1. Michael Ludden Level 3 Project AH2
1
Synthetic and Kinetic Studies of Migratory Insertion Reactions using
varying Molybdenum Complexes
Michael Ludden
Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK.
Email: mludden1@sheffield.ac.uk
Abstract
A variety of Molybdenum complexes were synthesised from a Molybdenum carbonyl dimer
starting material and characterised using 1
H NMR, 13
C NMR and IR spectroscopy. From these
complexes, kinetic measurements were taken for the reaction between [CpMo(CO)3R]
(R = Me, Et) and a phosphine nucleophile, PR3, using the software TimeBaseTM
. The effects of
varying individual aspects of the reaction under pseudo-first-order conditions (with respect to
the nucleophile) were investigated, and it was found that changing the R group, the solvent and
the temperature of reaction all produced a change in the rate constant, Kobs. Activation
parameters for the reaction were calculated and compared to literature values, with a negative
ΔSǂ
value obtained indicating an associative mechanism.
Introduction
A migratory insertion reaction involves a
metal complex with unsaturated co-
ordinated ligands and a nucleophilic X-type
ligand effectively undergoing nucleophilic
attack by either an incoming nucleophile, L,
or a co-ordinating solvent such as THF.1 This
prompts migration of the X-type ligand,
commonly an alkyl group, into one of the
unsaturated ligands cis to it, hence the term
‘migratory insertion’ (see Fig. 1). It is
possible for the insertion itself to occur two
ways: either 1,2-insertion or 1,1-insertion.
The process of migratory insertion forms a
new carbon-carbon or carbon-hydrogen
bond. It is this characteristic that makes it
1
I. S. Butler, F. Baloso, R. G. Pearson, Inorganic Chemistry, 1967,
6, 2074.
2
L. S. Hegedus, Transition Metals in the Synthesis of Complex
Organic Molecules, University Science Books, Sausalito, CA,
1999.
very useful when combined with a catalyst
in industrial applications, as it allows alkyl
fragments to have useful functional groups
added to them.2 A well-known example of
this is Alkene Hydrogenation using
Wilkinson’s Catalyst, [RhCl(PPh3)3].3
A process that utilises carbonyl migratory
insertion, and is more relevant to the topics
discussed in this report, is the Monsanto
process. This involves the carbonylation of
methanol to produce acetic acid and is a
process used industrially on a phenomenal
scale, with ca. 7 million tonnes being
produced annually.4
The rate at which migratory insertion will
occur depends on many different
experimental factors, most of which can be
varied reasonably easily. The rate
determining step in the overall mechanism
is the migration of the X-type ligand itself,
3
J. A. Osborn, G. Wilkinson and J. F. Young, Chem. Commun.
(London), 1965, 17.
4
A. Haynes, in Catalytic Carbonylation Reactions, ed. Beller and
Matthias, Springer, Berlin, 2006, ch. 4.
Figure 1 - Solvent assisted migration of alkyl group into
neighbouring CO ligand

2. Michael Ludden Level 3 Project AH2
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followed by the incoming L-type ligand
binding (which is relatively much quicker).
This is detailed in Fig. 2, seen below.
This report aims to investigate the kinetics
of migratory insertion reactions with
Molybdenum carbonyl complexes by
performing a series of experiments to
determine the rate law for the reaction, the
activation parameters ΔHǂ and ΔSǂ, and the
dependence of rate on factors such as the
solvent and nature of the incoming ligand.
Results and Discussion
1. Synthesis and Characterisation of
[CpMo(CO)3Me]
The first compound produced in the lab was
the methyl-group containing complex
[CpMo(CO)3Me]. This was done by a simple
reduction of a molybdenum carbonyl
dimer, [CpMo(CO)3]2 using Super Hydride
and then reacting the anion produced with
methyl iodide to add the methyl ligand.
After purification, the crystals produced
were small and bright yellow in colour. A
yield of 62% was calculated for this
5
G. Miessler, D. Tarr, Inorganic Chemistry, Pearson Education, St
Quezon City, 2007.
synthesis. The crystals were determined to
be the anticipated product through IR and
NMR characterisation – note the singlet
peak at ≈ 0.4 ppm in the 1H NMR spectrum
indicating the presence of the methyl
ligand. The protons on the methyl group are
very shielded due to them being bound to
the electropositive Molybdenum centre,
hence the lower than usual ppm shift (0.4
compared to 0.9).
A similar situation is seen in the 13C NMR
spectrum. The methyl group is again at a
much lower ppm shift than would usually
be expected – in this case it is at -22 ppm
(lit. value -29.8 – 23.5 ppm).5 The peak for
the Cp ring can clearly be seen at 92 ppm
(lit. value 91 – 129 ppm) and the upwards
pointing triplet at 77 ppm is that of the
solvent, CDCl3.
The IR spectrum recorded for this complex
shows two carbonyl peaks; the one at
around 1942 cm-1 is due to both the A’ and
A” stretching bands overlapping whilst the
peak at 2024 cm-1 is the symmetric
vibration. Note the absence of any peaks
lower down the spectrum, in the ketone
C=O stretch region. This points to all the
carbonyl ligands being terminal and triple-
bonded as in free CO; this is what would be
expected for this compound.
