C-X bonds in complex molecules. 

To rationalize observations such as these, Pearson presented the concept of hard and soft acids and bases (HSABs), designating polarizable acids and bases as soft and nonpolarizable acids and bases as hard. Much of the hard-soft distinction depends on polarizability, the degree to which a molecule or ion is distorted by interaction with other molecules or ions. Electrons in polarizable molecules can be attracted or repelled by charges on other molecules, forming slightly polar species that can then interact with the other molecules. The HSAB concept is a nseful gnide to explain acid-base chemistry and other chemical phenomena. Pearson stated, Hard acids prefer to bind to hard bases, and... 

The Pearson concept can be transferred to metal surfaces. In this case, in addition to the character of the bonding electrons, the crystallographic structure of the metal surface and the surface topography must be taken into account. For cubic close-packed structures the most prominent crystallographic surfaces are the (111), (100), and (110) interfaces. The (111) face is the closest packed structure and has the hardest surface topography followed by the (100) and (110) interfaces (the softest one). The hard-soft character of a polycrystalline... 

Pearson, R. G. (1972). [Quantitative evaluation of the HSAB (hard-soft acid-base) concept]. Reply to the paper by Drago and Kabler (1972). Inorg. Chem. 11, 3146. 

The phenomenon can also be interpreted qualitatively according to Pearson as hard-hard and soft-soft interactions. One can similarly make use of the Ahrland concept [Ah 66] of the interaction of reaction partners belonging to groups A and B,... 

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 . 

Acidity of Lewis Acids. In contrast to the quantitative scale of acid strength established for Brqnsted acids a similar scale for Lewis acids has not been developed. Perhaps the best known of the various approaches is the hard and soft acids and bases (HSAB) principle introduced by Pearson, defining hardness and softness as specific qualities of acids and bases (16). Hard acids have small acceptor atoms, their outer electrons are not easily excited, and bear considerable positive charge. Soft acids, in turn, have large acceptor atoms, the outer electrons are easily excited, and possess lower positive charge. A selection of typical hard and soft acids is given in Table 3. The HSAB concept predicts that hard acids prefer to associate with hard bases and soft acids with soft bases... 

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... 

The observation that silica facilitates reduction, while on aliunina reduction of Cr is more difficult, supports the idea of a support controlled redox behavior. Thus, the supports take actively part in the redox chemistry of Cr. This can be explained with the hardness-softness concepts, first introduced by Pearson the higher the Al-content of the support, the harder the support, the less polarizable or susceptible for electron fluctuations, tbe more difficult reduction [13,14]. 

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