Quantification of steric interactions in phosphine ligands from single crystal X-ray diffraction data. Crystal structures of (η5-C 5H4Me)Mo(CO)2(PR3)I (R3 = PhMe2, PhEt2, Et3)

Article abstract of DOI:10.1016/j.jorganchem.2005.10.032

Distorted square pyramidal complexes of molybdenum (η⁵- C₅H₄Me)Mo(CO)₂(PR₃)I (R₃ = PhMe₂ (2a); PhEt₂ (3a) and Et₃ (4a)) have been synthesized and the structures of the lateral (cis) isomers have been determined by X-ray diffraction. The cone (Θ) and solid (Ω) angles as well as the angular profiles of the phosphine ligands in the complexes have been computed using the program steric. Values for the crystallographic cone and solid angles calculated for 2a, 3a and 4a are Θ (129°, 135°and 139°) and Ω (2.73, 2.99 and 2.93 sr), respectively. A search of the Cambridge Structural Database (CSD) was made for piano stool, 5- and 6-coordinate complexes containing the title phosphine ligands. Results from this study show a wide range of sizes for each of the ligands and even the seemingly simple PPhMe₂ ligand exhibited a wide range of values for the cone (113-137°) and solid (2.49-3.07 sr) angles. These observations have been rationalized and related to the possible group conformations from the crystallographic data.

Full text of DOI:10.1016/j.jorganchem.2005.10.032

Journal of Organometallic Chemistry 691 (2006) 890–897
www.elsevier.com/locate/jorganchem
Quantification of steric interactions in phosphine ligands from
single crystal X-ray diffraction data. Crystal structures of
(g5-C5H4Me)Mo(CO)2(PR3)I (R3 = PhMe2, PhEt2, Et3)
Muhammad D. Bala, Olalere G. Adeyemi, David G. Billing,
*
Demetrius C. Levendis, Neil J. Coville
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg 2050, South Africa
Received 5 September 2005; received in revised form 17 October 2005; accepted 21 October 2005
Available online 5 December 2005
Abstract
Distorted square pyramidal complexes of molybdenum (g5-C5H4Me)Mo(CO)2(PR3)I (R3 = PhMe2 (2a); PhEt2 (3a) and Et3 (4a))
have been synthesized and the structures of the lateral (cis) isomers have been determined by X-ray diffraction. The cone (H) and solid
(X) angles as well as the angular profiles of the phosphine ligands in the complexes have been computed using the program STERIC. Values
for the crystallographic cone and solid angles calculated for 2a, 3a and 4a are H (129ꢀ, 135ꢀ and 139ꢀ) and X (2.73, 2.99 and 2.93 sr),
respectively. A search of the Cambridge Structural Database (CSD) was made for piano stool, 5- and 6-coordinate complexes containing
the title phosphine ligands. Results from this study show a wide range of sizes for each of the ligands and even the seemingly simple
PPhMe2 ligand exhibited a wide range of values for the cone (113–137ꢀ) and solid (2.49–3.07 sr) angles. These observations have been
rationalized and related to the possible group conformations from the crystallographic data.
ꢁ 2005 Elsevier B.V. All rights reserved.
Keywords: Piano-stool molybdenum complexes; Phosphine ligands; CSD search; Steric interactions
1. Introduction
gies [6], ab initio calculations [2], molecular mechanics [7]
and X-ray crystallographic structures [8]. The Tolman con-
cept [3] has the advantage of being easy to use and
understand.
The need to predict the relative reactivity of organic
ligands and metal fragments has been the driving force
behind the application of theoretical models and physical
probes in chemistry and catalysis. In this respect, the study
of steric effects is important since the numerous sizes and
conformations of ligands and organic groups play a role
in determining the rate and course of chemical transforma-
tions [1,2]. In organometallic chemistry the Tolman cone
angle (H) [3], the solid angle (X) [4,5] and the Brown ER
[6] values have been used. The Tolman cone angle in partic-
ular, is widely accepted and freely quoted as the standard
for gauging steric interactions and often compared with
values obtained by techniques using ligand repulsive ener-
Over the years deviations from H have been recorded
[9] and adjustments to the Tolman values have been pro-
posed and documented for flexible ligands capable of
assuming multiple conformations such as phosphites and
PEt3 [10]. It is such perceived shortcomings of H that
led to the development of alternative quantification tech-
niques such as X [11] and the introduction of ligand pro-
files [12] that takes into account the actual shape and
conformation of a ligand. Simply defined H is cylindri-
cally symmetrical and rigid, while X is best visualized as
the area of the ‘‘shadow’’ cast by a ligand on the inside
of a unit sphere (Fig. 1).
