Inorganic Reaction Mechanisms
By
Prof. Ramadan M. Ramadan
http://rmramadan-notes.pbworks.com
Oxidation Reduction Reactions: Inner
and Outer Sphere Processes
Electron Transfer Reactions
Electron transfer reactions can be divided into two
categories:
a) Electron transfer without any chemical change. It thus
called electron transfer processes.
b) Electron transfer including chemical change. This type
of electron transfer is known as normal oxidation
reduction processes.
In electron transfer reactions there are two mechanisms:
outer sphere mechanism and inner sphere mechanism.
Outer Sphere Electron Transfer
In principle, all outer sphere mechanisms involve electron transfer from a reductant to an oxidant
with the coordination shells or spheres of each staying intact.
The reactant becomes involved in the outer or second coordination sphere of the other reactant and
an electron flows from the reductant to oxidant.
Such a mechanism is established when rapid electron transfer occurs between two substitution-inert
complexes.
So, the two involved reactants have slower substitution reactions than the electron transfer.
In principle, all outer sphere mechanisms involve electron transfer from a reductant to an oxidant
with the coordination shells or spheres of each staying intact.
The reactant becomes involved in the outer or second coordination sphere of the other reactant and
an electron flows from the reductant to oxidant.
Such a mechanism is established when rapid electron transfer occurs between two substitution-inert
complexes.
So, the two involved reactants have slower substitution reactions than the electron transfer.
These reactions proceed without a chemical change.
These electron transfer reactions can be followed using radioactive isotopes.
Example 1:
Example 2:
Inner Sphere Electron Transfer
An inner sphere mechanism is the one in which the reductant and oxidant
share a ligand in their inner or primary coordination spheres, and the electron
being transferred across a bridging group formed in the intermediate.
For example,
In the reactants, Co(III) is inactive complex while Cr(II) complex is active.
In products the [Cr(H2O)6]2+ complex is inactive while the Co(II) complex is
active.
It is found that the bridging ligand X transfer quantitatively from [Co(NH3)5X]2+
to [Cr(H2O)5X]2+.
There are some ligands which can act as a bridging ligand in these
reaction:
The following mechanism is proposed for the above example:
The formation of the dinuclear intermediate with the
presence of a bridge from the X group is the only chemical
rout which can explain the reaction.
In the reactions occur between Cr2+ and Cr3+ and between
[Co(NH3)5X]2+ and CrX2+ which undergo inner sphere
mechanism, the rate of reaction is found to decrease in
the order:
X = I- > Br- > Cl- > F-
This order can be explained according to the relation
between the electron transfer and the polarizability of
the group.
The question which may arise: Is the mechanism includes
only the transfer of the bridging group from one metal to
another or this group facilitates the electron transfer?
There is no experimental proof for this explanation.
For example, in reactions containing two bridging groups,
it is found that only one electron is transferred.
On the other hand, in reactions of Pt(IV)/Pt(II), two
electrons are transfer although there is only one ligand
which is suppose to transfer.
An example shows that the change of one of the ligands can
change the type of electron transfer mechanism.
1- The reduction of hexaamminecobalt(III) complex using
hexaaquo-chromium(III) proceeds slowly (k = 10-3 M-1s-1).
This reaction undergoes by an outer sphere electron transfer
mechanism.
2- If one of the ammine groups in the cobalt complex is
substituted by a
chloride ion, the reduction reaction rate is highly increased
(k = 6 x 105 M-1s-1).
The mechanism of the reaction changes to the inner sphere
electron transfer due to the ability of the chloride group to
form a bridging bond.
In the inner sphere electron transfer, there are two proposals to
explain how the electron transfers from the reductant to
oxidant as soon as the dinuclear intermediate is formed
especially in the case of organic ligands as bridging groups.
a) The chemical mechanism:
In this mechanism, it is proposed that the electron transfers to
the bridging group which is reduced into a free radical anion
and then the electron transfers to the oxidizing metal ion.
b) The tunnel mechanism:
The transfer of the electron is explained by quantum
mechanics.
It is proposed that it passes through a tunnel penetrating the
barrier represented by the bridging group.
