Chemistry Reference
In-Depth Information
of a single-atom ligand capable of bridging, being replete with lone pairs of electrons; many
examples of M Cl M coordination exist. Another type commonly met is an ambivalent
ligand with two different donor atoms available, such as thiocyanate (SCN
−
), which offers
both a N and a S donor, and is able to form M SCN M linkages.
The basic process can be exemplified by the reaction of the cobalt(III) complex
[Co(NH
3
)
5
(NCS)]
2
+
(the oxidant) with the chromium(II) complex [Cr(OH
2
)
6
]
2
+
(the re-
ductant), where thiocyanate acts as the bridging ligand. The overall reaction in aqueous
acidic solution is shown in Equation (5.58).
[Co
III
(NH
3
)
5
(
N
CS)]
2
+
+
[Cr
II
(OH
2
)
6
]
2
+
(
5H
+
)
[Cr
III
(OH
2
)
5
(
S
CN)]
2
+
+
+
→
Co
2
+
aq
+
5(NH
4
+
)
(5.58)
The original inert Co(III) complex with N-bound thiocyanate has been reduced to a labile
Co(II) complex, which has dissociated into free protonated ligands and the Co(II) aqua ion.
Not only is the labile Cr(II) centre oxidized to an inert Cr(III) complex, but an S-bonded
thiocyanate group has been introduced into its coordination sphere. The only source of
this ligand is the original Co(III) complex. While this anion may have been captured
from solution following dissociation of the reduced Co(II) species, the observation that
it is bonded entirely by the S-donor group suggests that it may have been transferred in
a 'handshake' operation where it is shared between the cobalt and chromium centres; an
intermediate of the type
Cr(OH
2
)
5
]
4
+
}
is presumed to form, where one
water group in the labile Cr(II) complex has been displaced and replaced by the thiocyanate
S atom, as the N atom is already attached to the Co centre. The overall mechanism that has
been proposed is shown in Figure 5.21, for a general bridging ligand represented as X
[(H
3
N)
5
Co
NCS
{
Y.
L
L'
L
L'
bridge
formation
L
L
X
L'
L'
L'
L'
L'
L'
L
L
X
III
III
II
II
Co
Cr
Co
Cr
L
Y
L
Y
H
2
O
L
L
L'
L'
labile ligand
dissociation
electron transfer
L
L'
L
L'
L
L
L'
L'
L'
L
L
X
L'
III
L'
L'
III
II
II
Co
Cr
Co
Cr
Y
L
X
Y
L
OH
2
L'
L
L'
L
labile complex
dissociation
H
+
bridge
breaking
H
2
O
Co
2+
aq
+ 5(LH
+
)
Figure 5.21
Electron transfer between Co(III) and Cr(II) complexes by the
inner sphere
mechanism, involving
bridge formation in the intermediate. Transfer of the bridging group from the oxidant to the reductant
is not required, but is characteristic of the mechanism where it does occur.