Another point worth expanding on is the
presence of what could be called ‘satellite’
peaks on the IR spectrum for this
compound and the ethyl and iodide
compounds also. The reason for these very
low intensity peaks is the presence of the
13C isotope in the sample. With a natural
abundance of 1%, this matches up with the
relative intensities of the peaks seen on the
spectrum. They are seen at a slightly lower
wavenumber due to the slightly higher
Figure 2 - Reaction pathway for a migratory insertion
reaction involving an incoming ligand, L.
Figure 3 - The two-step reaction schematic for the
synthesis of [CpMo(CO)3Me].

3. Michael Ludden Level 3 Project AH2
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mass of 13C – as the CO molecule can be
treated as two atoms on a vibrating ‘spring’,
the frequency of vibration will decrease
when the mass of one of the carbon atoms
is increased.6 This explains why the
satellites are seen at a lower wavenumber
as opposed to a higher one.
Upon studying the characterisation data, it
was decided the product was indeed
[CpMo(CO)3Me].
2. Synthesis and Characterisation of
[CpMo(CO)3Et]
The next complex to be synthesised was
chosen to be [CpMo(CO)3Et], as it could be
synthesised following the route for the
previous compound but by substituting
methyl iodide for iodoethane.
This reaction afforded small, dark green
crystals with a yield of 54%. This was only
for the crude product, however, as the
vacuum sublimation step (detailed further
in the experimental) was very time-
consuming and was therefore not
performed for this compound. The spectra
were still collected as before.
The indications of the ethyl group being
present in the complex echoed those seen
in the methyl complex both in the 1H and
13C NMR spectra.
The IR spectra of this complex is near-
identical to that of the methyl complex.
Again, two distinct peaks are seen, but this
time at 1936 and 2019 cm-1, about 5
wavenumbers lower than those seen in the
6
H. Haas, R. K. Sheline, J. Chem. Phys, 1967, 47, 2996.
methyl complex. This difference can be
explained by bond strengths and the impact
of the bound ligand. An ethyl ligand is a
stronger electron donor than a methyl
ligand due to inductive effects, and so will
increase the electron density on the metal
centre. This leads to the bond between the
metal centre and the bound CO ligands
becoming stronger due to increased
backbonding (and stronger forward
bonding, taking synergic bonding into
account).7 In turn, this means the C≡O bond
is weakened, resulting in a lower vibrational
frequency in the IR spectrum. The
difference in this case is about 5 cm-1, as
mentioned earlier.
The NMR spectra obtained for this complex
also lent weight to the notion the synthesis
had been successful without the additional
purification step.
Whilst the 1H NMR spectrum for the ethyl
complex was quite cluttered with what
appeared to be contaminants such as water
and silicone grease, the triplet at 1.45 ppm
and the neighbouring quartet at 1.7 ppm
are indicative of a CH3 and CH2 group
respectively. This points very strongly to the
presence of an ethyl ligand, and further
agrees with a successful synthesis.
7
F. A. Cotton, C. S. Kraihanzel, J. Am. Chem. Soc., 1962, 84 (23),
pp 4432–4438.
Figure 4 - The complex
[CpMo(CO)3Et]
Figure 5 - The forward and backbonding seen between a
metal centre and any bound CO ligands. Both of these will
be strengthened by the presence of a donor ligand eg. Et

4. Michael Ludden Level 3 Project AH2
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The 13C NMR spectrum continues to
support a successful synthesis through the
peaks at -5 ppm and 20 ppm being typical
of a metal-bound ethyl group. The negative
ppm value again is due to the shielding
provided through bonding to the metal
centre.8 The large peak at 92 ppm shows
the Cp ring is still present in the product
also. After investigating the product’s
obtained spectra, it was deemed to be
[CpMo(CO)3Et], as anticipated.
3. Synthesis and Characterisation of
[CpMo(CO)2(COMe)(PPh3)]
As a large section of this experiment would
involve chemical kinetic investigations
producing compounds similar to
[CpMo(CO)2(COMe)(PPh3)], it seemed
sensible to synthesise and analyse this
product, to give some spectra to compare
the kinetic results to.
This product was synthesised by reaction of
the methyl complex synthesised earlier in
the experiment, [CpMo(CO)3Me], with
triphenylphosphine, PPh3. This was done
over a long period of time to ensure the
reaction would go to completion. A bright
yellow precipitate was formed overnight,
and this was filtered off and washed with
light petroleum. This gave dull yellow
crystals, with a calculated yield of 65%.
Initially the plan was to recrystallise this
precipitate, to purify the product and
8 Y. Ruiz-Morales, G. Schreckenbach, T. Ziegler,
Organometallics, 1996, 15 (19), pp 3920–3923.
9
A. G. Orpen; N. G. Connelly, “Structural systematics: the role of
P-A σ* orbitals in metal-phosphorus π-bonding in redox-related
obtain a more accurate yield. However,
upon attempted recrystallisation no
crystals were formed. The solvent of the
solution was removed under reduced
pressure and crystals of the original, crude
product were afforded. It is likely the
recrystallisation failed as too much hot
solvent was added to the impure crystals.