*
Furthermore, it has been shown that phosphine ligands
show a variable degree of intermeshing especially in
Corresponding author. Fax: +27 11 717 6749.
E-mail address: ncoville@aurum.chem.wits.ac.za (N.J. Coville).
0022-328X/$ - see front matter ꢁ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.10.032

M.D. Bala et al. / Journal of Organometallic Chemistry 691 (2006) 890–897
891
2.1. Molecular structure
The IR stretching frequencies for the CO ligands in
these complexes all occur within a narrow band at
1955(3) and 1870(2) cmꢀ1 and there is no clear distinction
between the two isomers with respect to peak positions.
A characteristic feature that clearly distinguishes the two
isomers is the relative intensity of the two carbonyl absorp-
tion bands. In all the diagonal isomers the symmetric CO
absorption band located at higher wave number is usually
weaker in intensity, while the reverse is true for all the lat-
eral isomers [17].
1
The Cp region of the H NMR spectra readily differ-
entiates and can be used to quantify the two isomers of
complexes 2, 3 and 4. Thus, the lat isomer shows four
resonances for the non-equivalent protons and two reso-
nances (further downfield) for each pair of the two
equivalent protons in the diag isomer. The phosphine
ligands exhibited characteristic 1H resonances typical of
the respective Me, Et or Ph components of the ligands.
All the complexes crystallized only as the lat isomer.
Further characterization of the diag isomers was
achieved by elemental analysis (see Supplementary Table
S1).
Fig. 1. The relationships between the cone (H) and the solid (X) angles.
crowded environments [13]. The combination of a ligandꢀs
ability to assume a variable conformation depending on the
space and environment available and its compressibility
potential has led to a large spread in the calculated values
of H [14–16].
2.2. Crystallography
In earlier studies, we [14] and others [15,16] have
attempted to evaluate the effects of the ligand environment
on a series of ligands. In this manuscript, a continuation on
the evaluation of environmental interactions of a metal–
ligand set on a series of ligands of different flexibility/com-
pressibility is presented. Here we report a study of the
synthesis and characterization by spectroscopic and single
crystal X-ray crystallography of some four-legged piano
stool complexes of molybdenum containing three com-
monly encountered tertiary phosphine ligands PPhMe2,
PPhEt2, PEt3 that potentially have a varying degree of flex-
ibility and steric bulk. Also, a CSD search was conducted
on these same three ligands. Computations of H, X, and
the radial profiles of the ligands from the crystallographic
data were performed on the three new complexes and the
data compared with similar structures reported in the liter-
ature as documented in the CSD.
The crystal structures of the three lateral isomers 2a, 3a
and 4a have been determined and the crystallographic data
and selected bond lengths and angles are presented in
Tables 1 and 2. In addition, Figs. 2–4, respectively, repre-
sent the ORTEP diagrams showing the atom numbering
scheme of the crystal structures.
The bond lengths and angles are within limits
recorded for piano-stool complexes having similar ligand
arrangements around molybdenum as listed in Table 4.
The crystallographically measured Mo–P and P–C bond
lengths for all the three complexes are within very nar-
˚
row margins of 2.49–2.51 and 1.82–1.83 A, respectively,
hence comparison of data from the steric calculations
are not influenced by variations in bond lengths between
complexes containing different ligand moieties. The pro-
gram STERIC utilises atomic coordinates from diffraction
data for calculating the steric parameters, hence the cal-
culated variation in H and X between the three ligands
may be observed from the differences in corresponding
bond angles, atomic coordinates and equivalent isotropic
displacement parameters in 2a, 3a and 4a. In order to
make the calculations the program reads the parameters
2. Results and discussion
The title compounds 2, 3, and 4 were obtained in rela-
tively low to moderate yields, due generally to material
loss on the long column required for isomer separation
and isolation. In general the lateral (cis) isomers that elute
first are obtained in higher yields when compared to the
diagonal (trans) isomers. The pure isomers were initially
characterized by IR and NMR spectroscopy, and product
composition confirmed by elemental analysis or X-ray
crystallography. Detailed spectroscopic and analytical
data are presented in the supplementary material (see
Table S1).
in the form of .cif, .res or .ins files. For 2a, 3a and
*
*
*
4a the calculated values of H (129ꢀ, 135ꢀ and 139ꢀ,
respectively) and X (2.73, 2.99 and 2.93 sr, respectively)
correspond well with their observed crystallographic
configurations (see ORTEP diagrams) in relation to the
various conformations available that rationalises the wide
spread in H and X values observed from the CSD data
(see later).

