for example in this sentences :
several metal ions in a solution could be first chelated to form metal complexes .
A chelating agent, is a compound that it has a number of free electron pairs higher than two. These electrons can occupy the empty orbtitals of a metal and so form various covalent bonds with the metal (Chelate).This structre is different than that of a complexe, since a complexe is formed by covalent bond between a metal and ligand whose number of free electron pairs is lower tor equal than two, for example NH3, H2O, etc.
A chelate is formed in one step, but a complexe is formed on several steps.
Chelation is the formation of a metal-ligand complex by a ligand that can form more than one ligand-metal bond.
For example, 2,2'-bipyridine is composed of two pyridine molecules bridged at the 2-position; both pyridinium nitrogen atoms can ligate a metal, and because two ligand-metal bonds are formed by one ligand, it is known as a bidentate ligand. Another well-known chelating agent is ethylenediaminetetraacetic acid (EDTA); this is a hexadentate ligand that has applications in medicine and biology, for example, the sequestration of heavy metals.
Also, because chelating ligands form several bonds to a metal center, the formation of these complexes is often thermodynamically favourable compared to formation of a complex with similar monodentate ligands. This difference occurs because when six monodentate ligands form an octahedral complex, there is an overall loss of entropy, i.e., seven molecules become one complex. In contrast, only three bidentate molecules would be required to form an octahedral complex, resulting in less loss of entropy and more favourable thermodynamics.
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