Synthesis, reactivity and structural characterization of some heterometallic complexes containing naked arsenic ligands

Article abstract of DOI:10.1039/a905688b

The reactions of [Co₂(μ-As₂)(CO)₆] with [Mo₂Cp₂(CO)₄] (Cp = η⁵-C₅H₅) and with [(OC)₄CoMCp(CO)₃] [M = Mo or W] gave, respectively, the

Full text of DOI:10.1039/a905688b

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Synthesis, reactivity and structural characterization of some
heterometallic complexes containing naked arsenic ligands†
Katherine V. Adams,a Nick Choi,b Gráinne Conole,b John E. Davies,a Jason D. King,‡a
Martin J. Mays,*a Mary McPartlinb and Paul R. Raithbya
a Department of Chemistry, Lensfield Rd., Cambridge, UK CB2 1EW
b School of Applied Chemistry, University of North London, Holloway Rd., London,
UK N7 8DB
Received 14th July 1999, Accepted 21st September 1999
The reactions of [Co2(µ-As2)(CO)6] with [Mo2Cp2(CO)4] (Cp = η5-C5H5) and with [(OC)4CoMCp(CO)3] [M = Mo
or W] gave, respectively, the known product [Mo2(µ-As2)Cp2(CO)4] 1 and the new heterobimetallic complexes
[(OC)3Co(µ-As2)MCp(CO)2] [M = Mo 2 or W 3]. The mixed-metal complexes have been treated with PPh3 to
give the substituted complexes [(Ph3P)(OC)2Co(µ-As2)MCp(CO)2] [M = Mo 4 or W 5]. Complex 2 reacted with
[Cr(CO)5]ؒTHF to give [(OC)3Co{µ-As2[Cr(CO)5]2}MoCp(CO)2] 6. Reaction of [MoWCp2(CO)4] with [Co2(µ-As2)-
(CO)6] does not lead to the complex [MoW(µ-As2)Cp2(CO)4] although this can be obtained by co-thermolysis
of [Mo2Cp2(CO)6] and [W2Cp2(CO)6] with grey arsenic which affords three bimetallic products, [MMЈ(µ-As2)-
Cp2(CO)4] (M = MЈ = Mo 1; M = Mo, MЈ = W 7; M = MЈ = W), and four trimetallic products, [M2MЈ(µ3-As)-
Cp3(CO)6] (M = MЈ = Mo; M = Mo, MЈ = W 8; M = W, MЈ = Mo 9; M = MЈ = W). The new heterometallic
complexes 2, 3, 5, 6 and 9 have been characterized by X-ray diffraction analyses.
whether M–M or As–As bond cleavage would be the preferred
mode of reaction of M2As2 complexes.
Introduction
Spectroscopic and vapour pressure studies of arsenic in the gas-
eous state reveal the presence of diatomic As2, a homologue of
N2.1 While this is unstable in the condensed phase with respect
to tetrahedral As4 in which three single As–As bonds replace
the formal As–As triple bond, stabilization of the As2 species at
transition metal centres has been achieved. Thus complexes
containing an As2 unit attached to two,2–9 three2,10 and
four3,8,11,12 transition metal atoms have been synthesized by a
variety of methods, as have polymetallic clusters with more
than one As2 ligand.7,13,14 However, previous studies have not
addressed the possibility of synthesizing heterobimetallic com-
plexes containing the diarsenic ligand. In this paper we report
new synthetic routes by which such species containing molyb-
denum or tungsten with cobalt can be prepared. We also report
the synthesis of some new heterotrimetallic clusters containing
the µ3-As ligand.
Reaction of [Co2(µ-As2)(CO)6] with [Mo2Cp2(CO)4] in reflux-
ing toluene for six hours gave orange [Mo2(µ-As2)Cp2(CO)4] 1
in 15% yield. The identity of 1 was established by comparison
of the IR spectroscopic data obtained with those previously
reported for this complex.2,3,5,6b The FAB mass spectrum dis-
played the expected molecular ion peak. Thus, under these
conditions, the reaction between these reagents which would
have paralleled that described above involving alkynes, namely
the generation of an octahedral tetrametallic complex with
cleavage of the Co–Co bond, does not occur. Rather, the As2
ligand appears to be ‘transferred’ from the dicobalt to the
dimolybdenum unit. This unusual reaction has only been noted
once before,17 in the transfer of a P2 ligand from [Co2(µ-P2)-
(CO)6] to [W2(OiPr)6(py)2], generating [W2P2(OiPr)6(py)].
