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Journal of Organometallic Chemistry 601 (2000) 34 42

From molecules to aggregates: crystal structures of molybdenum

binuclear complexes
Patrcia Pinto a, Eva Barranco a, Maria Jose Calhorda a,b,*, Vitor Felix a,c,
Michael G.B. Drew d

Instituto de Tecnologia Qumica e Biologica (ITQB), Quinta do Marques, EAN, Apt 127, 2781 -901 Oeiras, Portugal
Departamento de Qumica e Bioqumica, Faculdade de Ciencias da Uni6ersidade de Lisboa, 1749 -016 Lisbon, Portugal
Departamento de Qumica, Uni6ersidade de A6eiro, 3810 -193 A6eiro, Portugal
Department of Chemistry, Uni6ersity of Reading, Whiteknights, Reading RG6 6AD, UK
Received 15 November 1999; received in revised form 21 December 1999

The new complexes [MoCl(h3-C3H5)(CO)2(H2biim)], [NnBu4][{Mo(h3-C3H5)(CO)2}2(m-Cl)2(m-OMe)], and [PPN][{Mo(h3C3H5)(CO)2}2(m-Cl)3] (PPN is Ph3PN+PPh3) were synthesized and the two binuclear compounds were characterized by singlecrystal X-ray diffraction. Although an anion [{Mo(h3-C3H5)(CO)2}2(m-Cl)3] has been described in the literature, being formed
both in solution and in the solid, another conformer was present in the crystal of [PPN][{Mo(h3-C3H5)(CO)2}2(m-Cl)3], exhibiting
an eclipsed arrangement of the allyl groups. The same arrangement was found for the [{Mo(h3-C3H5)(CO)2}2(m-Cl)2(m-OMe)]
anion. Cyclic voltammetry data show that the eclipsed conformer of [{Mo(h3-C3H5)(CO)2}2(m-Cl)3] remains in solution, as the
other one, with staggered allyl groups, is formed when dissolving the parent compound [MoCl(h3-C3H5)(CO)2(NCMe)2] and
exhibits a distinctive pattern. Density functional theory (DFT) calculations showed the two conformers to have very similar
energies, both for this and for the related anion [{Mo(h3-C3H5)(CO)2}2(m-Cl)2(m-OMe)]. The crystal structure of [PPN][{Mo(h3C3H5)(CO)2}2(m-Cl)3] exhibits double channels formed by the PPN cation which are occupied by chains of the binuclear anions.
2000 Elsevier Science S.A. All rights reserved.
Keywords: Molybdenum; DFT calculations; Allyl complexes; Crystal structures; Binuclear complexes

1. Introduction
Organometallic complexes are very versatile species,
which can be used as building blocks to design molecular solids [1]. Their main advantage over more traditional organic precursors lies in their increased
versatility. Indeed, a change in metal allows the coordination sphere to vary as well, leading to different
shapes and charges for the complex, which may have a
determining effect in a crystallization process. Moreover, reduction or oxidation reactions can be undergone by many metal centers, changing the electronic
richness of the metal but not, in many cases, the
stoichiometry of the complex. Units containing more
than one metal atom are even more interesting, if the
* Corresponding author. Fax: +351-21-4411277.
E-mail address: (M.J. Calhorda)

communication between those metal centers can be

turned on or off by redox processes or some other
method. This behavior holds a high potential for applications in many fields.
In an attempt to understand the factors responsible
for the properties of solids, binuclear complexes were
used as precursors. The metal atoms may be linked by
bridging ligands or exhibit an unsupported metalmetal
bond. In many binuclear complexes, where the metal
centers are connected by bridging ligands, such as
Fe2(CO)9, the existence of a metalmetal bond has been
discussed widely, as it is required in order for each
metal to achieve a formal 18 electron count [2]. In other
examples, the metal centers are saturated, the
metalmetal distances are long, so that there is no
direct interaction between them. In this work, using as
a precursor the well-known allyl complex [MoCl(h3C3H5)(CO)2(NCMe)2] [3], we introduce two complexes

0022-328X/00/$ - see front matter 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 0 ) 0 0 0 1 8 - 8

P. Pinto et al. / Journal of Organometallic Chemistry 601 (2000) 3442

of this latter family, containing either three chloride or

two chloride and one methoxy bridges, describe their
crystal structures, and discuss their electronic structure.

