What is different about macrocyclic ligand complexes

What is different about macrocyclic ligand complexes  

A very large number of synthetic, as well as many natural, macrocycles have now been studied in considerable depth. A major thrust of many of these studies has been to investigate the unusual properties frequently associated with cyclic ligand complexes. In particular, the investigation of spectral, electrochemical, structural, kinetic, and thermodynamic aspects of macrocyclic complex formation have all received considerable attention. 

There are good reasons for nature choosing macrocyclic derivatives for the important complexes just mentioned - enhanced kinetic and thermodynamic stabilities are bestowed on the respective complexes by the 

Since 1960, a very large number of other synthetic macrocycles has been prepared and this has resulted in a great increase in interest in all aspects of the chemistry of macrocyclic systems. From about this time there has also been enhanced interest in the role of metal ions in biological systems and many such bioinorganic studies have involved complexes of both natural and synthetic macrocycles. Thus there has been an element of cross-fertilization between these two developing areas, viz  

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