Ligand substitution reactions

Classification Of Mechanisms, Ligand Substitution In Octahedral Complexes Without Breaking Metal-ligand Bond, Substitution Reaction In Square Planar Complexes, Factors Which Affect The Rate Of Substitution, Trans Effect (Labilizing Effect), Theories and applications Of Trans Effect

1. Ligand substitution reactions Pallavi Kumbhar MSc Part 1

2. CONTENTS Introduction Classification Of Mechanisms Ligand Substitution In Octahedral Complexes Without Breaking Metal-ligand Bond Substitution Reaction In Square Planar Complexes Factors Which Affect The Rate Of Substitution Trans Effect ( Labilizing Effect) T heories Of Trans Effect Applications Of Trans Effect

3. INTRODUCTION One of the most general reactions observed in coordination compounds is substitution, or replacement of one ligand by another. Ligand substitution reaction involves the exchange of one ligand from another with no change in oxidation state at the metal center. [ ML n X ] + Y = [ ML n Y ] + X X is the leaving group and Y is the entering group. The mechanism of a ligand substitution reaction is the sequence of elementary steps by which the reaction takes place. Dissociative (D) Associative (A) Interchange (I)

4. CLASSIFICATION OF MECHANISMS  DISSOCIATIVE REACTION MECHANISM: [ ML n X ] [ ML n ] [ ML n Y ] It is a two-step reaction F ormation of an intermediate C oordination number of the intermediate is lower than that in the starting complex This reaction corresponds to SN1 mechanism for organic compounds -X Slow +Y Fast 2) ASSOCIATIVE REACTION MECHANISM: [ ML n X ] [ ML n XY ] [ ML n Y ] It is a two-step reaction F ormation of an intermediate C oordination number of the intermediate is higher than that in the starting complex  +Y Slow -X Fast

5. 3) INTERCHANGE REACTION MECHANISM: [ ML n X ] + [Y] [Y-- ML n -- X] [ ML n Y ] Simultaneous bond formation between the metal and entering group occurs with bond cleavage between the metal and the leaving group. The reaction corresponds to SN2 reaction in organic chemistry N o intermediate is formed here In case of substitution reactions in 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 [ML 6 ], suggesting an associative mechanism.

6. 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. FOR EXAMPLE: The rapid conversion of carbonate ammine cobalt(III) complexes into the corresponding aquo complexes by the addition of excess acid. [(NH 3 ) 5 -Co-O-CO 2 ] + + 2H 3 18 O [(NH 3 ) 5 -Co-OH 2 ] 3+ + CO 2 + 2H 2 18 O MECHANISM: When the reaction is performed with 18 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.

7. SUBSTITUTION REACTION IN SQUARE PLANAR COMPLEXES Usually transition metals with d 8 configuration show square planar geometry. Eg : Ni 2+ , Pt 2+ , Rh + , Ir + , Au 3 + Follows associative mechanism Pt 2+ substitution reaction follows retention in configuration. Geometry of intermediate- Trigonal Bipyramidal. MECHANISM:

8. FACTORS AFFECTING THE RATE OF SUBSTITUTION 1) Natur e Of Entering Ligand The rate of substitution is proportional to the nucleophilicity of entering group. Soft ligands are more effective nucleophile for soft complexes and vice versa. The ordering is consistent with Pt(II) being a soft metal center. For Pt(II) complexes, more polarizable or soft ligands are good nucleophile. Therefore, increasing order of nucleophilicities of incoming ligand for Pt(II) complexes is: OH - < H 2 O < Cl - < NH 3 < alkenes < C 6 H 5 NH 2 < Br - < NO 2 < N 3 <SCN - < I - < R 3 P Nature Of Entering Ligand Nature Of Leaving Group Nature Of Solvent Trans Effect

