Stability of metal complexes-kns.pptx note

Chemistry

1. Stability of metal complexes Presented by G. PAVANI LECTURER IN CHEMISTRTY

2. CONTENTS

3. THERMODYNAMIC AND KINETIC STABILITY OF METAL COMPLEXES To define the stability of the complex compound formed in the solution, two types of the stability concept can be used which are given below Thermodynamic stability concept of the complexes When the stability of the complexes formed in the solution is defined by the thermodynamic parameters like bond energy, stability constant or formation constant then such type of the stability concept is called as thermodynamic stability concept of the complexes.

4. According to the thermodynamic stability concept, complex compound can be deluded into 2 different types Stable complexes: Those complexes which exhibit very high formation constant in the solution are known as stable complexes. Unstable complexes: Those complexes which exhibit low formation constant in the solution are known as unstable complexes.

5. Kinetic stability concept of the complexes When the stability of the complexes formed in the solution is defined by the kinetic parameter then the stability concept is called as kinetic stability concept of the complexes. According to the kinetic stability concept, complex compounds in the solution can be divided into two types Inert Complex: Those complexes which exhibit very low or negligible rate of replacement reaction in the solution are known as inert complexes . Labile Complex: Those complexes which exhibit very high rate of replacement reaction in the solution are known as labile complexes.

6. Relationship between thermodynamic and kinetic stability concept

7. Stepwise formation of the complexes and stepwise formation constant

8. Relationship between stepwise formation constant and overall formation constant To give the relationship between stepwise formation constants and overall formation constant, let us consider the formation of ML 3 complex by the stepwise formation method and overall formation method.  According to stepwise formation method:

9. Thus, from the above equation, it is observed that the product of stepwise formation constant is always equal to the overall formation constant for any particular complex. β 4 = K 1 x K 2 x K 3 x K 4

10. The equilibrium constant β of a reversible reaction shows the Gibb’s energy change ( Δ G) of that reaction. This indicates enthalpy change ( Δ H) and the entropy change( Δ S) RT ln β = - Δ G=- Δ H+T Δ S R= Ideal Gas Constant T= Kelvin Temperature β= Equilibrium constant or Formation Constant Δ G =Change in Gibbs free energy Δ H = Enthalpy change Δ S = Entropy change Relation Between Formation Constant and Thermodynamic Parameters

11. FACTORS AFFECTING THE STABILITY OF THE COMPLEXES There are two different factors which can affect the stability of complexes formed in the solution given as below: Nature of the central metal ion (CMI) Nature of the ligands

12. Nature of central metal atom (CMA) Charge on the CMA: Metal ion having high charge density forms stable complexes. Charge density means ratio of the charge to the radius of the ion. Thus, smaller the size and higher the charge of the metal ion, complexes are more stable. This is because a smaller, more highly charged ion allows closer and faster approach of the ligands and greater force of attraction results in to stable complex. In general, greater the charge on the central metal ion, greater is the stability of the complex.   Stability α +ve oxidation state of CMI

13. Electronegativity: This is another factor which determines stability of a complex. We can classify the metal ions in to two types: Class ‘a’ metals: These are electropositive metals and include the alkali metals, alkaline earth metals, most of the non-transition metals and those transition metals having only a few d-valence electrons (such as Sc, Ti , V). Such metals have relatively few electrons beyond an inert gas core.

14. Class ‘b’ metals: These are less electropositive heavy metals such as Rh, Pd, Ag, Ir , Pt, Au, Hg, Pb, etc. These have relatively large number of d electrons. Class ‘a’ metals, which attract electrons weakly, form most stable complexes with the ligands having electronegative atom such as nitrogen, oxygen and fluorine. Class ‘b’ metals form most stable complexes with π acceptor ligands containing P, S, As, Br and I. Stability of the complexes is increases with the increase in the electronegativity of CMA. Stability α Electronegativity

15. Polarizing power: With the increases in the polarizing power of CMA, stability of complexes also increases. Stability α Polarizing power of CMA

