Pyridine co-ordinated

Figure 8-16. A valence bond representation of a co-ordinated pyridine. The positive charge is delocalised and the 2- and the 4-positions of the ligand develop electrophilic character.

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. 

Figure 8-17. Gillard s proposal for the formation of pseudo-bases of co-ordinated pyridine ligands. It was suggested that the formation of these species allowed a simple explanation for the observed pH dependence of a variety of reactions of complexes of pyridine derivatives.

Figure 8-28. A mechanism for the deuteration of bpy complexes involving nucleophilic attack by the hydroxide ion. The hydroxide attacks the most electrophilic 4- and 6-positions of the co-ordinated pyridine ring. Specificity for the 3-position may be associated with steric relief as a result of the formation of an new sp3 hybridised carbon centre.

X-Ray structures of the corresponding cobalt(II), nickel(II) and zinc(ll) derivatives of 61 confirm that each of these complexes adopts a related helical geometry in which the co-ordination sphere is filled by six nitrogen donors from two essentially planar (meridionally co-ordinated) pyridine-2,6-diyldiimine units belonging to the macrocycle. 

On irradiation into a band corresponding to co-ordinated pyridine, U02(py)a-(N08)2 undergoes photosolvation [reaction (56)].162 163 Pyridine photoproducts are... 

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