Nucleophilic Reactions of Co-ordinated Pyridines

Rate equations of this type are normally associated with the formation of associative intermediates or the involvement of deprotonated ligand forms in reactions with two (or more) competitive pathways. However, in the case of octahedral metal complexes of ligands such as 2,2 -bipyridine or 1,10-phenanthroline, such mechanisms do not appear to be likely. Associative mechanisms would involve seven-co-ordinate intermediates, which are likely to be sterically strained and electronically disfavoured on ligand field grounds. Furthermore, this type of ligand does not appear to contain any strongly acidic protons which are likely to be involved in reactions with aqueous hydroxide ion (but see later). 

The suggestion has been made by Gillard that all of these observations may be conveniently explained in terms of nucleophilic attack by hydroxide ion on the co-ordinated ligand, which is rendered more electrophilic by co-ordination to the charged metal ion. 

The specific proposal is that the hydroxide attacks the 2- or the 4-position of the co-ordinated pyridine to form a hydroxy-substituted 1,2- or 1,4-dihydropyridine (Fig. 8-17). These hydroxy species are known as pseudo-bases. This behaviour is fully in accord with the known behaviour of A-alkylpyridinium cations. Although the pseudo-bases of simple A-alkylpyridinium cations are not dominant solution species under aqueous conditions, those derived from a variety of other nitrogen heterocycles are readily formed and are well-known. The suggestions had the advantage of linking the apparently divergent fields of heterocyclic and co-ordination chemistry by explaining some well-documented anomalies in the reactivity of pyridine complexes. 

It is convenient to separate the total electron density at each atom into a- and 71-components. It is likely to be the 7t-density that will be important in reactions with nucleophiles, since in an orbitally controlled reaction (Chapter 1) the donor orbital of the incoming nucleophile will initially interact with the lowest vacant 7i -orbital. The overall pattern of charge alternation is repeated in both the 7t- and the a-electron densities, and nucleophiles are expected to attack at the 2- or 4-positions. This is exactly the pattern that is seen in 

Many of the studies purporting to show nucleophilic attack upon co-ordinated heterocyclic ligands have been performed with complexes of 5-nitro-l,10-phenanthro-line, 8.20. Unfortunately, this ligand proves to be a less than optimal choice, as nitroalke-nes undergo facile reactions with nucleophiles and, in contrast to simple pyridines, the free ligand reacts with hydroxide ion. Hydroxide ion and other nucleophiles react with 8.20 at the 6-position to give the stabilised anion 8.21 (Fig. 8-20). It is almost certain that the majority of the reactions of 5-nitro-l,10-phenanthroline complexes with hydroxide 

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