Ligand substitution reactions in octahedral coordination complexes typically proceed via two main mechanisms: associative A and dissociative D pathways.1. Associative mechanism A : In this mechanism, the incoming ligand approaches the complex and forms a bond with the central metal ion before the departure of the original ligand. This leads to the formation of an intermediate with a higher coordination number usually heptacoordinated . The original ligand then leaves the complex, and the coordination number returns to six. The associative mechanism is more common for complexes with vacant coordination sites or with labile ligands.2. Dissociative mechanism D : In this mechanism, the original ligand departs from the complex first, creating a vacant coordination site. The incoming ligand then binds to the central metal ion, restoring the octahedral geometry. The dissociative mechanism is more common for complexes with strong-field ligands or with a high oxidation state of the central metal ion.The size, charge, and electronic properties of the incoming ligand can significantly influence the reaction rate and selectivity of the substitution process:1. Size: Larger incoming ligands may have steric hindrance, making it more difficult for them to approach the central metal ion and form a bond. This can slow down the reaction rate, especially in the associative mechanism.2. Charge: The charge of the incoming ligand can affect the stability of the intermediate species formed during the substitution process. For example, a negatively charged ligand may stabilize a positively charged intermediate, leading to a faster reaction rate. Conversely, a positively charged ligand may destabilize the intermediate, slowing down the reaction.3. Electronic properties: The electronic properties of the incoming ligand, such as its donor strength and ability to form strong bonds with the central metal ion, can influence the reaction rate and selectivity. Strong-field ligands can promote the dissociative mechanism by stabilizing low-coordination intermediates, while weak-field ligands may favor the associative mechanism by forming weak bonds with the central metal ion.In summary, the mechanism of ligand substitution reactions in octahedral coordination complexes depends on the nature of the complex and the incoming ligand. The size, charge, and electronic properties of the incoming ligand can significantly influence the reaction rate and selectivity of the substitution process.