1. Inorganic Chemistry
Reaction Mechanismes

2. Inorganic Reaction Mechanisms
Introduction
Inorganic Reaction Mechanisms
Electron Transfer Reactions
Substitution Reactions of Tetrahedral Complexes
Case Study of an Inorganic Reaction Mechanism
Insertion Reactions
Substitution Reactions of Octahedral Complexes
Substitution Reactions of Square Planar Complexes
Introduction

3. EVERYONE SAY THIS OUT ALOUD TOGETHER
Introduction
EVERYONE SAY THIS OUT ALOUD TOGETHER
THE TRANS EFFECT
THE EFFECT OF A LIGAND, UPON THE RATE OF LIGAND REPLACEMENT, OF THE GROUP TRANS TO ITSELF

4. Introduction
The Kinetic Model

5. The Kinetic Model Introduction Free Energy TS 1 TS 2 Intermediate
Reaction Coordinate
Free
Energy
Reactants
TS 1
TS 2
Intermediate
Products
The intermediate is stable to some extent and can be detected using spectroscopy

6. Substitution Reactions General Classification of Reaction Pathways
Introduction
Substitution Reactions General Classification of Reaction Pathways

7. Two more advanced mechanisms
Three Processes
Introduction
Associative A Detectable Intermediate Interchange I Transition State Dissociative D Detectable Intermediate No intermediates are detected but it can determine the character of substitution
Two more advanced mechanisms
As Y attacks M, X has substantially dissociated from the coordination sphere
As Y attacks M, X remains bonded to M

8. Inorganic Reaction Mechanisms
Substitution Reactions of Square Planar Complexes
Inorganic Reaction Mechanisms
Electron Transfer Reactions
Substitution Reactions of Tetrahedral Complexes
Case Study of an Inorganic Reaction Mechanism
Insertion Reactions
Substitution Reactions of Octahedral Complexes
Substitution Reactions of Square Planar Complexes
Introduction

9. EVERYONE SAY THIS OUT ALOUD TOGETHER
Substitution Reactions of Square Planar Complexes
EVERYONE SAY THIS OUT ALOUD TOGETHER
THE TRANS EFFECT
THE EFFECT OF A LIGAND,
UPON THE RATE OF LIGAND REPLACEMENT,
OF THE GROUP TRANS TO ITSELF

10. Substitution Reactions Square Planar Complexes (IA)
Substitution Reactions of Square Planar Complexes
Substitution Reactions Square Planar Complexes (IA)

11. Substitution Reactions of Square Planar Complexes
Due to the associative mechanism
STEREOCHEMISTRY IS CONSERVED IN BOTH PATHWAYS

12. Substitution Reactions of Square Planar Complexes
Role of solvent
[Y] = NHMe2
kobs
Hexane
Methanol
The methanol can attack the metal centre due to the lone pair on oxygen
The concentration of methanol does not effect the observed rate
Hexane can not coordinate

13. Substitution Reactions of Square Planar Complexes
Some evidence supporting associative mechanism Stable Five-Coordinate Pt(II)
Shape?
Trigonal Bipyramidal
R = Me
Favours Cl- intermediates. As is more electropositive than N or P.

14. Evidence for solvent role
Substitution Reactions of Square Planar Complexes
Evidence for solvent role
Test to whether the solvent has a role in the substitution
In water, trap the intermediate using a base. Hydroxyl ligand is formed

15. Substitution Reactions of Square Planar Complexes
Activation Magnitudes for substitution of square planar complexes Pt/Pd(II)

16. The Entering Group Y, Nucleophile
Substitution Reactions of Square Planar Complexes
Factors which can influence the rate of substitution of square planar complexes
Nature of the:
Entering group Y
Leaving group X
Metal M
Spectator ligands L
Steric effect
The Trans Effect
The Entering Group Y, Nucleophile
Rates are strongly influenced by the nucleophile and increases along the series:
PR3 > I- > N3- > NO2- > Br- > Py > NH3 > Cl- > H2O > HO-
This order is similar to nucleophilicity in SN2 reactions

