Download Fe(H2O)6

1.
Explain the differences in the rate constants for the following self-exchange, electron
transfer reactions:
[Fe(H2O)6]2+ + [Fe(H2O)6]3+ Æ [Fe(H2O)6]3+ + [Fe(H2O)6]2+ k = 4 M-1 s-1
[Fe(bpy)6]2+ + [Fe(bpy)6]3+ Æ [Fe(bpy)6]3+ + [Fe(bpy)6]2+
k > 106 M-1 s-1
[Co(NH3)6]2+ + [Co(NH3)6]3+ Æ [Co(NH3)6]3+ + [Co(NH3)6]2+ k = 10-6 M-1 s-1
(Hint: bpy = bipyridyl, a strong-field ligand, [Co(NH3)6]3+ is diamagnetic).
As no bridging ligands are present, all of these self-exchange, electron transfer reactions occur
via the outer sphere mechanism. The rate determining step involves making the bond lengths
in the oxidant and reductant equal. Slow reactions result when this step involves considerable
excitation corresponding to the oxidant and reductant having very dissimilar bond lengths.
Fast reactions result when this step involves minimal excitation corresponding to the oxidant
and reductant having similar bond lengths.
All of the reactions involve a M2+ complex as the reductant and a M3+ ion as the oxidant. In
each case, the M3+L bonds will be shorter due to the higher charge density on the metal.
Further differences can occur due to the differences in the dn configurations – occupation of
the antibonding eg* orbitals on the metal will cause a lengthening of the bonds.
For the [Fe(H2O)6]2+ / [Fe(H2O)6]3+ reaction, the complexes contain high spin d6 and d5 ions
respectively:
eg
eg
t2g
t2g
high spin Fe3+
high spin Fe2+
There is no difference in the occupation of the eg* orbitals so that the difference in the bond
lengths is due to the charge alone. The result is a relatively fast reaction.
For the [Co(NH3)6]2+ / [Co(NH3)6]3+ reaction, the complexes contain high spin d7 and low spin
d7 ions respectively:
eg
eg
t2g
t2g
high spin Co2+
low spin Co3+
(NH3 is a stronger-field ligand than H2O and the shorter Co3+N bond lengths increase ∆oct
sufficiently to lead to a low spin arrangement).
The Co2+ ion has an additional two electrons in the eg* orbitals so that its bonds are
lengthened. Note that Co2+ also has a lower charge so this effect leads to even longer bonds.
The larger difference in the bond lengths of the oxidant and reductant, compared to the iron
reaction, lead to a higher activation energy and a much slower reaction.
In fact, the differences in the bond lengths are:
[Fe(H2O)6]2+ / [Fe(H2O)6]3+ 13 pm
[Co(NH3)6]2+ / [Co(NH3)6]3+ 22 pm
The bipyridyl ligand is a strong-field ligand as it uses the delocalized and empty π* orbitals to
π-accept. For the [Fe(bpy)6]2+ / [Fe(bpy)6]3+ reaction, the complexes contain low spin d6 and
d5 ions respectively:
eg
eg
t2g
t2g
low spin Fe2+
low spin Fe3+
There is no change in the occupation of the eg* orbitals, just in the high spin Fe2+ / Fe3+
system. However, the Fe2+ system with its (t2g)6 configuration has an optimal electron count
for π-bonding with the bipyridyl ring: it is an 18 e- complex. Thus, even though Fe3+ has a
high charge, the Fe-N bond lengths are the same (within the accuracy of an X-ray structure) in
the two complexes. The result is a very low activation energy and a very fast electron transfer
reaction.
It is likely that the π* orbitals of the rings also facilitates electron transfer as this can occur via
the delocalized orbitals with greater overlap between the orbitals on the two complexes.
2.
The rate of reduction of [Co(NH3)5(H2O)]3+ by Cr2+(aq) is seven orders of magnitude
slower than reduction of its conjugate base, [Co(NH3)5(OH)]3+ by Cr2+(aq). The rates of
the reduction of the same cobalt complexes by [Ru(NH3)6]2+ differ by only a factor of 10.
Explain these observations. (Hint: OH- is able to bridge)
The large difference in the rates of the reactions of [Co(NH3)5(H2O)]3+ with [Cr(H2O)6]2+ and
[Co(NH3)5(OH)]2+ with [Cr(H2O)6]2+ is due to a change in the reaction mechanism.
[Cr(H2O)6]2+ is a labile complex: the Jahn-Teller distorted geometry of the high-spin d4 ion
leads to long and weak axial bonds which are readily replaced by other ligands.
[Co(NH3)5(OH)]2+ contains the OH- ligand which is able to form an additional bond to a
second metal. The reaction of these two complexes proceeds via the formation of an inner
sphere complex:
[Co(NH3)5(OH)]2+ with [Cr(H2O)6]2+
[Co(NH3)5-OH-Cr(H2O)5]4+
The close approach of the two metals ions facilitates e- transfer which probably proceeds via
the orbitals of the ligand.
[Co(NH3)5(H2O)]3+ does not possess any ligands capable of bridging in this way. Thus,
although [Cr(H2O)6]2+ is labile, there is no ligand on the other complex in the reaction which
can form an inner sphere reaction. Electron transfer must occur through the slower, outer
sphere route.
[Ru(NH3)6]2+ contains a low spin d6 Ru2+ ion. (The lower pairing energy and larger ∆oct values
for 4d and 5d metals ensure that all their complexes are low spin). This leads to a large LFSE.
As this LFSE is partially lost in exchanging ligands, this complex is inert. Thus, although
[Co(NH3)5(OH)]2+ possesses a ligand capable of forming a bridged complex, the other complex
in the reaction is inert and no inner sphere complex forms. Electron transfer must occur
through the slower outer sphere route for both [Co(NH3)5(OH)]2+ and [Co(NH3)5(H2O)]3+ and
there is very little difference in the rate. (The small difference in the rate is probably due to the
higher charge of Co(NH3)5(H2O)]3+ which leads to greater repulsion with the [Ru(NH3)6]2+ ion.