Electron transfer reactions, Exams of Applied Chemistry

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Electron Transfer Reactions of Transition

Metal Complexes

Chapter 20 and 26

November, 2010

Electron -Transfer Processes

simplest involve only transfer of electrons and can be monitored by isotopic tracers

[^56 Fe 3+^ (CN) 6 ]3-^ + [^59 Fe 2+^ (CN) 6 ]4-^ → [^56 Fe 2+^ (CN) 6 ]4-^ + [^59 Fe 3+^ (CN) 6 ]3-

two classes of electron transfer reactions were defined by Taube (Nobel Prize in chemistry 1983)

in an outer-sphere mechanism , electron transfer occurs without a covalent linkage formed between the reactants

in an inner-sphere mechanism , electron transfer occurs via a covalently bonded bridging ligand

Kinetic data can sometimes distinguish between these two cases, but often, it is not possible to distinguish between inner- and outer-sphere mechanism

1 st^ two steps identical to 1st two steps of Eigen-Wilkins mechanism

Other connecting ligands: OH-, NCS-,

Pyrazine, C 4 H 4 N 2 , 4,4-bipyridine , C 4 H 4 N 2 act as

Creutz-Taube electron transfer bridges;

[Fe 3+^ (CN) 6 ]3-^ + [Co2+^ (CN) 6 ]3-^ → [Fe 2+^ (CN) 6 ]4-^ + Co3+^ (CN) 5 H 2 O]2-

The complex (CN) 5 Fe - C≡N – Co(CN) 5 ] 6-^ can be isolated as a Ba salt

The -CN-^ in the transition complex Fe-CN-Co is not transferred to Co

For [Co(NH 3 ) 5 Cl] 2+^ + [V(H 2 O) 6 ] 2+^ or

[Co(NH 3 ) 5 Br] 2+^ + [V(H 2 O) 6 ] 2+^ or

[Co(CN) 5 (N 3 )] 2+^ + [V(H 2 O) 6 ] 2+

Bridging group has little effect on k, rate is similar to H 2 O

exchange, and rate determining step is H 2 O

leaving, not bridge forming step

[Co III^ (NH 3 ) 5 Br]2+^ + [V II^ (H 2 O) 6 ] 2+^ ⇒^ [Co III^ (NH 3 ) 5 ▬ Br ▬ [VII^ (H 2 O) 6 ] 4+^ fast

[Co III^ (NH 3 ) 5 ▬ Br ▬ [VII^ (H 2 O) 6 ]4+^ ⇒ [Co II^ (NH 3 ) 5 ] 2+ ▬ Br ▬ [VIII^ (H 2 O) 6 ] 3+^ fast

[Co II^ (NH 3 ) 5 ▬ Br ▬ [VIII^ (H 2 O) 6 ] 4+^ ⇒ [Co II^ (NH 3 ) 5 ] 2+^ + [VIII^ Br (H 2 O) 5 ] 2+^ + H 2 O rate determining

X = conjugated organic anions (π e-) , rate is faster (k larger);

X= CH 2 , CH 2 =CH 2 , CH 2 =CH 2 =CH 2 , CH≡CH, C(O)

See 26.10 (25.10); NC 5 H 4 —Y —NC 5 H 4 Y = conjugated, or short (CH 2 ) or flexible, leads to faster e- transfer; M-M’ closer ⇒outer-sphere mechanism

Conjugated bridging anions lead to faster inner –sphere electron transfer

e.g. - O 2 CCH 2 =CH 2 CO 2 -^ oxalate anion

N X^ N^

N

Outer-sphere mechanism

When both reactants are kinetically inert, e -^ transfer must take place by tunneling or outer-sphere mechanism

[ 56 Fe III^ (CN) 6 ]3-^ + [ 59 Fe II^ (CN) 6 ]4-^ → [ 56 Fe II^ (CN) 6 ]4-^ + [ 59 Fe III^ (CN) 6 ]3-^ ∆G ≈

In a self-exhange reaction as above (right and left hand side of the eq. are identical); only e- transfer and no net chemical reaction occurs.

Although ∆r G ≈ 0, activation energy ∆G #^ is needed to overcome

electrostatic repulsion between ions of like charge, to stretch and shorten bonds, so they are equivalent in the transition state

Franck-Condon approximation: electron transfer is much faster

than molecular vibration;

electronic transitions take place in a stationary nuclear framework

What do the different electron configurations and bond length

suggest about outer-sphere mechanism?

Reactants must approach close

enough for e- transfer

Important restriction:

Franck-Condon Principle

(a molecular electronic transition

is much faster than an electronic vibration)

During e- transfer, nuclei are essentially

stationary; electron transfer between

complexes with different bond length

can occur only between their

vibrationally excited states

with identical structures-

Fe3+

Fe2+ Electron transfer

M-L bond distances are different in oxidized and reduced complexes: e.g., Co3+-L shorter than Co2+-L first law of thermodynamics would be violated, if e -^ transfer occurred directly to [Co3+L 6 ] from [Co2+L 6 ] followed by relaxation to Co 2+^ and Co3+, respectively with loss of energy (∆G=0)

Due to Franck-Condon restriction

The activation E required to reach these vibrational states, varies for each system, hence the self-exchange rate varies:

t2g^5 eg^1 t2g^5 e (^) g^2

t (^) 2g^5 e (^) g^2 t (^) 2g^6

[Co(NH 3 ) 6 ] 3+^ + [Co(NH 3 ) 6 ] 2+^ → [Co(NH 3 ) 6 ] 2++ [Co(NH 3 ) 6 ] 3+,

Electron transfer can occur only at the intersection of the left and right potential curve, where the nuclear coordinates of the ox. and red. forms are the same; that state is reached by ∆G #^ ; if the ox. and red form have different eletronic configuration, ∆G #^ is larger

[Co(NH 3 ) 6 ] 3+

[Co(NH 3 ) 6 ] 2+

The differences in rates is expressed by Marcus (Nobel in 1983) Equation:

kET = νNκe e-^ ∆G# /RT

νN = nuclear frequency factor; frequency of attaining transition state

κe = probability that electron transfer will occur (0 to 1) depends on the overlap of donor and acceptor orbitals

∆G #^ = 1/4λ(1 + ∆G 0 /λ) for self-exchange ∆G^0 =

0

For cross-reactions ∆G 0 ≠ 0

∆G^0 = -zFE 0 (obtained from std. E 0 of redox couple λ = reorganization energy – E required to move the nuclei of reactant to the position of the product, just before electron transfer λ depends on: changes in M-L bond length (inner- sphere reorganization energy) re-orientation of solvent molecules around the complex (outer-sphere reorganization energy)