Pearson’s HSAB principle

The appearance and dominance of this second-order effect can be interpreted through the strong polarization of the acceptor and donor atoms. It should also be noted that a special approach to softness matching [11,19,51] has been adapted in this study. According to this method, which was originally derived from Pearson s HSAB principle, [17] the most favorable interaction between the sites A and B... 

Environmentally relevant nucleophiles (Lewis bases) may be classified according to Pearson s HSAB principle, as hard, soft, or borderline (possessing intermediate hard/soft character) as follows (Larson and Weber, 1994) ... 

In conclusion it can be stated that Klopman s E parameters describe the softness of ions better than Pearson s a = /r parameter. It can also be said that one of the shortcomings of Pearson s HSAB principle is that it cannot predict AH or AG as an explicit function such as the sum of two products of two parameters, one characterizing the acid and the other the base. In spite of the limitations, the HSAB principle is useful in rationalizing and understanding the behaviour of metal ions. 

Owing to its relative simplicity, Pearson s HSAB principle is often used to predict the run of different acid-base interactions on the background of media... 

Textbooks refer to Chatt s class (a) and class (b) acceptors, a concept subsequently developed in Pearson s HSAB principle. In contrast to this perceived wisdom, the complex [Col3(SbPh3)2], which contains a hard metal centre and extremely soft iodide and SbPh3 ligands, is quite stable.Its formation is thought-provoking. 

Klopman attempted to quantify Pearson s HSAB principle using frontier molecular orbital (FMO) theory (Klopman 1968), with the following equation ... 

Datta D (1992) On Pearson s HSAB principle. Inorg Chem 31(13) 2797-2800... 

On the conceptual side, the powers of DFT have been shown to be considerable. Without going into detail, I mention only that the Coulson work referred to above anticipated in large part the formal manner in which DFT describes molecular changes, and that the ideas of electronegativity and hardness fell into place, as do Ralph Pearson s HSAB and Maximum Hardness Principles. 

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] ... 

In principle, this constitutes a strong point for the HSAB treatment. In practice, it is its Achille s heel. The reason is that Pearson s rule (a hard reagent attacks preferentially a hard site) should apply only to the reactive species in the transition state. 

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. 

Among these ligand systems, the pseudohalide ions NCX (X = S, Se) are of permanent interest [1,5,7,11,19,21,25-41]. To explain coordination modes in the complexes of these ligands, the HSAB principle (details in Sec. 1.2.2) was first applied [42,43] the hard Pearson acids mostly bind the hard nitrogen atom and the soft Pearson acids bind the soft sulfur or selenium atoms. This approach, related with hard-soft interactions, was also used later on [1,2,6,7,11]. 

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]. 

Later workers have correlated the classification of elements in class (a) and class (b) with Pearson s principle of hard and soft acids and bases (HSAB principle) (see Hard Soft Acids and Bases) on the basis that class (a) metal ions are hard acids and class (b) are soft acids. Borderline elements in the Ahrland-Chart-Davies classification tend to be harder in the higher oxidation states and softer in their lower oxidation states. 

As cyanide ions operate as ambident nucleophiles, alkylation reactions may generate isonitriles as well as nitriles (equation 2). A whole range of parameters is responsible for the outcome of reactions of this type and their particular role together with special counter influences is not easily evaluated. There is a large and growing number of papers on this topic, but one can concentrate here on a few selected review articles.Suffice it to say that Komblum s seminal article s from 1955 is still of special importance in this field. Pearson s principle of soft and hard acids and bases (HSAB) proved to be particularly helpful in the interpretation of experimental results. ... 

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