Although alternative mechanisms are possible, the transfer of
the P2 ligand as a discrete unit from one bimetallic fragment to
another was suggested as the most likely pathway.18 In the reac-
tion leading to 1 considered here the lone pairs on the arsenic
atoms, which are known to be weak donors,12 could attack the
electronically unsaturated [Mo2Cp2(CO)4]. Complete transfer
to the new bimetallic fragment could then occur with decom-
position of the displaced Co2(CO)6. In practice a complex mix-
ture of decomposition products is obtained which could not be
separated.
Results and discussion
(a) Reaction of [Co2(ꢀ-As2)(CO)6] with [Mo2Cp2(CO)4]
(Cp ؍ ꢁ5-C5H5)
It has been shown recently that the complexes [Mo2(µ-RCCRЈ)-
Cp2(CO)4] [R = RЈ = H or Me; R = H, RЈ = Me, Ph or CO2Me]
undergo reaction with [Co2(CO)8]15 to yield the complexes
[Co2Mo2(µ4-RCCRЈ)(µ-CO)4Cp2(CO)4] and with [Ru3(CO)12]16
to give [Mo2Ru4(µ3-CR)Cp2(CO)12]. The former reaction occurs
with cleavage of the Mo–Mo bond whereas in the latter case
it is cleavage of the C–C bond that yields the bis-alkylidyne
product. Given the structural and electronic similarity between
alkyne-bridged and As2-bridged complexes, it was of interest
to investigate the reactions of a representative example of the
latter class of complex with other metal carbonyls to see
Effectively, the [Co2(µ-As2)(CO)6] complex simply acts as a
carrier of the As2 unit which is otherwise only available in
arsenic vapour at high temperatures. Thus the process may be
represented formally as the reaction of As2 with [Mo2Cp2-
(CO)4]. Viewed in this way it is not dissimilar to the thermolytic
reaction of [Mo2Cp2(CO)6] with elemental arsenic,2 during
which As2 may be formed as an intermediate.
(b) Reaction of [Co2(ꢀ-As2)(CO)6] with [(OC)4CoMCp(CO)3]
[M ؍ Mo or W]
† Supplementary data available: rotatable 3-D crystal structure diagram
in CHIME format. See http://www.rsc.org/suppdata/dt/1999/3679/
‡ Present address: Department of Chemistry, The University of
Montana, Missoula, 59812, Montana, USA.
Having established that [Co2(µ-As2)(CO)6] acts as a source of
‘free’ As2, it was of interest to see whether As2 could be ‘trans-
ferred’ to the mixed-metal complexes [(OC)4CoMCp(CO)3]
J. Chem. Soc., Dalton Trans., 1999, 3679–3686
This journal is © The Royal Society of Chemistry 1999
3679

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Co(µ-PhCCMe)WCp(CO)2],20 which both adopt a structure
Table 1 Selected bond lengths (Å) and angles (Њ) for [(OC)3Co(µ-As2)-
MCp(CO)2] [M = Mo 2 or W 3]
analogous to that of 2 and 3, the metal–metal bond lengths
have been determined to be just 2.692(1) and 2.693(1) Å,
respectively. The lengths of ligand-bridged Co–Mo and Co–W
bonds in other bi- and tri-metallic complexes typically lie in the
ranges 2.60–2.7519,21 and 2.55–2.76 Å,20,22 respectively. The long
Co–Mo bond found in [CpCoMo(CO)3(C8H8)]23 [2.998(1) Å] is
attributed to the steric requirements of the large bridging ligand
and it seems likely that the As2 unit exerts a similar influence in
2 and 3.