2. Results and discussion

2.1. Chemical results

The allyl complex [MoCl(h3-C3H5)(CO)2(NCMe)2]
(1) [3] has been used widely as a convenient source of
molybdenum(II) derivatives, because it contains two
labile nitrile ligands. The reaction with 2,2%-biimidazole
(H2biim) led to the substitution of the two nitriles, to
afford a neutral [MoCl(h3-C3H5)(CO)2(H2biim)] complex (2), similar to the known 2,2%-bipyridyl and
phenanthroline derivatives, which could also be obtained in this way [4]. The IR spectrum exhibits the
nCO stretching modes at 1944 and 1854 cm 1 and some
new ones, assigned to the coordinated 2,2%-bisimidazole.
The 1H-NMR spectrum shows one broad peak for this
ligand (d= 7.48), and three allyl peaks at d = 4.09,
3.53, and 1.41, integrating as 4:1:2:2, respectively. This
complex is extremely insoluble, which prevented its
recrystallization and cocrystallization with suitable Hbond acceptors to interact with the two NH groups.
On the other hand, deprotonation of the coordinated
ligand leads to a new organometallic ligand with two
nitrogen donor atoms [MoCl(h3-C3H5)(CO)2(biim)]2 ,
which can be used to prepare binuclear complexes by
reaction with, among others, the molybdenum precursor 1. The same reaction can be achieved by direct
reaction of [MoCl(h3-C3H5)(CO)2(NCMe)2] with the
lithium bisimidazolato, stabilized by TMEDA. Attempts to grow crystals of the resulting product led to
orangegreenish crystals, which did not contain the
nitrogen ligand. Their formulation was determined by
means of single-crystal X-ray diffraction as [NnBu4]-

Fig. 1. A PLUTON view of the binuclear anion [{Mo(h3C3H5)(CO)2}2(m-Cl)3] (4) with the atomic numbering scheme


[{Mo(h3-C3H5)(CO)2}2(m-Cl)2(m-OMe)] (3) (see below).

The IR spectrum of this complex exhibits the typical
nCO stretching modes at 1921 and 1809 cm 1, shifted
by 23 and 44 cm 1 relative to those of the parent
compound, as well as bands assigned to the CH
vibrations at 2962, 2875, and 2808 cm 1. The peaks in
the 1H-NMR spectrum could not be assigned fully, as
the large number of protons belonging to the NnBu4
counter ion cover the allylic protons.
A related anionic binuclear complex, [PPN][{Mo(h3C3H5)(CO)2}2(m-Cl)3] (4), was obtained upon recrystallization of the product from the reaction between
[MoCl(h3-C3H5)(CO)2(NCMe)2] and [Au2Cl2PPN]
(PPN is Ph3PN+PPh3) and its nature elucidated by
single-crystal X-ray diffraction. The two nCO stretching
modes were observed at 1861 and 1928, 1950 cm 1,
with the strongest CH stretching frequency at 3057
cm 1. For this complex, the peaks of the allyl ligand
are well separated from those of phosphines of the
counterion in the 1H-NMR spectrum. Anions with this
composition have been described before [5], but the
spectroscopic data (IR and 1H-NMR) are different. We
shall discuss these aspects later.

2.2. Crystallography
The crystal structures of complexes [NnBu4][{Mo(h3C3H5)(CO)2}2(m-Cl)2(m-OMe)] (3) and [PPN][{Mo(h3C3H5)(CO)2}2(m-Cl)3] (4) were determined by X-ray
diffraction analysis. The unit cell of complex 4 is built
up from an asymmetric unit composed of one dimeric
cation [Ph3PN+PPh3].
The crystal structure of complex 3 consists of the
(3) and the [NnBu4]+ cation, which is strongly disordered. All attempts to find a disorder model for
[NnBu4]+ were unsuccessful (see below) and a detailed
analysis of the structural features of complex 3 is as a
consequence limited. However, the structure of complex
anion is determined unequivocally, provides a detailed
characterization of the coordination sphere of two
metal centers, and allows a broader discussion of the
conformational features of the apparently flexible
[{M(h3-allyl)(CO)2}2(m-X)3] species (M= Mo(II) or
W(II)) [5,6]. For comparison purposes, the molecular
structures of the two related binuclear anions 4 and
3 will be discussed together.
Molecular diagrams, including the crystallographic
labeling schemes, are shown in Figs. 1 and 2 for
binuclear anions 4 and 3, respectively. Selected bond
lengths and angles are listed in Table 1.
In both complexes, each molybdenum center is coordinated to two carbonyl groups and one allyl ligand in
a h3-mode. The two independent Mo(h3-C3H5)(CO)2
moieties are bridged by three chlorine atoms in 4 and



Fig. 2. A PLUTON view of the binuclear anion [{Mo(h3C3H5)(CO)2}2(m-Cl)2(m-OMe)] (3) with the atomic numbering
scheme adopted.
Table 1
Selected bond lengths (A, ) and angles () for [NnBu4][{Mo(h3C3H5)(CO)2}2(m-Cl)2(m-OMe)] (3) and [PPN][{Mo(h3-C3H5)(CO)2}2(m-Cl)3] (4) a

3 [X= O(6)]

4 [X = Cl(3)]

Bond lengths



Bond angles



The bond lengths and angles involving the carbonyl groups are
average values.