17. Substitution Reactions of Square Planar Complexes
A Nucleophilicity Scale
The Leaving Group X
In all substitution mechanisms, including predominately associative, the stronger the metal bond with the leaving group the slower the substitution reaction
For association reactions the effect of the leaving group depends on the extent of breaking in the transition state
In general, good entering groups (nucleophiles) are poor leaving groups:
NO3 > H2O > Cl > Br > I > SCN > NO2 > CN

18. Substitution Reactions of Square Planar Complexes
The Metal M
The greater the tendency for the metal to form a five coordinate complex, the greater the association of the transition state and hence the faster substitution reaction
Bimolecular rate enhancement ( Ni > Pd > Pt )
The Spectator Ligands L
Steric Hindrance
Bulky spectator ligands make it difficult for entering and leaving groups to move around the system
Substitution is increased when smaller, less bulky, spectator ligands are present

19. Substitution Reactions of Square Planar Complexes
The Spectator Ligands L
The Trans Effect
THE EFFECT OF A LIGAND, UPON THE RATE OF LIGAND REPLACEMENT, OF THE GROUP TRANS TO ITSELF
Not to be confused with trans influence which is a ground state effect phenomenon concerned primarily with bond length changes
Termed a LABILIISING effect, emphasising kinetic nature
Different trans ligands T exert different labialising influence on X
The kinetic effect has a practical consequence in controlling the stereochemistry if substitution products at square planar complexes
Chloride is a stronger trans-labialising ligand than amine

20. Substitution Reactions of Square Planar Complexes
Weaker Trans-Labile Ligand
Stronger Trans-Labile Ligand
F- , H2O , -OH
NH3 , py
Cl-
Br-
I- , SCN- , NO2- , SC(NH2)2 , Ph-
SO32-
PR3 , AsR3 , SR2 , CH3-
H- , NO , CO , CN- , C2H4

21. Substitution Reactions of Square Planar Complexes
The Spectator Ligands L
The Trans Effect
Destabilisation of Ground State
T2 has a greater effect trans effect than T1 because it destabilises the ground state
H- & Cl- donates negative charge to metal centre which weakens the bond trans to itself.
Raises ground state energy, lowers the activation energy and makes the transition state more accessible
Stabilisation of the Transition State
T2 has a greater trans effect than T1 because it stabilises the transition state
CO can withdraw electron density on metal centre due to backbonding, so lowers the transition state energy

22. Substitution Reactions of Square Planar Complexes
Trans Influence
Substitution Reactions of Square Planar Complexes
Purely thermodynamic phenomenon
Ligands can influence the ground state properties of the groups to which they are trans
Such properties include
Metal-Ligand bond lengths
Vibrational frequency or force constants
NMR coupling constants
The trans influence series is based on structural data:
R- | H =PR3 CO | C=C | Cl- | NH3
Strong Effect Large Effect Little / No Effect
The Cis Effect
Much weaker than the Trans Effect

23. Substitution Reactions of Square Planar Complexes
Test Question
Define the trans effect. 10% Definition ‘The effect of a ligand upon the rate of ligand replacement of the group trans to itself, primarily refers to substitution at group VIII M(II) square planar complexes ’ A simple diagram would aid definition.

24. Substitution Reactions of Square Planar Complexes
Test Question
Hydride and carbon monoxide are both strong trans effect ligands, discuss how their trans effect operates %
Hydride lowers the activation energy for substitution at eg Pt(II) by destabilising the ground state, it places more electron density onto the metal centre, weakening the bond trans to it more so than does eg Cl-.
Carbon monoxide lowers the activation energy by stabilising the TS. It relieves the increased electron density brought in by the lone pair/negative charge on the incoming ligand by accepting this into pi* anti bonding orbitals.

25. Substitution Reactions of Square Planar Complexes
Test Question
Briefly discuss how the trans effect can provide a means of kinetically controlling the nature of the isomer formation in Pt(II) substitution chemistry. [10%] The selective formation of cis or trans isomers of [PtCl2(NH3)2] can controlled by either starting with [Pt (NH3)4]2+ and reacting with Cl- which gives exclusively the trans-product or by starting with [Pt Cl4]2- whch gives the cis- product. This is a consequence of Cl- being a greater trans effect than has NH3. Scheme as in notes would aid clarity.

