Hard bases definition

Model Definition. The HSAB model classifies Lewis acids (electrophiles) and bases (nucleophiles) as either "hard" or "soft." Hard acids and bases are relatively small, and exhibit low polarizability and a comparatively low tendency to form covalent bonds. Soft acids and bases have the opposite characteristics (24). Stated simply, the model postulates that hard acids react most readily with hard bases, and soft acids react most readily with soft bases (26). 

Just like Sanderson s electronegativity equalization principle, the Hard and Soft Acids and Bases principle was originally introduced without strong theoretical basis. Nevertheless, it was used widely from its formulation on. The principle states that hard acids prefer to coordinate with hard bases and soft acids with soft bases [82], In 1983, Parr and Pearson provided a definition for the chemical hardness [25]... 

Ash analytical methods may thus be used to test for evidence of protein-based matter including waterfowl down and feathers. But the characterization or even identification of bird species can hardly be definitely determined on the basis of chemical element percentage alone. One reason is the difficulty of collecting a chemically representative sample from a fairly non-homogeneous natural product. 

Note that this Principle is simply a restatement of the experimental evidence which led to Table 1.2. It is a condensed statement of a very large amount of chemical information. As such it might be called a law. But this label seems pretentious in view of the lack of a quantitative definition of hardness. HSAB is not a theory, since it does not explain variations in the strength of chemical bonds. The word prefer in the HSAB Principle implies a rather modest effect. Softness is not the only factor which determines the values of A/Z° in Equation (1.1). There are many examples of very strong bonds between mismatched pairs, such as H2, formed from hard H+ and soft H. H2O, OH and 0 are all classified as hard bases, but there are great differences in their base strength, by any criterion. 

Concepts of acidity and basicity are, in practice, defined and evaluated by their utility. Since overly formd definitions can be restrictive the concepts of acidity evolve towaids more comprehensive definitions. For example the Lewis definition includes the Broensted definition simply regarding the proton as an electron acceptor. Because the interaction of Broensted acids and bases in solutions involves a common process, protic transfer, scales of acidity can be established, for example the Hammett [1] acidity function. For Lewis acid-base interaction there is no common process to provide a unique basis for comparisons of acid strength. Experimentally, the strength of a Lewis acid depends upon the particular Lewis base. The classification of acids and bases as hard or soft in the principle of hard and soft acids and bases (HSAB principle) clarifies the interactions of Lewis acids and bases [2a]. Strong interactions occur between hard acid and hard base, or between soft acid and soft base, hi the hard-hard interaction there is a considerable electrostatic contribution to bonding and in the soft-soft interaction there is a major covalent contribution to bonding. The use of density functional analysis has clarified the concepts of hardness and softness and an empirical ranking of Lewis acids, based on local hardness is, proposed [2c]. 

The concept of softness has also been introduced, and the softness parameter S has been defined as the reciprocal of hardness, viz., 5 = if = 2(7 - A). The DFT-based definition for the softness parameter is also through the concept of local softness, as given by... 

FIGURE 3.4 Graphical correlations between the values of the molecular chemical hardness tj and the maximum hardness index Y by employing the Tables 3.4-3.7 for the Lewis acids and bases of Tables 3.1 and 3.2, in the case of experimental finite-difference (FD) based definition of atomic chemical hardnesses of Table 3.3, in left and right pictures, respectively (Putz, 2008c). 

For the atomic level, the experimental values based on the ionization potential and electron affinity definitions for electronegativity and chemical hardness were considered, see Eqs. (3.346) and (3.362), respectively. Nevertheless, the AIM level was formed by appropriate averaging of atoms-in-molecule summation for each of the considered reactivity indices, see Eqs. (4.15b), (3.252) and (3.248), and along of their MOL counterparts of Eqs. (4.15c), (3.346), and (3.362) the polarizability-, electronegativity- and chemical hardness- based aromaticity definitions were formulated with the associate qualitative trends established by Eqs. (4.15a), (4.16), and (4.17), respectively. Yet, for MOL level of computations all major quantum chemical methods for orbital spectra computation were considered, and implemented in the current application for some basics. Because of the quantum observable character of polarizability the related aromaticity scale was considered as benchmark for actual study and it offered the possibility in formulating... 

Instead, when chemical hardness based absolute aromaticity is particularized out of the general definition (4.18) with the help of AIM and CFD Eqs. (3.248) and (3.376), respectively... 

The usefulness of these definitions is that there is a general principle governing the interactions of hard and soft acids and bases, namely that hard acids prefer to coordinate with hard bases and soft acids prefer to coordinate with soft bases. Clearly these are qualitative definitions and a qualitative principle, and there are many intermediate cases and exceptions. Nevertheless the ideas are useful in that they bring a certain limited order into the otherwise varied and rather incomprehensible behaviour of Lewis acids and bases. 

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