Question

Which is a polydentate ligand?
A.EDTA
B.Ethylene diamine
C.Oxalate ion
D.CO

Answer 1

Hint: Ligands have an important role in coordination chemistry , A ligand is an ion or molecule which binds to the central metal atom to form a complex in coordination chemistry. Mostly ligands can be negative ions, positive ions or neutral molecules. But generally ligand is an electron donor which donates electron pairs to the central metal atom and act as a lewis base, where metal atoms act as lewis acid by accepting the electron pair.

Complete answer:
Any ion/molecule attached to a central metal atom in a coordination compound to form a coordinate covalent bond is called ligand. A coordination compound contains a central metal atom/ion that is surrounded by a number of oppositely charged ions/neutral molecules .They do not dissociate into simple ions when dissolved in water.
A general form of coordination compound is
 $\left[\text{M}{\left(\text{L}\right)}_{\text{n}}\right]$ $\text{n = 2,4,6}$
where $\text{M}$ is the central metal atom, $\text{L}$ is the ligand
 First we learn about different types of ligands .On the basis of the number of sites, ligands can be classified as monodentate , bidentate and polydentate ligands.
1.Mono or unidentate ligand: In monodentate ligand one donor atom is present and can coordinate with a metal ion at only one site in a complex.
Example:CO
2. Bidentate ligands : Bidentate ligands occupy two sites of the metal ion , that is ligands with two donor atoms
Example: Ethylenediamine, oxalate ions
3.Polydentate ligands: When more than two donor atoms are present then the ligand is called polydentate ligand. Polydentate ligands are further classified into tridentate, tetradentate, hexadentate etc.
Example: EDTA (Ethylene diamine tetra acetate) has six donor atoms.
From the above explanations it is clear that ethylene diamine , oxalate ion comes under the bidentate ligand category whereas CO is monodentate ligand.
EDTA having six donor atoms comes under polydentate ligand category.

Hence, option A is correct.

Note:
Ligands can be also classified based on their complex structure that are chelating agents and ambidentate ligands. where the chelating agents come under polydentate ligands, which form a ring structure around the central metal atom , EDTA is a chelating agent.