by two chlorine atoms and one methoxy group in 3,

giving rise to a pseudo-octahedral geometry around
each molybdenum atom. The overall geometry can be
described as a distorted confacial bioctahedron, with
the centroids of the allyl ligands occupying two sites of
this coordination polyhedron. The face shared by the
two octahedra is defined by the three bridging ligands.
The average metal-to-C3centroid distances are 2.036
and 2.040 A, for complexes 4 and 3, respectively. In

the coordination sphere of 3, the two allyl ligands are

trans to the methoxy group leading to angles between
the C3centroid and oxygen atom O(6) of 169.5 and
170.3, respectively. In complex 4, the average value of
the two MoCl distances involving the bridging chlorine Cl(1) [2.553(4) A, ], trans to the allyl ligands, is
slightly shorter than that involving the other two chlorine bridges, trans to the carbonyl ligands [2.595(4) A, ].
Average MoCl distances in 3 are 2.640(5) A, , average
MoCO bond distances are 1.95(2) A, in 4 and 1.90(5)
in 3, while the corresponding OCMoCO angles are
78.6(6) in 4 and 78.2(1) in 3. The carbonyl groups
are coordinated linearly to the molybdenum atoms, the
average MoCO angles being 178(1) and 176(3), for
4 and 3, respectively. These geometries agree well
with those found for the anion 4 in the complex
[Mo(CO)2(h3 - C3H5)(MeCN)3][{Mo(h3 - C3H5)(CO)2}2(m-Cl)3] (5) [5] and [{W(h3-C3H5)(CO)2}2(m-Cl)3] anion (6) [6], although different isomers are present, as
will be discussed further below.
The MoO(bridge) distances of 2.111(12) and
2.098(13) A, in 3 are similar to those found for the
binuclear complex [{Mo(h7-C7H7)}2(m-Cl)2(m-OH)]
(7), where the two molybdenum centers are also
bridged by one oxygen and two chlorine atoms [7].
Cotton and Ucko introduced several structural parameters to characterize the geometric distortions in bioctahedral structures such as those discussed above [8].
Among them, the parameter b 70.53 reflects the
axial distortion, away from the octahedron, of the
central core of [{Mo(C3centroid)(CO)2}2(m-Cl)3] in 4
and [{Mo(C3centroid)(CO)2}2(m-Cl)2(m-OMe)] in 3
along the axis defined by the two metal centers. b is the
MLM bond angle (L =Cl or OMe) and takes the
value of 70.53 for an ideal confacial bioctahedron.
Thus, b 70.53 is equal to 0 for an ideal bioctahedral
structure. Negative and positive values will be expected
for contracted and elongated bioctahedral structures,
respectively. Negative values for the b 70.53 parameter should be helped by the formation of a metalmetal
bond, while positive values reflect the absence of such
interactions. For the anion 4, values of 16.9(1), 14.7
(1) and 15.6(1) for the b70.53 parameter and a
MoMo distance of 3.526(1) A, were found. In anion
3, the b70.53 parameter has the small values of
8.4(1) and 9.4(2), calculated here from the two
MoClMo angles. The MoMo distance of 3.372(2)
A, is 0.15 A, shorter than for 4. The binuclear anions
[{M(h3-C3H5)(CO)2}2(m-Cl)3] [M= Mo(II) or W(II)]
in complexes 5 and 6 have b 70.53 parameters [14.5,
16.5 and 16.5 in 5 and 16.5, 17.5 and 18.5 in 6] and
MM distances [3.531 A, in 5 and 3.539 A, in 6] similar
to those found for anion 4 [5,6]. Therefore, this
analysis suggests the absence of metalmetal (M =
Mo(II) or M =W(II)) bonds in these anionic com-



2.3. Voltammetric studies

Fig. 3. The two conformations adopted by the [{Mo(h3C3H5)(CO)2}2(m-Cl)3] anion in complexes 4 and 5: eclipsed (e, left)
and staggered (s, right).

plexes, in agreement with the formal 18 electron count

for each d2 metal atom. In fact, the four MoMo
distances reported for these complex anions are clearly
too long to be ascribed to molybdenummolybdenum
The relative orientation of the two allylic fragments
in each binuclear unit can be characterized by the
rotation angle (l) defined by the centroids of these
ligands and the two molybdenum atoms. When the
centroids are eclipsed fully along the MoMo vector,
the l angle will be 0, while the trans staggered arrangement will lead to a l angle of 180. l takes the values
of 5.0 in 4 and 6.9 in 3, indicating that, in the
complexes under study, the two allyl ligands are almost
eclipsed. Interestingly, both anion 4 in complex 5 and
anion 6 in complex 6 exhibit intermediate conformations with l angles of 127.6 and 117.6, respectively.
Fig. 3 shows the two conformations adopted by the
and 5.

