HSAB applications

Several alternative attempts have been made to quantify Lewis-acid Lewis-base interaction. In view of the HSAB theory, the applicability of a scale which describes Lewis acidity with only one parameter will be unavoidably restricted to a narrow range of struchirally related Lewis bases. The use of more than one parameter results in relationships with a more general validity ". However, a quantitative prediction of the gas-phase stabilities of Lewis-acid Lewis-base complexes is still difficult. Hence the interpretation, not to mention the prediction, of solvent effects on Lewis-add Lewis-base interactions remains largely speculative. 

The hard-soft acid-base principle is not restricted to the usual types of acid-base reactions. It is a guiding principle that for all types of interactions species of similar electronic character interact best. We have already seen some applications (such as the relative strength of HF and HI) of this principle, which we will continue to call HSAB, but we now consider a number of other types of applications. 

One of the simplest applications of the HSAB principle is related to solubility. The rule "like dissolves like" is a manifestation of the fact that solute particles interact best with solvent molecules which have similar characteristics. Small, highly charged particles or polar molecules are solvated best by solvents containing small, highly polar molecules. Large solute particles having low polarity are solvated best by solvent molecules having similar characteristics. Consequendy, NaCl is soluble in water, whereas sulfur, S8, is not. On the other hand, NaCl is insoluble in CS2, but S8 dissolves in CS2. 

The application of the HSAB principle is of considerable importance in preparative coordination chemistry in that some complexes are stable only when they are precipitated using a counterion conforming to the above rule. For example, CuCls3 is not stable in aqueous solution but can be isolated as [Cr(NH3)6][CuCl5]. Attempts to isolate solid compounds containing the complex ion Ni(CN)s3 as K3[Ni(CN)5] lead to KCN and K2[Ni(CN)4]. It was found, however, that when counterions such as Cr(NH3)63+ or Cr(en)33+ were used, solids containing the Ni(CN)53 anion were obtained. 

We have already used the HSAB principle as it applies to linkage isomers in metal complexes. This application to bonding site preference can also be used to show the behavior of other systems. For example, the reactions of organic compounds also obey the principles when reacting with nucleophiles such as SCN- or N02 ... 

Application of the local HSAB principle to determine the preferred site of attack... 

These two paths are normally associated with different barrier heights introducing, thus, a regio-selectivity in the cycloadditive process. The path associated with the lower energy barrier should be preferred, and the corresponding cycloadduct will be dominant. Now, direct application of HSAB at the local level is not possible here, because it has to be satisfied for both the termini simultaneously. A softness matching criteria, thus, needs to be defined for the multisite interaction that measures the extent of the fulfillment of local HSAB principle. A quantity (A.v) can, thus, be defined to measure the softness matching criteria for the two paths in a least square sense, and the minimum value of this quantity should be preferable [27] ... 

Applications of local HSAB principle have been used for the determination of the softer regions in Si clusters by using Ga as probe atom [30a], or the site for H-atom adsorption on Si clusters. In the latter case, the isomer predicted by the Fukui function was found but it is not always the most stable one. The use of the reactivity indices is only valid when the adsorption process does not induce strong deformation of the cluster [30b]. 

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. 

Prediction of interaction between metal clusters with oxide surface The HSAB principle classifies the interaction between acids and bases in terms of global softness. In the last few years, the reactivity index methodology was well established and had found its application in a wide variety of systems. This study deals with the viability of the reactivity index to monitor metal cluster interaction with oxide. Pure gold cluster of a size between 2 and 12 was chosen to interact with clean alumina (100) surface. A scale was derived in terms of intra- and intermolecular interactions of gold cluster with alumina surface to rationalize the role of reactivity index in material designing [43]. 

Geerlings, P. and De Proft, F. 2000. HSAB principle Applications of its global and local forms in organic chemistry. Int. J. Quantum Chem. 80 227-236. 

Baeten, A., Tafazoli, M., Kirsch-Volders, M., and Geerlings, P. 1999. Use of the HSAB principle in quantitative structure-activity relationships in toxicological research Application to the genotoxicity of chlorinated hydrocarbons. Int. J. Quantum Chem. 74 351-355. 

The major disadvantage of the HSAB principle is its qualitative nature. Several models of acid-base reactions have been developed on a quantitative basis and have application to solvent extraction. Once such model uses donor numbers [8], which were proposed to correlate the effect of an adduct on an acidic solute with the basicity of the adduct (i.e., its ability to donate an electron pair to the acidic solute). The reference scale of donor numbers of the adduct bases is based on the enthalpy of reaction. A//, of the donor (designated as B) with SbCb when they are dissolved in 1,2-dichloroethane solvent. The donor numbers, designated DN, are a measure of the strength of the B—SbCb bond. It is further assumed that the order of DN values for the SbCb interaction remains constant for the interaction of the donor bases with all other solute acids. Thus, for any donor base B and any acceptor acid A, the enthalpy of reaction to form B A is ... 

