Pearson s concept

The high affinity of metal for sulfur is clearly demonstrated by the enormous variety of metal sulfide minerals in nature. Qualitatively this might be understood using Pearson s concept of hard and soft acids and bases. It is better described, however, by the covalency of the metal-sulfur bond, particularly with the soft metals. 

A similar picture holds for other nucleophiles. As a consequence, there might seem little hope for a nucleophile-based reactivity relationship. Indeed this has been implicitly recognized in the popularity of Pearson s concept of hard and soft acids and bases, which provides a qualitative rationalization of, for example, the similar orders of reactivities of halide ions as both nucleophiles and leaving groups in (Sn2) substitution reactions, without attempting a quantitative analysis. Surprisingly, however, despite the failure of rate-equilibrium relationships, correlations between reactivities of nucleophiles, that is, comparisons of rates of reactions for one carbocation with those of another, are strikingly successful. In other words, correlations exist between rate constants and rate constants where correlations between rate and equilibrium constants fail. 

Thus we can understand easily enough that the reaction between bromine and ethylene giving dibromoethane is exothermic—it replaces one n bond (C=C) with two a bonds (C—Br) at the expense of a weak a bond (Br—Br). However, it is not always obvious how strong the bonds will be when one molecule combines with another to form a single new molecule, or what happens to the energy if we exchange parts of one molecule with parts of another. A useful addition to understanding this sort of problem has been Pearson s concept of hard and soft acids and bases (HSAB). 

The Au-NCS/Au-SCN ratios of a series of LAu (thiocyanate) complexes in solution have been measured and the authors (4) claim that their results support Pearson s concept of antisymbiosis (2Sa), but it has been pointed out that the ligands used give ratios in an order which does not follow that accepted for the trans effect (18). 

According to Pearson s concept [16a], small compounds, which are not easily polarized, are classified as hard , while large and easily polarized compounds are classified as soft (Table 3). 

Hard and soft acids and bases (Pearson s concept)... 

HARD AND SOFT ACIDS AND BASES (PEARSON S CONCEPT)... 

This finding is easily rationalized using Pearson s concept of hard and soft acids and bases (see Section 2.14). Whenever NCN is coordinated by chemically hard cations (for example, Ca +, with rj = 19.52 eV, see Table 2.3) there results a considerable amount of ionic bonding between the cation and a symmetrical N=C=N anion. Chemically softer cations such as Pb + (8.46 eV) or Ag+ (6.7 eV) will bond more covalently to one end of the NCN unit, inducing the less symmetrical (N-C=N ) cyanamide shape this species must be chemically softer. Figure 3.41(b) shows the PbNCN structure with a seven-... 

Finally, let us not forget that there is a single, simple frame of understanding which correctly, albeit qualitatively, predicts the energetic order of mercury carbodiimide and mercury cyanamide, and it is based on chemical ideas, namely Pearson s concept of hard and soft acids and bases soft prefers soft and hard prefers hard. The absolute softness of Hg + (7.7 eV, see Table 2.3) lies between those of Pb and Ag+ such that a cyanamide anion and not a carbodiimide anion will be the preferred bonding partner for Hg +, just as for Pb + and Ag+. I cannot refrain from noting that I find this quite remarkable. 

Nevertheless, the term cosmotropic only means that the if is energetically strongly hydrated and chaotropic that it is energetically weakly hydrated. Note that this classification is not in phase with Pearson s concept of hard and soft i(Mis. Ag and Hg " are soft ions, but their enthalpies of hydratiOTi —483 and — 1,853 kJ mol respectively are more negative than those of the hard irais of similar size, Na and with values... 

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