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reacton mechanisms
Typology: Exams
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simplest involve only transfer of electrons and can be monitored by isotopic tracers
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
The -CN-^ in the transition complex Fe-CN-Co is not transferred to Co
[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
e.g. - O 2 CCH 2 =CH 2 CO 2 -^ oxalate anion
Outer-sphere mechanism
When both reactants are kinetically inert, e -^ transfer must take place by tunneling or outer-sphere mechanism
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.
electrostatic repulsion between ions of like charge, to stretch and shorten bonds, so they are equivalent in the transition state
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:
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
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)