The cyclic voltammetry of solutions of the binuclear

complexes 4 and 3 was studied in several solvents, as
well as that of 2, and the precursor 1 for comparison.
[MoCl(h3-C3H5)(CO)2(NCMe)2] (1) has been shown to
exhibit one quasi-reversible wave in MeCN, while three
waves are observed in CH2Cl2. These differences have
been traced to the lability of the nitrile ligands, which
lead to the formation of two different complexes, assigned as [{Mo(h3-C3H5)(CO)2}2(m-Cl)3] and [Mo(h3C3H5)(CO)2(NCMe)3]+ [9]. The first is apparently the
same anion as in 3.
The results of cyclic voltammetry in MeCN and
CH2Cl2 (see details in Section 4) are collected in Table
2 and are comparable to those obtained for 1 in a
published study [9].
Indeed, the same quasi-reversible process is observed
in MeCN, as well as three processes in CH2Cl2. The
E1/2 values differ because a different reference electrode
is being used. On the other hand, none of the waves
observed in the cyclic voltammogram of 4 coincides
with those of 1, showing that the anion 4 present in
solution is not the same as that formed by the dissociation of 1 in CH2Cl2. The previous results from the
crystal-structure determination suggest that the conformer formed upon dissolving 4 is the eclipsed, as
found in the crystal, while dissociation of 1 leads to the
staggered conformer. This will be discussed again in the
next section. The effect of substituting the Cl bridging
ions by a methoxide can be detected in the cyclic
voltammogram. Indeed, both solutions exhibit a quasireversible wave (only the E1/2 values differ from one
solvent to the other) and other two irreversible oxidations not showing their counterpart. The absence of this
counterpart is more pronounced in 3 than in 4. The E1/2
values cannot be compared, as the order is inverted
when the solvent changes. Finally, the 2,2%-bisimidazole
complex 2 can be compared with the bipy and phen

Table 2
Cyclic voltammetric data for the allyl complexes 1, 2, 3, and 4




[MoCl(h3-C3H5)(CO)2(H2biim)] (2)


Only Ea.
Only Ec.

910 a
1164 a













820 a
436 b








Table 3
Relative energies of the two isomers of [{Mo(h3-C3H5)(CO)2}2(mCl)2(m-X)] (X =Cl, OMe) in kcal mol1

Eclipsed (e) Staggered (s)




Fig. 4. DFT optimized structures of the [{Mo(h3-C3H5)(CO)2}2(mCl)3] anion in the two conformations: eclipsed (e, left) and staggered
(s, right).

derivatives described in the literature [9], as the ligand

also binds through two nitrogen atoms. For
[{MoCl(h3-C3H5)(CO)2(L-L)Cl] (LL = bipy, phen) one
single one-electron oxidation has been observed and
assigned to a quasi-reversible process. When LL =
2,2%-bisimidazole, there is one irreversible, but complete,
wave (the ipa/ipc ratio is ca. 1.8), as well as two other
incomplete processes, in MeCN. The poor solubility in
CH2Cl2 prevented any voltammogram being obtained.

2.4. Theoretical calculations

The observation of more than one isomer of the
complexes, led us to perform density function theory
(DFT) calculations [10] (ADF program [11], see Section
4 for details) in order to find out their relative stability
and to explore the possibility of the existence of isomers
for the other anionic complex [{Mo(h3-C3H5)(CO)2}2(m-Cl)2(m-OMe)] (3).
It was seen in Fig. 3 that one isomer exhibits an
eclipsed conformation of the two allyls. The threefold
kind of symmetry, typical of the pseudo octahedral
arrangement, can in principle lead to three isomers, for
l 0, 120, and 240, respectively. The symmetry of the
anions 4 and 3, however, allows only two independent isomers. If one allyl is allowed to rotate by ca.
120 relative to the other, the structure of these anions
is observed. Further rotation of 120 leads to an identical arrangement. There are only two isomers, which
will be described as eclipsed and staggered for clarity.
The relative energies given in Table 3 result from a
full geometry optimization without any symmetry