26. Inorganic Reaction Mechanisms
Substitution Reactions of Octahedral Complexes
Inorganic Reaction Mechanisms
Electron Transfer Reactions
Substitution Reactions of Tetrahedral Complexes
Case Study of an Inorganic Reaction Mechanism
Insertion Reactions
Substitution Reactions of Octahedral Complexes
Substitution Reactions of Square Planar Complexes
Introduction

27. EVERYONE SAY THIS OUT ALOUD TOGETHER
Substitution Reactions of Octahedral Complexes
EVERYONE SAY THIS OUT ALOUD TOGETHER
THE TRANS EFFECT
THE EFFECT OF A LIGAND,
UPON THE RATE OF LIGAND REPLACEMENT,
OF THE GROUP TRANS TO ITSELF

28. Substitution Reactions Octahedral Substitution (ID)
Substitution Reactions of Octahedral Complexes
Substitution Reactions Octahedral Substitution (ID)
Rates similar to water exchange rates, suggests dissociation of water is important
Rates insensitive to nucleophilicity of Y
A or IA pathways unlikely
Rates sensitive to charge on anion
Ion-pair formation
Rates strongly dependent on nature of M
Normally D or ID mechanism

29. Substitution Reactions of Octahedral Complexes
The D Mechanism
Substitution Reactions of Octahedral Complexes
Two elementary steps:
Complex gains enough energy to break the M-X bond completely
Intermediate is attacked by incoming Y
The ID Mechanism
As the M-X bond starts to break, M begins to form a new bond with Y
The entering group Y must be present in the region around the complex when the M-X bond begins to lengthen
Thus before substitution occurs Y must enter the outer sphere
Ligands swap over in rate determining step before X leaves

30. Substitution Reactions of Octahedral Complexes
Factors which can influence the rate of substitution of octahedral complexes
Nature of the:
Entering group Y
Metal M
Leaving group X
Spectator ligands L
Electronic Effects
Jahn-Teller Effect
The Entering Group Y
Expect that rate of a dissociative substitution would be insensitive to the entering group Y
By varying the nature of the entering group the effect is much smaller
The Metal M
The relatively small 3d transition metals do not sterically easily accommodate 7 coordination, D or ID mechanisms dominate their substitution chemistry

31. Substitution Reactions of Octahedral Complexes
The Leaving Group X
Reactions proceeding via I or ID mechanisms are expected to be sensitive to the nature of the leaving group X since the M-X bond is to be cleaved
Experimental results for the aquation reaction below shows influence of X on rate can span 5 orders of magnitude
The strength of the M-X bond is crucial in the magnitude of the activation energy in the rate determining step
X NO3- I- Cl- F- N3- NCS- K
The Spectator Ligands L
In D or ID mechanism the coordination number is decreased in the transition state
If the metal centre is surrounded by bulky ligands then steric strain is released on attaining the transition state
Enhancement is then given on the substitution rate

32. Substitution Reactions of Octahedral Complexes
Electronic Effects
The five coordinate transition state in the dissociative mechanism can be stabilised if the remaining ligands can donate more electron density to the metal
This is possible if there is a π- donor group CIS to the leaving group
This does not happen if it is trans.
Jahn-Teller Effect
The most labile transition metal ions are Cu(II) and Cr(II) and this is due to Jahn-Teller distortion
Octahedral d9 and d4 high spin complexes are unstable because of the two long metal ligand bonds

33. Substitution Reactions of Octahedral Complexes
Base Hydrolysis
Replacement of ligand by HO-
Often faster than acid hydrolysis
Sensitive to nature of entering group
Only observed when deprotonatable group present
First step is removal of proton from acidic ligand
Dissociative Conjugate Base Mechanism (DCB)
Garrick Mechanism
Addition of charge makes it easier to remove the chloride

