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Ligands such as ethylenediamine H2NCH2·CH2NH2 (abbreviated as en) and oxalate ion C2O2-4 (abbreviated as ox) contain two or more coordination atoms, with these two coordination atoms separated by 2 to 3 other atoms. When they coordinate to the same metal ion, they often form ring-shaped structures that include the metal ion. These metal complexes can be regarded as metal chelating compounds, abbreviated as metal chelates. These ligands are called chelating ligands; they are polydentate (or polyfunctional) ligands. The highest number of teeth found in polydentate ligands is 14. The most common coordinating atoms in such ligands are oxygen and nitrogen, followed by sulfur; in addition, phosphorus, arsenic, and others are also present. When the chelate is a neutral molecule, it is called an intramolecular complex salt, also simply referred to as an internal complex salt, and was noticed by people quite early on. For example, the glycine ion H2NCH2COO-2 (abbreviated as gly) is a representative of this class, and can form intramolecular complex salts such as [Cu1(gly)2] and [Co(gly)3]. The term \"intramolecular complex\" is not used very often these days; it originally referred to molecules that contain coordinate bonds. When polydentate chelating ligands replace the water molecules in hydrated complex ions, each chelating ligand can replace two or more water molecules. As a result, the total number of particles increases, the disorder of the system rises, and the entropy value increases. According to the equilibrium relationship ΔG = ΔH – TΔS = –2.303RTlgK, changes in the ligands have little effect on ΔH; therefore, the greater ΔS is after chelation, the smaller ΔG becomes, and the larger K_eq is, making the chelate more stable. Metal chelates necessarily contain ring structures that include the metal; such rings are called chelate rings. When an n-valent ligand coordinates with a metal ion, at least (n-1) chelate rings are formed. The greater the number of chelation rings formed in a chelate, the higher its stability; generally, five-membered and six-membered rings are more stable than rings with other numbers of members. This effect, by which chelates gain special stability due to the formation of a chelate ring, is called the chelation effect. Studying the structure of chelates is one of the important topics in coordination stereochimistry. Metal chelates formed from organic reagents often have distinctive colors; they are insoluble or poorly soluble in water, but many are readily soluble in organic solvents. Therefore, the organic reagents commonly used as precipitants and extractants for metal ions are mostly chelating ligands, and the resulting chelates are for the most part equivalent to intramolecular complex salts. For example, the anhydrous Cu(II) complex formed from 8-hydroxyquinoline C9H7NO is a planar coordination molecule. The red Ni(II) complex [Ni(C4H7N2O2)2] formed from succinimide is also a planar coordination molecule. The dark red Fe(III) complex [Fe(C5H7O2)3] formed with acetopropion and CH3COCH2COCH3 is a hexacoordinate octahedral coordination molecule. In the 1960s, it was discovered that macrocyclic polyethers (crown ethers and hole ethers) are a new type of chelating ligand. Due to their unique structure, macrocyclic polyethers possess certain properties that differ from those of common chelators, such as high selectivity and greater stability for some of the chelates they form (the macrocyclic effect or hyperchelation effect). Therefore, it has found applications in fields such as element separation and analysis, organic synthesis, and bionorganic chemistry, and is receiving increasing attention. Furthermore, among naturally occurring metal chelates such as vitamin B12 and chlorophyll, there are many complex compounds that play important roles in biochemistry. It is the ligands that primarily determine the properties of these chelates, which is a significant difference from simple inorganic complexes. The chemistry of natural chelators, including biochemical reactions involving metal ions, is also one of the latest areas of development in coordination chemistry. In summary, metal chelates are widely used in analytical and separation tasks such as the precipitation of metal ions, solvent extraction, ion exchange, and colorimetric quantification, as well as in applications like water softening and fabric dyeing. Some special metal chelates also have special applications.