The coordination number of a transition metal complex refers to the number of ligands atoms, ions, or molecules that are directly bonded to the central metal ion. It plays a significant role in determining the stability, reactivity, and magnetic properties of the complex.1. Stability: Generally, a higher coordination number leads to a more stable complex. This is because the higher number of ligands results in a larger number of bonds, which in turn leads to a greater stabilization of the metal ion. Additionally, complexes with higher coordination numbers tend to have more symmetrical structures, which also contributes to their stability.2. Reactivity: The reactivity of a transition metal complex is influenced by the coordination number as well. Complexes with lower coordination numbers are usually more reactive, as the metal ion has more vacant coordination sites available for bonding with other ligands or reacting with other species. On the other hand, complexes with higher coordination numbers are generally less reactive, as the metal ion is already saturated with ligands, leaving fewer available sites for reactions.3. Magnetic properties: The magnetic properties of a transition metal complex depend on the number of unpaired electrons in the d-orbitals of the metal ion. As the coordination number varies, the arrangement of ligands around the metal ion can cause changes in the splitting of d-orbitals, which in turn affects the number of unpaired electrons and the magnetic properties of the complex.For example, in octahedral complexes coordination number 6 , the d-orbitals split into two sets with different energy levels: the lower-energy t2g set and the higher-energy eg set. If the complex has unpaired electrons in the eg set, it will exhibit paramagnetic behavior, meaning it will be attracted to an external magnetic field. On the other hand, if all the electrons are paired, the complex will be diamagnetic and will not be attracted to a magnetic field.In contrast, in tetrahedral complexes coordination number 4 , the d-orbitals split into two sets with different energy levels: the lower-energy eg set and the higher-energy t2g set. The magnetic properties of these complexes will also depend on the presence of unpaired electrons in the d-orbitals.In summary, the coordination number of a transition metal complex affects its stability, reactivity, and magnetic properties. Higher coordination numbers generally lead to more stable and less reactive complexes, while the magnetic properties depend on the arrangement of ligands and the presence of unpaired electrons in the d-orbitals.