The application of the HSAB concept to solutions leads to the rule that hard solutes dissolve in hard solvents and soft solutes dissolve in soft solvents (Pearson, 1987). For example, benzene is considered a very soft solvent since it contains only a basic function. Contrary to benzene, water is a very hard solvent, with respect to both its basic and acidic properties, ft is the ideal solvent for hard bases and hard acids. The hardness of water is reduced by the introduction of alkyl substituents in proportion to the size of the alkyl group. In alcohols, therefore, softer solutes become soluble. 

R. G. Pearson, Hard and soft acids and bases—the evolution of a concept. Coord. Chem. Rev. 100, 403-425 (1990) R. G. Pearson, Absolute electronegativity and hardness. Application to inorganic chemistry. Chem. Br. 31, 444-447 (1991) R. G. Pearson, Recent advances in the HSAB concept. J. Chem. Educ. 64, 561 (1987). 

The HSAB principle predicts that the equilibrium should lie to the right, because the hard acid CH,CO should have a greater affinity for the hard base RO than for the soft base RS. Indeed, thiol esters are easily cleaved by OR" or hydrolyzed by dilute base (OH is also a hard base).113 Another application of the rule is discussed on p. 349.114... 

Although there are no reported applications for the cross-linker, its reactivity and use is similar to that of sulfo-HSAB. The commercial availability of the reagent provides additional options for spacer length to study the interactions between two proteins or other molecules. 

Another property that characterizes solvents is their softness, in terms of the HSAB concept (Pearson 1963), according to which the interactions of soft solvents are strongest with soft solutes, of hard solvents with hard solutes, but are weaker for hard solvents with soft solutes and vice versa. The applicability of the softness property takes into account that it is superimposed on the more general electron pair donation property discussed above. In fact, it can replace (Marcus 1987) the notion of the family dependence of the P scale, expressed by the , parameter (Kamlet etal. 1985). A few quantitative scales have been... 

The HSAB principle can be considered as a condensed statement of a very large amount of experimental information, but cannot be labelled a law, since a quantitative definition of the intuitive concepts of chemical hardness (T ) and softness (S) was lacking. This problem was solved when the hardness found an exact, and also an operational, definition in the framework of the Density Functional Theory (DFT) by Parr and co-workers [2], In this context, the hardness is defined as the second order derivative of energy with respect to the number of electrons and has the meaning of resistance to change in the number of electrons. The softness is the inverse of the hardness [3]. Moreover, these quantities are defined in their local version [4, 5] as response functions [6] and have found a wide application in the chemical reactivity theory [7],... 

Two proofs for the HSAB principle were provided under the restriction of a common chemical potential of the reaction partners [83, 84]. Later on, a local HSAB principle was provided by Gazqu z and Mendez [85], They showed that the interaction between two chemical species will not necessarily occur through their softest atoms, but through those whose softnesses are approximately equal. In Section 4.2, an intuitive application of the HSAB concept is provided, followed by an application of the local HSAB principle in the interpretation of regioselectivity in Diels Alder reactions. 

The aim of specific poisoning is the determination of the chemical nature of catalytically active sites and of their number. The application of the HSAB concept together with eight criteria that a suitable poison should fulfill have been recommended in the present context. On this basis, the chemisorptive behavior of a series of hard poisoning compounds on oxide surfaces has been discussed. Molecules that are usually classified as soft have not been dealt with since hard species should be bound more strongly on oxide surfaces. This selection is due to the very nature of the HSAB concept that allows only qualitative conclusions to be drawn, and it is by no means implied that compounds that have not been considered here may not be used successfully as specific poisons in certain cases. Thus, CO (145, 380-384), NO (242, 381, 385-392, 398), and sulfur-containing molecules (393-398) have been used as probe molecules and as specific poisons in reactions involving only soft reactants and products (32, 364, 368). 

The second chapter deals with quantum chemical considerations, s, p, d and f orbitals, electronic configurations, Pauli s principle, spin-orbit coupling and levels, energy level diagrams, Hund s mles, Racah parameters, oxidation states, HSAB principle, coordination number, lanthanide contraction, interconfiguration fluctuations. This is followed by a chapter dealing with methods of determination of stability constants, stability constants of complexes, thermodynamic consideration, double-double effect, inclined w plot, applications of stability constant data. 

A very important finding is that hard cations (e.g., Be2+, Al3+, Ti4+) prefer ligands with hard donor atoms (e.g., F, O, Cl), and soft cations (e.g., Ag+, Cd2+, Tl+) prefer ligands with soft donor atoms (e.g., I, Te, As). This fact has a myriad of applications, because it can be advantageously used for analysis, selective reactions, metal ion separation, removal, recovery, etc. It is an application of the Hard Soft Acid Base (HSAB) principle enunciated by Pearson at the end of the 1960s. 

In one way or another, the HSAB Principle has found many applications in most... 

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