892
M.D. Bala et al. / Journal of Organometallic Chemistry 691 (2006) 890–897
Table 1
Crystal data and structure refinement data for compounds 2a, 3a and 4a
Identification code
Empirical formula
Formula weight
Temperature (K)
2a
C16H18IMoO2P
496.11
293(2)
0.71073
3a
C18H22IMoO2P
524.17
293(2)
0.71073
4a
C14H22I MoO2P
476.13
293(2)
0.71069
˚
Wavelength (A)
Crystal system, space group
Unit cell dimensions
Orthorhombic, P212121
Monoclinic, C2/c
Monoclinic, P21/n
˚
a (A)
7.9752(8)
11.7571(12)
34.466(4)
7.5872(10)
7.5828(9)
16.0537(18)
˚
b (A)
˚
c (A)
19.172(2)
15.4454(19)
14.4459(16)
b (ꢀ)
–
96.501(2)
4013.0(9)
8, 1.735
2.278
95.272(2)
1751.1(3)
4, 1.806
2.599
3
˚
Volume (A )
1797.6(3)
4, 1.833
2.537
960
Z, calculated density (mg/m3)
Absorption coefficient (mmꢀ1
F(000)
)
2048
928
Crystal size (mm)
H range for data collection (ꢀ)
Limiting indices
0.42 · 0.38 · 0.17
2.03–28.30
0.26 · 0.10 · 0.08
2.38–25.00
0.32 · 0.24 · 0.14
1.90–28.27
ꢀ10 6 h 6 10,
ꢀ14 6 k 6 15,
ꢀ23 6 l 6 25
12,579/4447 (0.0185)
99.9
0.6723 and 0.4155
Full-matrix least-squares on F2
4447/0/194
ꢀ40 6 h 6 40,
ꢀ10 6 h 6 10,
ꢀ21 6 k 6 11,
ꢀ19 6 l 6 19
11,990/4319 (0.0204)
99.5
0.7123 and 0.4901
Full-matrix least-squares on F2
4319/0/173
ꢀ9 6 k 6 9,
ꢀ18 6 l 6 12
10,566/3523 (0.0296)
99.8
Reflections collected/unique (Rint
)
Completeness to theta = 25.00 (%)
Maximum and minimum transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
0.8388 and 0.5889
Full-matrix least-squares on F2
3523/1/208
1.033
1.027
1.015
Final R indices [I > 2r(I)]
R indices (all data)
Largest difference in peak and hole (e A
R1 = 0.0230, wR2 = 0.0539
R1 = 0.0246, wR2 = 0.0547
1.032 and ꢀ1.136
R1 = 0.0314, wR2 = 0.0716
R1 = 0.0464, wR2 = 0.0764
0.769 and ꢀ0.832
R1 = 0.0322, wR2 = 0.0679
R1 = 0.0429, wR2 = 0.0720
1.469 and ꢀ1.118
ꢀ3
˚
)
Table 2
Selected bond lengths (A) and angles (ꢀ) for 2a, 3a, and 4a
˚
2a
3a
4a
Mo–P1
P1–C3
P1–C4
P1–C21
2.494(8)
1.823(4)
1.826(4)
1.834(3)
Mo–P1
P1–C3
P1–C5
P1–C21
2.507(1)
1.840(4)
1.834(4)
1.828(4)
Mo–P1
2.511(1)
1.832(3)
1.835(4)
1.831(4)
P1–C3
P1–C5
P1–C7
Mo–P1–C3
Mo–P1–C4
Mo–P1–C21
117.29(15)
111.30(16)
119.00(11)
Mo–P1–C3
Mo–P1–C5
Mo–P1–C21
115.01(14)
117.86(14)
112.40(13)
Mo–P1–C3
Mo–P1–C5
Mo–P1–C7
113.37(13)
115.79(14)
117.73(14)
2.3. Quantification of steric parameters
< PEt3) in the value of Hc can be accounted for by consid-
ering the ligand conformation and the lack of a rigid group
(e.g., phenyl) that imposes constraints on free rotation
around the P–C bond for the PEt3 ligand. Ernst and co-
workers have previously shown that the conformer
(Fig. 5(a)) in which the three arms of the PEt3 ligand are
in a C3v conformation (used to compute the Tolman cone
angles) is an idealized non-existent specie. This is due to
the repulsive van der Waals forces that will build up
between adjacent members. They revised the Tolman cone
angle values to match the crystallographically observed
species (Fig. 5(b)), which is the sterically more demanding
bent-back conformer/or variants thereof. Hence a value of
H = 137ꢀ has been determined for the PEt3 ligand taking
into consideration its true conformation, which is similar
to the crystallographically determined data here (Hc =
139ꢀ) [19].
