The formal definition of a transition element:
An ❌ which forms at least one ❌ ion with a ❌ ❌ of electrons.

Complex ions with multidentate ligands are called ❌. Chelates can be used to effectively remove ❌ metal ions from solution. If you add a hexadentate ligand such as ❌ to a solution of a transition metal salt, the EDTA will replace all six water ❌ in the aqua ion, for example:
[Cu(H2O)6](2+) + EDTA(4-) --> [CuEDTA](2-) + 6H2O
In this equation, ❌ species are replaced by ❌. This increase in the number of particles causes a significant increase in ❌ which drives the reaction to the right. for this reason chelate complexes with ❌ ligands are favoured over complexes with ❌ ligands. This is known as the chelate effect.

When the ligands bond with the transition metal ion, there is ❌ between the electrons in the ligands and the electrons in the d orbitals of the ❌. That raises the energy of the d orbitals.
However, because of the way the d orbitals are arranged ❌, it doesn't raise all their energies by the same amount. Instead, it splits them into ❌, one group of 2 orbitals with a slightly ❌ energy level than the other 3.
The size of the energy gap between them varies with the nature of the transition metal ion, its ❌ (whether it is 3+ or 2+, for example), and the nature of the ❌.
When ❌ is passed through a ❌ of this ion, some of the energy in the light is used to ❌ an electron from the lower set of orbitals into a space in the upper set. The amount of energy absorbed in order to promote this electron corresponds to a ❌ of the visible light. We therefore see a combination of the other colours in the spectrum, which often shows as the ❌ colour of the one absorbed.