34. Substitution Reactions of Octahedral Complexes
Test Question
Briefly discuss the following. [15%]
(i) The influence of the leaving group in substitution reactions of
square planar complexes
first transition series octahedral complexes
A metal ligand bond begins breaking in the TBP TS therefore the stronger this bond the slower is the substitution reaction. For example NO3- is weakly bound, it is a labile ligand and its complexes Pt(II) are rapidly substituted whereas SCN- is tightly bound and its corresponding substitution reactions are very slow.
Again bond breaking takes place in the TS the stronger the M-X bond the slower is the substitution reaction. For example, [Co(NH)5X]2+ aquates 5 orders of magnitude faster for X = NO3- than for SCN-

35. Substitution Reactions of Octahedral Complexes
Test Question
Briefly discuss how the steric properties of spectator ligands can influence the rate of substitution of
(a) Octahedral
and (b) Square planar complexes [10%]
As TS becomes less crowded by departure of leaving group then steric strain relieved in the GS is relieved. This leads to a lower activation energy and faster substitution kinetics. An example would aid discussion.
The converse to above, higher steric strain in TS on going from 4 to 5 coordinate can raise activation energy decreasing substitution rate.

36. Substitution Reactions of Octahedral Complexes
Test Question
Discuss the mechanism of substitution of [Co(NH3)5Cl] + by hydroxide. What happens if another ligand such as SCN - is present when hydroxide is added to the aqueous solution of the complex?
Garrick Mechanism, Dissociative, conjugate base mechanism D-CB. A key point is that substitution shows first order dependence on complex and first-order on [OH]-concentrations, ie overall 2nd order kinetics. This is explained by hydroxide removing a proton from an ‘acidic’ NH3 ligand. This decreases the net charge on the complex making the electrostatic separation of Cl- from an 1+ charged complex easier (faster) than from the 2+ charged parent complex. Following Cl- dissociation hydroxide attacks the five coordinated intermediate. In the presence of SCN- competive quenching of this intermediate can occur giving both OH- and SCN- substitution products after the re-protonation step. A scheme as below would aid discussion.

37. Substitution Reactions of Octahedral Complexes

38. Substitution Reactions of Octahedral Complexes
Test Question
Substitution one dinitrogen ligand of trans-[Mo(N2)2(dppe)2)] by MeCN to give trans-[Mo(N2)MeCN)(dppe)2)] shows a first order dependence on the concentration of the complex and a zero order dependence on concentration of the incoming ligand under pseudo-first order conditions. Use the steady state approximation to derive an expression for the rate equation consistent with these observations and a mechanism involving loss of dinitrogen as the rds. Briefly explain why the rate of alkylation of dinitrogen by MeBr in the same complex to give a diazenide shows identical kinetics.25%

39. Substitution Reactions of Octahedral Complexes

40. Substitution Reactions of Octahedral Complexes
Since:
k2 [L] >> {k-1 [N2]
This reduces to :
dP / dt = k1[A] = - dA/dt
Thus k1 = kobserved
Substitution Reactions of Octahedral Complexes
Applying the steady-state approximation
dI /dt = 0
leads to an expression containing the concentration of I
k1 [A] = k-1 [I] [N2] + k2 [I][L]
Rearranging gives
[I] = k1 [A] / {k-1 [N2] + k2 [L]}
In terms of I the rate of formation of the product P is
dP /dt = k2 [I][L]
Substitute for [I] gives
dP /dt = k2 [L][k1[A] / {k-1 [N2] + k2 [L]}
= [k1 [A] / {k-1 [N2]/ k2 [L] + 1]}
The rate of reaction of the complex with MeBr shows the same kinetics because the rate determining step is the same, that is the loss of a dinitrogen ligand to give a five coordinate intermediate, the subsequent reaction of which with MeBr is fast.

