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Ligand substitution reactions

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Description, Mechanism, Types of Ligand Substitution reactions in Octahedral and Square Planar Complexes

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Ligand substitution reactions

  1. 1. Ligand Substitution Reactions (PAPER II- INORGANIC) -Jaiswal Priyanka Balister MSc - I
  2. 2. Contents 1. Introduction 2. The classification of mechanisms 3. Ligand substitution in octahedral complexes • The evidence for dissociative mechanisms • Ligand substitution in Octahedral complexes without breaking metal- ligand bond 4. Ligand substitution in square planar complexes • Associative Mechanism at D4h Centers • Factors Which Affect The Rate Of Substitution 5. Trans effect 6. Theories of trans effect • Electrostatic Polarisation Theory • Π Bonding Theory • Molecular Orbital Theory for σ & π trans effects 7. Application of trans effect 8. Bibliography
  3. 3. Introduction  One of the most general reactions exhibited by coordination compounds is that of substitution, or replacement, of one ligand by another.  Ligand substitution involves the exchange of one ligand for another, with no change in oxidation state at the metal center [MLxX] + Y = [MLxY] + X X is the leaving group and Y is the entering group.  Metal complexes that undergo substitution reactions with t1/2= 1 min. at 25 C are called kinetically labile. If t1/2 > 1 min., the complex is kinetically inert (H. Taube).  Examples: Cr(III) complexes are generally inert. Cu(II) complexes are generally very labile.
  4. 4. The classification of mechanisms •The mechanism of a ligand substitution reaction is the sequence of elementary steps by which the reaction takes. • Types of Substitution Mechanisms: 1)Dissociative (D) 2)Associative (A) 3)Interchange (I) 1. Dissociative Reaction Mechanism MLxX = MLx + X MLx + Y = MLxY  two-step pathway  formation of an intermediate  coordination number of the intermediate is lower than that in the starting complex  corresponds to SN1 mechanism for organic compounds
  5. 5. The classification of mechanisms 2. Associative Reaction Mechanism MLxX + Y = MLxXY MLxXY = MLxY + X  two-step pathway  formation of an intermediate  coordination number of the intermediate is higher than that in the starting complex 3. Interchange Reaction Mechanism MLxX + Y = Y--MLx--X = MLxY + X  Bond formation between the metal and entering group is concurrent with bond cleavage between the metal and the leaving group.  corresponds to SN2 reaction in organic chemistry  no intermediate
  6. 6. Ligand substitution in octahedral complexes ML6 + Y = ML5Y + L • For substitution reactions of octahedral metal complexes the following is very often observed: At high concentration of Y, the rate is independent of [Y], suggesting a dissociative mechanism. At low concentrations of Y, the rate depends on [Y] and [ML6], suggesting an associative mechanism.
  7. 7. The evidence for dissociative mechanisms a) The rate of reaction changes only slightly with changes in the incoming ligand. If dissociation is r.d.s., the entering group should have no effect at all on the reaction rate. b) Decreasing negative charge or increasing positive charge on the reactant complex decreases the rate of substitution. Larger electrostatic attraction between the positive metal ion & the negative ligand should slow the dissociation. c) Steric crowding on the reactant complex increases the rate of ligand dissociation. When ligands on the reactant are crowded, loss of one of the ligands is made easier. d) Activation energies & entropies are consistent with dissociation.
  8. 8. Ligand substitution in Octahedral complexes without breaking metal-ligand bond • In these substitution reactions, ligand exchange takes place without the cleavage of metal-ligand bond. • The metal-ligand bond is preserved. • Example – The rapid conversion of carbonate ammine cobalt(III) complexes into the corresponding aquo complexes by the addition of excess acid. • When the reaction is performed with O labelled water, it is found that it is the O-O bond that breaks, rather than Co-O bond. • The observation that reactions involving fission of Co- O bonds are invariably slow, confirms this result. 18
  9. 9. Mechanism • Kinetic studies show the reacting species is the bicarbonato complex [Co(NH ) CO H]. • It is formed by the attack of proton on oxygen atom bonded to Co. • Decarboxylation of this bicarbonate complex then occurs as the rate determining step. • This step is then followed by protonation to give the aquo complex. 3 35 2+
  10. 10.  Ligand substitution in square planar complexes • ML3X + Y ML3Y + X Initial attack by the entering group at a square planar Pt(II) centre is from above or below the plane. Nucleophile Y then coordinates to give a trigonal bipyramidal intermediate species which loses X with retention of stereochemistry). Mechanism :
  11. 11. Associative Mechanism at D4h Centers ML3X + Y ML3Y + X [PtCl4]2- + NH3 [PtCl3(NH3)]- + Cl- The incoming ligand (colored blue) approaches a vacant axial site of the square planar complex to form a square pyramidal intermediate (or transition state). Intramolecular rearrangement via a trigonal bipyramid generates a different square pyramidal structure with the incoming ligand now in the basal plane. (This motion is closely related to Berry Pseudorotation). The reaction is completed by the leaving group departing from an axial site with the stereochemistry being retained during the substitution process.
