Atoms which make bonds with the central atom are called ligands.The number of ligands participating directly in the bonding with the central atom is called the coordination number

The outer sphere (counter ion) is the aggregate of all the ions situated outside the inner coordination sphere.Monodentate ligands contain one donor atom: H2O, NH, OH, CT

Bidentate ligands contain two donor atoms and form two coordination bonds with the central atom.These are anions SO, CrO, molecules of ethylenediamine (HN-CH-CH-NH2), aminoacetic acid (H2N-CH-COOH).The most frequently used ligands are those with donor atoms of oxygen, nitrogen and sulphur

Depending on the number of donor atoms we can distinguish ligands with different dentation.It forms complexes with many metals owing to the oxygen atom of each of the four carboxylic groups and to both nitrogen atoms.