The coordination number (CN) of a metal ion in a complex is the count of ligand donor atoms directly bonded to the metal. It determines the spatial arrangement around the central metal ion.

Ambidentate ligands, like NO₂⁻ and SCN⁻ ions, have two different donor atoms. NO₂⁻ can coordinate through nitrogen or oxygen, while SCN⁻ can coordinate through sulfur or nitrogen, offering flexibility in metal binding.

Chelate ligands, like EDTA⁴⁻, use two or more donor atoms simultaneously to bind a metal ion, forming more stable complexes than those with unidentate ligands. The denticity of the ligand is the number of such ligating groups.

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.

Ligands are ions or molecules bound to the central atom/ion in a coordination entity. They can be simple ions like Cl⁻, small molecules like H₂O or NH₃, or even larger molecules such as proteins. Examples include Cl⁻, H₂O, NH₃, H₂NCH₂CH₂NH₂, ...