The change in ligands can significantly affect the electronic and magnetic properties of transition metal complexes. This is mainly due to the interaction between the metal ion and the ligands, which can alter the energy levels and electron distribution in the complex. The strength and nature of this interaction depend on the type of ligand and its electronic properties.There are two primary ways in which ligands can affect the electronic properties of transition metal complexes:1. Crystal Field Theory CFT : In CFT, ligands are considered as point charges that interact with the d-orbitals of the metal ion. The interaction between the ligands and the metal ion causes the degeneracy of the d-orbitals to be lifted, resulting in a splitting of energy levels. The magnitude of this splitting depends on the nature of the ligands and their arrangement around the metal ion. Strong-field ligands, such as cyanide and carbon monoxide, cause a larger splitting, while weak-field ligands, such as water and halides, cause a smaller splitting. This splitting affects the electronic configuration of the complex and can lead to changes in its color, redox potential, and reactivity.2. Ligand Field Theory LFT : LFT is an extension of CFT that takes into account the covalent nature of the metal-ligand bond. In LFT, the interaction between the metal ion and the ligands is described by molecular orbitals formed by the overlap of the metal d-orbitals and the ligand orbitals. The nature and strength of these interactions depend on the symmetry and energy of the orbitals involved. As a result, the electronic properties of the complex are influenced by the type of ligands and their arrangement around the metal ion.The magnetic properties of transition metal complexes are determined by the number of unpaired electrons in the d-orbitals. The change in ligands can affect the magnetic properties by altering the electronic configuration of the complex. For example, strong-field ligands can cause a greater splitting of the d-orbitals, leading to a low-spin configuration with fewer unpaired electrons and lower magnetic properties. In contrast, weak-field ligands can result in a high-spin configuration with more unpaired electrons and higher magnetic properties.Quantum chemistry methods, such as density functional theory DFT and ab initio calculations, can be used to predict the changes in electronic and magnetic properties of transition metal complexes upon ligand substitution. These methods are based on the principles of quantum mechanics and involve solving the Schrödinger equation for the electrons in the complex. By calculating the energy levels and electron distribution in the complex, these methods can provide insights into the electronic structure, bonding, and magnetic properties of the complex. Moreover, they can help in understanding the relationship between the structure and properties of transition metal complexes, which is essential for designing new materials and catalysts with desired properties.