Intramolecular Attack by Co-ordinated Hydroxide

It is very often extremely difficult to demonstrate that a metal-co-ordinated hydroxide ion is involved in a particular reaction. Studies of kinetic behaviour provide one of the most powerful tools for the determination of reaction mechanisms. It is not, however, always easy to distinguish between intra- and intermolecular attack of water or hydroxide. The most unambiguous studies have been made with non-labile cobalt(m) complexes, and we will open this discussion with these compounds. 

We saw in Chapter 3 that the hydrolysis of chelated amino acid esters and amides was dramatically accelerated by the nucleophilic attack of external hydroxide ion or water and that cobalt(m) complexes provided an ideal framework for the mechanistic study of these reactions. Some of the earlier studies were concerned with the reactions of the cations [Co(en)2Cl(H2NCH2C02R)]2+, which contained a monodentate amino acid ester. In many respects these proved to be an unfortunate choice in that a number of mechanisms for their hydrolysis may be envisaged. The first involved attack by external hydroxide upon the monodentate A-bonded ester (Fig. 5-62). This process is little accelerated by co-ordination in a monodentate manner. 

The second mechanism requires a preliminary displacement of chloride by the oxygen of the ester to give a chelated complex which may be attacked by external hydroxide as seen in Chapter 3. In practice, the displacement of chloride from cobalt(m) is very slow and this mechanism proceeds by the SNlcb mechanism, in which loss of chloride ion is aided by deprotonation of the amine. The first step involves deprotonation of the en ligand followed by chloride loss to give a five co-ordinate intermediate (Fig. 5-63). 

5 Stabilisation of Anions and the Reactions of Co-ordinated Ligands with Electrophiles 

This is followed by a rapid process in which the oxygen atom of the ester becomes coordinated to the metal (Fig. 5-64). 

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Figure 5-71. The hydrolysis of the tridentate ligand 5.33 is accelerated by co-ordination to a metal ion. The two reaction involves intramolecular attack by co-ordinated hydroxide.

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