Many chemical groups act as ligands that attach to metal ions. The donor atom is the exact point of contact on that ligand. Common donor atoms include nitrogen, oxygen, sulfur, and phosphorus.
Halogens like chlorine can also act as active donor atoms. These atoms act as Lewis bases because they provide the electrons. The central metal acts as a Lewis acid because it accepts the electrons.
Students often mix up the entire ligand with the donor atom. The ligand is the whole vehicle, but the donor atom is the steering wheel. Only the donor atom physically touches the metal ion.
Some simple ligands have only one single donor atom. They can only grab the metal at one specific spot. Water and ammonia are good examples of these simple ligands.
Other complex ligands contain multiple donor atoms in the same molecule. They can bite onto the metal at several different points at once. Chemists call these special molecules chelating ligands.
EDTA is a very famous and useful chelating ligand. It has six different donor atoms that wrap around a metal ion. This creates very stable ring structures called chelate rings.
The number and type of donor atoms determine the final molecular shape. They also control how tightly the metal stays bound in the solution.