Pearson concept

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)...

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... 

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. 

In the reaction of O-Iithium-N,N-bis(silyl)hydF0xylaimde with trimethylchloiostannan N,0-bis(ter/-butyldimethylsilyl)-N-(trimethylstannyl)hydroxyIamine was fonned (Eq. 5). In contrast to the expected N,N-bis(siIyI)-0-stannylhydroxylamme the reaction product was N,0-bis(silyl)-N-stannylhydroxylamine. The migration of the stannyl group from the oxygen to the nitrogen atom can be explained with the Pearson concept. A solid-state structure was elucidated by X-ray diffraction method (Fig. 6). 

R. G. Pearson, J. Am. Chem. Soc. 85 3533 (1963) T. L. Ho, Hard and Soft Acids and Bases in Organic Chemistry, Academic Press, New York, 1977 W. B. Jensen, The Lewis Acid-Base Concept, Wiley-Interscience, New York, 1980, Chapter 8. 

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... 

Pearson extended these concepts to a wide range of acids and bases and proposed the... 

This concept of Chatt and his coworkers was developed further by Pearson (1963, 1966, 1968a,b) in his theory of hard and soft acids and bases. Hard acids correspond with class (a) acceptors and soft acids with class (b) acceptors. 

The concept of hard and soft acids and bases ( HSAB ) should also be mentioned here. This is not a new theory of acids and bases but represents a useful classification of Lewis acids and bases from the point of view of their reactivity, as introduced by R. G. Pearson. 

Pearson, R. G. (1997). Chemical Hardness. Wiley-VCH, New York. This book is devoted to applications of the concept of hardness to many areas of chemistry. 

In this chapter, a survey of the enormously broad area of reactions of coordination compounds will be presented, and some of the basic mechanisms of the reactions will be presented. However, reactions of coordination compounds is such a very broad area that this chapter (as would be the case of any chapter) can present only the basic concepts and an elementary introduction to the field. More detailed coverage will be found in the references listed at the end of the chapter. The classic books in the field are Basolo and Pearson (1974) and Wilkins (1991), which present excellent and detailed reviews of the literature. We begin the chapter by illustrating some of the synthetic methods that have been useful for synthesizing coordination compounds. 

The next step is the identification of the concept of chemical hardness, 17, with the second derivative of the energy with respect to the number of electrons, formulated by Parr and Pearson [14]... 

This concept was introduced qualitatively in the late 1950s and early 1960s by Pearson, in the framework of his classification of Lewis acids and bases, leading to the introduction of the hard and soft acids and bases (HSAB) principle [19-21]. This principle states that hard acids prefer to bond to hard bases and soft acids to soft bases. In many contributions, the factor of 1/2 is omitted. The inverse of the hardness was introduced as the softness S=l/rj [22]. A third quantity, which can be expressed as a derivative with respect to the number of electrons is the Fukui function, was introduced by Parr and Yang [23,24] ... 

To the extent that the N+ correlation is successful it means that the pattern of nucleophilic reactivity is not influenced by the nature of the electrophilic center at which substitution takes place. On the other hand, according to the concepts of the theory of hard and soft acids and bases (HSAB) as applied to nucleophilic substitution reactions (Pearson and Songstad, 1967) one would expect that a significant change in the HSAB character of the electrophilic center as an acid should lead to changes in the pattern of nucleophilic reactivity observed. Specifically, in substitutions occurring at soft electrophilic centers, soft-base nucleophiles should be more reactive relative to other nucleophiles than they are in substitutions at harder electrophilic centers, and in substitutions at hard electrophilic centers hard-base nucleophiles should appear relatively more reactive compared to other nucleophiles than they do in substitutions at softer electrophilic centers. 

As mentioned in the Introduction, no structural information on these species was available for more than 40 years after the discovery of the first Zintl metal cluster anions, since no pure crystalline phases could be isolated and characterized structurally. Nevertheless, early efforts to rationalize the observed formulas and chemical bonding of these intermetallics and related molecules utilized the Zintl-Klemm concept [75, 76] and the Mooser-Pearson [77] extended (8 — N) rule. In this rule N refers to the number of valence electrons of the more electronegative metal (and thus anionic metal) in the intermetallic phases. 

Pearson, RG, Recent Advances in the Concept of Hard and Soft Acids and Bases , J. Chem. Educ. 63,687, 1986. 

We underline these results and the implied concepts quoting from a comprehensive review on this subject (Simon 1983). We remember indeed that, ever since it was experimentally possible to determine atomic distances in molecules and crystals, efforts have been made to draw conclusions about the nature of the chemical bonding, and to compare interatomic distances (dimensions) in the compounds with those in the chemical elements. Distances between atoms in an element can be measured with high precision. As such, however, they cannot be simply used in predicting interatomic distances in the compounds. In a rational procedure, reference values (atomic radii) have to be extracted from the individual (interatomic distances) measured values. Various functions have been suggested for this purpose. In the specific case of the metals it has been pointed out that interatomic distances depend primarily on the number of ligands and on the number of valence electrons of the atoms (Pearson 1972). 

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... 

Later on, Pearson [75] introduced the concept of hard and soft acid and bases (HSABs) hard acids (defined as small-sized, highly positively charged, and not easily polarizable electron acceptor) prefer to associate with hard bases (i.e., substances that hold their electrons tightly as a consequence of large electronegativities, low polarizabilities, and difficnlty of oxidation of their donor atoms) and soft acids prefer to associate with soft bases, giving thermodynamically more stable complexes. According to this theory, the proton is a hard acid, whereas metal cations may have different hardnesses. 

Early efforts to rationalize the observed formulas and chemical bonding of Zintl ions and related species used initially the Zintl-Klemm concept [10, 11] and subsequently the Mooser-Pearson [12] extended (8 — AO rule. In this rule, refers... 

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