Hard and soft acid base principle

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... 

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... 

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. 

However, the strength of Lewis acid-base interaction can be expressed in energy terms, such as the exothermic molar heat, —for the equilibrium (III) of adduct formation. The enthalpy term is preferred because entropy effects accompanying the formation of coordinative bonds are difficult to determine. Various models have been proposed for the theoretical estimation of the enthalpy term based on molecular properties of reactants and are reviewed in Ref 5. The most significant developments have been the hard and soft acid-base principle of Pearson [6], the E C equation of Drago and Wayland [7], the donor and acceptor numbers of Gutmann [8], and the perturbation theory of Hudson and Klopman [9]. 

One of the most important factors influencing the corrosion rate is the formation and protective ability of the corrosion products formed. The specific corrosion products formed are dependent on the peuticipating dissolved metal ions and the access to anions solved in the aqueous layer. Formation of the film of corrosion products take place in a sequence of consecutive steps— dissolution, coordination, reprecipitation. When the dissolution step is acid-dependent, coordination is based on the hard and soft acid base principle (i.e., hard acids are preferably coordinated with hard bases and soft acids are preferably coordinated with soft bases). Acids or bases with tightly held valence electrons that are not easily distorted are hard acids or bases. Acids or bases having valence electrons that are easily polarized or removed are considered to be soft acids or bases. 

Reactions.—Some examples of new methods for ether cleavage have been dealt with in an earlier section (Protection of Alcohols). Aliphatic and aromatic methyl ethers can be cleaved efficiently by an aluminium halide-ethanethiol combination the process has been rationalized according to the hard and soft acid-base principle. Several methods for the (presumed) in situ preparation of trimethylsilyl iodide, a known reagent for the cleavage of ethers (2,131), have been disclosed recently in an effort to circumvent the expense and moisture sensitivity of MeaSil. (Some of these methods have been mentioned earlier in this Report in connection with the conversion of alcohols into alkyl iodides.) Reports include two on trimethylsilyl chloride-sodium iodide, one on phenylseleno-trimethylsilane-iodine [equation (18)], and three on hexamethyldisilane-iodine [equation (jq)] 102,142,143 method has the advantage of... 

Stability of complexes Pearson (1963) explained the relative stability of complexes on the basis of hard and soft acid-base principle. Thus, a complex AS is most stable when both A and B eire either hard or soft. 

Shoeib T, Gorelsky SI, Lever ABP, Siu KWM, Hopkinson AC (2001) When does the hard and soft acid base principle apply in the gas phase Inorg Chim Acta 315(2) 236-239... 

T. L. Ho, Hard and Soft Acids and Bases Principle in Organic Chemisty, Academic Press, New York, 1977. 

These concepts play an important role in the Hard and Soft Acid and Base (HSAB) principle, which states that hard acids prefer to react with hard bases, and vice versa. By means of Koopmann s theorem (Section 3.4) the hardness is related to the HOMO-LUMO energy difference, i.e. a small gap indicates a soft molecule. From second-order perturbation theory it also follows that a small gap between occupied and unoccupied orbitals will give a large contribution to the polarizability (Section 10.6), i.e. softness is a measure of how easily the electron density can be distorted by external fields, for example those generated by another molecule. In terms of the perturbation equation (15.1), a hard-hard interaction is primarily charge controlled, while a soft-soft interaction is orbital controlled. Both FMO and HSAB theories may be considered as being limiting cases of chemical reactivity described by the Fukui ftinction. 

The low stability of the complex 3.6 is consistent with the hard and soft acids and bases principle of Pearson (1963, 1968 Parr and Pearson, 1983 theoretical aspects Pearson, 1989 Chatteraj et al., 1991 monograph Ho, 1977). According to that principle hard acids will tend to complex with hard bases and soft acids with soft bases. Water is a hard base, whereas the nitrosyl ion is classified by Pearson as a borderline acid with a tendency to be soft. 

Nucleophilic catalysis is also observed with iodide ions. Fluoride ion does not form nitrosyl fluoride under diazotization conditions, as is to be expected from Pearson s hard and soft acids and bases principle which was discussed briefly in Section 3.2. More recently, nucleophilic catalysis has also been shown to occur with thiocyanate ion (SCN ), thiosulfate ion (HS2Of), dimethyl sulfide, and thiourea (H2NCSNH2) or its alkyl derivatives (see below). 

The principle of hard and soft acids and bases and the problem of competitive coordination in complex compounds. A, D. Garnovskii, D. A. Osipov and S. B. Bulgarevich, Russ. Chem. Rev. (Engl. Transl), 1972, 41, 341-359 (441). 

The concept of hard and soft acids and bases can be used to interpret many trends in chemical reactivity. These trends are summarized in the hard-soft acid-base principle (HSAB principle), an empirical summary of results collected from many chemical reactions studied through decades of research. 

C21-0023. State the hard-soft acid-base (HSAB) principle. Define and give examples of hard and soft acids and bases. 

Pearson, R. G. (1968a). Hard and soft acids and bases, HSAB. Part I. Fundamental principles. Journal of Chemical Education, 45, 581-7. 

Another feature of the metal ions that are typically involved in cementitious bonding in AB cements is that most of them fall into the category of hard in Pearson s Hard and Soft Acids and Bases scheme (Pearson, 1963). The underlying principle of this classification is that bases may be divided into two categories, namely those that are polarizable or soft, and those that are non-polarizable or hard. Lewis acids too may be essentially divided into hard and soft, depending on polarizability. From these classifications emerges the useful generalization that hard acids prefer to associate with hayd bases and soft acids prefer to associate with soft bases (see Section 2.3.7). 

The hard-soft acid-base principle is not restricted to the usual types of acid-base reactions. It is a guiding principle that for all types of interactions species of similar electronic character interact best. We have already seen some applications (such as the relative strength of HF and HI) of this principle, which we will continue to call HSAB, but we now consider a number of other types of applications. 

In Chapter 9, the hard-soft acid-base principle was discussed, and numerous applications of the principle were presented. This principle is also of enormous importance in coordination chemistry. First-row transition metals in high oxidation states have the characteristics of hard Lewis acids (small size and high charge). Consequently, ions such as Cr3+, Fe3+, and Co3+ are hard Lewis acids that bond best to hard Lewis bases. When presented with the opportunity to bond to NH3 or PR3, these metal ions bond better to NH3, which is the harder base. On the other hand, Cd2+ bonds better to PR3 because of the more favorable soft acid-soft base interaction. 

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