Ligands

A ligand is an ion or molecule that can donate a pair of electrons to a central metal atom or ion to form a coordinate covalent bond, resulting in the formation of a coordination compound. Ligands play a crucial role in determining the structure, reactivity, and properties of coordination compounds.

1.0What is Ligand?

A ligand is an ion or molecule that donates a pair of electrons to a central metal atom or ion, forming a coordination complex through a coordinate covalent bond.

The term "ligand" originates from the Latin word "ligare," meaning "to tie" or "to bind," reflecting its role in binding to metal centres. Ligands can be classified as anions (negatively charged), cations (positively charged), or neutral molecules.

As Lewis bases, ligands donate electron pairs to the central metal atom, which acts as a Lewis acid by accepting these electron pairs.

The nature of bonding between a metal and a ligand can range from covalent (electron sharing) to ionic (electrostatic attraction), influencing the stability and properties of the coordination complex.

2.0Characteristics of Ligands

  1. Electron Donor: Ligands act as Lewis bases, which means they are electron pair donors. They provide a lone pair of electrons to the central metal atom or ion (which acts as a Lewis acid, or electron pair acceptor).
  2. Bonding: The bond formed between a ligand and the central metal ion is a coordinate covalent bond (also known as a dative bond), where both electrons in the bond originate from the ligand.
  3. Versatility: Ligands can be neutral molecules (such as H₂O, NH₃) or anions (such as Cl⁻, CN⁻). Some ligands can even be cations (though less common, like NO⁺ in certain contexts).

3.0Classification of Ligands

Ligands are classified based on several criteria, including their charge, denticity (the number of donor atoms), and the type of donor atom(s) involved.

  1. Based on Charge:
  • Neutral Ligands: These ligands have no charge. Common examples include:
    • Ammine (NH₃)
    • Aqua (H₂O)
    • Carbonyl (CO)
    • Phosphine (PR₃)
  • Anionic Ligands: These ligands carry a negative charge. Common examples include:
    • Chloro (Cl⁻)
    • Cyano (CN⁻)
    • Hydroxo (OH⁻)
    • Nitrate (NO₃⁻)
  • Cationic Ligands: These ligands are positively charged, though less common. An example is: Nitrosyl (NO⁺)
  1. Based on Denticity (Number of Donor Atoms):
  • Monodentate (Unidentate) Ligands: These ligands have a single donor atom that coordinates to the central metal ion. Each monodentate ligand forms one coordinate bond with the metal.

Examples: Water (H₂O), Ammonia (NH₃), Chloride (Cl⁻), Cyanide (CN⁻).

  • Bidentate Ligands: These ligands have two donor atoms that can coordinate to a single metal ion simultaneously, forming two bonds. This often results in the formation of a five- or six-membered ring with the metal.
  • Polydentate (Multidentate) Ligands: These ligands have multiple donor atoms that can coordinate to the same metal ion, forming more than two bonds. Polydentate ligands are often referred to as chelating agents because they can "chelate" the metal ion, forming very stable ring-like structures.
  1. Based on the Type of Donor Atom:
  • Anionic Ligands: Ligands that donate electrons from negatively charged donor atoms.

Examples: Chloride (Cl⁻), Hydroxide (OH⁻), Sulfate (SO₄²⁻).

  • Neutral Ligands: Ligands that donate electrons from neutral donor atoms.

Examples: Ammonia (NH₃), Water (H₂O), Phosphine (PH₃).

  • Ambidentate Ligands: Ligands that have two different donor atoms but can only coordinate through one donor atom at a time.

Examples: Nitro (NO₂⁻), which can coordinate through nitrogen (as in nitro, -NO₂) or through oxygen (as in nitrito, -ONO); Thiocyanate (SCN⁻), which can coordinate through sulfur (as thiocyanato, -SCN) or nitrogen (as isothiocyanato, -NCS).

4.0Types of Ligand Coordination

  1. Terminal Ligands: These ligands are bonded to only one metal atom. Most common ligands in coordination chemistry act as terminal ligands.

Examples: Water (H₂O), Ammonia (NH₃), Chloride (Cl⁻).

  1. Bridging Ligands: These ligands can coordinate two or more metal atoms simultaneously, acting as a "bridge" between them.

Examples: Hydroxo (OH⁻), Carbonyl (CO), Nitrosyl (NO).

5.0Importance of Ligands in Coordination Compounds

  1. Stability and Formation: Ligands play a crucial role in the stability and formation of coordination compounds. Polydentate ligands, due to their ability to form multiple bonds with the central metal ion, typically result in more stable complexes (chelate effect).
  2. Geometry and Structure: The type and number of ligands directly influence the geometry of the coordination complex. For example, six monodentate ligands often lead to an octahedral geometry, while four ligands can result in either tetrahedral or square planar geometry depending on the metal and ligands involved.
  3. Electronic Properties: Ligands affect the electronic properties of the central metal ion, including its oxidation state, electron configuration, and magnetic properties. For example, strong field ligands like CN⁻ can cause a large splitting of the d-orbitals in transition metals, resulting in low-spin complexes.
  4. Reactivity and Catalysis: The nature of the ligand can greatly influence the reactivity of the coordination compound. In catalysis, ligands are crucial in modulating the activity, selectivity, and stability of metal catalysts.
  5. Spectroscopic Properties: Ligands affect the absorption spectra of coordination compounds. Changes in the ligand field, induced by different ligands, can lead to varying colors in transition metal complexes due to d-d electronic transitions.
  6. Biological Importance: Ligands are vital in biological systems. For instance, in hemoglobin, the heme group (a multidentate ligand) binds to the iron ion, facilitating oxygen transport in the blood. Similarly, ligands in chlorophyll play a critical role in photosynthesis.

Examples of Ligands and Their Complex Type:

Ligand

Type

Complex Example

Water (H₂O)

Neutral, Monodentate

[Cr(H₂O)₆]³⁺

Ammonia (NH₃)

Neutral, Monodentate

[Co(NH₃)₆]³⁺

Chloride (Cl⁻)

Anionic, Monodentate

[PtCl₄]²⁻

Ethylenediamine (en)

Neutral, Bidentate

[Ni(en)₃]²⁺

Oxalate (C₂O₄²⁻)

Anionic, Bidentate

[Fe(C₂O₄)₃]³⁻

Ethylenediaminetetraacetic acid (EDTA)

Anionic, Hexadentate

[Ca(EDTA)]²⁻

Frequently Asked Questions

Ligands stabilize metal ions in coordination complexes by donating electron pairs to form coordinate covalent bonds. This interaction influences the chemical properties, geometry, reactivity, and color of the metal complexes.

A ligand is an ion or molecule that binds to a central metal atom or ion, donating a pair of electrons to form a coordination complex. Ligands act as Lewis bases, meaning they donate electron pairs to the metal, which serves as a Lewis acid.

Ligands bind to metal atoms or ions through coordinate covalent bonds, where the ligand donates a pair of electrons to the metal. The metal, acting as an electron acceptor, forms a bond with the ligand's electron pair.

Common ligands include: Water (H₂O): A neutral, monodentate ligand. Ammonia (NH₃): A neutral, monodentate ligand. Chloride (Cl⁻): An anionic, monodentate ligand. Ethylenediamine (en): A neutral, bidentate ligand. Oxalate (C₂O₄²⁻): An anionic, bidentate ligand. Ethylenediaminetetraacetic acid (EDTA): A hexadentate ligand.

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