Both IR and NMR spectra were run for the
crude product to confirm its identity. The IR
spectra for this complex differs from those
seen before as there is a new carbonyl
environment present. As seen previously,
there are still two carbonyl peaks visible at
the higher end of the spectrum – this time
at 1938 and 1855 cm-1. These are lower
than the peaks in this region for both the
methyl and ethyl complexes; this is likely
due to the replacement of an electron
donating X-type ligand (Me, Et) with a L-
type ligand, PPh3. The PPh3 ligand acts as
both a σ-donor and a π-acceptor, in the
same manner as a CO ligand.9 This results in
stronger M-C bonds and this in turn results
in weaker C≡O bond, and a lower
wavenumber.
The IR spectrum also displays a peak around
1620, lower than any seen previous. The
acyl ligand, COMe, is responsible for this
peak. As the ligand contains only a C=O
double bond, as opposed to a triple bond
seen in the other ligands, it is at a lower
wavenumber in a region more commonly
associated with ketones, aldehydes and
other such carbonyl containing compounds.
The presence of this peak confirms that the
product was likely to be
[CpMo(CO)2(COMe)(PPh3)].
The 1H NMR spectrum for this complex did
not yield much information as the sample
pairs of M-PA3 complexes”, Organometallics, 1990, 9 (4): 1206–
1210.
Figure 6 - The reaction scheme for the synthesis detailed
above

5. Michael Ludden Level 3 Project AH2
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was of an insufficient concentration. The
lower end of the spectrum is cluttered with
unidentifiable peaks, many of which could
be contaminants such as grease or water.
Further up the spectrum, however, are
peaks which can be labelled as the Cp ring,
seen at 5.0 ppm, and the phenyl rings of the
triphenylphosphine ligand, seen at roughly
7.5 ppm.
The 13C NMR spectrum also suffered from a
lack of quality due to another low
concentration sample. The signal-to-noise
ratio for the spectrum is poor and the peak
for the methyl group is not apparent. What
the spectrum does possess, however, is the
peak indicating the Cp ligand and numerous
separate environments for the phenyl
groups on the PPh3 ligand. The Cp ligand is
the peak seen at 97 ppm and if the
expansion is studied closely, three distinct
peaks can be identified, all related to
carbons in each phenyl ring. The only
disadvantage to this expansion is that the
fourth peak, a doublet with a large J value,
is not observable amongst the noise. This
missing doublet would relate to C1, seen in
Fig. 7, as it would have the largest J value.
This is because phosphorus contains spin-
active nuclei with spin = ½, and so 13C can
couple to it, hence the doublets observed
on the spectrum. The nearer the carbon to
the coupled phosphorus, the larger the
splitting will be.
The presence of phosphorus in this
compound meant that for the first time in
this experiment, 31P NMR could be used.
The spectrum obtained contained a single
peak, which was expected for this
compound, as there was only one
phosphorus atom present, in the PPh3
ligand. This contributed evidence to suggest
the synthesis had been completed
successfully.
4. Synthesis and Characterisation of
[CpMo(CO)3I]
The final compound synthesised was one
containing an atom of Iodine as a ligand.
This was done by the relatively
straightforward synthesis route of reacting
the molybdenum dimer with a
stoichiometric quantity of I2. The relevant
equation is shown in Eqn. 1, below.
[CpMo(CO)3]2 + I2 → 2[CpMo(CO)3I]
Equation 1 - The reaction between the molybdenum
dimer and Iodine
This reaction produced a small quantity of
dark purple crystals, with a calculated yield
of 36%. Characterisation was carried out on
these crystals without further purification.
The IR spectrum for this compound is
similar to that of both the methyl and ethyl
complexes, but with one notable
difference; the peaks that originally
overlapped on both aforementioned
spectra are visibly separate for this
complex. An explanation for this
observation is the electronegativity of
Iodine – this property means electron
density is pulled away from the metal
centre, producing weaker backbonding to
the CO ligands. This gives a higher CO bond
order and therefore stronger bonds, hence
Figure 7 - A diagram of the product complex with labelled
13C environments on the phenyl ring. All 3 phenyl rings
can be considered the same.

6. Michael Ludden Level 3 Project AH2
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the higher wavenumbers linked to the
peaks (a shift of about 20 cm-1).
The 1H NMR spectrum yields little other
than peaks seen before in other complexes.
The peak at 5.6 ppm indicates the presence
of the Cp ring and there is little else shown
on the spectrum, as expected.
The 13C NMR spectrum gives something
more to discuss in the more distinct CO
environments present in this complex.
There are two distinct CO peaks, leading to
the conclusion that two environments are
present. Looking at the predicted structure
of the product, this is in agreement with the
spectra obtained; two of the CO ligands can
be considered ‘cis’ to the electronegative
Iodine ligand whereas the third is opposite
and can be considered ‘trans’.10 Of course,
these terms are only used loosely as the Cp
ring causes distortion meaning the
remaining ligands are not truly ‘trans’ or
‘cis’. The ratio of the peaks also matches up
with the 2:1 cis to trans ratio of the
complex.
All the evidence detailed here pointed to
the complex synthesised indeed being
[CpMo(CO)3I].