  12. 12. Factors Which Affect The Rate Of Substitution 1) Role of the entering group 2) Role of the leaving group 3) Effect of the Metal Center 4) The trans effect Role of the Entering Group: • The rate of substitution is proportional to the nucleophilicity of entering group i.e. for most reactions of Pt(II), the rate constant increases in the order : H2O <NH3 = py < Br- < I- < CN- • The ordering is consistent with Pt(II) being a soft metal center.
  13. 13. • [Pt(dien)X]+ + py [Pt(dien)(py)]+ + X- • In H2O at 25o C the sequence of lability is : H2O > Cl- >Br - > I- > N3 - > SCN- > NO2 - > CN- a spread of over 106 in rate across series. The Role Of The Leaving Group:
  14. 14. Effect of the Metal Center • The order of reactivity of a series of isovalent ions is: Ni(II) > Pd(II) >> Pt(II) • This order of reactivity is the same order as the tendency to form 5- coordinate complexes. • More readily is the formation of a 5-coordinate intermediate complex, the greater is the stabilization of the transition state and so the greater is the bimolecular rate enhancement. M (II) Ni k = 33 M-1 sec-1 Pd k = 0.58 M-1 sec-1 Pt k = 6.7 x 10-6 M-1 sec-1
  15. 15. Trans effect • The trans effect is best defined as the effect of a coordinated ligand upon the rate of substitution of ligands opposite to it. • Or The ability of a ligand in a square planar complex to direct the replacement if the ligand trans to it. • It was recognized by Werner (1893) and elaborated by Chernuyaev (1926). • By comparing a large number of reaction rates Langford and Grey set up a trans directing series. • In the trans-directing series, the ligands are arranged in the increasing order of trans effect as follows: H2O < OH- < F- ≈ RNH2 ≈ py ≈ NH3< Cl- < Br-< SCN- ≈ I- ≈ NO2- ≈ C6H5- < SC(NH2)2≈ CH3-< NO ≈ H- ≈ PR3 < C2H4 ≈ CN- ≈ CO
  16. 16. Theories of trans effect • In order to explain the trans effect, several theories have been proposed. Some of them are as listed : 1. Electrostatic Polarisation Theory 2. Π Bonding Theory 3. Molecular Orbital Theory for σ & π trans effects
  17. 17. 1. Electrostatic Polarisation Theory • This theory is primarily concerned with effect in the ground state. • It was given by A.A. Grinberg. • According to polarization theory, the ligand, by electrostatic effects, weakens the bond trans to it and facilitates substitution in that position. • Support for this theory is demonstrated by looking at the trans directing series. CN- > NO2 - > I- = SCN- > Br- > Cl- > py > NH3 > H2O • The more polarizable ligands such as SCN-, and I- and the ligands containing π-clouds e.g. CN- are high in the series.. • Less polarizable ligands such as ammonia or water are lower in the series. • Additional support comes from the observation that Pt(II) complexes demonstrate a more pronounced trans effect than those of the less polarizable Pd(II) and Ni(II) cations.
  18. 18. Electrostatic Polarisation Theory • It explains the kinetic trans effect in square planar Pt(II) complexes.
  19. 19. 2. Π Bonding Theory • In the trigonal plane of the 5-coordinate transition state or intermediate, a π-bonding interaction can occur between a metal d-orbital (e.g. dxy) and suitable orbitals (p atomic orbitals, or molecular orbitals of π-symmetry) of ligand L2(the ligand trans to the leaving group) and Y (the entering group). • These 3 ligands and the metal center can communicate electronically through π-bonding only if they all lie in the same plane in the transition state or intermediate. • In other words, strong trans directing ligands are those capable of forming strong back bonding (π-acceptors), for example, CO, CN-, olefins etc. or strong σ-donors such as CH3-, H-, PR3 etc. Strong π-donors such as H2O, OH-, NH2-, etc. are weak in their trans directing properties.
  20. 20. 3. Molecular Orbital Theory for σ & π trans effects • The dx2-y2 orbital of Pt is involved in σ bonding in the square-planar geometry, & both the dxz & dyz orbitals can contribute to π bonding in the TBP transition state (5-coordinate species ). • Any feature that stabilizes the TBP lowers the activation energy barrier & speeds the reaction. • TBP intermediate is stabilized by π-bonding. •σ-bonding effects (ground-state effects) : The Pt-X bond is influenced by the Pt-T bond, because both use the same Pt px & d2-y2 orbitals. When the Pt-T σ bond is strong, it uses a larger part of these orbitals & leave less for the Pt-X bond (weaker & higher in energy at its ground state; lower Ea).
  21. 21. Application of trans effect 1. Synthesis of isomers of Pt(NH3)2Cl2
  22. 22. 2. Preparation of cis and trans PtCl2I(py) from PtCl4 2- , I- and py.
  23. 23. Bibliography 1. Inorganic Chemistry (4th ed.). Shriver, D. F.; Atkins, P. W. (2001). Oxford University Press 2. Inorganic Chemistry (3rd ed.). Miessler, G. L.; Tarr, D. A. (2003). Pearson Prentice Hall 3. Chemical Principles (5th ed.). Zumdahl, Steven S (2005). Houghton Mifflin Company 4. Inorganic Chemistry: Principles of Structure and Reactivity, James E.Huheey, Ellen A.Keiter, Richard L.Keiter, Okhil K.Medhi, Pearson Education, Delhi, 2006 5. Mechanisms of Inorganic Reactions : F. Basolo and R. Pearson,, London, 1958. 6. Inorganic Chemistry ,By William W. Porterfield 7. Advances in Inorganic Chemistry, Volume 34, By A. G. Sykes
  24. 24. Thank You..!!!

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