The As(1)–As(2) bond length of 2.305(1) Å in complex 2 lies
between that found for the corresponding bond in the homo-
metallic complexes [Co2(µ-As2)(CO)5(PPh3)]4a [2.273(3) Å, the
shortest known] and [Mo2(µ-As2)Cp2(CO)4]5 [2.311(3) Å]. Des-
pite the similarity of the metal–metal (see above) and metal–
arsenic (see below) bond lengths in 2 and 3, the As(1)–As(2)
bond length of 2.322(2) Å in 3 is significantly longer than the
As–As bond found in 2 or in either of the homometallic
examples cited above. In both 2 and 3 the As(1)–As(2) bond is
significantly shorter than that found in gaseous As4 [2.44 Å],24
(AsMe)5 [2.428(8) Å]25 or (AsPh)6 [2.456(5) Å]26 and shorter
also than the mean of those found in the related species
[As3Co(CO)3] (2.372(5) Å).4b This would seem to imply a degree
of multiple-bond character in the As–As bonds of 2 and 3. The
trend of decreasing As–As distance on sequential substitution
of metal carbonyl fragments into As4 can be rationalized by
considering a delocalized molecular orbital model, first applied
to [As3Co(CO)3], in which the metal system acts as an ‘electron
sink’ for the arsenic ligand.4b,27
M = Mo 2
M = W 3
M(1)–Co(1)
M(1)–As(1)
M(1)–As(2)
Co(1)–As(1)
Co(1)–As(2)
As(1)–As(2)
M(1)–Ccarbonyl
Co(1)–Ccarbonyl
C–Ocarbonyl
2.825(1)
2.579(1)
2.630(1)
2.420(1)
2.394(1)
2.305(1)
1.99(1)
1.783(8)–1.787(9)
1.134(8)–1.143(9)
1.38(1)–1.43(1)
2.821(2)
2.582(1)
2.632(1)
2.429(2)
2.406(2)
2.322(2)
1.99(1)
1.77(1)–1.80(1)
1.12(2)–1.16(1)
1.39(2)–1.43(2)
C–CCp
Co(1)–M(1)–As(1)
Co(1)–M(1)–As(2)
As(1)–M(1)–As(2)
M(1)–Co(1)–As(1)
M(1)–Co(1)–As(2)
As(1)–Co(1)–As(2)
M(1)–As(1)–Co(1)
M(1)–As(1)–As(2)
Co(1)–As(1)–As(2)
M(1)–As(2)–Co(1)
M(1)–As(2)–As(1)
Co(1)–As(2)–As(1)
52.95(3)
51.91(3)
52.51(3)
58.30(3)
59.83(3)
57.21(4)
68.75(3)
64.88(3)
60.85(4)
68.25(3)
62.61(3)
61.95(4)
53.21(5)
52.22(4)
52.88(4)
58.34(5)
59.84(5)
57.41(6)
68.45(5)
64.67(5)
60.80(6)
67.93(5)
62.45(4)
61.80(6)
In both complexes 2 and 3 the diarsenic unit is twisted to
place As(2) closer to cobalt and As(1) closer to molybdenum/
tungsten [For 2 Co(1)–As(1) 2.420(1), Co(1)–As(2) 2.394(1),
Mo(1)–As(1) 2.579(1), Mo(1)–As(2) 2.630(1) Å; for 3 Co(1)–
As(1) 2.429(2), Co(1)–As(2) 2.406(2), W(1)–As(1) 2.582(1),
W(1)–As(2) 2.632(1) Å]. The similarity of the Mo–As and
W–As bond lengths is noteworthy and consistent with the small
difference in the covalent radii of these metal atoms.
The presence of the cyclopentadienyl ligand on the same side
of the complex as As(1) and effectively trans to As(2) may be a
contributing influence to the selective extension of the M–As(2)
bond in these complexes. However, twisting of the diarsenic
unit is also a feature of the known homometallic analogues of
the new complexes.13 The frequency of this observation in a
range of compounds suggests a primarily electronic rather than
steric cause.
Fig. 1 The molecular structure of [(OC)3Co(µ-As2)MCp(CO)2] [M =
Mo 2 or W 3] including the atom numbering scheme. Hydrogen atoms
omitted for clarity.
[M = Mo or W], thus leading to heterobinuclear µ-As2 com-
plexes. The above mixed-metal complexes, like [Mo2Cp2(CO)4],
are known to react with alkynes under mild conditions to give
alkyne-bridged complexes.
