Kinetics Flashcards
What is meant by the term inert?
These are complexes that are thermodynamically unstable (high in energy) but survive for long periods of time (at least a minute)
What is labile?
These are complexes that undergo rapid equilibration
What is the general formula for ligand substitution in an octahedral complex?
ML6 + X -> ML5X + L
How does a lower activation energy increase the rate of reaction?
It stabilises the transition state
Destabilised the ground state
This makes a short energy route
What does the rate of substitution depend on?
It depends on the metal ion
What are the 2 possible mechanism for ligand substitution?
Sn1- 2 step (dissociative)
1) -L
2) +M
Forms a 5 coordinate intermediate
SN2- 1 step (associative)
1) +M
Forms a 7 coordinate transition state
What affects the rate of substitution?
The oxidative state of the metal
Distribution of the 3d electrons
How does oxidation state of the metal affect rate?
Rates are slower for ions with higher charge
This is because the metal ligand bonds are harder to break which makes sn1 harder
There is also less space of attack by incoming ligand as the ion is smaller which makes SN2 harder
How does the distribution of the 3d electrons affect the rate?
The rate drops when electrons go into t2g orbitals as they lie in between the ligands as there are less repulsions so the M-L bonds are stronger (SN1 is harder)
SN2 also becomes harder as the electrons in t2g orbitals protect the metal from attack by ligand
And vice versa
Why are d4 and d9 complexes more favourable?
This is because they are distorted
Kinetically distorted systems are more favourable because they have a lower activation energy
Thermodynamically distortion stabilises then
Kinetically is labilises then and sub reactions go faster
Describe the rates for octahedral low spin complexes
Rate: d3>d4>d5>d6<
Explain why K4[Fe(CN)6] is inert d6 and K3[Fe(CN)6] d5 is toxic
K4[Fe(CN)6] d6 has a slower rate of LS so it is kinetically inert and harmless
K3[Fe(CN)6] d5 has a faster rate of LS so it is labile (exchanges CN-) and toxic
How can coordination chemistry of cobalt be used as anti cancer agent
Co3+ d6, low spin octahedral (d6 has a low rate of LS so it’s inert and drug is not released)
Inside tumour, Co3+ is reduced to Co2+ d7 (1 electron goes to eg)
d7 has a much faster rate of LS so it’s labile and drug is released
