Identification of Stereochemical Isomers of [Mo(CO)4(L)2] by
Infra-Red Spectroscopy
Courtney Arnott
February 6, 2002
Background
Group theory involves the use of flowcharts and character tables to predict the
number of bands in an FT-IR spectrum based on the symmetry of the molecule.
Molecules can be classified into different groups on the basis of their rotations around
different axes, which will predict their behavior in the IR. There are two main categories
of point groups: axial systems, which involve one main axis, and cubic/icosahedral
groups.1 This lab was concerned solely with axial systems, which are organized onto a
flowchart (see Figure 1).
A character table is a numerical representation of the rotational behavior of a
molecule around different axes, derived from spherical harmonics equations. The
notation of the character table is based on Mulliken’s convention. Each group has a set
number of designations, called “irreps”, labeled with A, B, and E. A and B are
nondegenerate, while E is doubly degenerate. The irreps are further identified with the
use of the subscripts g and u, which denote inversion symmetry. Each irrep is examined
for rotational modes around a particular axis and assigned an integer value. 1 Reduction of
the character table for the particular molecule’s point group allows a mathematical
determination of the number of IR active vibrational modes expected for cis- and transconfigurations of the molecule.
By synthesizing the two configurations and examining their IR spectra in the
range of the particular ligand of interest (in this case, CO), we can confirm the
configuration of each molecule. The more symmetrical a molecule, the fewer bands will
be visible in the IR spectra. Thus, the cis- configuration will show more bands than the
trans.
After synthesis and thermal isomerization, the cis- and trans- isomers of
[Mo(CO)4(PPh3)2] can be confirmed through group theory calculations and IR
spectroscopy. The reduction of character tables shows that the cis- configuration should
display four IR bands in the carbonyl region (1750-2100 cm-1), while the transconfiguration should display only one band in the carbonyl region.
Experimental
Reagents
Molybdenum Hexacarbonyl (Aldrich), Toluene (Mallinckrodt), Piperidine (Fisher
Scientific), Petroleum Ether (EM Science), Dichloromethane (Aldrich),
Triphenylphosphine (Aldrich), Methanol (Aldrich)
All reactions were carried out using Schlink Techniques under N2. IR data was obtained
using a Mattsen 4020 Galaxy Series FT-IR spectrophotometer.
Synthesis of molybdenum carbonyl complex [Mo(CO)4(pip)2]
Synthesis was carried out according to the technique described in Becket2, p. 111.
1 g [Mo(CO)6] was added to 40 mL dry toluene, and 10 mL piperidene was added. The
solution was refluxed for 2 hours and a yellow precipitate allowed to form. This
precipitate was filtered, washed with cold petroleum ether, and dried on a vacuum line.
This intermediate product was used for further synthesis as described below.
Synthesis and thermal isomerization of [Mo(CO)4(PPh3)2]
Synthesis was carried out according to the technique described in Becket2, p. 111.
0.5 g of [Mo(CO)4(pip)2] was partially dissolved in 20 mL dry CH2Cl2 and 0.75 g PPh3
was added. This was refluxed for 15 minutes, during which time an orange solution
should form. However, the loss of solvent during an excess of heat caused the CH2Cl2 to
boil off, leaving the product to thermally isomerize to the configuration that should have
been obtained in section C (determined to be the trans- configuration). This product
([Mo(CO)4(PPh3)2], trans- configuration) was redissolved in methanol and collected on a
frit over a vacuum line. The pale yellow product described in B (determined to be the cisconfiguration) was then resynthesized according to the technique described in Becket1
above, with the addition of a condenser tube attached to the top of the reflux system and
the heat carefully monitored to prevent solvent loss.
Data/Observations/Calculations
The synthesis did not proceed according to the reaction scheme outlined in
Beckett2 due to excessive refluxing at a very high temperature. The heat caused the
solvent to boil off (because no condenser tube was used), which allowed the temperature
of the material to rise high enough to thermally isomerize the compound to the transconfiguration. The reaction system was exposed to excess air due to pressure problems
with the air outlet setup. Thus, the reaction was not carried out completely under a N2
purge as stated in the synthesis directions.
Tables
Compound
Mo(CO)4(pip)2
Melting Point Yield
% Yield
249°C
1.1374 g 75%
Mo(CO)4(PPh3)2 (trans-)
191°C
0.909 g
29.1%
Mo(CO)4(PPh3)2 (cis-)
182°C
0.32 g
10.2%
Table 1. Compound Data
C2v
E
C2
v(xz)
'v(yz)
A1
1
1
1
1
A2
1
1
-1
x2, y2, z2
-1
z
Rz
xz
yz
B1
1
-1
1
-1
x, Ry
B2
1
-1
-1
1
y, Rx
xy
Table 2. C2v Character Table (cis- configuration)
D4h
E
2C4
C2
2C2'
2C2"
i
2S4
h
2v
2d
A1g
1
1
1
1
1
1
1
1
1
1
A2g
1
1
1
-1
-1
1
1
1
-1
-1
A3g
1
-1
1
1
-1
1
-1
1
1
-1
x2-y2
A4g
1
-1
1
-1
1
1
-1
1
-1
1
xy
Eg
2
0
-2
0
0
2
0
-2
0
0
A1u
1
1
1
1
1
-1
-1
-1
-1
-1
A2u
1
1
1
-1
-1
-1
-1
-1
1
1
A3u
1
-1
1
1
-1
-1
1
-1
-1
1
A4u
1
-1
1
-1
1
-1
1
-1
1
-1
Eu
2
0
-2
0
0
-2
0
2
0
0
Table 3. D4h Character Table (trans- configuration)
Figures
See Figure 1 for group theory flowchart, and Figures 2-5 for FT-IR spectra data.
x2+y2, z2
Rz
(Rx, Ry)
z
(x, y)
(xz, yz)
Calculations

