Chemistry Reference
In-Depth Information
SiW
11
O
39
Mn] interestingly only shows one additional absorption band at 1261 nm after
the reaction with CO. For the underivatized (Q
7
)
8
[
D
-SiW
11
O
39
] additional absorptions
occur at 1922, 1882 and 1872 nm after the reaction with CO.
When the electron density on a metal center bound to a CO-ligand increases, stronger
S
-back-bonding to the CO ligand is enabled which weakens the C-O bond by donation of
more electron density into the formally empty carbonyl
S
*-orbital. Therefore the M-CO
bond strength is increased, making it more double-bond-like, and causes the resonance
structure M=C=O to assume more importance.
d
x
X
CO
(cm
-1
)
compound
free CO
2143
d
10
[Ag(CO)]
+
2204
d
10
Ni(CO)
4
2060
d
10
[Co(CO)
4
]
-
1890
d
10
[Fe(CO)
4
]
2-
1790
d
6
[Mn(CO)
6
]
+
2090
d
6
Cr(CO)
6
2000
d
6
[V(CO)
6
]
-
1860
Table 7
Overview of some homoleptic d-
metal CO complexes with the wavenumbers
for their carbonyl IR-vibration.
This effect is reflected in a weakened carbonyl IR-vibration e.g. the d
10
complexes
[Fe(CO)
4
]
2-
and [Ag(CO)]
+
differ by over 400 cm
-1
(see table 7). In addition a difference
between equally charged complexes of different d-electron configuration is the
consequence e.g. 60 cm
-1
between Ni(CO)
4
and Cr(CO)
6
.
Taking this well known relationship as basis it has to be assumed that the CO is not
coordinating to the hetero d-metal as all additional peaks in the obtained spectra
appear at practically the same wavenumber and do not show any dependency of the
heterometals charge density. Reaction at a different position of the cluster, as was
already assumed based on the missing shift of the d-d-absorption of the UV-Vis spectra,
seems to take place. Complexes of tungsten with CO are not uncommon (one just has to
think of the
Kubas
-complex) and in principle reaction of the CO with the addenda atoms
W
6+
could be possible in a reductive carbonylation reaction with W(CO)
6
as the product.
The second IR-band appearing at around 1261 cm
-1
could then be explained to stem
from the oxidized reducing agent necessary for such type of reaction which would
concretely be an oxidation product of the Q
7
. However the IR-vibration of W(CO)
6
is
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