2. Lecture 7: M-M bonds d-bonds and bonding in metal clusters
Schedule
Lecture 7: M-M bonds d-bonds and bonding in metal clusters
Lecture 8: Rates of reaction Ligand-exchange reactions, labile and inert metal ions
Lecture 9: Redox reactions Inner and outer-sphere reactions
Demos: charles law (2.3), CO2 density (2.1), star wars (4.3)
Brief mention of 3 states of matter.

3. Summary of Last Lecture
Quadruple bonds
Eclipsed geometry due to d-bond
Clusters
Consider total number of pairs of metal electrons and number of metal-metal connections
Bond order is number of pairs / number of edges
Carbonyl clusters
Use 18 e- rule to work out how many bonds have to be formed
Today’s lecture
Ligand-substitution reactions

4. Recap - Kinetics transition state H‡ (forward) H‡ (backward)
DG (forward)
= -DG (backward)

5. Recap - Kinetics
In the majority of reactions a series of such events occurs
in this case, each event is called a elementary step
the collection of these is the stoichiometric or reaction mechanism
the elementary step with the largest activation energy is called the rate determining step (or rds)
The reaction can only proceed if the reactants have at least this energy
The details of the rds are known as the intimate mechanism.

6. Ligand Substitution Reactions
In these reactions, one ligand is exchanged for another
Common examples include:
Aquation or acid hydrolysis: substitution of a ligand by H2O
[Ni(en)3]2+ + H2O  [Ni(en)2(H2O)]2+
Anation: substitution of H2O by an anion
[Co(NH3)5(H2O)]2+ + Cl-  [Co(NH3)5Cl]+
Two stoichiometric mechanisms need to be considered:
L5M + X + Y
dissociatve (d)
L5MX + Y
L5MY + X
L5MXY
associatve (a)

7. Dissociative (d) Mechanisms
If the stoichiometric mechanism is dissociative, there are again two possible intimate mechanisms
D mechanism
M-X bond breaks before Y attaches k1
L5MX
L5M + X
5-coordinate intermediate can, in theory, be isolated
equivalent to SN1
k-1 k2
L5M + Y
L5MY
Interchange Id mechanism
Before M-X bond fully breaks, M-Y bond begins to form
No intermediate
equivalent to SN2 Y M X

8. Associative (a) Mechanisms
If the stoichiometric mechanism is associative, there are two possible intimate mechanisms
A mechanism
M-Y bond forms before M-X bond breaks k1
L5MX + Y
L5MXY
7-coordinate intermediate can, in theory, be isolated
k-1 k2
L5MXY
L5MY + X
Interchange Ia mechanism
Before M-Y bond is fully made, M-X bond begins to break
No intermediate
equivalent to SN2 Y M X X

9. Interchange Mechanisms: Id vs Ia
Both the Id and Ia mechanism involve the interchange of X and Y without a definite intermediate
Id mechanism
In the transition state, bond breaking is more important than bond making
The activation energy is determined by M-X bond strength M X Y
Interchange Ia mechanism
In the transition state, bond formation is more important than bond breaking
Activation energy is determined by the crowding in the transition state M X Y

10. very slow: kinetically inert fast: kinetically ‘labile’
Water Exchange
The simplest substitution is the exchange of coordinated water with the solvent.
Rate varies over at least 16 orders of magnitude depending on the metal.
rate constant (s-1)
very slow: kinetically inert
fast: kinetically ‘labile’

11. Water Exchange
Activation energy for dissociate mechanisms (D and Id) determined by:
M-X bond strength
M+-X weaker than M2+-X so M+ faster
M2+-X weaker than M3+-X so M2+ faster
The larger Mn+ is, the weaker its bonds so faster
Change in LFSE between 6-coordinate L5MX and 5 coordinate L5M
Insensitive to nature of nucleophile Y
Activation energy for associative mechanisms (A and Ia) determined by:
Crowding in 7-coordinate transition state
Small Mn+ give ore crowded transition states and slower reactions
Change in LFSE between 6-coordinate L5MX and 7 coordinate L5MXY
Sensitive to nature of nucleophile Y

12. Water Exchange for Transition Metals
M3+ exchange water slower than M2+ whether mechanism as d or a
M3+ are smaller than M2+
M3+-X are stronger than M2+-X
Loss of LFSE is greatest for dn with large LFSE
slow for d3 and d8 and low spin d4, d5, d6, d7
-LFSE / oct
2.4
-+0.6 Doct
1.2
-0.4 Doct 10 5 1 2 3 4 6 7 8 9

13. Water Exchange for Cu2+ and Cr2+
These two ions are much more labile than their charge and LFSE indicates
Cr2+ (d4)
Cu2+ (d9)
Jahn-Teller distortion:
long axial bonds are weak and easily replaced
AJB – lecture 1, JKB – Lecture 6

14. Experimental Tests of Mechanism
Dissociative mechanisms (D or Id):
H‡ large (bond breaking)
S‡ large and positive (increasing number of molecules)
V‡ large and positive (increasing number of molecules)
Insensitive to nucleophile
Highly sensitive to leaving group
rds
L5MX
L5M + X
rds
Associative mechanisms (A or Ia):
H‡ small (no bonds broken)
S‡ large and negative (decreasing number of molecules)
V‡ large and negative (decreasing number of molecules)
Sensitive to nucleophile
Insensitive to leaving group
L5MX + Y
L5MXY

15. Square Planar Complexes
For square planar complexes, substitution occurs via an A mechanism
it is stereospecific
cis-platin
cis product
H‡ small
S‡ large and negative
V‡ large and negative

16. Summary By now you should be able to....
Describe the steps in the dissociative and associative mechanisms
Account for the wide variation in the rates of ligand exchange for
metals of different oxidation states and
metals of different dn configuration
Next lecture
e- transfer reactions

17. Practice