Coordination complexes of the transition metals can undergo a variety of reactions. Among these are — Organic Chemistry Chemistry Question
Kinetics and Mechanisms of Isomerization of an Octahedral Metal Complex
Coordination complexes of the transition metals can undergo a variety of reactions. Among these are electron transfer, substitution, rearrangement, and reaction at a coordinated ligand. Some of these reactions have been thoroughly studied and their mechanisms are generally well understood. This question examines the kinetics of the isomerization of a six-coordinate complex and uses the steady state approximation to develop rate laws for two possible pathways of reaction.
The cis isomer of the cation [Co(en)2Cl2]+ (where en = ethylenediamine) can be converted to the trans isomer in the presence of Cl– ion by two possible mechanisms: a) Associative and b) Dissociative.
[VISUAL]
Associative Mechanism
[VISUAL]
Dissociative Mechanism
H2N NH2 = ethylenediamine, NH2–CH2CH2–NH2
Assoc. Intermed.
k-1 Dissoc. Intermed.
For each of the mechanisms above derive the rate law using the steady state approximation.
Model Answer
Associative mechanism:
k1 step Rate = k1 [Cl–]
k–1 step Rate = k–1 [Assoc. Intermed.]
k2 step Rate = k2 [Assoc. Intermed.]
Overall rate = k2 [Assoc. Intermed.]
Steady state:
d[Assoc.intermed.]/dt = k1[Cl-] - k-1[Assoc. Intermed.] - k2[Assoc. Intermed.] = 0
k1[Cl-] = (k-1 + k2)[Assoc. Intermed.]
[Assoc. Intermed.] = k1[Cl-] / (k-1 + k2)
Overall rate = k1 k2 [Cl-] / (k-1 + k2)
Dissociative mechanism:
k1 step Rate = k1
k–1 step Rate = k–1 [Dissoc. Intermed.] [Cl–]
k2 step Rate = k2 [Dissoc. Intermed.] [Cl–]
Overall rate = k2 [Dissoc. Intermed.] [Cl–]
Steady state:
d[Dissoc.intermed.]/dt = k1 - k-1[Dissoc. Intermed.][Cl-] - k2[Dissoc. Intermed.][Cl-] = 0
k1 = (k-1 + k2)[Dissoc. Intermed.][Cl-]
[Dissoc. Intermed.] = k1 / ((k-1 + k2)[Cl-])
Overall rate = k1 k2 / (k-1 + k2)
Show what happens to each of the rate laws:
(i) when the first step is rate-limiting,
(ii) when the second step is rate-limiting.
Model Answer
For the associative mechanism —
If the 1st step is rate-determining:
k2 + k–1 ≈ k2 (because k2 >> k–1)
Rate = k1 [Cl–]
If the 2nd step is rate-determining:
k2 + k–1 ≈ k–1 (because k–1 >> k2)
Rate = (k1 k2 / k-1) [Cl-] = Keq k2 [Cl-]
For the dissociative mechanism —
If the 1st step is rate determining, k2 is large and so Rate = k1
If the 2nd step is rate determining, k1 is large and so Rate = Keq k2
Derive an equation for the observed rate constant, kobs, in each of the four cases.
Model Answer
kobs = Rate /
For associative mechanism:
- 1st step is rate determining: kobs = k1 [Cl–]
- 2nd step is rate determining: kobs = Keq k2 [Cl–]
For dissociative mechanism:
- 1st step is rate determining: kobs = k1
- 2nd step is rate determining: kobs = Keq k2
Is it possible to tell which is the rate-determining step in the associative mechanism based on the observed rate law?
Model Answer
The associative mechanism is the 1st order in Cl–, but it is not possible to tell which step is rate-determining.