5. Kinetics of Migratory Insertion reactions
of [CpMo(CO)3R] (where R = Me, Et)
The kinetic experiments for the complexes
synthesised involved producing samples of
both the incoming phosphine/phosphite
and the metal complex dissolved in the
chosen solvent. Initially, a background
spectrum was run of the phosphine (or
phosphite) solution using TimeBaseTM IR
software, and then the solution of metal
complex was mixed with the
phosphine/phosphite solution. Upon
10
L.J. Todd, J.R. Wilkinson, J.P. Hickey, D.L. Beach and K. W.
Barnett, Journal of OrganometaIlic Chemistry, 1978, 154, p. 151-
157.
mixing, the software was set to run in the
region of 20+ spectra over a given time, to
allow observation of the reaction
proceeding. The focus of the spectra was
the carbonyl region, between 2200 and
1500 cm-1, as this allowed both the
decrease in reactant and increase in
product to be seen through the IR spectra;
as the alkyl complex carbonyl peak
decreases the acyl complex carbonyl peak
would increase. Once the time set for the
experiment had elapsed or the reaction was
deemed to have gone to completion, the
data was exported to KaleidagraphTM and
from this the rate constant was calculated.
y = ((m2 − m3) e−(m1mo)
) + m3
Equation 2 - The equation used in Kaleidagraph to
calculate the line of best fit for the data gathered.
The equation shown above was used to
produce a trendline to fit the data obtained
from the spectra. Absorbance was plotted
against time and this produced a graph
displaying an exponential curve, as seen in
the example graph, Fig. 8.
Figure 8 - An example of the graphs produced by
Kaleidagraph. This particular example is for the methyl
complex reacting with PMe3 in THF at 48.8 °C

7. Michael Ludden Level 3 Project AH2
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Equation 2 could be related to another
equation, labelled as equation 3, to
determine a value for the rate constant, k.
𝐴 = (𝐴∞ − 𝐴0) 𝑒−𝑘 𝑡
+ 𝐴0
Equation 3 - The modified version of Eqn. 2; this links the
recordable values for x and y - these being time and
absorbance, respectively.
By comparing the two equations it can be
seen that the rate constant, k, can be
compared to m1 in the first equation (Eqn.
2). KaleidagraphTM produced a value for m1
in the table seen above the graph, and
these were taken and recorded using
Microsoft ExcelTM to compare the values for
different complexes, solvent, incoming
ligand and varying temperature. These will
now be looked at in more detail
individually.
5.a. Effect of complex (variation of R
group)
Two complexes were compared:
[CpMo(CO)3Me] and [CpMo(CO)3Et]. All
factors other than temperature and the
alkyl substituent were kept constant
throughout this sub-experiment.
As Table 1 displays, the rate constants for
the ethyl complex are a factor of 22.5 to
27.5 times larger than those for the methyl
complex. The difference in k with increasing
11
S. P. Nolan, R. L. de la Vega, S. L. Mukerjee, and C. D. Hoff,
Inorg. Chem. 1986, 25, p. 1160-1165.
temperature is roughly double for both
complexes.
It is likely that the rate constant is larger for
the ethyl complex because of the increased
steric strain of the ethyl complex. As ethyl is
a bulkier ligand than methyl, incorporating
it into a CO ligand to form an acyl ligand will
relieve this steric strain, and give a more
stable product. It seems logical, therefore,
that the rate constant for ethyl will be
higher.11
It was also suggested that due to the ethyl
complex’s ability to undergo β-elimination,
it would be more unstable than the methyl
complex (which cannot undergo β-
elimination). This lower stability results in a
more reactive species and therefore a
faster rate of reaction.
Past studies on migratory insertion
involving alkyls report that the σ-bond
strength of the alkyl-Mo bond may also
affect the rate; ethyl forms a weaker bond
to the Mo centre and also reacts quicker.12
5.b. Effect of incoming ligand (variation of
phosphine/phosphite)
The next variable tested was the nature of
the incoming ligand that promoted
migratory insertion on the complexes. Four
different ligands – two phosphines and two
phosphites – were tested and the rate
constants compared.
12
P. J. Craig and M. Green, J. Chem. Soc (A), 1968, p. 1978-1981.
Complex Temp. / K Kobs/s-1
[CpMo(CO)3Me] 311.0 1.73x10-4
[CpMo(CO)3Me] 321.2 3.94x10-4
[CpMo(CO)3Et] 311.0 4.76x10-3
[CpMo(CO)3Et] 321.2 8.77x10-3
Complex Nucleophile Kobs/s-1
[CpMo(CO)3Me] PPh3 3.94x10-4
[CpMo(CO)3Me] PCy3 3.89x10-4
[CpMo(CO)3Me] P(OEt)3 3.40x10-4
[CpMo(CO)3Me] P(OMe)3 3.50x10-4
Table 1 - Data for the variation of alkyl
substituent at two different temperatures.
Table 2 - Data for the variation of the
incoming nucleophile.

