Inorganic Chemistry: d-Block Elements - SS3 Chemistry Past Questions and Answers - page 2

Ligands play a crucial role in transition metal complexes by coordinating with the central metal ion. A ligand is a molecule or ion with one or more lone pairs of electrons that can form coordinate covalent bonds with the metal ion. The coordination of ligands around the metal ion leads to the formation of a coordination complex.

Influence on Properties and Reactivity:

1.    Coordination Number: The number of ligands coordinated to the central metal ion determines the coordination number of the complex. Different coordination numbers result in varying geometries (e.g., octahedral, tetrahedral, square planar), which influence the complex's stability and reactivity.

2.    Color and Spectroscopy: Ligands can cause d-d electron transitions in the metal ion, leading to the observation of vivid colours in coordination complexes. The absorption and emission of specific wavelengths of light provide valuable information about the complex's electronic structure through spectroscopic techniques.

3.    Stability and Solubility: The presence of ligands can significantly impact the stability and solubility of transition metal complexes. Some ligands form very stable complexes, while others may lead to less stable species or even precipitate the complex from the solution.

4.    Redox Properties: Certain ligands can influence the redox properties of the metal centre in a complex. The presence of strong ligands may stabilise a specific oxidation state, while weaker ligands may facilitate redox reactions by allowing the metal to change its oxidation state more readily.

5.    Catalysis: Transition metal complexes with specific ligands can act as catalysts in various chemical reactions. The presence of ligands can modify the reactivity and selectivity of the catalyst, enabling efficient and selective transformations.

The coordination number in transition metal complexes refers to the number of ligands directly coordinated to the central metal ion. It determines the spatial arrangement of ligands around the metal and influences the complex's structure, stability, and reactivity.

Significance in Structure:

1.    Octahedral Coordination (CN = 6): In octahedral complexes, six ligands surround the central metal ion, leading to a symmetric arrangement. This coordination number is common for transition metals with d2sp3 hybridization, such as metal ions in the 3d series.

2.    Tetrahedral Coordination (CN = 4): Tetrahedral complexes have four ligands coordinated around the metal ion, resulting in a tetrahedral arrangement. This coordination number is typical for transition metals with dsp^2 hybridization, such as metal ions in the 4d series.

3.    Square Planar Coordination (CN = 4): Square planar complexes have four ligands in a flat square arrangement around the metal ion. This coordination number is observed in certain transition metals with d^8 electron configuration, such as metal ions in the 4d and 5d series.

Significance in Stability:

The coordination number significantly affects the stability of the complex. For instance, octahedral complexes with six ligands are generally more stable than tetrahedral complexes with four ligands. This is because octahedral coordination allows for greater electrostatic interactions between the metal ion and ligands, leading to stronger bonding.

Moreover, the coordination number is also influenced by the size of the ligands and the size of the metal ion. The coordination geometry is optimised to minimise repulsions between ligands, which contributes to the overall stability of the complex.

In conclusion, the coordination number is a critical factor in determining the structure and stability of transition metal complexes. Different coordination numbers result in distinct geometries and reactivity, allowing for a wide range of applications in various fields, including catalysis, material science, and medicinal chemistry.