I6.1
EXPERIMENT I6
SUBSTITUTION REACTIONS OF METAL CARBONYLS
Among the stable, isolated, organotransition metal compounds which have been
studied, metal carbonyls are well represented.
INFRARED SPECTROSCOPY
Infrared spectroscopy of metal carbonyls has proved to be a valuable and
convenient source of information concerning both structure and bonding. The high
intensity of the bands attributable to the carbon-oxygen stretching mode in a metal
carbonyl makes the I.R. technique particularly convenient. It is often possible to infer
the symmetry of the arrangement of the CO groups from the number of CO
stretching bands observed in their spectrum. The procedure consists of first
determining from symmetry requirements how many CO bands ought to appear in
their spectrum for each possible structure. The experimental observations are then
compared to predictions, and thus structures which disagree are discarded. In
favourable cases only one structure remains. This procedure is reasonably reliable,
however, due regard must be given to the possibilities that bands may be weak or
superimposed.
A consideration of the expected CO stretching vibrations for cis and trans-ML2(CO)4
is shown below.
O
C
cis
L
CO
L
C
O
L
not I.R.
L
C
O
L
O
C
CO
C
OC
C
O
L
C
O
CO
L
I.R.
C
O
L
CO
CO
L
O
C
O
CO
I.R.
OC
C
O
L
L
C
O
L
CO
CO
O
C
O
CO
I.R.
OC
C
L
L
C
O
L
trans
C
O
C
O
OC
C
O
not I.R.
I.R. (same frequency)
I.R.
O
C
CO
L
I6.2
For the cis-isomer, all four are distinct and can absorb infrared radiation. For the
trans-isomer, two are equivalent and have the same frequency, forming a
degenerate vibration, only this one can absorb infrared radiation.
Thus a simple measurement of the number of CO stretching frequencies allows a
decision concerning the degree of substitution and whether a complex is the cis or
trans isomer.
Data for octahedral complexes are listed below in Table 1 and representative spectra
are shown at the end of this experiment.
TABLE 1
Octahedral
Complex
No.of CO Bands
I.R.
Raman
M(CO)6
1
2
M(CO)3L
3
4
cis-M(CO)4L2
4
4
trans-M(CO)4L2
1
2
fac-M(CO)3L3
2
2
mer-M(CO)3L3
3
3
cis-M(CO)2L4
2
2
trans-M(CO)2L4
1
1
In addition, the position of the CO stretching frequency also provides information
about the bonding modes and the extent of "back-bonding" in a metal carbonyl.
Terminal carbonyls of neutral or cationic complexes exhibit bands in the region 21251850 cm1 while bridging groups absorb in the range 1700-1850 cm1. For example,
the infrared spectrum of Fe2(CO)9 exhibits CO stretching frequencies at 2090, 2020,
1830 cm1. From this information alone it may be inferred that the structure must
contain both terminal and bridging carbonyls. In using the positions of the CO bands
to indicate the presence of bridging CO groups it is important to keep conditions in
mind: the frequencies of terminal CO groups can be quite low if (a) there are a
number of other ligands present in the complex that are good donors but poor acceptors or (b) the complex bears a net negative charge. Both of these effects tend
to increase the extent of back donation from metal to carbonyl (i.e., increase the
metal-carbon bond order) thus decreasing the carbon-oxygen bond order and CO
stretching frequency.
I6.3
A series of isolectronic species illustrating this trend (with their I.R. active CO
stretching frequency) is: Ni(CO)4 (2060 cm1); [Co(CO)4] (1890 cm1); [Fe(CO)4]2
(179 cm1).
EXPERIMENTAL
You will be required to prepare two substituted molybdenum carbonyl compounds
(consult your demonstrator to determine which of the experiments listed below must
be performed). As some of the experiments involve working under nitrogen you will
be introduced to the technique of synthesis under inert gas conditions.
The experiments are performed with the microscale kits.
I6.4
A.
PREPARATION OF Mo(CO)4(PPh3)2
Dissolve Mo(CO)6 (0.1 g), NaBH4 (0.05 g) and triphenylphosphine (0.24 g) in
ethanol (5 mL, heat up or add more ethanol if reactants do not dissolve
completely) contained in a round bottom flask connected to a reflux
condenser. Reflux this solution in air for 5 minutes. A yellow precipitate will
separate from the solution. Cool the flask in a salt-ice bath for several
minutes. Filter in air and wash the precipitate copiously with water. Then
wash with small portions of ice-cold ethanol or ether (1-15 mL total) and dry
on the water pump. Record your yields. Record the I.R. spectrum in
chloroform (or nujol) (2100 - 1700 cm1 region).
B.
PREPARATION OF Mo(CO)4 (DIPYRIDYL)
The 2,2'-dipyridyl is a bidentate ligand.
Set up the apparatus as shown below.
Degas toluene (5 mL) in a long necked round bottom flask with nitrogen.
Consult your demonstrator about this task. Add Mo(CO)6 (0.075 g) and
dipyridyl (0.045 g) to the flask and a boiling chip and reflux in the presence of
nitrogen for 60 minutes. Allow the solution to cool and increase the flow of
nitrogen during this process. (Why?) Filter the product in air. Wash with
petroleum ether (5 mL) and dry. Record the yield and the I.R. spectrum ion
nujol (2100 - 700 cm1 region).
I6.5
REPORT
Yields and interpretation of I.R. spectra must be reported.
QUESTIONS
1.
What are the molecular structures of the compounds you have prepared?
2.
Why should one normally work under an inert atmosphere when preparing
metal carbonyl derivatives?
3.
List all possible geometrical isomers for a complex Mo(CO)3(A)(B)(C).
4.
Draw structures for [(5-C5H5)Mo(CO)3]2, [(5-C5H5)Fe(CO)2]2, Fe3(CO)12 and
Os3(CO)12. (Consult the library for information.) Are isomers possible for the
first complexes? Explain.
REFERENCES
1.
R.J. Angelici, 'Synthesis and Technique in Inorganic Chemistry'. 2nd Edition,
W.B. Saunders Company, Philadelphia, 1977, p 144.
2.
D.M Adams, 'Metal-Ligands and Related Vibrations', Edward Arnold
Publishers, London, 1967.
3.
F.A. Cotton and G. Wilkinson, 'Advanced Inorganic Chemistry', 3rd Edition,
Interscience Publishers, New York, 1972.