16. Nature of ligand Size and charge of the ligands: In general Ligands with less charge and more size are less stable and form less stable coordination compounds. Ligands with higher charge have small size and form more stable compounds. With the increase in the – ve charge value of the ligand stability of complexes is increase i.e. Stability α –ve charge at the ligand For example: F - forms more stable complexes with Fe +3 than Cl - , Br - or I - . Thus, a small fluoride F - ion forms more stable Fe +3 complex as compared to the large Cl - ion. This is due to easy approach of the ligand towards metal ion. Similarly, a small di negative anion O 2- forms more stable complexes than does the large S -2 ion.

17. Basis character: Higher the basic character or strength of the ligand, higher will be the stability of coordination compounds. It is defined that a strong base or higher basic strength of the ligand means it forms more stable compounds or its donating tendency of electrons to central metal ion is higher.   Stability α Basis character of ligand For example: Aromatic diamines form unstable coordination compounds while aliphatic diamines form stable coordination compounds. Ligands like NH 3 , CN- etc. have more basic character and thus, they form more stable compounds.

18. Chelate effect: The term chelate effect is used to describe special stability associated with complexes containing chelate ring when compared to the stability of related complexes with monodentate ligands. The chelate effect can be seen by comparing the reaction of a chelating ligand and a metal ion with the corresponding reaction involving comparable monodentate ligands. We observed that complexes formed by chelating ligands such as ethylene diamine ( en ), ethylene diamine tetra acetic acid (EDTA), etc. are more stable than those formed by monodentate ligands such as H 2 O or NH 3 . This enhanced stability of complexes containing chelating ligands is called chelate effect.

19. Steric effect: Complexes containing less sterically hindered ligands have more stability than the complexes having satirically hindered ligand (this factor remains dominated over the basic character of the ligand concept). For example, NH 2 CH 2 CH 2 NH 2 ethylene diamine ( en ) forms more stable complexes than its substituted derivative (CH 3 ) 2 N CH 2 CH 2 (CH 3 ) 2 N.24

20. DETERMINATION OF COMPOSITION OF METAL COMPLEXES (JOB’S METHOD) The chemical composition of metal complexes is determined by Spectrophotometric methods. Among the spectrophotometric methods in use, Job's continuous variation method and mole-ratio method are very widely used. We shall learn about these methods now.

21. JOB’S METHOD Let a metal M react with ligand 'L' to form a complex as shown below. Here 'n' denotes the number of moles of ligand that binds with one mole of metal ion. The value of this is to be determined experimentally. In this method of Job, equimolar solutions of the metal ion and the ligand solution are prepared separately. These solutions are mixed as shown below and the mixed solutions are prepared. The total volume is constant (10mL). The necessary buffer solution of required pH must also be added in constant volume. M + nL ML n

22. Metal ion Solution (mL) 1 2 3 4 5 6 7 8 9 10 Ligand solution (mL) 10 9 8 7 6 5 4 3 2 1 In all the above mixed solutions the total number of mole is same. If the concentration of the mixed solution is 'C'. C = C M + C L C M =concentration of metal ion solution C L =concentration of ligand solution The mole fraction of ligand= C L /C = x The mole fraction of metal ion = C M /C = 1-x If the formula of the complex is ML n n= × =  

23. The wavelength at which the metal complex formed exhibits maximum absorbance must be first experimentally established. At this wavelength ( λ max ) the absorbance (A) of each of the mixed solution is measured with a spectrophotometer. A graph is drawn between the absorbance (A) and the mole fraction of the ligand.

24. The mole fraction of ligand corresponding to this maximum point is identified and it denotes x. If more than one complex is formed under the experimental conditions, this method may not be suitable.   A curve shown in the figure is obtained. The mole fraction of the ligand corresponding to the point of maximum absorbance is obtained from the graph by extrapolation method.

25. Successful Testing

26. Contact Us [email protected]