The ions or molecules bound to the central atom/ion in the coordination entity are called ligands. These may be simple ions such as Cl^(-), small molecules such as H_(2)O or NH_(3), larger molecules such as H_(2)NCH_(2)CH_(2)NH_(2) or N(CH_(2)CH_(2)NH_(2))_(3) or even macromolecules, such as proteins. When a ligand is bound to a metal ion through a single donor atom, as with Cl^(-), H_(2)O or NH_(3), the ligand is said to be unidentate. When a ligand can bind through two donor atoms as in H_(2)NCH_(2)CH_(2)NH_(2) (ethane-1, 2-diamine) or C_(2)O_(4)^(2-) (oxalate), the ligand is said to be didentate and when several donor atoms are present in single ligand as in N(CH_(2)CH_(2)NH_(2))_(3), the ligand is said to be polydentate Ethylenediamineteraacetate ion [EDTA^(4-)] is an important hexadentate ligand. It can bind through two nitrogen and four oxygen atoms to a central metal ion. What is meant by didentate ligand ? Give an example other than ethane-1,2-diamine.