Hard-soft-acids-bases model

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. 

Fundamental Difficulties with the Hard-Soft Acid Base Model... 

This chapter is intended to provide basic understanding and application of the effect of electric field on the reactivity descriptors. Section 25.2 will focus on the definitions of reactivity descriptors used to understand the chemical reactivity, along with the local hard-soft acid-base (HSAB) semiquantitative model for calculating interaction energy. In Section 25.3, we will discuss specifically the theory behind the effects of external electric field on reactivity descriptors. Some numerical results will be presented in Section 25.4. Along with that in Section 25.5, we would like to discuss the work describing the effect of other perturbation parameters. In Section 25.6, we would present our conclusions and prospects. 

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. 

Iwamoto et al. (54) studied the activity of a series of metalion exchanged zeolites for the water-gas shift reaction. The lower water-gas shift activity of the acidic cations was explained in terms of hard-soft acid/base properties. In this model, carbon monoxide, which is a soft base, interacts more strongly with soft acid sites. The adsorption of CO is generally considered to be the rate controlling step in the water-gas shift reaction. Cations of lower acidity are generally softer acids and as such adsorb CO more readily. This would lead to higher surface concentrations of CO, thereby increasing the water-gas shift acitivity of the sample. 

Furthermore, we have performed a vahdity test of the suggested formula for evaluating molecular hardness by the application of the global hardness data of the carbon containing poly atomic molecules in the real field of hard-soft acid-base reactions. We have noticed that the hardness data of this calculation can draw the exact picture of the model of the chemical reaction surfaces. However, the hardness data evaluated through the ansatz and operational and approximate formula of Parr and Pearson poorly correlate the same reaction surfaces. 

A detailed examination of the reactions of ambident nucleophiles has shown that hard-soft acid-base theory and the Klopman-Salem model based on charge- and orbital-control of a reaction are not able to predict the results found in many reactions involving ambident nucleophiles. The analysis of the reactions of many ambident nucleophiles with a variety of substrates indicates that Marcus theory, which derives... 

Auboiroux, M., F. Melon, F. Bergaya, and J. C. Touray. 1998. Hard and soft acid-base model applied to bivalent cation selectivity on a 2 1 clay mineral. Clays and Clay Minerals 46, no. 5 546-555. 

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. 

Contrary to hard and soft acid-base predictions, the first example of a vanadyl-thioether complex has been made (151).693 The crown thioether 1,4,7-trithiacyclononane forms a stable 1 1 coordination complex upon reaction with VC13 the presence of adventitious water likely explains the hydrolysis/oxidation of V111 to form VlvC)21. Complexes with a variety of bi- and tetradentate S-donor functionalities have been prepared as model complexes for nitrogen-ase.498,694 These complexes have some structural similarities to the binding site of nitrogenase, however, in contrast to the cofactor they fail to convert N2 to NH3. 

However, the strength of Lewis acid-base interaction can be expressed in energy terms, such as the exothermic molar heat, —for the equilibrium (III) of adduct formation. The enthalpy term is preferred because entropy effects accompanying the formation of coordinative bonds are difficult to determine. Various models have been proposed for the theoretical estimation of the enthalpy term based on molecular properties of reactants and are reviewed in Ref 5. The most significant developments have been the hard and soft acid-base principle of Pearson [6], the E C equation of Drago and Wayland [7], the donor and acceptor numbers of Gutmann [8], and the perturbation theory of Hudson and Klopman [9]. 

In our original work, we used an ionic-covalent model to interpret the E and C parameters. It has been suggested that our E and C parameters are a quantitative manifestation of the hard-soft model. "Softness (or hardness") can be considered (67) as a measure of the ratio of the tendency of a spedes to undergo covalent interaction to the tendency of the species to undergo electrostatic interaction. The relative "softness or hardness is depicted in the C/E ratio. The ratios for the acids and bases can be calculated from the data in Tables 3 and 4. If the ratio C/E is comparatively large, the add or base would be classified as type B or soft. Inasmuch as the relative ratios of C/E tells the relative importance of the two effects for various donors and acceptors, we agree that the hardness or softness discussed in the HSAB model is given by this ratio. 

The Hard and Soft Acids and Bases (HSAB) Model... 

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). 

The current understanding of the structure of ZDDP-based tribofilms derives mainly from XANES spectroscopy (Armstrong et al., 1997 Bancroft et al., 1997 Ferrari et al., 1999a Fuller et al., 1995, 1998 and 2002 Martin et al., 2001 Yin et al., 1993 and 1997a), infrared spectroscopy (Harrison and Brown, 1991 Lindsay et al., 1993 Willermet et al., 1992 and 1995b), and a comprehensive multi-technique (AES/XPS/XANES) approached on the basis of the hard and soft acids and bases (HSAB) principle (Martin, 1999, Martin et al., 2001 ). Most prominent models involve a two-layer structure. The layer in contact with the metal surface is thought to be composed of short-chain polyphosphates, and a top layer, consists of long-chain zinc polyphosphate. 

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