8. Michael Ludden Level 3 Project AH2
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Variation of the nucleophile had very little
effect on the rate constant, with a total
range of 0.54x10-4 between the smallest
and largest values. This is a great contrast
from the variation seen by varying the alkyl
ligand on the complex. The values are close
enough that it could be said with
confidence that there was no effect on the
rate constant by varying the incoming
nucleophile. There is a slight difference
seen between the phosphines and
phosphites – about 0.4x10-4 s-1 separates
them. This could be explained by the
presence of electronegative oxygen atoms
in phosphites, drawing electron density
away from the phosphorous atom and
causing the nucleophile to be weaker
overall. Other studies of this experiment
prior to this paper have reached the same
conclusion.13
The lack of variation of Kobs between ligands
points to the nucleophile co-ordination step
of the mechanism not being rate-limiting.
As migratory insertion is a two-step
process,14 it suggests that the methyl
migration to form an acyl ligand is in fact
the rate-limiting step.
5.c. Effect of ligand concentration (varying
the concentration of the nucleophile)
In addition to varying the nature of the
incoming nucleophile, the concentration
was also varied in an attempt to determine
whether the rate depended on this factor.
13
I. S. Butler, F. Baloso, R. G. Pearson, Inorganic Chemistry,
1967, 6, 2074.
Perhaps the most obvious first impression
from the data shown in Table 3 is that the
rate was faster when the nucleophile was at
the same concentration as the complex,
and that a 2:1 ratio produced an almost
identical value for Kobs as having the
nucleophile in excess. This was an
unpredictable outcome for this experiment,
but could potentially be explained by the
manipulation on the rate law caused by the
new concentrations used.
When the nucleophile is in excess, the rate
law is pseudo first order, and takes the form
seen in equation 4:
Rate = k′
[Complex]
Equation 4 - Rate law for the migratory insertion process.
And as 𝑘′
= 𝑘[𝑁𝑢], the nucleophile’s
concentration is not involved in the rate, as
it is considered a constant. When a 1:1 ratio
is used, however, the nucleophile is no
longer in excess and the rate law becomes
second order:
Rate = k[Nu][Complex]
Equation 5 - Second order rate law for the 1:1
[Nu]:[Complex] case.
This could explain the anomalously high Kobs
observed for this reaction.
Sadly, compared to the other variables
tested during this experiment, varying the
ligand concentration did not yield much
useful information.
14
M. J. Wax and R. G. Bergman, J. Am. Chem. Soc. 1981, 103, p.
7028-7030.
Complex [Nu]:[Complex] Kobs/s-1
[CpMo(CO)3Me] excess 1.80x10-3
[CpMo(CO)3Me] 1:1 2.95x10-3
[CpMo(CO)3Me] 2:1 1.82x10-3
Table 3 – Data for the variation of the
incoming nucleophile concentration.

9. Michael Ludden Level 3 Project AH2
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5.d. Effect of temperature (variation of
reaction temperature)
In this series of tests, the temperature of
reaction was varied between 30 and 60 °C
on the apparatus display. These
corresponded to a slightly lower actual
operating temperature due to heat loss in
the tubes etc. This was accounted for in the
data table:
Table 4 - The variation of the rate constant with changing
temperature.
It was found that an increase in
temperature resulted in an increase in Kobs.
The relationship between the two was
determined as exponential, as seen in the
Arrhenius equation (eqn. 6).
k = Ae
−EA
RT
Equation 6 - The Arrhenius equation linking a reaction's
rate constant and the temperature of reaction.
The results obtained from this section of
the experiment agree with previous studies
in this area.15 As the temperature is
increased, more molecules in the reaction
mixture will possess an energy greater than
EA, allowing the reaction to proceed.
Figure 9 shows the link between the two
values with an R2 value of 0.9976. This is a
very strong correlation and confirms that
the rate of reaction is dependent upon
temperature.
15
F. Calderazzo, F. A. Cotton, Inorg. Chem., 1962, 1 (1), p. 30–
36.
5.e. Effect of solvent (variation of solvent
reaction carried out in)
The reaction was carried out in co-
ordinating solvents, these being either THF
or MeCN. The co-ordinating effect of the
solvents means that the 16-electron
intermediate of the reaction is stabilised
and will react with the incoming
nucleophile at a higher rate than in non-co-
ordinating solvents.16 It was considered
that the solvent’s strength as a donor could
affect the rate of reaction, as the solvent co-
ordination occurs before the nucleophilic
attack of the new ligand.
As table 5 shows, the rate constant was
about 4.5 times lower in THF than in MeCN.
This observation agrees with theory
regarding donor strength; as MeCN is a
linear donor whereas THF is bulkier, MeCN
is expected to be a better donor. The ability
of MeCN to accept backbonding into its π*
16
A. Haynes, Longitudinal Ligands in Organometallic Chemistry,
CHM3104, University of Sheffield, 2015, p. 34-38.
Complex
Display
Temp/°C
Operating
Temp/°C
Kobs/s-1
[CpMo(CO)3Me] 30.0 28.8 6.56x10-5
[CpMo(CO)3Me] 40.0 38.0 1.73x10-4
[CpMo(CO)3Me] 50.0 48.2 3.94x10-4
[CpMo(CO)3Me] 60.0 58.0 9.94x10-4
Complex Solvent Kobs/s-1
[CpMo(CO)3Me] THF 3.94x10-4
[CpMo(CO)3Me] MeCN 1.80x10-3
y = 0.0489e0.0917x
R² = 0.9976
0
1
2
3
4
5
6
7
8
9
10
11
20 30 40 50 60
Kobs/s-1
Temperature / °C
Comparison of T and Kobs
Figure 9 - The relationship between T and Kobs in graphical
form.