The differences in energy are extremely small, especially for the complex with three chlorine bridges. It is
therefore not surprising that both isomers have been
found. Packing effects will be important in adding to
the stability of either form, the overall stability depending also on the counter ion. For the second complex,
only one compound is known and it adopts the arrangement with the lowest energy. As the two isomers
were characterized structurally for the derivatives of
[{Mo(h3-C3H5)(CO)2}2(m-Cl)3], 4 and 5, respectively,
they will be used to compare and discuss the results of
the calculations, as for the OMe bridged complex only
one, poor quality, crystal structure is available.
We emphasize in Fig. 4 the most important details
relative to the two structures. The MoCl distances
were all different, owing to the lack of symmetry constraints in the calculation, respectively, 2.650, 2.646 A,
in s, 2.553, 2.547 A, in the eclipsed form e, for the
MoCl bond trans to the allyl ligands, and 2.543, 2.551,
2.663, 2.643 A, in the staggered form s, 2.646, 2.665,
2.669, 2.661 A, in e for the others. The MoMo distance
was found to be 3.575 and 3.577 A, in s and e, respectively. These results show a good agreement with experimental distances, the calculated root mean square
(r.m.s.) values, excluding hydrogen atoms, between the
X-ray structure of 4 and cis-4 being 0.078 A, , and
between the X-ray structure of 5 and trans-4 0.064
A, . The r.m.s. value calculated for the X-ray structure of
3 and cis-3 is larger (0.120 A, ), owing to the poor
quality of the structure, but still reflects a good agreement. The energy differences are very small and
difficult to interpret, but show that both isomers correspond to local minima. We can, therefore, believe that
each isomer may be formed depending on the conditions, the staggered form being the thermodynamic
product obtained by dissociation of the mononuclear
precursor in CH2Cl2, while the serendipitous synthesis
of our binuclear complex 4 resulted in the less stable
eclipsed form, owing probably to kinetic reasons.
We also calculated the energy differences between the
anions and the oxidized neutral species, which are,
respectively, 3.83 eV for cis-4, 3.81 eV for trans-4,
and 3.75 eV for cis-3, in order to compare their
relative oxidation potentials, but the differences are
rather small. It is not surprising that the change in
solvent can invert the order, as found between cis-4
and cis-3. The difference between cis-4 and trans-4
is also small. However, the strong solvent dependence
restricts the value of such comparisons.

2.5. The crystal structures

After having studied the isolated anions, we shall
now examine their crystal structures, in an effort to
understanding the factors that govern the role of the
two conformational isomers and their packing systems.


A comparative study of the crystal structures of the

[NnBu4][{Mo(h3-C3H5)(CO)2}2(m-Cl)2(mOMe)] (3), [PPN][{Mo(h3-C3H5)(CO)2}2(m-Cl)3] (4),
[Mo(CO)2(h3 - C3H5)(MeCN)3][{Mo(h3 - C3H5)(CO)2}2(m-Cl)3]C6H6 (5) and [NnBu4][{W(h3-C3H5)(CO)2}2(mCl)3] (6) was carried out, and the coordinates were
taken from the Cambridge Crystallographic database
In the crystal structure of 3, layers of [{Mo(h3C3H5)(CO)2}2(m-Cl)2(m-OMe)] anions are intercalated
by layers of highly disordered n-butyl ammonium
cations. The crystal structure of 5 is composed of layers
of the complex species [Mo(CO)2(h3-C3H5)(MeCN)2]+
and [{Mo(h3-C3H5)(CO)2}2(m-Cl)3] intercalated by
layers of benzene solvent molecules. The solid-state
structure of 6 is afforded by co-crystallization of
[{W(h3-C3H5)(CO)2}2(m-Cl)3] anions and ordered nbutyl ammonium cations. These two block units intercalate without a special structural crystal order, cations
being surrounded by anions and vice-versa. The molecular assembling of anion [{Mo(h3-C3H5)(CO)2}2(mCl)3] and PPN in 4 results in the most interesting
solid-state structure of the group, as shown in Fig. 5.
The [Ph3PN+PPh3] cation units are aggregate in layers, forming open double channels. Each one of these
large channels accommodates two chains of [{Mo(h3C3H5)(CO)2}2(m-Cl)3] anions, which are related by a
crystallographic inversion center. The channels are
sheathed by CH groups pointing towards the carbonyl
groups of the anions.
We cannot draw conclusions from the different packing patterns in the two structures, because the change
of isomer is accompanied by a drastic change of counterion. It is clear from Fig. 5 that the cations determine
the main features of the crystal structure of 4. Their
presence is vital for forming the structure.


3. Conclusions
The complex [MoCl(h3-C3H5)(CO)2(H2biim)] (2),
with two NH groups in the coordinated H2biim ligand, appeared as a very promising precursor for the
formation of molecular solids, different units being
linked by hydrogen bonds. Such studies were unsuccessful, the main obstacle being its very poor solubility. On
the other hand, we managed to isolated a different
isomer of the well-known [{Mo(h3-C3H5)(CO)2}2(mCl)3] anion, and showed, by means of both cyclic
voltammetry and theoretical studies, that it does not
interconvert into the other. The packing of the eclipsed
isomer shows double chains of the dimer, but it is
determined by the PPN cation, rather than the anion.
4. Experimental

4.1. Synthesis
Commercially available reagents and all solvents
were purchased from standard chemical suppliers. All
solvents were used without further purification except
acetonitrile (dried over CaH2), dichloromethane (dried
over CaH2), and THF, which was distilled over
sodiumbenzophenone ketyl and used immediately.
2,2%-Bisimidazole (H2biim) [13] and [(PMDTLi)2biim
[14] were synthesized according to literature procedures.
H-NMR spectra were recorded on a Bruker AMX-300
(300 MHz) spectrometer in acetone-d6 (d 2.04), CD3CN
(d 1.93) and CD3OD (d 4.78 and 3.30) as solvents.
Elemental analyses were carried out at ITQB and Universidade de Zaragoza (PerkinElmer 2400). The IR
spectra were recorded on a Unicam Mattson 7000
FTIR spectrometer. Samples were run as KBr pellets.

