The degenerate d-orbitals in a spherical field environment split into two levels i. The splitting of the degenerate levels due to the presence of ligands is called the crystal-field splitting while the energy difference between the two levels eg and t2g is called the crystal-field splitting energy. Ligands which produce this effect are known as strong field ligands and form low spin complexes.
Chapter Chosen Coordination Compounds. Book Chosen Chemistry I. Subject Chosen Chemistry. Book Store Download books and chapters from book store. Currently only available for. Class 10 Class Coordination Compounds. Other complexes have a much more transient existence and may exist only in solution or be highly reactive and easily converted to other species. All metals form complexes, although the extent of formation and nature of these depend very largely on the electronic structure of the metal.
The concept of a metal complex originated in the work of Alfred Werner , who in was awarded the first Nobel Prize in Inorganic chemistry. A description of his life and the influence his work played in the development of coordination chemistry is given by G.
Complexes may be non-ionic neutral or cationic or anionic, depending on the charges carried by the central metal ion and the coordinated groups. The total number of points of attachment to the central element is termed the coordination number and this can vary from 2 to greater than 12, but is usually 6. The term ligand ligare [Latin], to bind was first used by Alfred Stock in in relation to silicon chemistry.
The first use of the term in a British journal was by H. Irving and R. Williams in Nature , , , in their paper describing what is now called the Irving-Williams series. For a fascinating review of the origin and dissemination of the term 'ligand' in chemistry see: W. If the enthalpy changes are similar, what causes the difference in the extent to which the two reactions happen? Entropy is most easily thought of as a measure of disorder. Any change which increases the amount of disorder increases the tendency of a reaction to happen.
Note: If you want a more formal approach to entropy, I'm afraid you will have to look elsewhere. You will find a gentle introduction to the mathematical side of entropy in my chemistry calculations book. If you look again at the two equililbria, you might notice that the 1,2-diaminoethane equilibrium does lead to an increase in the disorder of the system an increase in its entropy.
There are only two species on the left-hand side of the equation, but three on the right. Compare that with the other equilibrium. In this case, there is no change in the total number of species before and after reaction, and so no useful contribution to an increase in entropy. Here, we are increasing the number of species present from two on the left-hand side to seven on the right. You can get a major amount of increase in disorder by making this change. Reversing this last change is going to be far more difficult in entropy terms.
You would have to move from a highly disordered state to a much more ordered one. That isn't so likely to happen, and so the copper-EDTA complex is very stable. Complexes involving multidentate ligands are more stable than those with only unidentate ligands in them. The underlying reason for this is that each multidentate ligand displaces more than one water molecule. This leads to an increase in the number of species present in the system, and therefore an increase in entropy.
If this is the first set of questions you have done, please read the introductory page before you start. What is a stability constant? This can be written as an equilibrium reaction to show the overall effect: In fact, the water molecules get replaced one at a time, and so this is made up of a series of part-reactions: Although this can look a bit daunting at first sight, all that is happening is that first you have one, then two, then three, then four water molecules in total replaced by ammonias.
Individual stability constants Let's take a closer look at the first of these equilibria: Like any other equilibrium, this one has an equilibrium constant, K c - except that in this case, we call it a stability constant. The stability of coordination complex is an important factor that decides the stability and reactivity of a metal complex.
The stability of metal complex is governed by two different aspects such as thermodynamic and kinetic stabilities. The correlation between stability and reactivity of coordination compounds has been described in this chapter. This chapter also enlists the factors influencing the stability of metal complexes such as the nature of metal ions, ligands, bonding between metal ions and ligands, etc. In addition, the methods available for the determination of stability constants are given in detail.
Stability and Applications of Coordination Compounds. The stability of metal complex generally means that it exists under favorable conditions without undergoing decomposition and has a considerable shelf life period [ 1 ].
The stability of metal complexes can be explained with the help of two different aspects, namely, thermodynamic stability and kinetic stability [ 2 ]. 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.
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.
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 [ 1 ]. 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.
Thermodynamic stability of a complex refers to its tendency to exist under equilibrium conditions. It determines the extent to which the complex will be formed or be converted into another complex at the point of equilibrium. 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. The thermodynamic stability of complexes is represented by formation constant.
The formation constant is also known as stability constant, which is the equilibrium constant obtained for the formation metal complex [ 1 , 2 ]. In general, the metal complexes are not prepared from their corresponding starting materials in gaseous phase but are prepared in aqueous solution. When a ligand replaces water molecule from aqua complex ion, a new metal complex is formed and equilibrium is established as shown:. For simplicity, the above reaction can be written in generalized form as given:.
The equilibrium constant K f of the reaction is given by:. In the above equation, the concentration of water is not included. Since the solution is dilute, the water molecules which enter the bulk solution do not have much influence on the equilibrium constant. It is observed from Eq. Thus, large value of K f indicates that the ligand L binds to the metal ion more strongly than H 2 O and hence L is a stronger ligand than H 2 O.
If K f is less than 1. Thus stability constant is used as a measure of thermodynamic stability of the complex. The steady decrease in the value of stepwise formation constants from K 1 to K n is due to: Increase in the number of ligands in coordination sphere that causes to decrease the number of H 2 O molecules to be replaced and thus the probability of replacement of water molecules decreased.
The change in electronic structure of the metal ion causes the variation in the crystal field stabilization energy CFSE. The complex with higher CFSE value will be stable, and the equilibrium constant for that complex formation will be high. From Eqs. The formation constant describes the formation of a complex from metal cation and ligands.
Bjerrum defined that the formation of a metal complex in aqueous solution takes place by replacing the water molecule by another ligand L [ 5 , 6 ]. It is assumed that this reaction does not occur in a single step but occurs in several steps, and each step is characterized by its individual equilibrium constant called as stepwise formation constant K. For example, consider the formation of a complex [ML n ] formed by the following reactions:. By assuming the value of activity coefficients as unity, the equilibrium constant K 1 for the complex ML having one ligand L will be given as.
The equilibrium constants K 1 , K 2 , …, K n are known as stepwise formation constants. On the other hand, the equilibrium constant for the overall reaction may be considered as. For example, consider the product of stepwise formation constants K 1 , K 2 , K 3 , …, K n.
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