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.
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.
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).
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.
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).
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:
2. 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.
Examples: Img 3
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.
Examples: Img 4
3. Based on the Type of Donor Atom:
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 iso-thiocyanato, -NCS). Img 5
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).
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.
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.
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.
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.
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.
(Session 2025 - 26)