4.2. Preparation of [MoCl(h 3-C3H5)(CO)2(H2biim)] (2)

To a yellow solution of [MoCl(h3-C3H5)(CO)2(NCMe)2] in ethanol (0.311 g, 1 mmol) an equivalent
amount of 2,2%-bisimidazole (0.134 g, 1 mmol) was
added. After stirring overnight, an orange precipitate
formed, which was isolated, washed with diethyl ether
and dried under vacuum.
The yield was 0.276 g (76%). Anal. Calc. for
MoClC11H11N4O2: C, 36.4; N, 15.4; H, 3.1. Found: C,
36.4; N, 15.3; H, 3.2%. 1H-NMR (300 MHz, CD3OD,
25C): d 7.63 (s, 2H, H2biim), 7.48 (s, 4H, H2biim), 4.09
(m, H, H(allyl)meso ); 3.52 (d, 2H, H(allyl)syn ); 1.41 (d,
2H, H(allyl)anti ).

4.3. Preparation of [N nBu4][{Mo(h 3-C3H5)(CO)2}2 (m-Cl)2(m-OMe)] (3)

Fig. 5. Crystal packing diagram of 4 showing the open channels
formed by PNP cations. These channels accommodate the complex

To a yellow solution of [MoCl(h3-C3H5)(CO)2(NCMe)2] in ethanol (0.622 g, 2 mmol) in 20 ml of freshly



distilled THF, [(PMDTLi)2biim] (0.520 g, 1 mmol) was

added. After refluxing for 2 h, the solution was filtered
and evaporated to dryness. The remaining oil was
redissolved in THF and [NnBu4]Br was added (0.184 g,
2 mmol). The solution was filtered to separate excess
[NnBu4]Br and dried. The solid residue was recrystallized from methanol to give orange crystals. One crystal
was studied by single-crystal X-ray diffraction.
The yield was 0.177 g (36%). 1H-NMR (300 MHz,
CD3CN, 25C): d 3.08 (t, 8H, a NnBu4); 1.59 (m, 8H, b
NnBu4); 1.35 (m, 8H, g NnBu4); 0.97 (t, 12H, d NnBu4).

4.4. Preparation of
THF (0.031 g, 0.1 mmol) was cooled to 0C in an ice
bath. AuClPPN (0.081g, 0.1 mmol) was added. After it
dissolved, the solution turned green. It was stirred for
30 min and filtered over celite. The volume of the
yellow solution was reduced to 5 ml under vacuum and
hexane was added. The green oil was converted to light
green crystals after several additions of hexane. After
recrystallization from hexane, light green crystals were
obtained. One crystal was studied by single-crystal Xray diffraction.
The yield was 0.091 g (87%). Anal. Calc. for
C46H40Cl3O4P2NMo2: C, 53.6; N, 1,4; H, 3.9. Found:
C, 52.9; N, 1.5; H, 3.8%. 1H-NMR (300 MHz, CD3CN,
25C): d 7.57.7 (m, 20H, PNP), 3.61 (m, H, H(allyl)meso ); 1.78 (s, 2H, H(allyl)syn ); 1.40 (s, 2H,
H(allyl)anti ).

4.5. Crystal structure determinations

4.5.1. Crystal data
C27H49Cl2Mo2NO5 (3), M =730.45, triclinic space
group, P1( , a= 10.975(11), b =11.532(11), c=
14.014(16) A, , a = 80.53(1) b = 77.19(1), g=89.64(1),
V = 1705(3), Z=2, Dcalc. =1.423 g cm 3, F(000)=
752, m =0.924 mm 1.
C46H40Cl3Mo2NO4P2 (4), M = 1030.96, triclinic space
group, P1( , a = 10.060(14), b =15.360(17), c =
16.750(17) A, , a =117.04(1), b =95.31(1), g= 98.01(1),
V = 2247(5) A, 3, Z= 2, Dcalc. =1.524 g cm 3, F(000)=
1040, m =0.851 mm 1.
4.5.2. Data collection and processing
The X-ray single-crystal data for the two complexes
were collected using graphite-monochromated MoKa
radiation on a MAR research image plate system (l=
0.71073 A, ) at Reading University. The crystals were
positioned at 70 mm from the image plate. A total of 95
frames were measured at 2 intervals with a counting
time suitable for the diffraction pattern exhibited by the

complex under investigation. Data analysis was performed with the XDS program. [15] The intensities were
not corrected for absorption effects.