Calculations of the cone (Hc) and solid (Xc) angles for
2a, 3a, and 4a were based on the atomic coordinates
obtained from the crystallographic data. The actual ligand
conformations observed in the crystal structures were used
to correlate ligand steric parameters and radial profiles.
Also, it appears that the choice of the crystallographic
value of the M–P bond distance is not critical in the anal-
ysis. For example, after statistically evaluating thousands
of complexes from the CSD, Mingos and Muller [18] have
¨
˚
concluded that the distance of 2.28 A employed for earlier
calculations is smaller than the mean crystallographic M–P
bond distance for all the complexes studied [2.32(9) A] (see
also Table 4).
The cone angle increases with ligand chain length (cf.
PPhMe2 and PPhEt2). The trend (PPhMe2 < PPhEt2
˚

M.D. Bala et al. / Journal of Organometallic Chemistry 691 (2006) 890–897
893
P
M
P
M
a
b
Fig. 5. (a) Idealized extreme conformation adopted by Tolman;
(b) crystallographic conformation of the PEt3 ligand.
Results of calculations on the two steric measures Hc
and Xc, selected for this study are presented in Table 3.
The quantification of the variations in solid angle, and
the cone angle with distance from the metal, i.e., the solid
angle radial profile (Fig. 6), and the cone angle radial pro-
file (Fig. 7), respectively, are presented. The solid angle
radial profiles for 2a, 3a, and 4a all show two maxima at
ca. 1.7 A and at 3.6 A, then drops off to zero beyond 8 A
in the order PPhMe2 < PEt3 < PPhEt2. A similar plot for
the cone angle radial profiles has resulted in comparable
Fig. 2. ORTEP diagram of 2a showing the atomic numbering scheme.
˚
˚
˚
Table 3
Calculation of steric parameters from single crystal and CSD data
Parameter
PPhMe2
PPhEt2
PEt3
Crystallographic cone angle Hc (ꢀ)
Tolman cone angle H (ꢀ)a
129
122
63
135
136
17
139
132
39
Number of data points from CSD
Mean CSD cone angle (ꢀ)
Range of CSD cone angle (ꢀ)
Crystallographic solid angle Xc (sr) 2.73
Mean CSD solid angle X (sr)
122
113–137
134
140
126–140
2.99
2.97
129–156
2.93
3.06
2.74
Range of CSD solid angle X (sr)
2.49–3.07 2.80–3.11 2.79–3.50
a
Ref. [3].
Fig. 3. ORTEP diagram of 3a showing the atomic numbering scheme.
PPhMe
-------- PPhEt
......... PEt
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
2
4
6
8
10
Distance / Armstrong
Fig. 6. Solid angle radial profile of the ligands PPhMe2, PPhEt2 and PEt3
Fig. 4. ORTEP diagram of 4a showing the atomic numbering scheme.
in 2a, 3a and 4a.

894
M.D. Bala et al. / Journal of Organometallic Chemistry 691 (2006) 890–897
2.4. Statistical analysis of CSD data
- PPhMe
-- PPhEt
.. PEt
2.5
2.0
1.5
1.0
0.5
0.0
The variations in calculated cone Hc and solid angles Xc
for a range of metal complexes containing the three title
ligands are presented in Table 3, and summarized in the
form of histograms (see Supplementary material Figs. S1–
S3). A large spread in Hc data from the CSD has already
been demonstrated for phosphine [14] and phosphite ligands
[15]. For each ligand the high and low values are given in
Table 4. It is expected that in addition to the degrees of con-
formational freedom possible for the ligand, two other fac-
tors can play a role in determining Hc and Xc and the
amount of deviation from the mean. These factors are the
central metal (its relative position in the periodic table)
and the degree of coordination (congestion) around it [15].
For the same ligand environment, complexes of early transi-
tion metals have smaller cone and solid angles compared to
those of late transition metals and the cone and solid angle
values increase moving across a transition series. The chro-
mium complex 15 illustrates a case of ligand overcrowding
(3 CO and 3 PEt3 ligands) around a relatively small metal
(Cr) centre, yielding the smallest Hc and Xc values for the
PEt3 ligand. On the other extreme 16 with a 3-legged piano
stool ligand arrangement around a relatively large Ru metal
allows for the only PEt3 ligand to move freely thereby gener-
ating high Hc and Xc angles of 156ꢀ and 3.41 sr, respectively.
The PEt3 ligand is flexible and has a high degree of confor-
mational freedom (see Fig. 5(a) and (b)).
