Chemistry Reference
In-Depth Information
σ* (e
g
)
π (t
2g
)
π
π
Ru
III
Ru
II
Ru
II
Ru
III
+
+
σ* σ*
σ* (e
g
)
π
(t
2g
)
Co
III
Co
II
Co
II *
Co
II
Co
III
Co
III *
+
+
+
electron rearrangment from
excited state to ground state required
Figure 5.20
Electron transfer processes octahedral Ru
III/II
(top) and Co
III/II
(bottom) complexes.
successor assembly breaking up to form separated ions is a form of dissociation, but in this
case of whole complex ions.
If we return to ligand field theory, recall that the d electrons for an octahedral complex
lie in the
t
2g
and
e
g
∗
∗
respectively to equate with the
character of bonding in which they participate in a complex exhibiting both
levels, also designated as
and
-donor and
-donor/acceptor bonding character. For electron transfer, a metal d electron needs to move
from a location in a
∗
orbital on the other
metal ion. Generally, it will be more favourable for an electron to move between orbitals
of the same symmetry (or from like to like orbitals); that is
∗
or
orbital on one metal ion to a
or
∗
→
∗
→
or
transitions
∗
orbitals differ, and so the
electron transfer process will be affected by the nature of the donor and acceptor orbitals.
Models predict that the d
are energetically favoured. Further, the character of
and
∗
orbitals, thus more able
to interact with orbitals on a different metal complex, and as a result it is anticipated that
→
orbitals are more 'exposed' than d
∗
→
∗
electron transfer. Let's examine
this for a Ru(III)/Ru(II) and a Co(III)/Co(II) couple.
For Ru(III)/Ru(II), we have a process operating as defined in Figure 5.20, with a vacant
position in the
electron transfer should occur faster than
orbital of a Ru(III) ion able to accommodate an electron transferred from
the
process. Moreover, the bond lengths for
similar donor groups in Ru(III) and Ru(II) complexes are very similar (typically within 5
pm), so there is little solvent rearrangement required. As a result of efficient electron transfer
between
orbital of a Ru(II) ion-afavourable
→
orbitals along with small bond length changes following electron transfer, the
electron transfer reaction rate is predicted to be fast in this example. For Co(III)/Co(II), we
have a process operating (Figure 5.20) with vacant positions only in the
∗
orbitals of a
∗
low-spin Co(III) ion able to accommodate an electron transferred from the
orbital of a
∗
→
∗
process operates.
Co(II) ion; a less favourable