The co-thermolysis of [Co2(µ-As2)(CO)6] and [(OC)4CoM-
Cp(CO)3] [M = Mo or W] in refluxing THF produced, in addi-
tion to unchanged [Co2(µ-As2)(CO)6], the orange complexes
[(OC)3Co(µ-As2)MCp(CO)2] [M = Mo 2 or W 3] in 36 and 21%
yields respectively. Both 2 and 3 have been characterized by IR,
1H and 13C NMR spectroscopy, mass spectrometry, and micro-
analysis and by single crystal X-ray diffraction studies.
The molecular structures of the isomorphous complexes 2
and 3, as determined by X-ray crystallography, are represented
in Fig. 1. Selected bond lengths and angles are given in Table 1.
Both molecules feature an As2 ligand bonded ‘side-on’ to a
bimetallic fragment, producing a tetrahedral CoMAs2 unit in
each case. The cyclopentadienyl ligand is bonded to metal M
such that it is trans to the diarsenic ‘edge’.
Given the similarity of the covalent radii of molybdenum
and tungsten, it is perhaps unsurprising that the Co(1)–Mo(1)
bond length [2.825(1) Å] in complex 2 is virtually the same as
the Co(1)–W(1) bond length [2.821(2) Å] in 3. However, both
are long compared with corresponding ligand-bridged single
bonds found in many other complexes. For example, in the
complexes [(OC)3Co(µ-CF3CCCF3)MoCp(CO)2]19 and [(OC)3-
Twisting of the bridging ligands in the related complex
[Nb2(µ-PhCCPh)2Cp2(CO)2] was initially ascribed to non-
bonded interactions within the molecule,28 but examination of
molecular orbital diagrams for this complex reveals that the
HOMO and LUMO are close in energy and well separated from
the other bonding molecular orbitals.29 Under these circum-
stances a second-order Jahn–Teller distortion is expected.
Twisting of the bridging ligand is one possible modification.
The molecular orbital approach has been extended to molecules
of the type [Co2(µ-X2)(CO)6]30 and it seems likely that this
effect contributes to some extent to the As2 twisting observed in
2 and 3.
The FT-IR spectra of complexes 2 and 3 each exhibit four
strong absorptions in the CO stretching region. The band pat-
tern is very similar to that observed for [(OC)3Co(µ-RCCR)-
MoCp(CO)2]. The cyclopentadienyl groups give rise to signals
1
in the H and 13C NMR spectra of both 2 and 3. A weaker
sharp singlet at δ 224.1 is due to the two carbonyl carbon atoms
bonded to molybdenum, indicating that there is a fluxional
process in solution which renders the two carbonyl groups
equivalent.
The formation of complexes 2 and 3 shows that a ‘transfer’
process does occur when [Co2(µ-As2)(CO)6] reacts with
[(OC)4CoMCp(CO)3] [M = Mo or W], yielding the hetero-
bimetallic products, [(OC)3Co(µ-As2)MCp(CO)2] (M = Mo or
3680
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Table 2 Selected bond lengths (Å) and angles (Њ) for [(Ph3P)(OC)2-
Co(µ-As2)WCp(CO)2] 5
Molecule 1
Molecule 2
W(1)–Co(1)
W(1)–As(11)
W(1)–As(12)
W(1)–C(13)
W(1)–C(14)
Co(1)–As(12)
Co(1)–As(11)
Co(1)–C(12)
Co(1)–C(11)
Co(1)–P(1)
As(11)–As(12)
C–O
2.781(3)
2.583(3)
2.650(3)
1.95(2)
2.832(3)
2.576(3)
2.637(3)
1.95(2)
1.95(2)
1.95(2)
2.383(4)
2.431(4)
1.71(2)
2.380(4)
2.400(4)
1.70(3)
1.71(2)
1.72(2)
2.208(7)
2.309(5)
1.17(2)–1.20(3)
1.81(3)–1.84(3)
2.219(7)
2.310(4)
1.17(2)–1.20(3)
1.81(2)–1.84(2)
P(1)–Cphenyl
Co(1)–W(1)–As(11)
Co(1)–W(1)–As(12)
As(11)–W(1)–As(12)
W(1)–Co(1)–As(11)
W(1)–Co(1)–As(12)
As(11)–Co(1)–As(12)
W(1)–As(11)–Co(1)
W(1)–As(11)–As(12)
Co(1)–As(11)–As(12)
W(1)–As(12)–Co(1)
W(1)–As(12)–As(11)
Co(1)–As(12)–As(11)
W(1)–Co(1)–P(1)
53.73(10)
51.97(9)
52.43(9)
51.44(9)
52.58(9)
52.35(11)
58.96(10)
61.17(11)
57.32(13)
67.30(11)
65.31(11)
60.29(13)
66.86(11)
62.34(11)
62.39(13)
157.7(2)
58.29(10)
60.05(10)
57.79(12)
69.28(10)
65.07(10)
60.66(12)
68.52(11)
62.35(10)
61.55(12)
150.4(2)
Fig. 3 The molecular structure and atom numbering scheme of
[(OC)3Co{µ-As2[Cr(CO)5]2}MoCp(CO)2] formed by reaction
of complex 2 with [Cr(CO)5]ؒTHF. There is a slight disorder (60:40) of
some of the carbonyl oxygen atoms and the major component is
illustrated.
