Types of Ligands
Coordination Chemistry | Denticity & Properties
1. What is a Ligand?
Ligands act as Lewis Bases (Electron Donors), and the central metal acts as a Lewis Acid (Electron Acceptor).
2. Classification based on Charge
- Anionic (Negative): Ends in 'o'. E.g., $Cl^-$ (Chlorido), $CN^-$ (Cyanido), $OH^-$ (Hydroxido).
- Neutral: Special names. E.g., $H_2O$ (Aqua), $NH_3$ (Ammine), $CO$ (Carbonyl).
- Cationic (Positive): Rare, ends in 'ium'. E.g., $NO^+$ (Nitrosonium), $NH_2NH_3^+$ (Hydrazinium).
3. Classification based on Denticity
Denticity refers to the number of donor atoms in a single ligand molecule that bind to the central metal.
A. Monodentate (Unidentate)
Has only one donor atom. Examples: $H_2O, NH_3, Cl^-, CN^-, OH^-$.
B. Bidentate (Didentate)
Has two donor atoms and can bind at two points.
| Ligand | Symbol | Donor Atoms | Charge |
|---|---|---|---|
| Ethylenediamine | $en$ | 2 N | 0 |
| Oxalate ion | $ox^{2-}$ | 2 O | -2 |
| Glycinate ion | $gly^-$ | 1 N, 1 O | -1 |
C. Polydentate
Has more than two donor atoms.
- Hexadentate: EDTA (Ethylenediaminetetraacetate).
- Structure: It has 2 Nitrogen and 4 Oxygen donor atoms.
- Charge: $EDTA^{4-}$ has a charge of -4.
- Use: Treating lead poisoning (Chelating agent).
4. Special Types of Ligands
A. Chelating Ligands
Di- or polydentate ligands that use two or more donor atoms to bind to a single metal ion, forming a Ring Structure.
B. Ambidentate Ligands
Monodentate ligands with two different donor atoms but can use only one at a time.
- Nitro group ($NO_2^-$): Can bind through N (Nitro) or O (Nitrito).
- Thiocyanate ($SCN^-$): Can bind through S (Thiocyanato) or N (Isothiocyanato).
C. Flexidentate Ligands
Polydentate ligands that do not always use all their donor atoms. E.g., EDTA can act as tetradentate or pentadentate in certain conditions.
Practice Quiz
Test your knowledge on Ligands.
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