Compounds of Polydentate Ligands

Possible coordination for three-donor linear and cyclic ligands to square planar and octahedral metal complexes. The cyclic ligand is unable to bind all three donors in the square planar geometry. 

As the number of donors increase, so does the diversity in shape that may be met. We tend to classify ligands in terms of their appearance or basic framework - for example, they may be described as linear, branched, cyclic and cyclic with pendant chains. Nevertheless, despite the variations possible, these types of ligands are still regarded as basic or simple in character there exists an array of more complicated and specific shapes that we shall touch upon briefly below. 

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The trimethylaluminum complexes of tetraaza macrocycles contain AlMe2 groups and externally coordinated AlMes molecules [416-419] tetrathia macro-cycles [420, 421] are perhaps better described as simple coordination compounds of polydentate ligands, rather than host-guest complexes. This is suggested by the short Al-N distances (ca 2.0 A) and the external coordination of the metal. 

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