Chelate complexes (or chelates) refer to compounds in which a polydentate ligand (that has more than one free electron pair) binds to at least two binding points of the central atom. The ligand is called chelator in this case. The central atom is usually a double positively charged metal ion (e.g. Fe2+, Cu2+). Ligands and central atoms are bound via coordinative compounds (i.e. the binding electron pair is provided only by the ligand). Chelate complexes are more stable than the same complexes with monodentate, not interconnected ligands. This “chelation effect” has two causes: firstly, entropy decrease in the complex formation is lower, which results in a thermodynamic stabilization effect. Secondly, a chelate ligand can only separate from the central atom after dissolution of all bonds (that means that the chelator dissociates much harder from the metal ion). Therefore, chelate complexes are more stable than those with monodentate ligands. As a result, the likelihood of immediate recombination increases after the split. Heme, chlorophyll and vitamin B12 are examples from nature. Among other things, ethylenediaminetetraacetic acid (EDTA) is of importance in medicine. It is used in chelate therapy and is also applied for titrations in analytical chemistry (chelatometry).
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