Page 101 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN005M-206 June 15, 2001 20:25

Electrochemistry 179

TABLE III Apparent Metal-Ligand Covalent-Bond- The electrochemical reduction of permanganic acid
Formation Free Energies (−∆G BF ) for Several Man- [HOMn VII (O) 3 ], which is traditionally represented as a
ganese, Iron, and Cobalt Complexes
metal-centered electron transfer to change Mn 7+ to Mn ,
6+
Complex −∆G BF , kcal mol −1 a is another example of a ligand-centered process,
A. Manganese VII VI
+
(O) 3 Mn –OH + H O + e − Mn (O) 3 + 2H 2 O
3
III
(8Q) 2 Mn –8Q 6
◦
III
(acac) 2 Mn –acac 9 − G BF = 28 kcal mol −1 (pH 1). E , +1.45 V (106)
III
(PA) 2 Mn –PA 22
III
[(bpy) 2 Mn –bpy] 3+ >23 b Comparison of this with the reduction of free hy-
B. Iron droxyl radical (HO·) [Eq. (104)] provides a measure
III
(8Q) 2 Fe –8Q 15 of the HO–Mn VII (O) 3 bond energy [− G BF = (2.66 −
−1
III
(acac) 2 Fe –acac 23 1.45) 23.1 = 28 kcal mol ]. The other strong oxidants
IV
VI
3+
III
(PA) 2 Fe –PA 31 [(HO) 2 Cr (O) 5 and HOCe (OH 2 ) ] that are used for
5
2
III
[(bpy) 2 Fe –bpy] 3+ >29 b aqueous redox titrations are reduced by a similar path,
III
[(Ph 3 PO) 3 Fe –OPPh 3 ] 3+ >30 b
VI
VI
+
III
[(MeCN) 4 Fe –OH 2 ] 3+ 23 HO–Cr (O) 2 OCr (O) 2 OH + H O + e −
3
C. Cobalt (O) 2 Cr OCr (O) 2 OH + 2H 2 O
VI
V
III
(8Q) 2 Co –8Q 16 +1.30 V
III
(acac) 2 Co –acac 21 − G BF , 31 kcal mol −1 (pH 1) (107)
III
(PA) 2 Co –PA 35 IV
HO–Ce (OH 2 ) 3+ + H O + e −
+
III
[(bpy) 2 Co –bpy] 3+ >46 b 5 3
III
Ce (OH 2 ) 3+ + H 2 O
a 6
(− G BF ) = [E − − E − ] +1.66 V
1/2[ZnL /ZnL 2 (L·)] 1/2(ML /M(·L)L 2 )
3 3
× 23.1 kcal mol −1 . − G BF , 23 kcal mol −1 (pH 1). (108)
b
(− G BF ) = [E p,a (ZnL/ZnL + ·) − E p,a(ML/M−L + ) ] × 23.1kcal
mol −1 ; L = (bpy) 3 or (Ph 3 PO) 4 .
An important point in these electron-transfer reductions
is that the primary electron acceptor is the hydronium ion
+

e (H O), which is transformed to a hydrogen atom (H·) that
3
reacts with HO· (either free or bound via a covalent bond
to the metal center). Thus, in the reactions of Eqs. (103),
(104), and (106)–(108), the oxidant in each is the hydro-
H 2 O
+
H HO nium ion (H O) and the reduction potential is determined
3
by the H–OH bond energy (− G BF ) of the product H 2 O,
(100)
minus the metal–OH bond energy [Eqs. (106)–(108)].
−
−
Hence, reductive electrochemistry converts electrons (e ) Under alkaline conditions Mn VII O is reduced via di-
4
via the solution matrix at the interface to atoms and an- rect electron addition to one of the bound oxygen atoms,
ions. The solution outside the inner double layer never
VI
is exposed to an electron. Some examples of such inner- − OMn VII (O) 3 + e − − OMn (O) 2 O −
double-layer electron transfer include
◦
E , +0.55VvsNHE. (109)
◦
H 2 O + e − [H·] + HO − E , −2.93VvsNHE
(101) The extent of the stabilization of the oxygen atom in
−
Mn VII O is indicated by the reduction potential for a free
+
◦
H O + e − [H·] + H 2 O E , −2.10 V (102) 4
3
·O· atom,
2
III
+
(H 2 O) + Fe –OH + H O + e −
5 3
[·O·] + e − ·O − (E ) pH 14 , +1.43 V. (110)
◦
pH 1 II
Fe (OH 2 ) 2+ + H 2 O E , +0.71 V (103)
◦
H 2 O 6
In summary, the electron-transfer reactions for metals
pH 1
+
◦
[HO·] + H O + e − 2H 2 O E , +2.66 V (104) and metal complexes are ligand centered (or solvent cen-
3 H 2 O
tered). In each case the potential for the oxidation of free
2+ III ligand is decreased in the presence of metal or reduced-
− G BF (H 2 O) Fe –OH = [2.66 − 0.71] × 23.1
5
metal complex by an amount that is proportional to the
= 45 kcal mol −1 (105) metal-ligand bond energy (− G BF ).