Stabilisation of charged species

In order to facilitate the reaction, the charges that have been created by the reagents within the transition states and intermediates may be stabilised. It is the mechanisms that are available for such stabilisation that are covered in this chapter. 

The two main forms of charge stabilisation have already been introduced in the context of polarised bonds, i.e. the inductive and mesomeric effects. Other mechanisms exist, and these may be of overriding importance when they are 

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The exciplex emission is also affected by solvent polarity, where an increase in the solvent polarity results in a lowering of the energy level of the exciplex, at the same time allowing stabilisation of charged species formed by electron transfer (Figure 6.7). Thus, in polar solvents the exciplex emission is shifted to even higher wavelength and accompanied by a decrease in the intensity of the emission, due to competition between exciplex formation and electron transfer. 

This book is divided into three parts Basic Principles Mechanisms and Appendices. Within the first part, all the basic ideas that are needed in order to write organic mechanisms are discussed and explained. The areas covered are electron counting covalent bonding and polarisation shapes of molecules stabilisation of charged species thermodynamic and kinetic considerations and acid/base characteristics. In each case, the underlying principles will be highlighted and many of the common errors and misunderstandings will be explained so that not only do you know what to do, but you also know what not to do and why. 

In the next chapter, the stabilisation of charged species will be examined, and one of the most important mechanisms available involves the delocalisation of a lone pair in a manner that is similar to the stabilisation of the trans conformer of an ester or amide functionality. 

Such reactions are commonly found as a result of the decomposition of charge transfer excited states. For example, while excitation of the MC bands of Co(NH3)5X (X = Cl, Br, I) leads to photosolvation and the formation of Co(NH3)5(0112) and Co(NH3)4(OH2)X, shorter wavelengths yield the LMCT state which decomposes into Cc II) ions and halogen atoms. The quantum yield for the reaction is found to depend on the excitation energy, indicating a role for the initially formed radical pair (Eq. 5). This may reform the starting complex (Eq. 6) or decompose to the redox products stabilised by the solvent or some other species (Eq. 7). The Co(II) complexes eventually decomposes (Eq. 8). 

This type of reaction is related to the well-known SN2 reaction, widely encountered in organic chemistry. The essential feature of this type of a reaction is the presence of a leaving group attached to the sp3 carbon centre. The reactions may be controlled in a number of ways. The metal could either co-ordinate to the leaving group and stabilise the anionic species as it is formed, or it could co-ordinate to some other group and increase the S+ charge on the sp3 carbon atom and hence activate it towards the initial attack by the nucleophile (Fig. 4-36). 

The key feature of these reactions is the interplay between metal-centred and ligand-centred radical species, as discussed in Chapter 5. To summarise, the process involves generation of a nitrogen-centred radical, which is stabilised by charge transfer from the... 

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