In coordination chemistry, ligand substitution reactions involve the replacement of one ligand by another in a coordination complex. These reactions play a crucial role in understanding the reactivity and properties of coordination compounds. The mechanism of ligand substitution reactions can be classified into two main types: associative A and dissociative D mechanisms.1. Associative mechanism A : In this mechanism, the incoming ligand approaches the metal center and forms a bond with it before the departure of the original ligand. This results in the formation of an intermediate species with a higher coordination number. The intermediate then loses the original ligand, returning to the initial coordination number with the new ligand in place. This mechanism is more common in complexes with vacant coordination sites or those that can accommodate an additional ligand without significant steric hindrance.Example: The substitution of water by thiourea in hexaaquachromium III complex:[Cr H2O 6]3+ + SC NH2 2 [Cr H2O 5 SC NH2 2 ]2+ + H2O2. Dissociative mechanism D : In this mechanism, the original ligand departs from the metal center, creating a vacancy. The incoming ligand then binds to the metal center, filling the vacant site. This mechanism is more common in complexes with high coordination numbers or those that exhibit significant steric hindrance.Example: The substitution of water by chloride in hexaaquacopper II complex:[Cu H2O 6]2+ + Cl- [Cu H2O 5Cl]+ + H2OThe substitution of a ligand in a coordination complex can significantly affect its properties, such as stability, reactivity, color, and magnetic behavior. Some of the effects are:1. Change in stability: The substitution of a ligand can either increase or decrease the stability of a complex, depending on the strength of the metal-ligand bond. For example, replacing a weak-field ligand e.g., H2O with a strong-field ligand e.g., CN- can increase the stability of the complex.2. Change in reactivity: The reactivity of a complex can be influenced by the nature of the ligands. For example, substitution of a labile ligand easily replaced with an inert ligand difficult to replace can decrease the reactivity of the complex.3. Change in color: The color of a complex is determined by the energy difference between its d-orbitals, which can be affected by the ligand field strength. Substituting a ligand can change the ligand field strength, resulting in a change in color. For example, the substitution of water by ammonia in hexaaquacopper II complex changes its color from pale blue to deep blue:[Cu H2O 6]2+ + 4NH3 [Cu NH3 4 H2O 2]2+ + 4H2O4. Change in magnetic behavior: The magnetic properties of a complex are determined by the number of unpaired electrons in its d-orbitals. Substituting a ligand can change the electron configuration, resulting in a change in magnetic behavior. For example, replacing water with cyanide in hexaaquairon II complex changes its magnetic behavior from high-spin to low-spin:[Fe H2O 6]2+ + 6CN- [Fe CN 6]4- + 6H2OIn summary, ligand substitution reactions in coordination chemistry can occur via associative or dissociative mechanisms, and the substitution of a ligand can significantly affect the properties of a coordination complex, such as stability, reactivity, color, and magnetic behavior.