Intermolecular Attack by Co-ordinated Hydroxide

In the preceding section we discussed the use of co-ordinated hydroxide as an intramolecular nucleophile. It could also act as a nucleophile to an external electrophile. Over the past few decades, there has been considerable interest in the nucleophilic properties of metal-bound hydroxide ligands. One of the principal reasons for this relates to the widespread occurrence of Lewis acidic metals at the active site of hydrolytic enzymes. There has been a lively discussion over the past thirty years on the relative merits of mechanisms involving nucleophilic attack by metal-co-ordinated hydroxide upon a substrate or attack by external hydroxide upon metal-co-ordinated substrate. As we have shown above, both of these mechanisms are possible with non-labile model systems. 

However, we may also design model systems to study the reactions of co-ordinated hydroxide with external electrophiles. The simplest models utilise non-labile complexes with a single hydroxide ligand, such as [M(NH3)5(OH)]2+ (M = Co or Rh). Various electrophiles have been shown to react with such metal-bound hydroxide ligands, and some of these reactions are indicated in Fig. 5-73. 

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