Table 5 – Comparison of solvent effects on the rate
constant. Both solvents used were co-ordinating.

10. Michael Ludden Level 3 Project AH2
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orbital was also suggested to contribute to
its strength as a donor (THF does not have
this ability).17
Having the rate constants display this effect
lends weight to the proposal that the
mechanism is solvent-assisted and will
occur faster in more co-ordinating solvents.
It also points to the rate limiting step
involving solvent co-ordination, otherwise
the solvent would have little to no effect on
Kobs.
5.f. Effect of adding Lewis Acid (catalyst)
The final variation tested during this
experiment was addition of a catalyst in the
form of a Lewis Acid, AlCl3. It was predicted
that the rate would increase by a factor up
to 108 times upon addition of the catalyst.
The effect of the AlCl3 group is to polarise
the CO ligand and promote migration of the
alkyl group. Once migration has occurred
the AlCl3 group remains bound to the CO
ligand and donates a lone pair to the new
vacant site present on the metal centre,
negating the need for a co-ordinating
solvent to stabilise the 16-electron
intermediate.
17
K. Burger, Solvation, Ionic and Complex Formation Reactions
in Non-Aqeuous Solvents, Elsevier Scientific, Amsterdam, 1983.
Table 6 indicates how the results from this
experiment did not display the large
increase in rate predicted before the
readings were taken. There was no notable
difference in Kobs with the catalyst added –
in fact there was a slight decrease observed.
This was attributed to the aluminium
chloride reactant being aged and
potentially unreactive due to moisture
contamination (aluminium trichloride can
hydrolyse upon contact with water).
6. Activation Parameters for the reaction
Having calculated rate constants for the
reaction between the methyl complex
[CpMo(CO)3Me] and PPh3, it was possible to
calculate the activation parameters, ΔHǂ
and ΔSǂ, for this reaction. These parameters
are useful indicators of the reaction
mechanism, whether it be associative or
dissociative.
The rate constant is linked to the Gibbs’ free
energy of the reaction, and using the Eyring
equation produces equation 7, seen below:
𝑙𝑛 k =
−∆Hǂ
RT
+ 𝑙𝑛 (
kBT
h
) +
∆Sǂ
R
Equation 7 - The modified Eyring equation, allowing us to
calculate values for the enthalpy and entropy of reaction.
kB is Boltzmann’s constant.
This equation is conveniently in a form
comparable to the equation for a linear
plot, 𝑦 = 𝑚𝑥 + 𝑐. By plotting ln k on the y-
axis and 1/T on the x-axis, values for the
enthalpy and entropy of reaction can be
Complex Nucleophile Kobs/s-1
[CpMo(CO)3Me] PPh3 1.73x10-4
[CpMo(CO)3Me] PPh3/AlCl3 1.67x10-4
Figure 10 - A diagram of how the 16 electron
intermediate is stabilised by a co-ordinating solvent.
Here, THF is donating into the metal centre.
Figure 11 - AlCl3 acting as a catalyst during the migratory
insertion reaction.
Table 6 - Comparison of the rate constant with and
without the catalyst added.

11. Michael Ludden Level 3 Project AH2
11
calculated, allowing determination of the
mechanism for the reaction.
The linear trendline for this plot gives values
that can be incorporated into calculations
to produce the values for ΔHǂ and ΔSǂ. A
sample calculation for this will be provided
in the appendix to this report.
The first point to make relative to the data
shown in table 7 is the trend for the entropy
of activation. This experiment produced a
lower ΔSǂ value for the methyl complex
than for the ethyl complex; a trend not
18
I. S. Butler, F. Baloso, R. G. Pearson, Inorganic Chemistry,
1967, 6, 2074.
* This value is for reaction with P(n-OC4H9)3
19,19,20
S. P. Nolan, R. L. de la Vega, S. L. Mukerjee, and C. D.
Hoff, Inorg. Chem. 1986, 25, p. 1160-1165.
reflected in the literature. This could be due
to experimental error in the lab and/or the
quality of the ethyl complex sample; as it
was not purified, impurities may have been
present.
The negative value of ΔSǂ for both
complexes is indicative of an associative
mechanism.22 As the transition state is
more ordered, its entropy is lower and so
ΔSǂ would be expected to decrease. This
agrees with the notion mentioned earlier
involving solvent co-ordination in the rate
limiting step, which is itself an associative
process.
The data obtained from this experiment
does not correlate with the data found in
the literature for values of ΔHǂ. It is
reported in previous studies of this area
that the barrier for insertion involving metal
carbonyls is greater for larger R groups.23
This would assume a larger ΔHǂ value for
the ethyl complex, as the ethyl migrating
involves a larger group sterically relative to
methyl and therefore a larger barrier to
insertion.
All reactions carried out were thermally
assisted to encourage the reaction to run to
completion due to a lack of spontaneity
caused by a positive Gibbs’ free energy. The
positive status of both enthalpy values
indicates that the process is endothermic
and agrees with the prior statement.
