4. Answer the following questions.

v. What are ligands ? What are their types ? Give one example of each type.


In coordination chemistry, ligands are the species that surround the central metal atom or ion. They serve as electron pair donors and form coordinate bonds with the central metal ion. Ligands can be classified based on the number of electron donor atoms they possess:

i. Monodentate ligands: Monodentate ligands are characterized by having a single donor atom that shares an electron pair to create a coordinate bond with the central metal ion. Examples of monodentate ligands include Cl–, OH–, and CN–.

ii. Polydentate ligands: Polydentate ligands have two or more donor atoms that are capable of binding to the central metal ion. Depending on the number of donor atoms, polydentate ligands can be further categorized into:

a. Bidentate ligands: These ligands form bonds with the central metal through two donor atoms. For instance, ethylenediamine binds to the central metal atom through two nitrogen atoms. Similarly, the oxalate ligand (C₂O₄²⁻) utilizes the electron pair on each of its negatively charged oxygen atoms to form bonds with the central metal.

b. Hexadentate ligands: Hexadentate ligands create bonds with the central metal through six donor atoms. A classic example is the ethylenediaminetetraacetate ion (EDTA⁴⁻), which binds to the metal through electron pairs from four oxygen atoms and two nitrogen atoms.

iii. Ambidentate ligands: Ambidentate ligands possess two donor atoms, and they can choose either of these atoms to form a coordinate bond with the metal ion. Two common examples are:

a. NO₂⁻ ligand: The NO₂⁻ ligand can bind to the metal ion through either nitrogen or oxygen.

b. SCN⁻ ligand: The SCN⁻ ligand has two potential donor atoms, nitrogen and sulfur, and can form coordinate bonds in two different ways: M ← SCN⁻ or M ← NCS⁻.

This classification of ligands based on their donor atoms and bonding characteristics is essential in understanding the coordination chemistry of transition metal complexes.