Ligands and Coordination Number - SS2 Chemistry Lesson Note
Ligands and coordination numbers are fundamental concepts in the field of coordination chemistry. Ligands are molecules or ions that bind to a central metal ion or atom in a coordination compound, while coordination number refers to the number of ligands attached to the central metal.
Ligands in Coordination Compounds:
Ligands are molecules or ions that have one or more electron pairs available for forming coordinate bonds with a central metal ion or atom. Ligands act as Lewis bases, donating electron pairs to the central metal, which acts as a Lewis acid. The formation of coordinate bonds between the ligands and the metal ion or atom is a key characteristic of coordination compounds.
Classification of Ligands:
Ligands can be classified based on their charge, donor atoms, and binding modes:
● Charge: Ligands can be neutral (e.g., H2O, NH3, CO) or charged (e.g., Cl-, OH-, CN-). Charged ligands are called complex ions or coordination entities.
● Donor Atoms: Ligands are classified based on the atoms that donate electron pairs. Common donor atoms include nitrogen (N), oxygen (O), sulphur (S), and phosphorus (P). For example, ligands with one donor atom are known as monodentate ligands, while those with multiple donor atoms are called polydentate or chelating ligands.
● Binding Modes: Ligands can bind to the central metal ion through different modes, such as monodentate (binding through a single atom), bidentate (binding through two atoms), or bridging (binding to multiple metal ions).
Coordination Number in Coordination Compounds:
Coordination number refers to the number of ligands directly bonded to the central metal ion or atom in a coordination compound. The coordination number determines the geometry of the compound and plays a crucial role in its physical and chemical properties.
Common Coordination Numbers:
● Coordination number 2: Known as linear geometry, where the ligands are arranged in a straight line around the metal ion. Examples include [Ni(CO)4] and [Ag(NH3)2]+.
● Coordination number 4: Known as tetrahedral or square planar geometry, where the ligands surround the metal ion in a tetrahedral or square planar arrangement. Examples include [CuCl4]2- (tetrahedral) and [PtCl4]2- (square planar).
● Coordination number 6: Known as octahedral geometry, where the ligands form an octahedron around the central metal ion. Examples include [Cr(H2O)6]3+ and [Co(en)3]3+.
● Coordination number 8: Known as cubic or dodecahedral geometry, where the ligands occupy the vertices or faces of a cube or dodecahedron around the metal ion. Examples include [PtCl6]2- (octahedral) and [Co(NH3)8]3+ (cubic).
Factors Affecting Coordination Number:
The coordination number depends on several factors, including the electronic configuration of the metal ion, the size of the metal ion, and the size and charge of the ligands. The preferred coordination number for a metal ion is often determined by the balance between the repulsion of ligands and the stabilisation provided by the metal-ligand bonds.
Ligand Field Theory:
Ligand field theory (LFT) is a theoretical model used to explain the electronic structure and properties of coordination compounds. LFT considers the interaction between the ligands' electron orbitals and the d orbitals of the metal ion. This theory helps in predicting the colour, magnetic properties, and stability of coordination compounds based on the splitting of d orbitals caused by ligand-field effects.
Applications of Ligands and Coordination Number:
● Medicine: Ligands play a crucial role in the design and development of metal-based drugs, such as platinum-based anticancer drugs and iron-containing haemoglobin.
● Catalysts: Coordination compounds with specific ligands are utilised as catalysts in various chemical reactions, including industrial processes and environmental remediation.
● Materials Science: Ligands and coordination number are essential in the synthesis of materials with desired properties, such as magnetic materials, luminescent compounds, and molecular sieves.
● Bioinorganic Chemistry: Understanding ligands and coordination numbers is important for studying the structure and function of metalloproteins, such as haemoglobin and cytochromes.
Ligands and coordination numbers are fundamental concepts in coordination chemistry. Ligands are molecules or ions that bind to a central metal ion or atom through coordinate bonds. Coordination number refers to the number of ligands directly attached to the central metal. Ligands can be classified based on their charge, donor atoms, and binding modes. The coordination number determines the geometry and properties of coordination compounds. Understanding ligands and coordination numbers is crucial for various applications in medicine, catalysis, materials science, and bioinorganic chemistry.