Coordinated ligands Nucleophilic reactivity

Having established a very effective method for the synthesis of tricar-bonyl(T74-vinylketene)iron(0) complexes, Thomas has subsequently undertaken the most comprehensive study on the reactivity of these complexes to date. The reactions of 221 with phosphoramidate anions,90134 coordinating ligands such as phosphines3 and isonitriles,69,87,89,135,142,143 a variety of nucleophiles,86,89135142 phosphonoacetate anions,88,89 alkynes,108,109,144,145 and al-kenes146,147 have ah been investigated. Crucially, Thomas has also developed a method138 for the kinetic resolution of the vinylketene complexes (221) that ultimately yields enantiomerically pure samples of the complex. This... 

In contrast to electrophiles, nucleophiles are expected to react preferentially with n -bonded heteroaldehyde and -ketone ligands. The site of attack is expected to be the carbon atom of the E = C group, -coordinated ligands should be less reactive. 

The synthetic utility of reactions of coordinated ligands is an important and varied subject. It is based on the enhancement in reactivity of organic ligands as a consequence of metal coordination. For example, the metal can act as a super add and cause enhanced nucleophilic attack on coordinated carbonyl and imine ligands. The metal ion can also enable the ligand itself to act as a nucleophile, sometimes by direct activation, sometimes by protecting other parts of the ligand and sometimes by a combination of both. 

In transition metal-catalyzed reactions, the coordinating ability (nucleophilicity) and/or reactivity of anions towards the metal active center is often determinant. The coordination properties of anions have been the object of many studies [5]. These properties depend in large part on the nature of the anions themselves (size, charge), but also on the hardness of the metal center, its oxidation state, and its surrounding ligands. Here the qualitative Hard and Soft Acids and Bases (HSAB) concept developed by Pearson can be applied to classify some typical anions in respect of their softness or hardness to transition metal complexes (Figure 1). Ionic transition metal complexes of the type [L 1M]+X, are particularly suitable for use... 

Reactivity of a coordinated ligand can change significantly due to polarization effects that alter its acidity this is exemplified by the acidity of coordinated water rising markedly on binding to a metal ion, the deprotonated hydroxide form being an efficient nucleophile in reactions. 

This chapter, in previous volumes, concentrated on mechanistic studies of the stoichiometric reactions of coordinated a- and tt-hydrocarbons with nucleophiles and electrophiles. In order to provide a more comprehensive overview of the reactivity of coordinated ligands in general, related ligand reactions in classical coordination complexes are now also included. The stereospecificity of such processes and their potential for asymmetric synthesis has continued to attract increasing attention, and it is therefore appropriate to collect them all together in one chapter. There are three subsequent sections. The first is concerned with cobalt(III) complexes, and the second with complexes of other metals. The last section deals with the ligand reactivity of organometallic compounds. 

Such an intermediate can be formed because of the nucleophilic reactivity of a dioxygen molecule when complexed to iridium(I) and because of the known propensity of a coordinated carbonyl ligand to undergo such nucleophilic attack. 

Although polyenes are typically subject to electrophilic attack in the free state, they are rendered amenable to nucleophilic attack as coordinated ligands, a fascinating reversal in chemical reactivity usually referred to by the German term umpolung . It is important to note that nucleophiles (Nu ) usually add to the external face of an arene ligand (i.e., an exo- attack) and tend to reduce the hapticity of the ligands to which they add (Scheme 11). 

In contrast to the increase in reactivity of hydrocarbons with nucleophiles after coordination to electron-accepting metal centers, an increase in reactivity of unsaturated hydrocarbons with electrophiles is observed upon coordination to particularly electron-ridi metal centers. This contrasting reactivity is shown schematically in Figure 11.3. Olefin complexes of very electron-rich metal centers are best described as metallacyclopropane complexes, as noted in Chapters 1 and 2. As such, the olefin ligands in these complexes contain a large degree of M-C cr-bond character and react with electrophiles. Reactions of electrophiles with coordinated ligands are described in Chapter 12. 

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