Question

What are ligands and how are they classified? Give suitable examples.

Answer 1

Ligands are atoms or groups of atoms that contribute a single pair of electrons to the core metal atom or ion. The ligands can be classed as follows.

[A] Ligands are classified on the basis of charge:

1. Neutral ligands: These ligands are electrically neutral. These are typically molecular entities with one or more lone pairs of electrons. 
   For example, H₂O (water), NH₃ (ammonia), CO (carbonyl) and NO (nitrosyl).
2. Anionic ligands: These ligands are negatively charged. These anionic entities have one or more lone pairs of electrons.
   For example, Cl⁻ (chloride), CN⁻ (cyanide), OH⁻ (hydroxide) and SCN⁻ (thiocyanate).
3. Cationic ligands: Positively charged ligands are infrequently found in complexes.
   For example, NO⁺ (nitrosonium) and N₂H₅⁺ (hydrazinium).

[B] Ligands are classified on the basis of the number of donor atoms:

1. Unidentate ligands: They are also known as monodentate ligands, have only one donor atom and can form a single coordinate bond with the central metal or ion. These ligands can only connect to the center atom at a single site. They may be neutral or anionic. For example, amine and carbonyl.
2. Didentate or bidentate ligands: They are defined as those that have two donor atoms and can make two coordinate bonds with the central metal atom or ion. Such ligands can connect to the core metal atom or ion at two sites. For example, oxalato (ox) and glycino (gly).
3. Tridentate ligands: They are those with three donor atoms and the ability to create three coordinated bonds with the center metal atom or ion. For example, diethylenetriamine (dien) and aspartate ion (asp) are used.
4. Tetradentate ligands: They consist of four donor atoms. For example, Triethylenetetramine (trien).
5. Pentadentate ligands: These ligands refer to those with five donor atoms. For example, ethylenediaminetriacetate.
6. Hexadentate ligands: These ligands consist of six donor atoms. For example, ethylenediaminetetraacetato (EDTA).
7. Bridging ligands: These ligands are monodentate ligands that can connect to two or more metal ions at the same time. For example, OH⁻, Cl⁻.