Reactions of Co-ordinated Thiolate

One of the simplest and widely used methods of forming C-S bonds involves nucleophilic attack of a thiolate on a suitable C-centred electrophile such as an alkyl halide (Fig. 5-74). Co-ordinated thiolate ligands behave as nucleophiles in exactly the same manner, and the method has been extensively used for the preparation of thioethers and their metal complexes. The method has been particularly commonly utilised in the formation of macrocyclic ligands in templated syntheses (see Chapter 6). 

The selectivity observed in some of these alkylation reactions is great, and it is usually possible to selectively alkylate only the sulfur in a co-ordinated aminothiolate. A typical example is seen in the methylation of bis(2-aminoethylthiolato)nickel(n) with methyl iodide to yield the bis(methylthioethylamine)nickel(n) cation (Fig. 5-76). 

In general, these reactions are better characterised with chelated thiolate ligands rather than with monodentate thiolates, although many reactions giving rise to monodentate coordinate thioethers are known. A typical example is shown in Fig. 5-77, in which an iron(n) thiolate is converted to a thioether complex. 

As mentioned above, reactions of this type have been widely used in the synthesis of macrocyclic ligands. Indeed, some of the earliest examples of templated ligand synthesis involve thiolate alkylations. Many of the most important uses of metal thiolate complexes in these syntheses utilise the reduced nucleophilicity of a co-ordinated thiolate ligand. The lower reactivity results in increased selectivity and more controllable reactions. This is exemplified in the formation of an A -donor ligand by the condensation of biacetyl with the nickel(n) complex of 2-aminoethanethiol (Fig. 5-78). The electrophilic carbonyl reacts specifically with the co-ordinated amine, to give a complex of a new diimine ligand. The beauty of this reaction is that the free ligand cannot be prepared in a metal-free reac- 

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

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