41. Inorganic Reaction Mechanisms
Insertion Reactions
Inorganic Reaction Mechanisms
Electron Transfer Reactions
Substitution Reactions of Tetrahedral Complexes
Case Study of an Inorganic Reaction Mechanism
Insertion Reactions
Substitution Reactions of Octahedral Complexes
Substitution Reactions of Square Planar Complexes
Introduction

42. EVERYONE SAY THIS OUT ALOUD TOGETHER
Insertion Reactions
EVERYONE SAY THIS OUT ALOUD TOGETHER
THE TRANS EFFECT
THE EFFECT OF A LIGAND,
UPON THE RATE OF LIGAND REPLACEMENT,
OF THE GROUP TRANS TO ITSELF

43. Insertion Reaction (Migration)
Insertion Reactions
The entering group Y does not insert between the M-X bond but is bound CIS in the product
Generally dissociative, D, in character with the formation of the trigonal bipyramid intermediate as the rate determining step

44. Insertion Reactions How do they take place
The R group migrates towards the CO
Proceed with the retention of stereochemistry and accelerated by Lewis acids

45. Inorganic Reaction Mechanisms
Case Study of an Inorganic Reaction Mechanism
Inorganic Reaction Mechanisms
Electron Transfer Reactions
Substitution Reactions of Tetrahedral Complexes
Case Study of an Inorganic Reaction Mechanism
Insertion Reactions
Substitution Reactions of Octahedral Complexes
Substitution Reactions of Square Planar Complexes
Introduction

46. EVERYONE SAY THIS OUT ALOUD TOGETHER
Case Study of an Inorganic Reaction Mechanism
EVERYONE SAY THIS OUT ALOUD TOGETHER
THE TRANS EFFECT
THE EFFECT OF A LIGAND,
UPON THE RATE OF LIGAND REPLACEMENT,
OF THE GROUP TRANS TO ITSELF

47. Case Study of an Inorganic Reaction Mechanism

48. Apply Steady State Approximation
Case Study of an Inorganic Reaction Mechanism

49. Alkylation of dinitrogen ligand
Case Study of an Inorganic Reaction Mechanism
All proceed via the same rate of substitution

50. Alkylation of dinitrogen ligand – Mechanism
Case Study of an Inorganic Reaction Mechanism
MeBr coordinates and homolytically cleaves with formation of methyl radical
Attacks Mo(I) radical to form diazenide
Evidence : EPR

51. Inorganic Reaction Mechanisms
Substitution Reactions of Tetrahedral Complexes
Inorganic Reaction Mechanisms
Electron Transfer Reactions
Substitution Reactions of Tetrahedral Complexes
Case Study of an Inorganic Reaction Mechanism
Insertion Reactions
Substitution Reactions of Octahedral Complexes
Substitution Reactions of Square Planar Complexes
Introduction

52. EVERYONE SAY THIS OUT ALOUD TOGETHER
Substitution Reactions of Tetrahedral Complexes
EVERYONE SAY THIS OUT ALOUD TOGETHER
THE TRANS EFFECT
THE EFFECT OF A LIGAND,
UPON THE RATE OF LIGAND REPLACEMENT,
OF THE GROUP TRANS TO ITSELF

53. Substitution Reactions of Tetrahedral Complexes
Tetrahedral cyclopentadienyl complexes
Might expect dissociative mechanism
Experimentally 2nd order kinetics rate = k[Rhenium Complex]1[PMe3]1[CO]0
Ring Slippage
Now to apply the SSA again

54. Substitution Reactions of Tetrahedral Complexes

55. What if dissociative pathway?
Substitution Reactions of Tetrahedral Complexes

56. Tetrahedral Substitution
Substitution Reactions of Tetrahedral Complexes
Tetrahedral Substitution

57. Inorganic Reaction Mechanisms
Electron Transfer Reactions
Inorganic Reaction Mechanisms
Electron Transfer Reactions
Substitution Reactions of Tetrahedral Complexes
Case Study of an Inorganic Reaction Mechanism
Insertion Reactions
Substitution Reactions of Octahedral Complexes
Substitution Reactions of Square Planar Complexes
Introduction

58. EVERYONE SAY THIS OUT ALOUD TOGETHER
Electron Transfer Reactions
EVERYONE SAY THIS OUT ALOUD TOGETHER
THE TRANS EFFECT
THE EFFECT OF A LIGAND,
UPON THE RATE OF LIGAND REPLACEMENT,
OF THE GROUP TRANS TO ITSELF

