![]() (3) that the higher the value of K f, the greater will be the stability of the complex formed. Since the solution is dilute, the water molecules which enter the bulk solution do not have much influence on the equilibrium constant. In the above equation, the concentration of water is not included. When a ligand replaces water molecule from aqua complex ion, a new metal complex is formed and equilibrium is established as shown: In aqueous solution, a metal cation gets hydrated to give aqua complex of the type n+. ![]() In general, the metal complexes are not prepared from their corresponding starting materials in gaseous phase but are prepared in aqueous solution. The formation constant is also known as stability constant, which is the equilibrium constant obtained for the formation metal complex. The thermodynamic stability of complexes is represented by formation constant. In other words, thermodynamic stability of complexes is the measure of tendency of a metal ion to selectively form a specific metal complex and is directly related to the metal-ligand bond energies. It determines the extent to which the complex will be formed or be converted into another complex at the point of equilibrium. ![]() Thermodynamic stability of a complex refers to its tendency to exist under equilibrium conditions. Stability of complex compound is assigned to be its existence in aqueous solution with respect to its bond dissociation energy, Gibbs free energy, standard electrode potential, pH of the solution, and rate constant or activation energy for substitution reactions. ![]() For example, the system may not have sufficient energy available to break a strong bond, although once the existing bond is broken it could be replaced by new bond which is stronger than the older one. On the other hand, the complex is said to possess kinetic stability if it reacts with water to form a stable product and there is a known mechanism through which the reaction can proceed. Nevertheless, a metal complex is said to be stable if it does not react with water, which would lead to a decrease in the free energy of the system, i.e., thermodynamic stability. The stability of metal complexes can be explained with the help of two different aspects, namely, thermodynamic stability and kinetic stability. The term stability of metal complex cannot be generalized since the complex may be stable to one reagent/condition and may decompose in presence of another reagent/condition. The stability of metal complex generally means that it exists under favorable conditions without undergoing decomposition and has a considerable shelf life period. ![]()
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