4.5.3. Structure analysis and refinement

The positions of the molybdenum atoms were determined by direct methods and the positions of remaining
non-hydrogen atoms were obtained subsequently by
successive Fourier difference syntheses. The structures
were refined by full-matrix least-squares methods on F 2
using a weighting scheme with the standard form.
For complex 4 6871 independent reflections were
measured, which were used in the solution and refinement of the structure. Anisotropic thermal parameters
were used for all non-hydrogen atoms. The hydrogen
atoms were introduced in the process refinement at
ideal geometric positions with Uiso values equal to 1.2
times those of the atoms to which they were bonded.
The final refinement converged to R and wR2 values of
0.1061 and 0.2919 for the 4371 data with I\ 2s(I) and
0.1921 and 0.3457 for all unique hkl data.
A total of 5445 independent reflections were measured for complex 3, which are used in the structure
solution and refinement calculations. From the earliest
Fourier-difference maps calculated for this complex, it
was apparent that the NnBu4 cation was disordered.
The refinement of a trial model including anisotropic
thermal parameters for the nitrogen atom of the cation
and all non-hydrogen atoms of the complex anion gave
a high R value of 17% for the 2272 hkl data with
I\ 2s(I). Individual isotropic thermal parameters and
unit occupation factors were considered for the carbon
atoms of the NnBu4 cation. For all these carbon atoms
high values for the Uiso parameters were obtained, and
other alternative positions were discernible for all
atoms from the Fourier-difference map. These results
indicated definitively that the NnBu4 cation was
strongly disordered. Then, many trial refinements were
performed using different disorder models. Unfortunately, no trial model gave satisfactory results either in
terms of a reasonable overall geometry for the NnBu4
cation or a pronounced decrease of the R value. Consequently, the unsuccessful modeling of the contribution
of NnBu4 for the electronic density map led naturally to
a low quality of the molecular structure of anion in 3,
namely in the standard deviations of their geometric
parameters. However, the molecular geometry of the
complex anion is unambiguous and allows us to compare the different stereochemistries of [{Mo(h3C3H5)(CO)2}2(m-Cl)2(m-OMe)] and [{Mo(h3-C3H5)(CO)2}2(m-Cl)3].
All calculations required to solve and to refine the
structures were carried out with SHELXS and SHELXL
within the SHELX-97 package [16]. The molecular and
crystal packing diagrams were drawn with PLATON [17].


4.6. Electrochemistry
The electrochemical instrumentation consisted of a
BAS CV-50W voltammetric analyzer connected to
BAS/Windows data-acquisition software. All the electrochemical experiments were run under argon at room
temperature. Tetrabutylammonium hexafluorophosphate (Aldrich) was used as supporting electrolyte; it
was recrystallized from ethanol. Cyclic voltammetry
experiments were performed in a glass cell MF-1082
from BAS in a C-2 cell enclosed in a Faraday cage. The
reference electrode was Ag AgCl (MF-2079 from BAS)
and its potential was 44 mV relative to a SCE. The
reference electrode was calibrated with a solution of
ferrocene (1 mM) to obtain a potential in agreement
with the literature value [18]. The auxiliary electrode
was a 7.5 cm platinum wire (MW-1032 from BAS) with
a gold-plated connector. The working electrode was a
platinum disk (MF-2013 from BAS) with ca. 0.022 cm2
sealed in Kel-F plastic. Between each CV scan the
working electrode was electrocleaned, polished on diamond 1 mM and alumina cleaning with water
methanol and sonicated before use, according to
standard procedures. Solvents were dried as described

4.7. DFT calculations

Density functional calculations [10] were carried out
program [11] developed by Baerends et al. (release 2.3)
[19]. Vosko et al.s local exchange correlation potential
was used [20], with Beckes nonlocal exchange [21] and
Perdews correlation corrections [22]. Unrestricted calculations were performed for the paramagnetic complexes. The geometry optimization procedure was based
on the method developed by Versluis and Ziegler [23],
using the non-local correction terms in the calculation
of the gradients. using nonlocal exchange and correlation corrections. The structures of the complexes described above and others referred to in the text were
used to prepare input files for the full optimization of
the geometry of the anionic complexes. In all the calculations, a triple-z Slater-type orbital (STO) basis set was
used for Mo, 5s, 4p, 5p, 4d; triple-z STO augmented
with a 3d single-z polarization function were used for
C, 2s and 2p, O 2s and 2P, and H, 1s. A frozen core
approximation was used to treat the core electrons of C
(1s), O (1s), and Mo ([1 3]s, [1 3]p, 3d).