It is regrettable that the trends between
this experiment and previous studies did
not agree. A potential lack of data points
may have contributed to the activation
parameters calculated being different to
those found in the literature.
* These values are for reaction with PMePh2
22
A. Haynes, Transition Metal Reaction Mechanisms, CHM 2104,
University of Sheffield, 2014.
23
L. Luan, P. S. White, M. Brookhart, J. L. Templeton, J. Am.
Chem. Soc. 1990, 112, p. 8190.
Complex
∆S / J K-1
mol-1
∆S lit.
∆H / kJ
mol-1
∆H lit.
[CpMo(CO)3Me] -75.9 -104.618* 75.1 53.619*
[CpMo(CO)3Et] -138.0 -94.620 47.2 68.221*
y = -9.0319x + 14.627
R² = 0.9968
-16.0
-15.5
-15.0
-14.5
-14.0
-13.5
-13.0
-12.5
-12.0
3.00 3.10 3.20 3.30 3.40
ln(k/T)
103.(1/T) / K
Eyring Plot for the reaction of
[CpMo(CO)3Me] with excess PPh3 to
determine activation parameters
Table 7 - A comparison of the activation parameters
obtained using data from this experiment to values
quoted in the literature.
Figure 12 - Eyring plot produced using data from this
experiment. All factors other than temperature were kept
constant.

12. Michael Ludden Level 3 Project AH2
12
Conclusions
This experiment set out to characterise a
number of Molybdenum carbonyl
complexes synthesised in the lab and to
then perform kinetic investigations upon
the complexes to determine the rate law,
dependence of the rate on any factors of
the reaction and values for the activation
parameters, ΔHǂ and ΔSǂ. The complexes
that were attempted were all synthesised
successfully. These were:
1) [CpMo(CO)3Me]
2) [CpMo(CO)3Et]
3) [CpMo(CO)3I]
4) [CpMo(CO)2(COMe)(PPh3)]
Compounds 1) and 2) were then used for a
series of kinetic experiments involving a
phosphine nucleophile reacting with the
complex in solution through the mechanism
of migratory insertion. As the concentration
of the nucleophile greatly exceeded the
concentration of the complex, pseudo-first-
order conditions were apparent and this
allowed any dependences of the rate on
factors of the reaction to be highlighted. It
was found that the rate increased by a
factor of roughly 22 times (Kobs = 8.77x10-3
compared to 3.94x10-4 at 40°C) when the R
group present on the Molybdenum
complex was ethyl rather than methyl.
Other factors positively influencing the rate
include the solvent – MeCN produced a rate
constant 4.5 times greater than observed in
THF – and the temperature, which caused
an increase in an exponential manner, as
predicted by the Arrhenius equation.
Varying both the identity and the
concentration of the nucleophile had no
effect on the value of Kobs great enough for
them to be considered an influential factor.
Introducing a catalyst was predicted to
increase the rate of reaction but this was
not observed during this experiment due to
reasons discussed earlier.
The rate law was deemed to be first order
with respect to complex and zero order
with respect to the incoming nucleophile
after consideration of the above
information gathered from this experiment.
This was reinforced by the values calculated
for the entropy and enthalpy of activation –
both compounds displayed negative values
of ΔSǂ (-75.9 and -138.0 for R = Me, Et
respectively) meaning the mechanism for
the reaction was associative and solvent
assisted. In this case, migration of the alkyl
group occurs before the nucleophile binds
and is the rate-determining step. Much of
the evidence acquired from this study lent
weight to this being the situation for this
reaction. The calculated activation
parameters are an area that would require
further study, as the data from this
experiment did not correlate with trends
and values found in the literature.
Experimental
1. Synthesis and characterisation of
[CpMo(CO)3Me]
[CpMo(CO)3]2 (0.51 g, 1.04 mmol) was
dissolved in 25 cm3 dry THF under an inert
N2 atmosphere and constant stirring. To
this solution, Li[BEt3H] (3 cm3, 2.5 mmol)
was added by syringe. A colour change from
dark red to dark green after 20 minutes was
observed. Methyl iodide (0.93 cm3, 14.94
mmol) was added and left under constant
stirring for 1 hour. The solvent was then
removed under reduced pressure and the
resulting solid product extracted using pet.
ether (4 x 20 cm3 portions). After extraction,
the now colourless sample was filtered
through an Al2O3 plug under gravity and
more pet. ether (≈ 40 cm3) was run through

13. Michael Ludden Level 3 Project AH2
13
the column to remove the yellow
colouration attained by the solution. The
solvent was removed from the filtrate again
using reduced pressure, and the solid
obtained then further purified by vacuum
sublimation between 40 - 60 °C for
approximately 8 hours.
Produced bright yellow crystals (0.33 g, 62
%), found ʋmax/cm-1 (solution cell; THF) 2025
(C≡O stretch) 1942 (C≡O stretch); δH
(400MHz; CDCl3) 0.39 (3H, s, CH3) 5.30 (5H,
s, C5H5) 7.30 (s, CDCl3); δC (100MHz; CDCl3) -
22.2 (CH3) 30.9 (acetone impurity) 76.7,
77.0, 77.3 (CDCl3) 92.5 (C5H5).
