Question #e9fc3

Chelation is simply a type of bonding that requires the formation of dative covalent bonds, which you'll often see referred to as coordinate bonds, between one or more multidentate ligands and a metal cation.

The thing that stands out here is the fact that one or more ligands must be multidentate, meaning that they must form at least #2# dative covalent bonds with a metal cation to produce a coordination complex.

As you know, a dative covalent bond is very similar to a classic covalent bond. However, the thing to remember here is that when two atoms form a dative covalent bond, both bonding electrons are supplied by one of the two atoms.

Simply put, a dative covalent bond is formed when one atom donates a pair of electrons to bond with another atom.

Now, chelation takes place when you have a metal cation and at least one multidentate ligand called chelating agent.

A classic chelating agent to use as an example would be ethylenediamine (en), #"C"_2"H"_4("NH"_2)_2#.

Each molecule of en can form #2# dative covalent bonds with a metal cation due to the presence of those #2# lone pairs of electrons located on the nitrogen atoms. In this regard, en is called a bidentate ligand.

So, for example, you can have cobalt(III), #"Co"^(3+)#, as the metal cation and three en chelating agents.

This complex ion is called tris(ethylenediamine)cobalt(III). Notice that each en molecule forms #2# dative covalent bonds with the cobalt(III) cation.

You can also have something like this

In this case, one of the three en chelates has been replaced with two chloride anions, #"Cl"^(-)#. These anions are not chelating agents because they only form #1# dative covalent bond with the cobalt(III) cation.

The resulting complex ion is called trans-dichloro-bis(ethylenediamine)cobalt(III). You can also have the cis isomer, cis-dichlorobis(ethylenediamine)cobalt(III), which looks like this

So, to sum this up, chelation is a process that involves the formation of dative covalent bonds between at least one multidentate ligand and a metal cation.