0
2
4
6
8
10
Distance / Armstrong
Fig. 7. Tolman cone angle radial profile of the ligands PPhMe2, PPhEt2
and PEt3 in 2a, 3a and 4a.
profiles (very close total curve area) for all the three ligands
confirming its insensitivity to conformational changes. It is
interesting to note that the observed difference between the
calculated solid angles Xc and the maxima (Xmax) from the
solid angle radial profile further indicates a lack of symme-
try in the shapes of the ligands (implying that they cannot
be treated as symmetrical cones), since it has been shown
that Xc = Xmax for cylindrical cones [11].
In general, the crystallographically calculated steric
parameters (Xc and Hc) are larger than Tolmanꢀs (by about
7ꢀ; researchers have even proposed an addition of 10ꢀ to
the cone angles) [19a]. Our calculated values are within
the range of values published by Ernst and co-workers
[19] as a revision to Tolmanꢀs original data.
Interestingly even for the PPhMe2 ligand which theoret-
ically may at a first approximation seem uni-conforma-
tional a large spread in Hc and Xc values are observed. It
seems that the bulky phenyl group has a relatively variable
degree of freedom and is capable of assuming varied con-
formations depending on the size of the central metal and
degree of metal coordination. This becomes evident by
Table 4
The Hc and Xc data and references to some piano stool complexes containing the phosphine ligands PPhME2, PPhEt2 and PEt3 from the CSD highlighted
in the text
˚
No.
CSD Refcode
Complex
Hc (ꢀ)
Xc (sr)
M–P (A)
Ref.
Ligand
PPhMe2
5
6
LAMXOM
TOWSED
JISBIW
ROMHIK
QEMVIN
JUFCAO
(g5-C5H5)Mo(Cl)3(PPhMe2)2 Æ (CH2)4O
(g5-C5H5)MoCl(PPhMe2)3
(g5-C5Me5)RhBrPh(PPhMe2)
(g5-C5H5)MoH3(PPhMe2)2
(g5-C5H5)RuCl(PPhMe2)2
ðg2-C2H2Þðg5-C5H5ÞRuþðPPhMe2Þ2 ꢁ BFꢀ4
113
113
137
119
125
132
2.49
2.49
3.1
2.71
2.92
2.98
2.55
2.50
2.27
2.41
2.28
2.30
[20]
[21]
[22]
[23]
[24]
[25]
7
8
9
10
Ligand
PPhEt2
11
12
13
14
OCEPRE01
TCEPRH
GILNAQ
HEPRUP
Re(O)Cl3(PPhEt2)2
Rh(Cl)3(PPhEt2)3
(g5-C5H7)RuCl(PPhEt2)2
(g5-C5H5)Mo(CO)2I(PPhEt2)
126
140
134
135
2.96
3.10
3.09
2.99
2.48
2.32
2.32
2.51
[26]
[27]
[28]
[29]
Ligand
PEt3
15
16
17
18
ETPHCR
IKELAL
ZUWSOZ
YIXDIS
Cr(CO)3(PEt3)3
129
156
137
144
2.79
3.41
2.89
3.19
2.43
2.35
2.52
2.29
[30]
[31]
[32]
[33]
[g6-C6H4(C4H8OH)]RuCl2PEt3
(g5-C5H5)MoCl2(PEt3)2
(g5-C5H5)RuCl(PEt3)2

M.D. Bala et al. / Journal of Organometallic Chemistry 691 (2006) 890–897
895
comparing structures 8, 9 and 10 (Table 4). The 3 com-
plexes are structurally similar and all are variants of the
piano stool arrangement with slight chemical modifica-
tions. An analysis of the two extreme points in the data
range reveals that the PPhMe2 ligands in these complexes
exist in the conformations depicted as A and B in Figs. 8
and 9, respectively. In conformer A the phenyl group of
the PPhMe2 ligand is on a plane parallel to the plane of
the methyl groups while in conformer B its plane is perpen-
dicular to the plane of the methyl groups, with the conse-
quence that ligand steric parameters (cone and solid
angles) are lowest in the former conformation and highest
in the latter. These are the extreme points for which cone
angles of 113ꢀ and 137ꢀ were calculated corresponding to
solid angles of 2.55 and 3.07 sr, respectively. Therefore,
the unexpected wide spread in the CSD data for the
PPhMe2 ligand is due to the ability of the phenyl group
to twist on its plane and also tilt with respect to the methyl
groups between the two extreme conformations in order to
be accommodated within the lattice space available in a
given crystal volume. It is also this flexibility that leads to
situations where 2 or more ligands of the same moiety in
the same complex can each exist in a unique conformation
(for e.g., cf. the 2 PPhMe2 ligands in OCEPRE01 or the 3
PPhMe2 ligands in TCEPRH) [26,27]. The spread in the
CSD data points of complexes bearing the PPhEt2 ligand
is due to a combination of the factors discussed above,
i.e., the flexibility of the ethyl groups coupled to the ability
of the phenyl group to exist in conformations that tilt
towards A or B depending on the degree of congestion
(or freedom) in the crystal lattice.