59. Electron Transfer Reactions
May occur by either or both of two mechanisms
Outer Sphere Electron Transfer
Inner Sphere Electron Transfer
Outer Sphere Electron Transfer
In principle all outer sphere mechanisms involve electron transfer from reductant to oxidant with the coordination shells or spheres staying intact
One reactant becomes involved in the outer or second coordination sphere of the other reactant and an electron flows from the reductant to oxidant
Such a mechanism is established when rapid electron transfer occurs between two substitution inert complexes

60. Inner Sphere Electron Transfer
Electron Transfer Reactions
Inner Sphere Electron Transfer
Reductant and oxidant share a ligand in which their inner or primary coordination sphere
The electron is transferred across the bridging group
Undergoes aquation to hexaquo

61. Outer Sphere Electron Transfer Elementary Steps
Electron Transfer Reactions
Outer Sphere Electron Transfer Elementary Steps
1. Formation of precursor complex in a shared solvent cage
A + B  {A---B} KAB
2. Activation of precursor, electron transfer, and relaxation of successor complex
{A---B}  {A+---B-} kET
3. Products separate from solvent cage
{A+---B-}  A+ + B- fast
Important Factors
Solvent reorganisation
Electronic structure
Metal-Ligand reorganisation is small

62. Simplest Reaction Electron Self Exchange | Low Spin Ligand
Electron Transfer Reactions
Undergoes Outer Sphere Electron Transfer Mechanism
Requires the transfer of an electron from one eg orbital to the other
Note: Same ligand complex but always in constant exchange
Aromatic ligands give fast transfer
[Co(Phen)3]2+ + [Co(Phen)3]3+  [Co(Phen)3]3+ + [Co(Phen)3]2+

63. Electron Self Exchange | High Spin Ligand
Electron Transfer Reactions
Extremely slow due to change in multiplicity of the system
[Co(NH3)6]2+ + [Co(NH3)6]3+  [Co(NH3)6]3+ + [Co(NH3)6]2+

64. Electron Transfer Bond Length changes
Electron Transfer Reactions

65. Frank-Condon Barrier to Electron Transfer
Electron Transfer Reactions
Oxidant and reductant must reorganise coordination spheres before electron transfer so energies of the transition states are equal
If electron transfer took place at their equilibrium bond positions, then we would have a compressed Fe2+ ion and a stretched Fe3+ ion. These are in vibrationally excited states and would release energy which contradicts thermodynamics.
The reagents must match their energies before electron transfer can take place

66. Cross reactions and the Marcus Equation
Electron Transfer Reactions
K1,2
[MLn]x [MLn]y+
[MLn]y [MLn]x+
k1,1
k2,2
[MLn]y [MLn]x+
f12 = z21,2 / z1,1 . z2,2
k12 = (k11k22K12f12)1/2
Equilibrium constant, can consider as
driving force.
WITH APPROXIMATION f = 1
k12 = (k1k2K12)1/2
Can be calculated from
redox potentials of reactants
Electron-self exchange
rate constants
Rate constant

67. Past Exam Question Past Exam Questions
Square planar Pt(II) complexes usually undergo substitution by an associative, A, or intimate associative, Ia, mechanism. Discuss this observation. [15%]

68. Past Exam Question Past Exam Questions
Square planar Pt(II) complexes usually undergo substitution by an associative, A, or intimate associative, Ia, mechanism. Discuss this observation. [15%]

69. Past Exam Question Past Exam Questions
What is the principal stereochemical feature of ligand substitution at Pt(II) and how does this arise? [15%]

70. Past Exam Question Past Exam Questions
How does a bulky ligand, placed in the cis-position relative to the substituted ligand, influence the rate of substitution of a Pt(II) complex? [15%]

71. Past Exam Question Past Exam Questions
What is the trans-effect and how does it operate? [25%]

72. Past Exam Question Past Exam Questions
What is the trans-effect and how does it operate? [25%]

73. Past Exam Question
Past Exam Questions

74. Past Exam Question
Past Exam Questions

75. Past Exam Question
Past Exam Questions

76. Past Exam Question
Past Exam Questions