Soft nucleophiles, definition

S, I) are more polarizable (they are further away from the nucleus), and since larger soft species have, in general, lower solvation energies, they are better nucleophiles as compared to smaller hard species. (For definition of the term hard and soft nucleophiles, see Box 13.1.) Thus, we can qualitatively understand why, for example, nucleophilicity increases from F to Cl to Br to V (Table 13.3), and why HS- is a stronger nucleophile than OH". 

Our approach was outlined in the framework of the Hard-Soft Acid-Base theory (HSAB, Ref. 90). In a short definition, the HSAB theory states that hard nucleophiles prefer to react with hard electrophiles and soft nucleophiles prefer to react with soft electrophiles. 

Model Definition. The HSAB model classifies Lewis acids (electrophiles) and bases (nucleophiles) as either "hard" or "soft." Hard acids and bases are relatively small, and exhibit low polarizability and a comparatively low tendency to form covalent bonds. Soft acids and bases have the opposite characteristics (24). Stated simply, the model postulates that hard acids react most readily with hard bases, and soft acids react most readily with soft bases (26). 

The display of reactivities in Fig. 1 illustrates two points clearly. First, the definitions of Eqs. (33)—(35), taken from Parr and coworkers [8-12] but applied directly to averages taken with the ensemble of Eq. (28a), do not yield useful measures of reactivity for systems with discrete spectra, i.e., localized systems. The values of the local and global softnesses are either zero or infinity, and the electrophilic Fukui function of the N0-electron system is identical to the nucleophilic Fukui function of the (N0 + l)-electron system. 

It soon became clear that these two classes should be called hard and soft electrophiles, respectively. Since the terms nucleophile and electrophile refer to rates of reaction, by definition, the acid-base reaction involved is... 

Geerlings De Proft, 2008 Cardenas et al., 2009 Senet, 1996). One may obtain a condensed-to-atom variant and also for the electrophilic, nucleophilic and radical attacks in the usual way. Moreover, the inverse of tu(r,r ) may generate a hierarchy of nucleophilicity kernel. Unlike the previous formulations, the overall treatment here is general and analytic with hardly any bearing on the explicit form of E(N). The traditional operational definition of local softness and hardness contain the same potential information and they should be interpreted as the local abundance or concentration of their corresponding global properties. 

In the brief guidelines given above for what makes a good nucleophile and electrophile, we touched on the energy and accessibility of the electrophilic and nucleophilic orbitals. This brings us to another related concept, that of "hard" and "soft" acids and bases. In this definition, the acids and bases are best viewed as being of the Lewis type. Here we examine the "hardness" and "softness" of the acid and base to predict reactivity. In this analysis, the character of a nucleophile or electrophile is most often correlated with the polarizability of the species hard reactants are non-polarizable, whereas soft reactants are polarizable. The... 

Eq. 5.30 is a general relationship for the interactions of electrophiles and nucleophiles, and is not restricted to definitions and discussions of hard and soft acids and bases. It tells us that the relative nucleophilicity of several Lewis bases will depend upon which electrophile is used, because the c s and yS values will change for each different electrophile. Similarly, the relative electrophilicities of several Lewis acids will depend upon what nucleophile is used. We will see exactly such results when we explore quantitative scales for various nucleophiles and electrophiles, where the scales are highly dependent upon the particular reaction that is chosen to analyze relative reactivities (see Chapter 8). Eq. 5.30 nicely explains the reactivity trends for soft acids and bases. It predicts that the Eoveriap will be best for Lewis acids and bases that have electrophilic and nuclephilic orbitals of roughly the same energy, which is the cases for the soft acids and bases of Table 5.8. 

With this background in this review we will now talk about the basic theory of the reactivity index including definitions of local and global softness along with relative nucleophilicity and electrophilicity. We will as well cover the role of response function in deriving the excited state reactivity index theory, followed... 

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