From Table 4, it seems the substituent on the cyclopen-
tadienyl moiety has little effect in influencing the values of
Hc and Xc (cf. complex 2a with 7, 3a with 14, and 4a with
18). The data obtained is consistent with the crystallo-
graphic configuration of the Cp ring as lying at the apex
position of the pseudo seven-coordinate stool and the
group has little interaction with the rest of the ligands
and hence substitution on the ring has little effect on the
steric requirements of the phosphine ligands attached to
the central metal. As noted above the position of the metal
in the periodic table, has the most influence on the steric
requirement. The trend in the crystallographically deter-
mined cone and solid angles can be summarized as follows:
moving across a period or down a group of the periodic
table, ligand species of the same kind increase in the size
of calculated Hc and Xc [15].
3. Conclusion
The X-ray crystal structures of (g5-C5H4Me)Mo(CO)2-
(PR3)I (R3 = PhMe2, PhEt2, Et3) have been determined
and the cone and solid angles for the coordinated phos-
phines computed from the data. The metal fragment ꢁ(g5-
C5H4Me)Mo(CO)2Iꢀ attached to the PR3 ligands provides
a common ꢁholeꢀ with which to evaluate the size of the
PR3 ligands. These values were compared with data
obtained from a CSD search.
The CSD search revealed that the three ligands each
gave a wide range of steric values (Hc and Xc) with two
cone angles larger (PPhMe2, PEt3) and one similar
(PPhEt2) to the original Tolman values. All three values
are typical average values when compared to the average
CSD values computed. This suggests that while the
PPhMe2 and PEt3 ligands show expected (larger) values
than the Tolman values (due to the method of measure-
ment) the PPhEt2 ligand fortuitously shows the same value
as measured by Tolman. This arises from the ligand con-
formations measured (one Et group bent towards the
metal, and the phenyl ring in the plane of an Et C–C bond).
The implication is that a simple correction to modify the
Tolman values to increase the ligand size (e.g., cone angle)
to more correctly represent the actual ligand conformation
is not possible and that corrections must be done on a
ligand to ligand basis.
Fig. 8. Conformer A: Phenyl group on parallel plane to the plane of
methyl groups in the PPhMe2 ligand. Steric parameters are lowest in this
conformation.
4. Experimental
Fig. 9. Conformer B: Phenyl group on plane perpendicular to the plane of
methyl groups in the PPhMe2 ligand. Steric parameters are highest in this
conformation.
All operations involving the handling of air sensitive
materials were carried out under dry nitrogen using

896
M.D. Bala et al. / Journal of Organometallic Chemistry 691 (2006) 890–897
standard Schlenk techniques or a glove box. Solvents
were dried by conventional methods and distilled under
nitrogen prior to use. Mo(CO)6 and the three phos-
phines PR3 (R3 = Me2Ph, Et2Ph, Et3) were used as sup-
plied (Strem Chemicals). Trimethylamine N-oxide
dihydrate (Aldrich) was used as received. Solution IR
spectra were recorded (CH2Cl2) on a Bruker Vector
CSD using the search and retrieval program CONQUEST
1.6, Version 5.25 [38]. A search string for any single-
bonded transition metal to tertiary phosphine ligands
PR3 (R3 = Me2Ph, Et2Ph, Et3) was used. The CSD pro-
gram Mercury was used to manually screen the entries
based on the following criteria: (a) 3D atomic coordinates
of the ligand must include all the hydrogens and be of suf-
ficient quality (R 6 0.07); (b) ligand arrangement around
the metal should be piano stool or 5- & 6-coordinated
non-cyclopentadienyl based ligand systems, hence, metal
clusters, bimetallic compounds, solvated and ionic com-
pounds were excluded. The coordinates of all compounds
satisfying the above conditions were saved for input to
the in-house program STERIC which performs steric param-
eter calculations running on a Pentium 450 PC operating
on a RED HAT LINUX program as earlier described [9,14].
The program was based on the following covalent and
1
FTIR spectrometer in KBr cells. H NMR spectra were
recorded (CDCl3) at 300 MHz on a Bruker AC 300
spectrometer. Elemental analyses were conducted at the
Institute for Soil, Climate and Water, CSIR, Pretoria.
