Pearson’s hard/soft acid-base

Further examination of the results indicated that by invocation of Pearson s Hard-Soft Acid-Base (HSAB) theory (57), the results are consistent with experimental observation. According to Pearson s theory, which has been generalized to include nucleophiles (bases) and electrophiles (acids), interactions between hard reactants are proposed to be dependent on coulombic attraction. The combination of soft reactants, however, is thought to be due to overlap of the lowest unoccupied molecular orbital (LUMO) of the electrophile and the highest occupied molecular orbital (HOMO) of the nucleophile, the so-called frontier molecular orbitals. It was found that, compared to all other positions in the quinone methide, the alpha carbon had the greatest LUMO electron density. It appears, therefore, that the frontier molecular orbital interactions are overriding the unfavorable coulombic conditions. This interpretation also supports the preferential reaction of the sulfhydryl ion over the hydroxide ion in kraft pulping. In comparison to the hydroxide ion, the sulfhydryl is relatively soft, and in Pearson s theory, soft reactants will bond preferentially to soft reactants, while hard acids will favorably combine with hard bases. Since the alpha position is the softest in the entire molecule, as evidenced by the LUMO density, the softer sulfhydryl ion would be more likely to attack this position than the hydroxide. 

The utilization of Pearson s hard-soft acid-base theory to interpret the reactions of lignin has also been described in work from the Soviet Union by Zarubin and Kirysun (59). Beyond this specifically related paper, researchers in the Soviet Union have been quite active in the application of numerical methods to lignin-related problems (60,61). 

Woerpel and Nevarez demonstrated the synthetic potential of silaaziridines by selective insertion reactions (Scheme 7.52).123 Silver-catalyzed aldehyde insertion into the Si-N bond of 169b produced the N,0-cyclic acetal 180 as the cis isomer. In contrast to this process, insertion of tert-butyl isocyanide occurred into the weaker C-Si bond to afford imine 181. The authors rationalized the chemoselectivity for these two processes on the basis of Pearson s hard-soft acid-base theory 124-126 the more ionic Si-N bond reacted with harder benzaldehyde electrophile, whereas the more covalent Si-C bond reacted with the softer isocyanide. 

Pearson s Hard-Soft-Acid-Base (HSAB) priciple is that hard add-base combinations form readily and are generally ionic compounds. The other group of stable compounds and complex ions involves the interaction between soft acid and soft bases. For these, the bonding is primarily covalent with interpenetrating orbitals. The combinations hard acid with soft base, or vice versa, have little stability. 

This mixed ligand coordination where carboxylate and pyridyl groups are bound to the same cation is not common. In fact, one could argue that this is in contrast to Pearson s hard/soft acid-base distinction in that the uranyl cation is a hard acid and the dipyridyl linkers are softer in nature. Indeed, U02 -pyridyl coordination (2,2 -bipyridine, terpyridines, etc) is quite rare, even in molecular materials, yet in these extended topologies, it is likely that sterics play a considerable role and that the 4,4 -dipyridyl is uniquely suited to displace H2O groups and serve as a bridging moiety. This is mentioned in this context as... 

Formation constants for many metal complexes have been compiled by Ramunas Motekaitis and Art Martell, and these as well as techniques for measuring them in the laboratory will be covered in Chapters 3 and 8. One can, however, predict the relative stability of a desired complex based on simple bonding theories. Crystal field theory, as well as the Irving-Williams series and Pearson s hard-soft-acid-base theory (see the next section) enable us to predict what might happen in solution. 

In terms of Pearson s hard-soft acid-base principle, the soft Lewis acid copper ion is not compatible with the hard base H20 molecule, but a recent article provided the first example of a copper(I)-water bond (240). The novel copper complex with 2,3-diphenylquinoxaline (L 1), [Cu(L91)(H20)]C104, is diamagnetic and indefinitely stable in air in the solid state. Its structure consists of an infinite polymeric cationic chain in which adjacent metal centers are bridged by the aro-... 

Finally, it must be mentioned that acid-base interactions can also be analyzed in terms of Pearson s hard-soft acid-base (HSAB) principle [56,57]. At present, the application of this concept to solid-solid interactions and thus to adhesion is under investigation. 

Drago [Dr 73] considered how his concept could be correlated with Pearson s hard-soft acid-base theory [Pe 63b, Kl 68]. Setting out from the assumption that the terms E Eg and C Cg may serve as measures of the electrostatic and covalent interactions, respectively, and identifying the hard and soft interactions of the Pearson concept with the electrostatic and the covalent interactions, respectively, Drago wished to employ the C/E ratios for the acids or bases examined as a measure of the soft nature of the molecules. He considered that the higher the value of C/E, the softer is the given acid or base, whereas low value of C/E is indicative of a hard character. 

Dutta M (1999) The use of electronegativity and hardness towards understanding Pearson s hard-soft acid-base principle. Asian J Chem 11(1) 198-202... 

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