Why are chelating ligands often used to treat poisoning by heavy metals (e.g. mercury, lead)? Why are chelating ligands such as EDTA often used to treat poisoning by heavy metals (e.g., mercury, lead)?


I have a vibe that it has something to do with the lone pairs of electrons that are able to bond to the heavy metals to form a stable coordination complex but I'm not sure about the in-depth process or explanation behind this. 

Also, does it have to do with it being a polydentate ligand and not a monodentate ligand?
Lead, mercury and other heavy metals form complexes with both monodentate and polydentate ligands. But the complexes made with polydentate ligands like EDTA are much more stable than with monodentates. Usually metallic complexes become toxic if the central metallic ion (non-complexed) is released or not entirely surrounded by ligands. Monodentate complexes do not surround completely the central metallic ion, and they may be decomposed by the acidity of the stomach. Polydentate complexes resist to this acidity, and often they surround the metallic ions much better than monodentate. So if a person has ingested one of these heavy metal ions, its toxicity will be hidden by a complexing agent like EDTA.