Chemistry Reference
In-Depth Information
compounds are labile and prone to rapid reactions. This lability is put to good use in
synthesis, since it is convenient to use the Co(II) form to rapidly coordinate ligands ini-
tially, and then oxidize the mixture to the Co(III) form. This oxidation can often be achieved
by oxygen in the air alone, depending on the ligand environment and the redox potential
(
E
◦
) of the complex. For example in an aqueous ammonia/ammonium chloride buffered
solution, Co(II) reacts in a sequential manner essentially as in (6.26).
O
2
Co
2+
aq
+ NH
3
/NH
4
Cl [Co(NH
3
)
6
]
2+
[Co(NH
3
)
6
]
3+
excess
3+
2+
NH
(6.26)
OH
HO
OH
HN
NH
Co
Co
cobalt(III)
yellow
cobalt(II)
pale pink
HO
OH
HN
NH
OH
NH
Substitution reactions without redox chemistry being involved are available for Co(III),
but not commonly met. One example is the use of [Co
III
(CO
3
)
3
]
3
−
as a synthon, since
the chelated carbonate ion is readily displaced by other better chelating ligands such as
polyamines. An example of this type of reaction, where the entering polyamine (cyclam)
is a saturated and flexible macrocyclic tetraamine, is (6.27).
3-
O
1+
O
NH
O
O
HN
O
NH
HN
Co
O
Co
O
O
O
O
HN
NH
HN
O
NH
(6.27)
O
[Co(CO
3
)
3
]
3-
+ cyclam
[Co(CO
3
)(cyclam)]
+
+ 2 CO
3
2-
Where stronger oxidizing agents than air are required to take the Co(II) form to Co(III),
which usually applies where fewer N-donor and more O-donor groups are bound, hydrogen
peroxide is a particularly useful oxidizing agent, because it leaves no problematical products
to separate from the desired complex product. One of the problems with oxidation of
Co(II) to Co(III) compounds is that in some cases a bridged peroxo complex, featuring a
Co
III
O
2
2
−
Co
III
linkage, forms as a stable intermediate. One can envisage its formation
through a redox reaction whereby an oxygen molecule is reduced to peroxide ion, and two
Co(II) ions are oxidized to Co(III). The O
2
2
−
ion in the bridge can be readily displaced
by reaction with strong acid and heating, with the use of hydrochloric acid leading to
monomers with Co
III
Cl
−
components replacing the bridging group (6.28).
O
2
III
III
4+
O
-
O
-
Co
2+
aq
+ 5 L [L
5
Co(OH
2
)]
2+
L
5
Co
Co L
5
HCl
aq
L
4+
(6.28)
L
L
O
2+
L
L
Co
2 [Co (Cl)L
5
]
2+
III
L
L
Cl
L
O
L
L
Co
Co
L
L
L
1
2
(+ H
2
O + O
2
)
L
L
Many other metal complexes can be chemically oxidized to higher oxidation states with an
appropriate oxidizing agent. This can occur with complete preservation of the coordination
sphere (6.29), which means the oxidation-reduction reaction is reversible if a suitable
reducing agent is then employed for reduction of the oxidized form.
