Hexadentate

Hexadentate describes a ligand that can donate six electron pairs to a single metal ion, binding through six donor atoms. It is a subset of polydentate, or chelating, ligands, and typically enforces an octahedral or near-octahedral coordination sphere around the metal. The six donor atoms may be a combination of nitrogen, oxygen, sulfur, or other donors, positioned to form multiple chelate rings that stabilise the complex via the chelate effect.

Ethylene diamine tetraacetic acid (EDTA) is the prototypical hexadentate ligand. In its fully deprotonated form, EDTA4- binds through four carboxylate oxygens and two amine nitrogens, occupying six coordination sites and producing strong, often kinetically inert, metal complexes such as [Ca(EDTA)]2− or [Fe(EDTA)]−. Other hexadentate ligands include certain macrocyclic polyamines and polyaminocarboxylates designed for metal sequestration or catalysis.

Hexadentate ligands often confer enhanced stability compared to lower-denticity ligands because of the chelate effect and preorganization. The resulting complexes’ geometry is typically octahedral but can be distorted by the metal ion size, ligand bite angles, and protonation state. In solution, pH can control donor availability, affecting binding strength; for EDTA, higher pH generally favors metal binding due to deprotonation of carboxylate groups. Applications include metal ion sequestration in analytical chemistry, water treatment, and therapeutic chelation.