6
lengths and angles in these two molecules, thought to be due
to correlations between the two molecules in refinement. For
purposes of the following discussion the mean values of the
bond lengths and angles will be used.
From Fig. 2 it is clear that the phosphine has substituted the
axial carbonyl on the cobalt atom and now occupies this site.
The regiospecificity of this process has been observed previ-
ously in related reactions of Co–Mo and Co–W systems.21 The
phosphorus–cobalt bond is inclined at 25.9Њ to the projection of
the W–Co bond, placing it trans to the cyclopentadienyl ligand
across the Co–W axis.
Co-ordination of the phosphine brings about a very slight
decrease in the length of the Co–W bond from 2.821(2) Å in
complex 3 to 2.807(3) Å in 5. Within the limits of experimental
error, the As–As separation of 2.310(5) Å in 5 is virtually the
same as the As(1)–As(2) bond length in 3 [2.322(2) Å]. Simi-
larly, the mean W–As and Co–As bond lengths of, respectively,
2.612(3) and 2.399(4) Å for 5 are not significantly different from
those of the parent cluster 3 [W–Asmean 2.607(1) Å, Co–Asmean
2.418(2) Å].
Both the 1H and 13C NMR spectroscopic data of complexes 4
and 5 are similar to those obtained for the parent compounds 2
and 3. The broad singlet at δ Ϫ86.3 in the 31P-{1H} NMR spec-
trum of complex 4 (δ Ϫ97.8 in 5) is typical of a cobalt-bound
terminal phosphine.
Fig. 2 The molecular structure of [(Ph3P)(OC)2Co(µ-As2)WCp(CO)2]
5. Details as in Fig. 1.
W). The mechanism for this process is presumed to be similar to
that for the reaction of [Co2(µ-As2)(CO)6] with [Mo2Cp2(CO)4].
The initial step must be loss of CO from [(OC)4CoMCp(CO)3]
just as CO loss from [Mo2Cp2(CO)6] is required prior to reac-
tion with [Co2(µ-As2)(CO)6].
(d) Reaction of [(OC)3Co(ꢀ-As2)MoCp(CO)2] 2 with
[Cr(CO)5]ؒTHF
Vigorous stirring of [(OC)3Co(µ-As2)MoCp(CO)2] with
[Cr(CO)5]ؒTHF in THF at room temperature gave, in addi-
tion to unchanged [(OC)3Co(µ-As2)MoCp(CO)2], red-orange
[(OC)3Co{µ-As2[Cr(CO)5]2}MoCp(CO)2] 6 in 34% yield. Com-
plex 6 has been characterized by IR and 1H NMR spectroscopy,
mass spectrometry and microanalysis. Additionally, it has been
the subject of a single crystal X-ray diffraction experiment.
