Question

Which of the following is a tridentate ligand?
A ) $$\text{EDT}{\text{A}}^{4-}$$
B ) $${\left(\text{COO}\right)}_2^{2-}$$
C ) $$\text{dien}$$
D ) $${\text{NO}}_2^{-}$$

Answer 1

Hint: Ligands can be classified into monodentate, bidentate, tridentate, tetradentate, hexadentate, polydentate etc. These are based on the number of donor atoms present in the ligands. Monodentate, bidentate, tridentate, tetradentate, hexadentate, polydentate etc contain one, two, three, four, six and several donor atoms respectively.

Complete answer:
A tridentate ligand contains three donor atoms. When a tridentate ligand forms a complex with metal, all the three donor atoms of the tridentate ligand form bonds with metal ions.
Now we have to find out which of the following is a tridentate ligand. Let us discuss it one by one:
A ) Ethylenediaminetetraacetate ion $$\text{EDT}{\text{A}}^{4-}$$ is a hexadentate ligand. It contains six donor atoms. Out of these, two donor atoms are two nitrogen atoms of ethylene diamine and four donor atoms are oxygen atoms of four acetate groups.
B ) Oxalato ligand $${\left(\text{COO}\right)}_2^{2-}$$ is a bidentate ligand obtained from oxalic acid. It is a bidentate ligand with two donor atoms. Of two oxygen atoms present in each carboxylate group, only one oxygen atom is the donor atom.
C ) The ligand diethylene triamine $$\text{dien}$$ is a tridentate ligand. Its structural formula is $$\text{N}{\text{H}}_{\text{2}}\text{C}{\text{H}}_{\text{2}}\text{C}{\text{H}}_{\text{2}}\text{N}{\text{H}}_{\text{2}}\text{C}{\text{H}}_{\text{2}}\text{C}{\text{H}}_{\text{2}}\text{N}{\text{H}}_{\text{2}}$$.
All the three nitrogen atoms are donor atoms.
D ) The ligand nitrite ion $${\text{NO}}_2^{-}$$ is an ambidentate ligand. Ambidentate ligands have two donor atoms but at a time only one of them forms a bond with metal ions. Thus, in nitrite ions, either nitrogen or oxygen can form bonds with central metal ions, but both nitrogen and oxygen cannot form bonds at the same time.

Hence, the correct option will be the option C ).

Note: Ligands having more than one donor atom forms cyclic compounds with metal ions. These cyclic compounds are called metal chelates. When a coordination complex contains a chelating ligand, then it is more stable than the coordination complex containing only non chelating ligands.