Polydentate ligands mixed-metal complexation

The field has been the subject of several reviews of the properties, structures, and stability of numerous ligands and their complexes, including both mixed donor open-chain and macrocyclic polydentates. " Detailed reports of coordination chemistry that focus on the metal include many examples of mixed donor ligand systems. The nature of this article requires that only limited references are cited herein as examples, but these include some key reviews readers may access original work through these reviews or else by structure searching from compounds represented in line drawings that appear here. 

Central to the stability and chemistry of complexes formed by mixed donor ligands are two key concepts of coordination chemistry. The first is the chelate effect, which applies to all polydentate ligands, and reflects the increase in stability of a type of complex as monodentate donor molecnles are replaced by polydentates with donors linked by chelate rings. The hard-soft acid-base theory is particnlarly relevant to mixed donor ligands where donors of distinctly different character may bind to a central metal ion. The like prefers like concept means hard nonpolarizable donor atoms (N and O, for example) bond preferentially to hard nonpolarizable metal... 

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