Fig. 3 shows the molecular structure of complex 6; selected
bond lengths and angles are given in Table 3. The tetrahedral
CoMoAs2 core of the parent compound 2 is retained when the
two arsenic atoms each co-ordinate to a Cr(CO)5 unit. The As–
As bond length of 2.317(3) Å is slightly longer than that of
2.305(1) Å in 2, consistent with the higher co-ordination num-
bers at the arsenic centres. There is a significant shortening of
all the metal–arsenic bond lengths in 6 compared to those in 2
(see Table 1). This latter trend is explicable in terms of the
(c) Reaction of [(OC)3Co(ꢀ-As2)MCp(CO)2] [M ؍ Mo 2 or
W 3] with PPh3
Reaction of [(OC)3Co(µ-As2)MCp(CO)2] (M = Mo or W) with
an excess of PPh3 in refluxing benzene for 20 h gave, in addition
to starting material, red-orange [(Ph3P)(OC)2Co(µ-As2)MCp-
(CO)2] (M = Mo 4 or W 5) in 58 and 54% yields respectively.
1
Both complexes have been characterized by IR, H and 13C
NMR spectroscopy, mass spectrometry and microanalysis.
Complex 5 has been the subject of a single crystal X-ray diffrac-
tion study.
The molecular structure of complex 5 is presented in Fig. 2.
Selected bond lengths and angles are collated in Table 2. The
complex crystallizes with two molecules in the asymmetric unit.
There are significant differences between some of the bond
J. Chem. Soc., Dalton Trans., 1999, 3679–3686
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Table 3 Selected bond lengths (Å) and angles (Њ) for [(OC)3Co{µ-As2[Cr(CO)5]2}MoCp(CO)2] 6
Mo(1)–Co(1)
Mo(1)–As(2)
Co(1)–As(2)
As(1)–Cr(1)
Mo–Ccarbonyl
Cr–Ccarbonyl
2.874(4)
2.608(3)
2.357(4)
2.469(4)
1.91(3)–1.95(3)
1.80(2)–1.93(3)
Mo(1)–As(1)
Co(1)–As(1)
As(1)–As(2)
As(2)–Cr(2)
Co–Ccarbonyl
C–Ocobalt
2.557(3)
2.408(4)
2.317(3)
2.469(4)
1.72(3)–1.76(3)
1.14(2)–1.25(3)
Co(1)–Mo(1)–As(1)
As(1)–Mo(1)–As(2)
Mo(1)–Co(1)–As(2)
Mo(1)–Co(1)–C(1)
Mo(1)–Co(1)–C(3)
Mo(1)–As(1)–As(2)
Cr(1)–As(1)–As(2)
Co(1)–As(2)–As(1)
Cr(2)–As(2)–As(1)
52.24(9)
53.30(8)
58.80(9)
97.1(9)
Co(1)–Mo(1)–As(2)
Mo(1)–Co(1)–As(1)
As(1)–Co(1)–As(2)
Mo(1)–Co(1)–C(2)
Mo(1)–As(1)–Co(1)
Co(1)–As(1)–As(2)
Mo(1)–As(2)–Co(1)
Mo(1)–As(2)–As(1)
50.64(8)
57.08(9)
58.2(1)
148.0(7)
70.7(1)
59.8(1)
70.6(1)
96.6(8)
64.47(9)
129.6(1)
62.0(1)
62.23(9)
141.2(1)
enhanced π-acceptor character of the diarsenic unit in 6, result-
ing from its co-ordination to the strongly electron-accepting
Cr(CO)5 groups, which would produce an increased back
donation from the molybdenum and cobalt atoms to the As2
antibonding orbitals; this is apparently a larger effect than the
lengthening of the arsenic covalent radius produced by the
increase in co-ordination number. The Co(1)–Mo(1) bond
length of 2.874(4) Å in 6 is considerably longer than that of
2.825(1) Å in the parent compound 2, consistent with the over-
all loss of electron density from the two metal centres. The
twisting of the diarsenic unit observed in both 2 and 3 is also
seen in 6 where Mo(1) is closer to As(1) than to As(2) and Co(1)
is closer to As(2) than to As(1), both by 0.051 Å. The As(2)–
As(1)–Cr(1) angle of 129.6(1)Њ is much smaller than the As(1)–
As(2)–Cr(2) angle of 141.2(1)Њ a difference that at first sight
appears to be related to the different environments of the two
extremely bulky Cr(CO)5 groups. This explanation can, how-
ever, be ruled out because there are no close contacts between
the atoms of these groups and the rest of the molecule. It is
interesting that the mean As–As–Cr angle of 135.4Њ is within
the range observed for the C–C–R angle in ‘side-on’ co-
ordinated alkynes [130–150Њ].27,31
The formation of [M2(µ-As2)Cp2(CO)4] [M = Mo or W] from
[M2Cp2(CO)6] and grey arsenic has been proposed to involve
condensation of triply bonded (OC)2CpMoAs fragments2,34
and a similar process could occur in the reactions (a), (b) and
(e). A mechanism based on this proposal is presented in Scheme
1. If such a process operates there is the possibility that it could
Scheme 1 Possible route to complexes 1, 2 and 3 via triply bonded
intermediates (a) M = MЈ = MЉ = MoCp(CO)2, (b) M = MЉ = Co(CO)3,
MЈ = MCp(CO)2 [M = Mo or W], (e) MЉ = Co(CO)3, M = MoCp(CO)2
and MЈ = WCp(CO)2 or MЈ = MoCp(CO)2 and M = WCp(CO)2.
