Ligands (Lewis bases) with several binding sites are called chelates and form particularly stabile complexes: metal-chelates. The stability of a coordination compound increases with the number of binding centers on the ligands (chelate effect). Amino acids, peptides, and proteins contain many metal binding groups that make them excellent chelates. In proteins, besides peptide NH and C=O groups many side chains may serve as complex agents for metal ions. These include thiolate in cysteine, the imidazole ring of histidine, carboxylates of glutamic acid and aspartic acid, and the amino side chain of lysine.
The rationale behind the chelate effect is quite straightforward. As soon as a metal ion coordinates to one group in a multidentate ligand, the probability for coordination of other potential donor groups is enhanced. A favorable entropic factor further adds to the stability since chelation is accompanied by release of nonchelating ligands like water from the coordination sphere.
A closely related effect is termed the macrocyclic effect, which relates to the notion that a complex with a cyclic polydentate ligand has greater thermodynamic stability when compared with a similar noncyclic ligand. As a consequence, macrocyclic complexes occur widespread in nature, and are found in, e.g., crown ethers, cryptands (alkali metals), cytochromes (iron), chlorophyll (magnesium), and coenzyme-B12 (cobalt).
(1) With one or more free electron pairs
(2) Without free electron pairs but with n-binding electrons, e.g., ethylene, benzene
(a) No vacant orbitals to (b) Vacant orbitals that may (c) With further n-electrons receive electrons from receive n-electrons from the that may couple to the metal ion, e.g., H2O, metal, e.g., CN -, aromatic amines vacant metal orbitals, e.g.,
NH3, F- thiolates, phosphines
FIGURE 10.2 Classification of the different ligand types.
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