Complexes that undergo complete ligand exchange within 1 minute at 25 C are labile. Henry Taube ( ), Nobel laureate of 1983

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1 Complexes that undergo complete ligand exchange within 1 minute at 25 C are labile. Henry Taube ( ), Nobel laureate of 1983

2 Which d n configuration should provide inert octahedral complexes? Inert Complexes d 3 & d 8, low spin d 4 -d 6

3 Limiting Reaction Mechanisms for Ligand Substitution Reactions Reaction Profile D = Dissociative ~ S N 1 From C.N. = 6 to C.N. =5 k -1, k 2 E k 1 ML 5 +X + Y k 1 << k -1 < k 2 ΔE (E a ) ML 5 X + Y ML 5 Y +X ΔG Reaction coordinate What are the kinetics of reactions proceeding via the D mechanism?

4 Kinetics of reactions proceeding via the D mechanism k 1 << k -1 < k 2 Reaction Profile k -1, k 2 k 1 ML 5 +X + Y But: k -1 [X] << k 2 [Y], because k -1 < k 2 and (usually) [X] << [Y]. This leads to: d[ml 5 Y]/dt = k 1 [ML 5 X] ML 5 X + Y ML 5 Y +X The reaction is 1 st order in substrate and hence, the rate of substitution should be independent of Y.

5 A simple case of a reaction proceeding via the D mechanism The rate is independent on the identity of L, the entering ligand

6 Another case of a reaction proceeding via the D mechanism The rate is dependent on the identity of L, the leaving ligand

7 Kinetics of the two limiting reaction mechanisms for Ligand Substitution Reactions in octahedral complexes (r = dp/dt) Dissociative (D) ML 5 X k 1 slow ML 5 + X +Y k 2 fast ML 5 Y r = k 1 [ML 5 X] Associative (A) ML 5 X + Y k 1 slow ML 5 XY -X k 2 fast ML 5 Y r = k 1 [ML 5 X][Y]

8 A reaction proceeding via the A mechanism The identity of entering ligand influences the rate very much ΔS # is negative, consistent with expectation for 2 molecules forming 1 new complex

Summary of of ligand substitution (exchange) reaction mechanisms in octahedral complexes ML n X + Y ML n Y + X ML n X Dissociative (D) ML n Y ML n X Associative (A) ML n Y X ML n Y Y ML n XY X I d if

9 Summary of of ligand substitution (exchange) reaction mechanisms in octahedral complexes ML n X + Y ML n Y + X ML n X Dissociative (D) ML n Y ML n X Associative (A) ML n Y X ML n Y Y ML n XY X I d if dissociation is more important I a if association is more important Y assists the leaving ligand (X) Dissociative Interchange (I D ) Y strongly begins bond formation before X leaves Associative Interchange (I A )

10 Substitution reactions in square-planar complexes and the trans effect Trans effect: The ability of a ligand (T) to labilize a ligand trans to it (X) T = a trans-directing ligand The trans effect series

11 Synthetically useful consequences of the trans effect H 3 N NH 3 H 3 N NH NH 3 H 3 N NH 3 Trans only!! 2-2 NH NH 3 NH 3 Cis only!!

12 Synthetically useful consequences of the trans effect H 3 N NH 3 H 3 N NH NH 3 H 3 N NH 3 H 3 N NH 3 H 3 N NH NH NH 3 NH NH 3 NH 3 The trans effect series

13 The trans effect series and the spectrochemical series Task 1 Note similarities and differences between the two series

14 Task 2 Let s try understanding the trans effect by taking a closer look at a conceivable reaction mechanism Note 1: the energies of square pyramidal and trigonal bipyramidal complexes are very similar and they interconvert very fast. Note 2: π - accepting ligands prefer equatorial positions in trigonal bipyramidal complexes

15 Task 2 Note: The barrier for tbp to spy is very small

16 Task 1 + Task 2 σ- donation by T weakens the M-X bond, hence σ - donor strength: π - acceptors prefer equatorial positions in bpy complexes, hence π - acceptor series:

17 Conclusion Both σ- donors and π-acceptors are expected to be strong trans-directing ligands

18 Both σ- donors and π-acceptors are expected to be strong trans-directing ligands Important notes: a) Information about σ-donation is obtained from isolated complexes. b) Information about π-acceptance is obtained from isolated complexes. c) Information about preferred occupation in tbp complexes is obtained from isolated complexes. d) Conclusion: Information for a-c is acquired from thermodynamic data. But, information regarding trans-directing ligands is obtained from kinetic data. Remember that the transeffect is defined as The ability of a ligand (T) to labilize a ligand trans to it (X) Conclusion: a) All information acquired about the effect of ligands on thermodynamic properties of other ligands trans to them is called the trans-influence. b) Information acquired about the effect of ligands on kinetic properties of ligands trans to them is called the trans-effect.

Kinetics and reaction mechanisms in coordination compounds

Kinetics and reaction mechanisms in coordination compounds Square planar complexes Three pathways by which one ligand can replace another Nucleophilic substitutions in square planar complexes (with Y being