A complicated infrared spectrum is obtained for complex 6
with a large number of terminal ν(CO) absorptions Those
bands associated with the parent complex remain visible and
shifted only slightly, suggesting that some decomposition takes
place on dissolution of the adduct. It appears that the Co–Mo
bimetallic fragment is sufficiently basic to allow both arsenic
atoms to ligate a Cr(CO)5 fragment, forming an adduct which is
stable in the solid state.
lead to products other than those isolated, yet there is no
[(OC)3Co(µ-As2)MoCp(CO)2] recovered from the reaction of
[Mo2Cp2(CO)4] and [Co2(µ-As2)(CO)6] and no [Mo2(µ-As2)-
Cp2(CO)4] appears to be produced in reaction (e).
(e) Reaction of [MoWCp2(CO)4] with [Co2(ꢀ-As2)(CO)6]
(f) Reaction of [Mo2Cp2(CO)6] and [W2Cp2(CO)6] with grey
The mixed-metal complex [MoWCp2(CO)4] reacts with alkynes
under the same mild conditions as [Mo2Cp2(CO)4]32 which sug-
gested that this too might yield a new molybdenum–tungsten
complex with an As2 ligand on reaction with [Co2(µ-As2)(CO)6].
Reaction of [MoWCp2(CO)4] with [Co2(µ-As2)(CO)6] in reflux-
ing xylene, however, gave only [(OC)3Co(µ-As2)MoCp(CO)2] 2
and [(OC)3Co(µ-As2)WCp(CO)2] 3 in very low yield. The iden-
tities of the products collected were established by comparison
of infrared data to those obtained previously.
Although the desired diarsenic-bridged Mo–W complex was
not obtained from this reaction the products of the reaction
show that a MCp(CO)2 [M = Mo or W] fragment may substi-
tute a Co(CO)3 unit in [Co2(µ-As2)(CO)6]. This result provides
some support for the proposal that the syntheses of [Mo2-
(µ-As2)Cp2(CO)4] and [(OC)3Co(µ-As2)MCp(CO)2] [M = Mo
or W] from [Co2(µ-As2)(CO)6] occur by stepwise exchange of
Co(CO)3 units by MCp(CO)2 fragments rather than by a direct
transfer of the As2 ligand. Reactions of this type for cobalt
tetrahedranes are known to occur33 but mechanisms have not
been proposed.
arsenic
Most routes to tetrahedral transition metal complexes contain-
ing naked arsenic atoms employ arsenic-containing complexes
as starting materials but Ziegler et al.2,3,4 successfully syn-
thesized [M2(µ-As2)Cp2(CO)4] and [M3(µ3-As)Cp3(CO)6] by
thermolytic reaction of [M2Cp2(CO)6] [M = Mo or W] with
grey arsenic. This reaction was presumed to proceed via
generation of the triply bonded intermediate, AsMCp(CO)2
(M = Mo or W); eqn. (1). If such a triply bonded intermediate
is indeed involved then by refluxing [Mo2Cp2(CO)6], [W2Cp2-
(CO)6] and grey arsenic it ought to be possible to synthesize the
heterometallic µ-As2 complex [MoW(µ-As2)Cp2(CO)4].
