Download Electron-transfer processes

9C-1
Electron-transfer processes
H&S 26.5
Transition metal complexes participate in various types of “redox” chemistry, because of the
multiple oxidation states that are readily accessed by most of these metals.
In redox reactions electrons are transferred from one species to another.
Fe(CN)64- + IrCl62- -> Fe(CN)63- + IrCl63-
Electron-transfer processes
9C-2
H&S 26.5
For coordination complexes, there are two classes of electron-transfer mechanism:
1) INNER SPHERE MECHANISM: The two molecules become (temporarily) connected by some
bridging ligand, through which the electron is transferred. Often the bridging ligand itself is
also transferred.
e.g.
2) OUTER SPHERE MECHANISM: The exchange occurs between two separate coordination
spheres, which do not change upon reaction.
e.g.
As always, rely on kinetic studies to distinguish between mechanisms, but can be challenging,
(i) b/c of very high rates, and (ii) difficulty in tracking involvement of ligands.
9C-3
Inner-sphere electron transfer
H&S 26.5
The inner-sphere mechanism involves three distinct steps
1) A substitution reaction that leaves the oxidant and the reductant linked by the bridging ligand
2) The actual transfer of the electron
3) Separation of the products (frequently accompanied by transfer of the ligand)
9C-4
Inner-sphere electron transfer
H&S 26.5
How do we know the Cl– didn’t just dissociate, or wasn’t substituted by H2O prior to e– transfer?
The reaction rates are usually 2nd order overall (i.e. bimolecular), but it can be tricky to draw
conclusions about the relative rates of the individual steps.
In cases where a slow step is identified, it is usually the first step (briding ligand complex
formation) or the second step (e- transfer).
In this example the rate determining step is the 2nd step. The first step is very fast because it’s a
substation reaction at a very labile Cr(II) (d4, HS) center.
9C-5
Relative rates of inner-sphere electron transfer reactions H&S 26.5
However if we look at the corresponding reaction with V(II) instead of Cr(II),
the rate determining step becomes the initial substitution (1st step).
1. [Co(NH3)5Cl]2+ + [V(H2O)6]2+ ––> [(NH3)5Co(μ-Cl)V(H2O)5]4+
2. [(NH3)5Co(μ-Cl)V(H2O)5]4+ ––> [(NH3)5Co(μ-Cl)V(H2O)5]4+
3. [(NH3)5Co(μ-Cl)V(H2O)5]4+ ––> [Co(NH3)5(H2O)]2+ + [V(H2O)5Cl]2+
[Co(NH3)5Cl]2+ + [V(H2O)6]2+ ––> [Co(NH3)5(H2O)]2+ + [V(H2O)5Cl]2+
9C-6
Relative rates of inner-sphere electron transfer reactions H&S 26.5
Here’s an unusual example of an inner-sphere mechanism in which breaking of the µ-L bridge is
rate-limiting (3rd step). Dinuclear complex slowly cleaved by hydration.
9C-7 Inner-sphere electron transfer: nature of the bridging group H&S
Varying the ligand X in reactions where the rds is the
with the ability of X to act as a bridging ligand:
e–-transfer
26.5
step, gives an increase in rate
[Co(NH3)5X]2+ + [Cr(H2O)6]2+ + 5[H3O]+ ––> [Co(H2O)6]2+ + [Cr(H2O)5X]2+ + 5[NH4]+
Table 26.8, p.993
increase in k for F–<Cl–<Br–<I– correlates with increased ability of X– to act as bridge.
OH– comparable to Br–
k for H2O is (i) v. small and (ii) pH dependent - Consistent with not µ-OH2, but rather µ-OH–.
9C-8 Inner-sphere electron transfer: nature of the bridging group H&S
26.5
Bridging ligands commonly involved in inner sphere mechanisms include
halides, OH–, NCS–, pyrazine, 4,4’-bipyridine, pyrazolyl
For X=NCS– get a 70:30 mix of N- or S-bound Cr3+ product.
[Co(NH3)5(NCS)]2+ + [Cr(H2O)6]2+––> [Co(H2O)6]2+ + [Cr(H2O)5(NCS)]2+ + [Cr(H2O)5(NCS)]2+
Two possible bridging intermediates:
9C-9 Inner-sphere electron transfer: nature of the bridging group H&S
26.5
Electron transfer rate constants have been measured for the following reaction:
[Fe(CN)5(H2O)]3– + [Co(NH3)(L)]3+ ––>
L=
~k is large when X is conjugated
~k is smaller when X is short, saturated
~k increases again when X is long and flexible (e.g. saturated)
9C-10
Outer-sphere electron transfer reactions
H&S 26.5
When the complexes involved in redox chemistry are (relatively) kinetically inert to substitution
(no bridging possible), the electron transfer can occur via a tunnelling, or outer sphere,
mechanism.
For “self-exchange” reactions, there is no net reaction: G°~0
BUT rates vary widely! So must be due to differences in G‡
Table 26.9, p.994
9C-11
Outer-sphere electron transfer: mechanism
H&S 26.5
Reactants must get close to each other for the electron transfer to occur. Can consider this
close approach in terms of an “encounter complex” (similar to some octahedral ligand
substitution mechanisms).
So for self-exchange, expect contribution to G‡ for overcoming electrostatic repulsion
between complex ions of like charge.
Also expect contribution to G‡ from changes in solvation required for encounter complex to
form.
These factors alone are not enough to account for the huge variation in k values (see previous
slide) for self exchange reactions.
9C-12
Outer-sphere electron transfer: mechanism
H&S 26.5
Structural changes accompany the metals’ change in oxidation state: M-L bond lengths are
typically shorter for higher O.S. complexes, so the overall redox reaction includes changes in
bond lengths, even for self exchange reactions where G°≈0.
Franck-Condon principle: vibrations giving bond length changes occur many orders of
magnitude slower that the actual electron transfer.
E- transfer produces new oxidation state in vibrationally excited states.
These must relax to new equilibrium geometries with overall loss of energy.
This is not possible for G°≈0!
9C-13
Outer-sphere electron transfer: mechanism
H&S 26.5
“When the reactants have differing bond lengths, vibrationally excited states with equal bond
lengths must be formed in order to allow electron transfer to occur.”
9C-14
Outer-sphere electron transfer: mechanism
H&S 26.5
To maintain conservation of energy, vibrationally excited states with equal* bond lengths must
be formed before outer sphere e– transfer can occur. Franck-Condon restriction.
G‡ required to stretch/compress M-L bonds to reach this transition state varies according to
the system; hence the huge variation in self-exchange k values
(For outer-sphere e–-transfer reactions between complexes with different M and/or L (i.e. not
self-exchange) the excited state must achieve “appropriate intermediate bond lengths” for each
of the metal complexes involved)
9C-15
outer-sphere electron transfer rate examples
[Fe(CN)6]4– / [Fe(CN)6]3-
k ~ 105 L mol-1s–1
Reductant & oxidant are both low spin (d6, d5, respectively). Have very similar bond lengths,
despite difference in charge (Fe–N = 195, 192 pm respectively)
Also true for [Fe(bpy)3]2+/3+ pair: Fe–N = 197, 196pm, respectively. (k > 106)
[Co(NH3)6]2+ / [Co(NH3)6]3+
k ~ 10–6 L mol-1s–1
Reductant is high spin d7, oxidant is low spin d6.
The HS/LS difference gives a significant difference
in bond lengths, in addition to charge
considerations. Co–N = 211 pm vs 196 pm
Electrons in eg* orbitals have antibonding character, while those in t2g orbitals are non-bonding,
or slightly bonding (if π interactions).