Hard/soft acids and bases

Hammett equation(s) 78, 93, 148ff., 151 f., 153ff., 167f., 190, 193, 196, 297, 299, 308, 312, 375, 381, 392, see also Dual substituent parameter, and Quantitative structure-reactivity relationships Hammond postulate, in additions of nucleophiles to diazonium ions 157 Hard and soft acids/bases principle (Pearson) 49, 54, 109... 

Continuum effects indicated by hard and soft acid-base (Lewis acids) and bases. C. K. Jorgensen, Top. Curr. Chem., 1975,56,1-66 (210). 

The extent to which an atom or molecule s charge distribution is affected by an external electric field E (which could be due to an approaching reactant) is governed, to first order, by its polarizability a. It was really a to which Pearson was referring in his hard and soft acid-base theory, which rationalizes a large number of chemical reactions. The terms hard and soft refer, respectively, to low and high polarizability. 

Metalloenzymes, 33 40, see also Enzymes crystal structure, 44 230-258 DNA repair, 45 251 inhibitors, 36 40-42 molybdenum, 45 2, 53, 60-63 nuleic acid hydrolysis, 45 251-252 superoxide dismustases and, 45 130 zinc-containing, hard and soft acid-base behavior, 42 103-109 Metalloids... 

FMO calculations using PM3-C1 were used to investigate the regioselectivities obtained by the photochemical reactions between 2-pyridone and pcnta-2,4-dienoate.46 The hard and soft acid-base principle has been successfully used to predict product formation in Patemo-Buchi reactions.47 The 2 + 2-photo-cycloaddition of homobenz-valene with methyl phenylglyoxylate, benzyl, benzophenone, and 1,4-benzoquinone produced the corresponding Patemo-Buchi products.48 The photo-cycloaddition of acrylonitrile to 5-substituted adamantan-2-ones produces anti- and svn-oxetanes in similar ratios irrespective of the nature of the 5-substituent49... 

Hard-Soft Classification The hard and soft acid-base (HSAB) classification is used for rationalisation of differences, when two acids (Aj and A2) are competing for two bases (B and B2). 

Metal-inhibitor interactions can be viewed in terms of acid-base reactions. This type of rationalization has been done by Sastri in terms of the hard and soft acid-base principle (HSAB) at great length.18 The characteristics of hard acids, soft acids, hard bases and soft bases along with borderline acids are given below. 

Bismuth occurs in nature as Bi203 and as bismuth sulfide ores. This reflects the fact that Bi(ni) is an intermediate acid in the Pearson s classification of hard and soft acids-bases (see Hard Soft Acids and Bases). Interestingly, over-the-counter dmg formulations involve primarily oxygen-donor ligands snch as salicylate, citrate, and nitrate, while in vivo reactions... 

A and B Behavior of Metal Cations (Hard and Soft Acid-Base Rules)... 

For electron donors, a simple attempt at classifying hard and soft bases is illustrated Table 6.3. Such classification schemes reveal, not suipris-ingly, that hard and soft are not absolute, but gradually varying qualities. Pearson (1963) has proposed, in generalizing hardness and softness properties for Lewis acids and bases for many kinds of systems, the so-called HSAB (hard and soft acid-base) rules. 

The preferential interaction between [SiOAl] and Na+ at one hand and between [SiOB]" and TPA+ at the other can be understood on the basis of the hard and soft acid-base interaction. It is well known that hard acids accompany better hard bases and soft acids link preferentially to soft bases. As Na+ is a harder acid than TPA+, [SiOAl] is also a harder base than [SiOB]. The preferential interactions lead then to the TPA+-[SiOB] pairs, as it was demonstrated previously [22]. 

Contrary to hard and soft acid-base predictions, the first example of a vanadyl-thioether complex has been made (151).693 The crown thioether 1,4,7-trithiacyclononane forms a stable 1 1 coordination complex upon reaction with VC13 the presence of adventitious water likely explains the hydrolysis/oxidation of V111 to form VlvC)21. Complexes with a variety of bi- and tetradentate S-donor functionalities have been prepared as model complexes for nitrogen-ase.498,694 These complexes have some structural similarities to the binding site of nitrogenase, however, in contrast to the cofactor they fail to convert N2 to NH3. 

POLARIZABILITY AND HARD AND SOFT ACID-BASE THEORY... 

Section 2.4 Polarizability Hard and Soft Acid-Base Theory... 

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