5. Supplementary material
Crystallographic data for the structural analyses have
been deposited with Cambridge Crystallographic Data
Centre, CCDC nos. 136832 for compound 3 and


136833 for compound 4. Copies of this information

may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(Fax: +44-1223-336033; e-mail:
uk or www:

This work was supported by PRAXIS XXI under
project PRAXIS/PCNA/C/QUI/103/96. We thank Zara
Miravent Tavares for the elemental analysis (ITQB)
and cyclic voltammetric experiments. P.P. thanks ITQB
for a grant. M.J.C. and E.B. thank the TMR Transition
Metal Clusters in Catalysis and Organic Synthesis. V.F.
thanks the British Council and FCT for funding. The
University of Reading and EPSRC are thanked for
funds for the Image Plate system.

[1] (a) J.M. Lehn, Angew. Chem. Int. Ed. Engl. 29 (1990) 1304. (b)
D. Braga, F. Grepioni, Acc. Chem. Res. 27 (1994) 51. (c) M.J.
Calhorda, D. Braga, F. Grepioni, Transition metal clusters
the relationship between molecular and crystal structure, in: P.
Braunstein, L.A. Oro, P.R. Raithby (Eds.), Metal Clusters in
Chemistry, Wiley VCH, Weinheim, 1999. (d) D. Braga, F.
Grepioni, A.G. Orpen (Eds.), Crystal Engineering: from
Molecules to Crystals, to Materials, Kluwer, Dordrecht, 1999.
[2] (a) D. Braga, M.J. Calhorda, F. Grepioni, P.E.M. Lopes, E.
Tedesco, J. Chem. Soc. Dalton Trans. (1995) 1215. (b) C. Mealli,
D.M. Proserpio, J. Organomet. Chem. 386 (1990) 203. (c) E.
Hunstock, C. Mealli, M.J. Calhorda, J. Reinhold, Inorg. Chem.
38 (1999) 5053.
[3] (a) H. tom Dieck, H. Friedel, J. Organomet. Chem. 14 (1968)
375. (b) R.G. Hayter, J. Organomet. Chem. 13 (1967) P1. (c)
P.K. Baker, Adv. Organomet. Chem. 40 (1996) 45 and references
[4] C.G. Hull, M.H.B. Stiddard, J. Organomet. Chem. 9 (1967) 519.
[5] (a) H.D. Murdoch, R. Henzi, J. Organomet. Chem. 5 (1966) 552.
(b) M.G.B. Drew, B.J. Brisdon, M. Cartwright, Inorg. Chim.
Acta 36 (1979) 127.
[6] M. Boyer, J.-C. Daran, Y. Jeannin, Inorg. Chim. Acta 47 (1981)
[7] C. Couldwell, K. Prout, Acta Crystallogr. Sect. B 34 (1978)
[8] F.A. Cotton, D.A. Ucko, Inorg. Chim. Acta 6 (1972) 161.
[9] B.J. Brisdon, K.A. Conner, R.A. Walton, Organometallics 2
(1983) 1159.
[10] R.G. Parr, W. Yang, Density Functional Theory of Atoms and
Molecules, Oxford University, New York, 1989.
[11] AMSTERDAM DENSITY FUNCTIONAL (ADF) program, release 2.3,
Vrije Universiteit, Amsterdam, The Netherlands, 1995.
[12] F.H. Allen, J.E. Davies, J.J. Galloy, O. Johnson, O. Kennard,
C.F. Macrae, D.G. Watson, J. Chem. Inf. Comp. Sci. 31 (1991)
[13] H. Debus, Ann. Chem. 197 (1858) 199.
[14] B.F. Fieselmann, D.N. Hendrickson, G.D. Stucky, Inorg. Chem
17 (1978) 2078.
[15] W. Kabsch, J. Appl. Crystallogr. 21 (1988) 916.



[16] G.M. Sheldrick, SHELX-97, University of Gottingen, Germany,

[17] A.L. Spek, PLATON, a Multipurpose Crystallographic Tool,
Utrecht University, Utrecht, The Netherlands, 1999.
[18] I.V. Nelson, R.T. Iwamoto, Anal. Chem. 35 (1963) 867.
[19] (a) E.J. Baerends, D. Ellis, P. Ros, Chem. Phys. 2 (1973) 41. (b)
E.J. Baerends, P. Ros, Int. J. Quantum Chem. S12 (1978) 169.
(c) P.M. Boerrigter, G. te Velde, E.J. Baerends, Int. J. Quantum


Chem. 33 (1988) 87. (d) G. te Velde, E.J. Baerends, J. Comp.

Phys. 99 (1992) 84.
S.H. Vosko, L. Wilk, M. Nusair, Can. J. Phys. 58 (1980) 1200.
A.D. Becke, J. Chem. Phys. 88 (1987) 1053.
(a) J.P. Perdew, Phys. Rev. B 33 (1986) 8822. (b) J.P. Perdew,
Phys. Rev. B 34 (1986) 7406.
(a) L. Versluis, T. Ziegler, J. Chem. Phys. 88 (1988) 322. (b) L.
Fan, T. Ziegler, J. Chem. Phys. 95 (1991) 7401.