Crystallographic data were collected on a Bruker
SMART 1K CCD area detector diffractometer with graph-
ite monochromated Mo Ka radiation (50 kV, 30 mA). The
collection method involved x-scans of width 0.3ꢀ. Numer-
ical data pertaining to the experimental measurement and
details of the structure analyses are given in Table 1. Data
reduction was carried out using the program SAINT+ [34]
and data were corrected for absorption using the program
SADABS [34]. The structures were solved by standard Patter-
son procedures and refined by least-squares methods based
on F2. The SHELX-97 [35] suite of programs as incorporated
into WINGX [36] were used for all crystallographic computa-
tions. In the final stages of refinement hydrogen atoms were
geometrically fixed and allowed to ride on the respective
parent atoms.
˚
van der Waals radii (A): Mo (1.30, 1.30); P (1.10, 1.85);
C (0.77, 1.70); and H (0.37, 1.20). Mean crystallographic
˚
bond lengths (A) from the crystal structures are: Mo–P
(2.49, PPhMe2 ligand); Mo–P (2.51, PPhEt2 and PEt3
ligands); P–C (1.83); C–H (0.96, alkyl); C–H (0.93,
aromatic).
Acknowledgements
Financial support from the NRF, THRIP, and the Uni-
versity of the Witwatersrand is gratefully acknowledged.
4.1. Synthesis of (g5-C5H4Me)Mo(CO)3I (1)
The starting material 1 was prepared by the adaptation
of a standard procedure [37]. A dimethoxyethane suspen-
sion of Mo(CO)6 and C5H5Me was refluxed for 24 h,
cooled to room temperature and iodine was added to the
solution with continued stirring. After 3 h the solvent was
removed in vacuo. Extraction with hexane afforded the title
compound as red needles. IR mCO (cmꢀ1): 2039 (s), 1961 (s).
1H NMR (ppm): 5.22–5.24 (t, 2H, CpMe), 5.11–5.14 (t,
2H, CpMe), 2.12 (s, 3H, CpMe).
Appendix A. Supplementary data
Spectroscopic and elemental analysis data (Table S1) for
the title complexes and histograms (Figs. S1–S3) showing
the spread in data from the CSD are available. Also, crys-
tallographic data for the structural analysis has been
deposited with the Cambridge Crystallographic Data Cen-
tre, CCDC Nos. 281979, 281980 and 281981 for com-
pounds 2a, 3a, and 4a, respectively. Copies of this
information may be obtained free of charge from: The
Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ
UK; fax: +44 1223 336 033, e-mail: deposit@ccdc.cam.
ac.uk or http://www.ccdc.cam.ac.uk. Supplementary data
associated with this article can be found, in the online ver-
sion, at doi:10.1016/j.jorganchem.2005.10.032.
4.2. Synthesis of (g5-C5H4Me)Mo(CO)2(PR3)I
(R3 = PhMe2, PhEt2, Et3)
All the compounds were prepared via a common route.
Typically the reaction of 1 with a 5–10-fold excess of ligand
PPhMe2 and trimethylamine N-oxide in dichloromethane
at room temperature afforded 2 as a mixture of the lat (a,
cis) and diag (b, trans) isomers. Isomer separation was
achieved by dissolving the crude material in CH2Cl2 fol-
lowed by mixing with a small quantity of silica gel. The yel-
low powder remaining after solvent removal was
chromatographed on a column of silica (2 · 60 cm), eluting
with 1:1 toluene/hexane mix solvent.
References
[1] D.J. Darensbourg, A.H. Graves, Inorg. Chem. 18 (1979) 1257.
[2] J.T. DeSanto, J.A. Mosbo, B.N. Storhoff, P.L. Bock, R.E. Bloss,
Inorg. Chem. 19 (1980) 3086.
[3] C.A. Tolman, Chem. Rev. 77 (1977) 313.
[4] A. Immirzi, A. Murzo, Inorg. Chim. Acta 25 (1977) L41.
[5] (a) K.W. Bagnall, L. Xing-Fu, J. Chem. Soc., Dalton Trans. (1982)
1365;
(b) D. White, N.J. Coville, Adv. Organomet. Chem. 36 (1994) 95.
[6] (a) T.L. Brown, K.J. Lee, Coord. Chem. Rev. 128 (1993) 89;
(b) T.L. Brown, Inorg. Chem. 31 (1992) 1286;
4.3. CSD data retrieval and steric parameter calculations
The information utilized for the statistical analysis was
retrieved from the January 2004 updated edition of the
(c) M.L. Caffrey, T.L. Brown, Inorg. Chem. 30 (1991) 3907;
(d) M.-G. Choi, T.L. Brown, Inorg. Chem. 32 (1993) 5603.

M.D. Bala et al. / Journal of Organometallic Chemistry 691 (2006) 890–897
897
[7] M. Chin, G.L. Durst, S.R. Head, P.L. Bock, J.A. Mosbo, J.
Organomet. Chem. 470 (1994) 73.
