Unidentate ligands, like Cl⁻ or H₂O, bind through a single donor atom. Didentate ligands, such as ethane-1,2-diamine or oxalate, bind through two donor atoms. Polydentate ligands, like N(CH₂CH₂NH₂)₃, have several donor atoms in a single ligand.
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Unidentate ligands bind to a metal ion through a single donor atom (e.g., Cl⁻ in [NiCl₂(H₂O)₄]). Didentate ligands use two donor atoms for binding (e.g., ethane-1,2-diamine in [Ni(en)₃]²⁺). Polydentate ligands employ multiple donor atoms (more than two) in a single ligand molecule (e.g., EDTA⁴⁻ in [Cu(EDTA)]²⁻). The number of donor atoms determines the denticity of the ligand. Ethylenediaminetetraacetate ion (EDTA⁴⁻) is an example of a hexadentate ligand, forming six coordinate bonds with the central metal ion (e.g., Cu²⁺).