Pearson’s HSAB

Both the Edwards equation and Pearson s HSAB concept take as primary determinants of nucleophilicity the polarizability and basicity. A two-term equation of Bartoli and Todesco" uses these ideas also, but as a measure of polarizability the... 

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... 

The differences in reactivity between the nitrogen and the sulfur atom in OZTs reveal that most reactions can be interpreted with reference to Pearson s HSAB theory.56 In the case of l,3-oxazolidine-2-thiones, one may consider the nitrogen atom as a harder basic center than the softer sulfur atom. 

This pronounced double selectivity has been explained in terms of Pearson s HSAB theory. According to this procedure, A-methylcarbamates have been prepared from primary aliphatic and aromatic amines, either at reflux temperature of... 

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. 

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) ... 

At present, these theoretical calculations are either too demanding with regard to their data requirements (24) or too simplistic (301 to be used to predict absolute rates of reaction between sulfur nucleophiles and haloaliphatic substrates in natural waters. However, they provide a solid theoretical justification for using Pearson s HSAB model to explain trends in reactivity among different nucleophiles toward haloaliphatic substrates. 

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 ... 

As in the case of a conventional (liquid) solution, the ion association dynamics in a polymer electrolyte will be driven by the relative strength of the different interactions present, namely cation-anion, cation-polymer, and anion-polymer, which can be understood in the conceptual framework of the Pearson s HSAB model [152-154]. In the most general situation, oxygen atoms in a polyether can be considered as hard bases. This means that they interact strongly with hard acids (i.e., cations such as Li, Mg, Na, Ca, or Cu ). However, the cation-polymer interaction win be weak with soft cations Ag, Sn, Pb, or Hg ". In the case of anions, the main characteristic that seems to control the... 

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

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