Stability constants Flashcards
What is a lewis base?
Ligand
Electron pair donor
What is a lewis acid?
Metal
Electron pair acceptor
What is binding affinity expressed as?
A numerical value
What is kinetic stability?
The rate of reaction
Whether something is labile or inert
What is thermodynamic stability?
The position of equilibrium which is measured using the equilibrium constant
Is there a correlation between kinetic and thermodynamic stability?
No
What is used to show kinetic stability?
small k
What is used to show thermodynamic stability?
large K
What do you measure the thermodynamic affinity of?
the ligand binding to the metal
What equation is used to calculate K?
K = [Products]/[reactants]
What is the binding of the ligand like when the value of K is large?
Tight binding
What is the binding of the ligand like when the value of K is small?
Weak binding
What value of K is favoured?
Large value of K
Due to stability constants usually being large, what is the more convenient way to measure them?
Use Log10(K)
What does K not tell you anything about?
Rate
There is no relationship to the rate of reaction
A large K does not mean ligands bind fast
What symbol is used for the formation constant?
β
What would the general equation for K1 be?
K1 = [MLⁿ⁺] / [Mⁿ⁺][L]
What would the general equation for K2 be?
K2 = [ML₂ⁿ⁺]/[MLⁿ⁺][L]
What would the general formation constant be?
β = [ML₂ⁿ⁺]/[Mⁿ⁺][L]²
How would you generate the formation constant?
Look at the starting and end concentrations and the amount of ligands used
Intermediates aren’t included
If you expand the formation constant, what can you get?
The two K constants generated
β = K1 x K2
What is the general order for the values of the stability constants?
K1>K2>K3>K4
What are the three influencing factors for the value of β?
1) Sterics
2) Statistics
3) charge
Which factor is the most important?
Sterics
How do sterics effect β?
When you substitute a new ligand its usually bigger than the ligands that are already bound
This means if any more are added, they will be repelled by the larger ligand already substituted.
The repulsion causes the reaction to become more difficult
What are statistics and how do they influence β?
As more ligands bind, this reduces the amount of variable sites available for the new ligands to bind
When is charge a problem and effects β?
When anionic ligands are used
What happens when you add 3 charged ligands to a positive complex?
The first one - Attracted to the complex to binds easily
The second one - binding to a neutral complex so there is no charge issue
The third one - binding to a negatively charged complex meaning that the anion will be repelled
Which metal is an exception, having an unfavourable change in co-ordination number? Explain.
Hg
When a third ligand is added, K3 is low and K4 is high which doesn’t follow the trend
What is the order for K values for spin states when adding large ligands such as bipy and phen?
K1>K2
What is a chelate ring?
Ring made up of ligands, usually bidentate
Why are chelate rings preferred?
They are more stable
Bidentate ligands bind tigheter to a metal than two similar monodentate ligands
How do formation constants for bidentante ligands compare to monodentant ligands?
They are significantly larger
How does the dissociation of monodentate ligands compare to the dissociation of bidentate ligands?
Monodentate ligands dissociation from the metal ion
For bidentate, the dissociated end cannot migrate away from metal ion. Migration requires simultaneous breaking of two bonds which is very unlikely
How does the entropy of monodentate ligands compare to the entropy of bidentate ligands?
There is higher disorder and higher entropy for a reaction with bidentante ligands
2 molecules to 3 whereas monodentate ligands are 3 to 3
What is the macrocyclic effect?
Multidentate ligands binding to one metal
What are crown ethers?
Multiple oxygens joined to the metal centre
Different metals match different cavity sizes
How is the best fit for a crown ether determined?
Highest K value
What is the chelate effect mostly?
Entropic in nature