Percent yield, Synthesis A
Mo(CO)6 = 263 g/mol, pip = 85 g/mol, Mo(CO)4(pip)2 = 377 g/mol
 1mol 
 = .004 mol starting material
1.06 g 
 263g 
 1mol 
 = .003 mol product
1.1374 g 
 377 g 
.003mol
x 100 = 75% yield
.004mol

Percent yield, Synthesis B (to trans- compound)
Mo(CO)4(pip)2 = 377 g/mol, PPh3 = 261 g/mol, Mo(CO)4(PPh3)2 = 729 g/mol
 1mol 
 1mol 
 + .75 g 
 = .0043 mol starting materials
.5 g 
 377 g 
 261g 
 1mol 
 = 1.25x10-3 mol
.909 g 
729
g


3
1.25 x10 mol
x 100 = 29.1% yield
.0043mol

Percent yield, Synthesis B (to cis- compound)
Mo(CO)4(pip)2 = 377 g/mol, PPh3 = 261 g/mol, Mo(CO)4(PPh3)2 = 729 g/mol
 1mol 
 1mol 
 + .75 g 
 = .0043 mol starting materials
.5 g 
 377 g 
 261g 
 1mol 
 = 4.39x10-4 mol
.32 g 
729
g


4.39 x10 4 mol
x 100 = 10.2% yield
.0043mol
Results/Conclusions
Analysis of the IR spectra for each synthesis did not show the expected bands in
the carbonyl range. However, bands were observed at approximately 3100-3300 cm-1, in
a similar configuration to what was expected. See Figures 3 and 4 for IR data. The
product that was predicted to be a cis- configuration (which is more kinetically favored in
synthesis) showed one band in this higher region, as well as one clear band in the CO
region (see Figure 3). The product that was heated to thermally isomerize the ciscompound to the trans- compound (which is more stable and thermodynamically favored)
showed 4 bands in the 3100-3300 cm-1 region and a broad band in the CO region (see
Figure 4). When a more dilute sample was examined in the IR, the broad CO band
resolved into two clearer, but still wide, bands (see Figure 5). The band on this IR shows
that the sample is still extremely concentrated, and thus the data cannot be resolved. Had
a dilute enough sample been run on the IR, the broad CO band would have resolved into
4 separate bands.
The resolved peaks in the 3100-3300 cm-1 region suggest a symmetry that point to
a synthesis and thermal isomerization. The peaks are in the region of aromatic rings, and
could be due to the addition of tri-phenyl phosphene rings. However, the stereochemistry
of this molecule is too complex to verify the group theory prediction through the use of
character tables.
Two sources of error must be taken into account when analyzing the results. It is
possibly that the solvents were not dried over the molecular sieve thoroughly, which
would introduce water into the reaction system. In addition, the lack of a complete N2
environment could have affected the reaction as well. However, the similarity of the
peaks in the IR data verifies that a cis- isomer and a trans- isomer were both correctly
synthesized and identified, as predicted by group theory.
In order to minimize error in future research, a condenser tube should be used on
all reactions and the heat should be monitored carefully to avoid solvent loss. A better
setup of the N2 atmosphere and careful drying of all solvents over molecular sieves would
assure that the correct compound is synthesized. If time had allowed in this experiment,
the synthesis and IR analysis would have been repeated and a more dilute sample
analyzed in order to attempt to correctly synthesize the CO compounds and resolve the
peaks in the CO region.
References
1. Taylor, Peter R. “Chem 249: Symmetry, Structure, and Spectroscopy.” (Course
lecture notes, University of California, San Diego). Internet,
http://www.sdsc.edu/QC/Lectures.pdf, January 9, 2000 (accessed 4-3-02).
2. Becket, Michael A. “Identification of Stereochemical (Geometrical) Isomers of
[Mo(CO)4(L)2] by Infra-Red Spectroscopy.”