Bonding HSAB principle

Once acids and bases have been classified as hard or soft, a simple rule can be given hard acids prefer to bond to hard bases, and soft acids prefer to bond to soft bases (the HSAB principle)P The rule has nothing to do with acid or base strength but merely says that the product A—B will have extra stability if both A and B are hard or if both are soft. Another rule is that a soft Lewis acid and a soft Lewis base... 

Once acids and bases have been classified as hard or soft, a simple rule of the HSAB principle can be given hard acids prefer to bond to hard bases, and soft acids prefer to bond to soft bases. 

They indicated that the softness parameter may reasonably be considered as a quantitative measure of the softness of metal ions and is consistent with the HSAB principle by Pearson (1963, 1968). Wood et al. (1987) have shown experimentally that the relative solubilities of the metals in H20-NaCl-C02 solutions from 200°C to 350°C are consistent with the HSAB principle in chloride-poor solutions, the soft ions Au" " and Ag+ prefer to combine with the soft bisulfide ligand the borderline ions Fe +, Zn +, Pb +, Sb + and Bi- + prefer water, hydroxyl, carbonate or bicarbonate ligands, and the extremely hard Mo + bonds only to the hard anions OH and. Tables 1.23 and 1.24 show the classification of metals and ligands according to the HSAB principle of Ahrland et al. (1958), Pearson (1963, 1968) (Table 1.23) and softness parameter of Yamada and Tanaka (1975) (Table 1.24). Compari.son of Table 1.22 with Tables 1.23 and 1.24 makes it evident that the metals associated with the gold-silver deposits have a relatively soft character, whereas those associated with the base-metal deposits have a relatively hard (or borderline) character. For example, metals that tend to form hard acids (Mn +, Ga +, In- +, Fe +, Sn " ", MoO +, WO " ", CO2) and borderline acids (Fe +, Zn +, Pb +, Sb +) are enriched in the base-metal deposits, whereas metals that tend to form soft acids... 

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 ... 

One apparent contradiction to the HSAB principle involves the stability of the complexes [Co(NH3)5NCS]2+ and [Co(CN)5SCN]3-. In the first of these, the thiocyanate ion is bonded to Co3+ through the nitrogen atom, as expected. However, in the second complex, SCN- is bonded to Co3+ through the sulfur atom, and this arrangement is the stable one. The difference between these... 

Local HSAB principle can also be used to calculate the relative homolytic bond dissociation energies (BDE). For the homolytic dissociation of para-substituted phenols ... 

These descriptors have been widely used for the past 25 years to study chemical reactivity, i.e., the propensity of atoms, molecules, surfaces to interact with one or more reaction partners with formation or rupture of one or more covalent bonds. Kinetic and/or thermodynamic aspects, depending on the (not always obvious and even not univoque) choice of the descriptors were hereby considered. In these studies, the reactivity descriptors were used as such or within the context of some principles of which Sanderson s electronegativity equalization principle [16], Pearson s hard and soft acids and bases (HSAB) principle [17], and the maximum hardness principle [17,18] are the three best known and popular examples. 

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] ... 

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 ... 

A more refined but still debated in the literature notion is Pearson s Hard and Soft Acids and Bases (HSAB) principle [9,41], which quantifies energy changes to second order according to which hard (soft) acids (electron pair acceptors) prefer to interact with hard (soft) bases (electron pair donors). Soft likes soft relates to covalent bonds being facilitated by high polarizabilities, while hard likes hard relates to a creation of predominantly electrostatic interactions. 

These effects, which have also been called symbiosis, double bond-no bond resonance, clustering, or geminal or anomeric effect [54], can also be explained in terms of the HSAB principle X3C+ will be harder than H3C+ if X is hard. Accordingly, X3C+ will have a higher affinity for the hard X- than H3C+ will have [54]. 

Infrared and NMR-spectral analysis, and x-ray diffraction data, testify [42-54] that in case of complexes of the already discussed pseudohalide ions, the competitive coordination can be explained by the HSAB principle hard Pearson acids are bound with hard N-center, and soft acids with soft X- donor (S, Se) centers. This situation allows us to obtain directly the coordination compounds of pseudohalides with a definite localization mode of the coordination bond, i.e., to carry out the regioselective synthesis on the basis of the higher stability of complexes which are obtained as a result of hard-hard or soft-soft interactions [2]. 

Romero, M. de L. and Mendez, F., The local HSAB principle and bond dissociation energy of p-substituted phenols, J. Phys. Chem. A, 107, 5874-5875, 2003b. 

According to the HSAB principle, hard acids react preferentially with hard bases and soft acids react preferentially with soft bases. It is clear from the above description of the various factors leading to the enunciation of the HSAB principle that the general concepts underlying the HSAB principle are not new. To a good approximation, hardness is synonymous with electrovalent bonding while softness is not the same as covalent bonding. 

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