2. Synthesis and characterisation of
[CpMo(CO)3Et]
[CpMo(CO)3]2 (0.50 g, 1.02 mmol) was
dissolved in 25 cm3 dry THF under an inert
N2 atmosphere and constant stirring. To
this solution, Li[BEt3H] (3 cm3, 2.5 mmol)
was added by syringe. Again, a colour
change from dark red to dark green was
observed. Iodoethane (0.8 cm3, 10 mmol)
was added and left under constant stirring
for 1 hour. The solvent was then removed
under reduced pressure and the resulting
solid product extracted using pet. ether (4 x
20 cm3 portions). The colourless solution
was filtered through an Al2O3 plug under
gravity, and more pet. ether (≈60 cm3) was
allowed to run through the column to
ensure maximum product extraction. The
solvent was then again removed under
reduced pressure, and the solid obtained
dried under vacuum for 10 minutes.
Dull green crystals were afforded (0.30g,
54%), found ʋmax/cm-1 (solution cell; THF)
2019 (C≡O stretch) 1937 (C≡O stretch); δH
(400MHz; CDCl3) 1.42 (3H, t, CH3) 1.70 (2H,
q, CH2) 5.30 (5H, s, C5H5) 7.29 (s, CDCl3); δC
(100MHz; CDCl3) -3.8 (CH2) 20.2 (CH3) 76.7,
77.0, 77.3 (CDCl3) 92.8 (C5H5) 227.7 (CO).
3. Synthesis and characterisation of
[CpMo(CO)3I]
[CpMo(CO)3]2 (0.20 g, 0.408 mmol) was
reacted with Iodine (0.104 g, 0.408 mmol)
in 25cm3 dry THF under an inert N2
atmosphere and left under constant stirring
for 1 hour. The solvent was removed under
reduced pressure and the solid obtained
was extracted using 4 x 20 cm3 portions of
pet. ether before being filtered using an
Al2O3 plug. The solvent was again removed
under reduced pressure and the solid
product dried under vacuum for 10
minutes.
Obtained dark red/purple crystals (0.11 g,
36%), found ʋmax/cm-1 (solution cell; THF)
2044 (C≡O stretch) 1977, 1962 (C≡O
stretch); δH (400MHz; CDCl3) 5.62 (5H, s,
C5H5) 7.29 (s, CDCl3); δC (100MHz; CDCl3)
76.7, 77.0, 77.4 (CDCl3) 94.2 (C5H5) 220.2,
236.3 (CO).
4. Synthesis and characterisation of
[CpMo(CO)2(COMe)(PPh3)]
[CpMo(CO)3Me] (0.13 g, 0.5 mmol) was
added to PPh3 (0.2 g, 0.76 mmol) in 10 cm3
MeCN under N2 for approximately 16 hours.
A yellow precipitate was formed which was
collected on a sinter and washed
thoroughly with pet. ether (3 x 15 cm3)
portions. The solid was dried over vacuum
and then a recrystallisation attempted
using boiling pet. ether. No crystals were
formed in the recrystallisation and so the
solvent was removed under reduced
pressure and the solid produced dried
through vacuum desiccation for 30
minutes.
Dull yellow crystals were produced (0.17 g,
65%), found ʋmax/cm-1 (solution cell; THF)
1938 (C≡O stretch) 1855 (C≡O stretch) 1620

14. Michael Ludden Level 3 Project AH2
14
(C=O stretch); δH (400MHz; CDCl3) 1.22 (3H,
s, CH3) 1.52 (water impurity) 2.17 (acetone
impurity) 5.0 (5H, s, C5H5) 7.29 (s, CDCl3)
7.47 (15H, m, C6H5); δC (100MHz; CDCl3)
76.7, 77.0, 77.3 (CDCl3) 96.7 (C5H5) 129.3 (d,
C2 of C6H5) 131.2 (s, C4 of C6H5) 133.8 (d, C3
of C6H5); δP (100MHz; CDCl3) 67.9 (PPh3).
5. Kinetic Measurements for CO migratory
insertion
0.2g PR3 was dissolved in 2cm3 solvent
(MeCN / THF) in a 5cm3 graduated cylinder.
The solution was made up to the mark by
adding more solvent. A sample of this
solution was placed in a solution cell and
allowed to equilibrate with the
temperature of the thermal jacket. A
background spectrum was then run using
TimeBaseTM software. The cell was rinsed
thoroughly with DCM and also the chosen
solvent in two separate processes.
2cm3 of the phosphine/phosphite solution
made up earlier was added to 5mg
[CpMo(CO)3R] and shaken to dissolve all
solid. A sample of this solution was again
placed in the solution cell and multiple
spectra were run over a period of time,
usually 3-4 hours. The reactions were
stopped prior to the full time elapsing if
they could be considered to have reached
completion.
The TimeBaseTM data for each individual
carbonyl peak was saved and exported to
KaleidagraphTM, where a first order
formation/regression graph was plotted
(for each peak). The rate constant for each
graph was calculated by the software and
recorded as a spreadsheet on Microsoft
ExcelTM.
Thanks go to the other members of this research group (J. Hill, H. Pan, J. Skade, L. Thomas and
L. Wattam) for their contribution and effort during the course of this study, and Dr. Haynes for
his time and guidance throughout.