The Complexes—Types of Bonding

2 -Terpyridine commonly behaves as a chelating terdentate ligand, with the majority of complexes exhibiting 1 1 or 1 2 metalrligand ratios. The 1 2 complexes are invariably based upon an octahedral geometry, and frequently exhibit local symmetry. These complexes are discussed at the appropriate points in the text. 

Numerous crystal structural analyses of complexes with the 1 1 metal ligand stoichiometry have been reported. These are discussed in the appropriate sections. A wide variety of geometries are adopted, but it is clear that the essentially planar terdentate ligand imposes a steric requirement such that uncommon geometries are favored. In particular, the distorted trigonal-bipyramidal (Fig. 9a), square-pyramidal (Fig. 9b), and pentagonal-bipyramidal geometries (Fig. 9c) are commonly encountered. 

The strain introduced into the ligand will be even higher in complexes of the higher oligopyridines, and it was at one time suggested that they could not exhibit their maximum denticity in monodentate complexes. It is now clear that quaterpyridine may act as a quaterdentate ligand, albeit with very unsymmetrical M—N distances. 

To date, the cyclometallated bonding mode has not been directly observed in complexes of terpy or higher oligopyridines. However, the unequivocal demonstration of a cyclometallated 2,2 -bipyridine moiety in iridium complexes suggests that such a possibility exists 71,130,131,145,233,419, 420, 478). There is H NMR and kinetic evidence to suggest that cyclometallated intermediates are not involved in the deuteration of [Rufterpylj] at the 3, 3, 5, and 5 positions 138). 

It is not unreasonable to expect alkali metal ions to form complexes with the moderately hard nitrogen donor atoms of pyridine ligands, and it is now becoming apparent that a rich chemistry may exist in this area. 

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