Acid-base soft chemistry

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. 

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... 

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. 

When painting a wall, better coverage is assured when the roller passes over the same area several times from different directions. It is the opinion of the author that this technique works well in teaching chemistry. Therefore, a second objective has been to stress fundamental principles in the discussion of several topics. For example, the hard-soft interaction principle is employed in discussion of acid-base chemistry, stability of complexes, solubility, and predicting reaction products. Third, the presentation of topics is made with an effort to be clear and concise so that the book is portable and user friendly. 

The identification given by Equation 2.6 is consistent with the hard and soft acids and bases principle [12,15-21] established originally by Pearson [22] to explain many aspects of Lewis acid-base chemistry. 

The rational design of chelating agents as antidotes requires a careful consideration of acid-base chemistry. Metal ions are Lewis acids, while the chelating agents or ligands are Lewis bases. The concepts of hardness and softness may be used to describe systematically the interaction between them. A hard metal cation is one that retains its... 

Chapter 9 Acid-Bose Chemistry 318 Acid-Base Concepts 318 Measures of Acid-Base Strength 330 Hard and Soft Acids and Bases 344... 

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

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],... 

In the 1963 Pearson introduced the concept of "hard and soft acids and bases" (HSAB) [1]. The root of the idea lies in the simple acid-base view of chemistry of Lewis and leads to the statement that "hard acids prefer to coordinate to hard bases and soft acids prefer to coordinate to soft bases". 

Studying acid-base properties of acids and bases, Pearson put forward the principle of hard and soft acids and bases according to which hard acids prefer to react with hard bases, while soft acids prefer to react with soft bases [12,13]. This principle has been widely used in chemistry, however, no quantitative characteristics of the hardness of acids and bases were known. 

Central to the stability and chemistry of complexes formed by mixed donor ligands are two key concepts of coordination chemistry. The first is the chelate effect, which applies to all polydentate ligands, and reflects the increase in stability of a type of complex as monodentate donor molecnles are replaced by polydentates with donors linked by chelate rings. The hard-soft acid-base theory is particnlarly relevant to mixed donor ligands where donors of distinctly different character may bind to a central metal ion. The like prefers like concept means hard nonpolarizable donor atoms (N and O, for example) bond preferentially to hard nonpolarizable metal... 

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. 

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. 

Acid-Base Chemistry (Hard-Soft-Acid-Base Theory)... 

This text uses several concepts and tools not present in most undergraduate organic chemistry texts to aid in understanding the most difficult sections of the course. Hard-soft acid-base theory is used to guide decisions and to explain and predict the dual reactivity of many species. Energy diagrams and surfaces are presented so that students have a physical model to help with the more complex decisions. 

Also, in Table 1.4, CF is shown as a harder acid than CH. These are examples of a very general phenomenon, first noted by Jorgensen and called by him the symbiotic effect. Soft bases attached to the same central acceptor atom make it a soft acid, and hard bases make it a hard acid. In coordination chemistry, symbiosis explains why some ligands, such as CN or phenanthroline, make a metal ion form strong complexes with other soft ligands, whereas F and H2O favor the bonding of other hard ligands. 

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. 

However, in many cases, electronegativity difference alone cannot account for the stability of the molecule. For example, according to the electronegativity criterion, the CsF molecule should be very stable as the electronegativity difference between Cs and F is very large. But the reaction enthalpy data indicate that Fil and CsF will react to form Csl and FiF. In order to predict the direction of acid-base reactions and to account for the stability of the products, Pearson introduced two parameters hardness and softness in the vocabulary of chemistry. The qualitative definitions of hard and soft acids and bases are as follows [28-32] ... 

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