Benniston L3 Flashcards
(25 cards)
How good is CFAE at predicting the water exchange rates for first row transition metals
Correctly predicts Ni and V have the smallest exchange rate but cannot explain why
Fails to correctly predict the order of Mn, Zn, Fe and Co
What are the limitations of CFAE
Does not predict everything accurately
Cannot distinguish between similar d-configurations with different behaviour
Breaks down when CFAE is 0
What is a substitution reaction
Involves the replacement of a ligand coordinated to a metal by a free ligand in solution or the replacement of a coordinated metal ion by a free metal ion
Describe oxidation state of the metal during a substitution reaction
There is no change in oxidation state of the metal ion during substitution, but a change in oxidation state may take place as a result
What are the two ways in which ligand interchange in metal complexes can occur
1) combination of solvolysis and ligation
2) simple interchange - no direct intervention of solvent
In which complexes is the different types of the ligand interchange found
Solvolysis and ligation mechanism is mainly seen in octahedral complexes whilst simple interchange is normally observed for square planar complexes
What are the 5 general mechanism for substitution
Associative
Interchange associative
Interchange
Interchange dissociative
Dissociative
Describe the associative mechanism
The entering group binds to the reaction centre before an bond weakening of the leaving group
Describe the dissociative mechanism
The leaving group has left rhe reaction centre prior to any interaction with the leaving group
Describe the interchange associative mechanism
The leaving group does not respond sufficiently fast to the approach of the entering group and remains tight up to the point of the leaving group going.
Describe the interchange mechanism
Both the ligand binding and leaving occur at the same time
- half in and out for both cases
Describe the interchange dissociative mechanism
The leaving group is sensitive to the approach of the entering group
What does a positive volume of activation mean
Dissociative mechanism
What does a negative volume od activation mean
Associative mechanism
What does a volume of activation value of zero mean
Interchange mechanism
What does a positive entropy of activation value mean
Dissociative mechanism
What does a negative entropy of activation value mean
Associative mechanism
What are the three categories of substitution in octahedral complexes
Three categories
a) solvolysis reactions
b) displacement of coordinated solvent
c) direct replacement Kd one group for another ( no solvent species)
When is the solvolysis substitution pathway important
Important when the solvent has a high affinity for a metal site
When is the direct replacement of one group pathway preferred
Preferred by low oxidation state complexes in solvents of low coordinating power
What type of reaction is solvolysis
Reactions have shown positive values for ∆S‡ and ∆V‡ consistent with a dissociative mechanism
What factors effect the rate of solvolysis
Steric crowding
Leaving group
Spectator ligands
How does steric crowding effect the rate of solvolysis
An increase in bulkiness of the coordinate ligands results in an increase in the reaction rate
How does the leaving group effect the rate of solvolysis
The tendency of the leaving groups to depart depends on the strength of the metal ligand bond. The stronger the bond the slower the reaction. If the metal ion centre is hard then the harder ligands will dissociate more slowly. If the metal centre is soft then softer ligands will dissociate more slowly