9. 2) Nature Of Leaving Group In dissociative mechanism the bond between metal and leaving group breaks in transition state. Therefore these reaction depends upon leaving group. In associative mechanism, the leaving group effect depends upon the degree of bond breaking in the transition state [Pt( dien )X] + + py [Pt( dien )( py )] 2+ + X - The order of rate of reaction as per leaving group (X): CN - < NO 2 < SCN - < N 3 - < I - < Br - < Cl - < H 2 O < NO 3 - Nature Of Solvent In the solvent path mechanism of substitution, direct displacement of X- takes place with water in the slow step. If the solvent has more tendency to coordinate with the metal ion, the contribution of the solvent path mechanism will increase in the overall rate of reaction. Trans-[Pt( py ) 2 Cl 2 ] + 36 Cl - DMSO, H 2 O, CH 3 OH, ROH : Good coordinating solvents, the rate of substitution does not depend upon concentration of Cl. CCl4 and C 6 H 6 : Poor coordinating slovents , the rate of substitution depends upon the concentration of Cl.

10. TRANS EFFECT (LABILIZING EFFECT) The trans effect is 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. By comparing a large number of reaction rates, Langford and Grey set up a trans directing series. H 2 O < OH - < F - ≈ RNH 2 ≈ py ≈ NH 3 < Cl - < Br - < SCN - ≈ I - ≈ NO 2 - ≈ C 6 H 5 - < SC(NH 2 ) 2 ≈ CH 3 - < NO ≈ H - ≈ PR 3 < C 2 H 4 ≈ CN - ≈ CO For example, consider the substitution reaction in Pt (II) square-planar complex [Pt(NO 2 )Cl 3 ] 2- with NH3 to yield [Pt(NO 2 ) Cl 2 (NH 3 )] - Theoretically, two reaction products are possible which as shown.

11. 1) ELECTROSTATIC POLARISATION THEORY It was given by A.A. Grinberg. According to polarization theory, the ligand, by electrostatic effects, weakens the bond which is trans to it and facilitates substitution in that position. This theory applies only in case of σ -donor ligands such as halogens. It is not applicable in case of Π donor ligands. For example, in case of [PtLX 3 ] complex, L is the leaving group and X 1 = X 2 = X 3 and X is more polarized than L . THEORIES OF TRANS EFFECT: In order to explain the trans effect, several theories have been proposed. Some of them are as listed : Electrostatic Polarisation Theory Π Bonding Theory Pt X 1 L X 2 X 3 Pt 2+ L - X 2 + + + - - - + + + - - -

12. 2) Π BONDING THEORY This theory was given by Chat in 1955 and Orgel in 1956. According to this theory vacant Π and Π * orbitals of the metal ( p yz and p xz ) forms M-L Π bond. In these cases a reduction of electron density on the metal , as a result of back donation donation from M- L. The formation of Π - bond in the complex increases the electron density in the direction of L and diminishes it in the direction of the ligand, X trans to L. Ligands which show Π -bonding trans effect are PR 3 , NO, CO, C 2 H 4 . Ligands which follow Π -bonding theory have more difference in energy (activation energy) between ground and transition state.

13. APPLICATIONS OF TRANS EFFECT It is prepared by substituting the two chloro groups of [PtCl 4 ] 2- by ammonia molecules. In the first step, any of the chloro group is substituted by ammonia randomly. But in the second step, the ammonia group preferentially substitutes the chloro group cis to the first ammonia. Synthesis of cis-platin complex C is-platin is an anti- tumour medication which is frequently used to treat ovarian and testicular cancer. Cis-platin could be made from treatment of tetraammineplatinum(II) with chloride salts , but this synthesis will result in the formation of trans-platin, a compound that has all of the nasty side effects of the cis isomer but with none of the therapeutic benefit.

14. REFRENCES Inorganic Chemistry (4th ed.). Shriver, D. F.; Atkins, P. W. (2001) James E.Huheey M. L. Tobe and J. Burgess, Inorganic Reaction Mechanism, Longman, 1999. chem.libretexts.org/Bookshelves/ Inorganic_Chemistry Google