[23] F. Abugideiri, J.C. Fettinger, B. Pleune, R. Poli, C.A. Bayse, M.B.
Hall, Organometallics 16 (1997) 1179.
[8] H.C. Clark, M.J. Hampden-Smith, Coord. Chem. Rev. 79 (1987) 229.
[9] J.M. Smith, N.J. Coville, L.M. Cook, J.C.A. Boeyens, Organomet-
allics 19 (2000) 5273.
[10] L. Stahl, R.D. Ernst, J. Am. Chem. Soc. 109 (1987) 5673.
[11] J.M. Smith, Ph.D. Thesis, University of the Witwatersrand, 1996, p.
134.
[12] G. Ferguson, P.J. Roberts, E.C. Alyea, M. Khan, Inorg. Chem. 17
(1978) 2965.
[13] A. Immirzi, A. Musco, B.E. Mann, Inorg. Chim. Acta 21 (1977) L37.
[14] J.M. Smith, N.J. Coville, Organometallics 20 (2001) 1210.
[24] S.T.N. Freeman, F.R. Lemke, C.M. Haar, S.P. Nolan, J.L. Petersen,
Organometallics 19 (2000) 4828.
[25] J.R. Lomprey, J.P. Selegue, J. Am. Chem. Soc. 114 (1992) 5518.
[26] V.S. Sergienko, M.A. Porai-Koshits, Koord. Khim. (Russ.), (Coor-
d.Chem.) 8 (1982) 251.
[27] A.C. Skapski, F.A. Stephens, J. Chem. Soc., Dalton Trans. (1973)
1789.
[28] J.R. Bleeke, D.J. Rauscher, Organometallics 7 (1988) 2328.
[29] H.L. Stokes, T.L. Smalley Jr., M.L. Hunter, M.E. Welker, A.L.
Rheingold, Inorg. Chim. Acta 220 (1994) 305.
[15] T.E. Muller, D.M.P. Mingos, Trans. Met. Chem. 20 (1995) 533.
¨
[30] A. Holladay, M.R. Churchill, A. Wong, J.D. Atwood, Inorg. Chem.
19 (1980) 2195.
[31] G. Marconi, H. Baier, F.W. Heinemann, P. Pinto, H. Pritzkow, U.
Zenneck, Inorg. Chim. Acta 352 (2003) 188.
[32] A.A. Cole, J.C. Fettinger, D.W. Keogh, R. Poli, Inorg. Chim. Acta
240 (1995) 355.
[33] M.E. Cucullu, L. Luo, S.P. Nolan, P.J. Fagan, N.L. Jones, J.C.
[16] (a) A.G. Orpen, L. Brammer, F.H. Allen, O. Kennard, D.G. Watson,
R. Taylor, J. Chem. Soc., Dalton Trans. (1989) S1;
(b) B.J. Dunne, R.B. Morris, A.G. Orpen, J. Chem. Soc., Dalton
Trans. (1991) 653;
(c) S.E. Garner, A.G. Orpen, J. Chem. Soc., Dalton Trans. (1993)
533.
´
[17] J.W. Faller, A.S. Anderson, J. Am. Chem. Soc. 92 (1970) 5852.
Calıbrese, Organometallics 14 (1995) 289.
[18] D.M.P. Mingos, T.E. Muller, J. Organomet. Chem. 500 (1995) 251.
¨
[19] (a) L. Stahl, W. Trakarnpruk, J.W. Freeman, A.M. Arif, R.D. Ernst,
Inorg. Chem. 34 (1995) 1810;
(b) L. Stahl, R.D. Ernst, J. Am. Chem. Soc. 109 (1987) 5673.
[20] F. Abugideiri, J.C. Gordon, R. Poli, B.E. Owens-Waltermire, A.L.
Rheingold, Organometallics 12 (1993) 1575.
[34] Bruker, SAINT+, Version 6.02, Bruker AXS Inc., Madison, WI,
1999.
[35] G.M. Sheldrick, SHELX-97, University of Go¨ttingen, Germany, 1997.
[36] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.
[37] R.B. KingOrganometallic synthesis, vol. 1, Academic Press, New
York, 1955.
[21] F. Abugideiri, J.C. Fettinger, D.W. Keogh, R. Poli, Organometallics
15 (1996) 4407.
[22] W.D. Jones, V.L. Kuykendall, Inorg. Chem. 30 (1991) 2615.
[38] CSD CONQUEST 1.6, Version 5.25 (January 2004): I.J. Bruno, J.C.
Cole, P.R. Edgington, M. Kessler, C.F. Macrae, P. McCabe, J.
Pearson, R. Taylor, Acta Crystallogr. B58 (2002) 389.