Reaction of [Mo2Cp2(CO)6] and [W2Cp2(CO)6] with grey
arsenic in refluxing p-xylene for 24 h gives three orange, crystal-
line, bimetallic products, [MMЈ(µ-As2)Cp2(CO)4] (M = MЈ =
3682
J. Chem. Soc., Dalton Trans., 1999, 3679–3686

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Table 4 Selected bond lengths (Å) and angles (Њ) for [MoW2(µ3-As)-
Cp3(CO)6] 9
the narrow range 3.097(2)–3.124(2) Å and are equal within
experimental error. This is consistent with the results for the
two separate molecules 2 and 3, where equivalent Co–Mo and
Co–W bonds lengths were virtually identical (see above). The
µ3-As atom is not exactly above the centroid of the triangle of
metal atoms but is displaced towards the ordered tungsten atom
W(2); the W(1)–As and Mo(1)–As bond lengths are not signifi-
cantly different from their mean value of 2.552(2) Å but the
W(2)–As bond length of 2.515(2) Å is markedly shorter. The
difference in metal–arsenic bond lengths may be related to
differences in the orientation of the M(CO)2Cp groups.
W(1)–W(2)
W(1)–As
W(2)–As
W(1)–C(1)
W(2)–C(3)
Mo(1)–C(5)
C–O
3.118(1)
2.557(2)
2.515(2)
2.00(2)
2.00(2)
1.96(2)
W(1)–Mo(1)
W(2)–Mo(1)
Mo(1)–As
W(1)–C(2)
W(2)–C(4)
Mo(1)–C(6)
3.124(2)
3.097(2)
2.547(2)
1.96(2)
2.00(2)
1.98(2)
1.13(2)–1.17(2)
W(2)–W(1)–As
Mo(1)–W(1)–As
W(1)–W(2)–As
W(2)–Mo(1)–W(1) 60.16(3)
W(1)–Mo(1)–As
Mo(1)–As–W(1)
51.45(4)
52.12(5)
52.69(5)
W(2)–W(1)–Mo(1) 59.50(3)
W(1)–W(2)–Mo(1) 60.34(3)
The 1H NMR spectrum of complex 8 has two sharp singlets
at δ 5.05 and 5.06, integrating to 10 and 5 protons, respectively.
These are attributable to the protons of the three cyclopenta-
dienyl rings, one of which coordinates to each metal. Thus
the first resonance corresponds to the two Mo-bound cyclo-
pentadienyls and the second to the single W-bound ligand.
A similar spectrum is obtained for complex 9, with two singlets
at δ 5.07 and 5.09, the first resonance corresponding to W-
bound Cp and having twice the integral of the second. All
three cyclopentadienyl ligands are in distinct environments
in the solid state structure of 9 so that a fluxional process
which renders equivalent the two Mo-bound Cp groups in
8 and the two W-bound Cp groups in 9 must be operating in
solution.
Mo(1)–W(2)–As
W(2)–Mo(1)–As
W(2)–As–W(1)
W(2)–As–Mo(1)
52.75(5)
51.80(5)
75.86(5)
75.45(6)
52.42(5)
75.46(6)
Two singlets are observed in the 13C NMR spectrum of com-
plex 8 at δ 89.5 and 87.3. Again these are due to the Cp ligands
and since the first signal appears twice as intense as the second
this is assigned to the Mo-bound ligands. In the analogous
spectrum for complex 9 the two Cp resonances occur at δ 88.6
and 87.8 with the second signal, which is twice as intense as the
first, being attributed to the W-bound ligands.
Clear, sharp singlets are also seen for the carbonyl carbon
resonances for both complexes. For a complex of the type
[M2MЈ(µ3-As)Cp3(CO)6], if the MCp(CO)2 and MЈCp(CO)2
groups are assumed to ‘spin round’ in solution then there
will be only one environment for the pair of carbonyls bound
to MЈ but the carbonyls bound to M are diastereotopic and
remain inequivalent. In the 13C NMR spectrum of complex 8
there are two Mo–CO resonances (δ 229.7, 223.2) and one
W–CO resonance (δ 214.0) but in the spectrum of 9 there
are two W–CO resonances (δ 210.8, 216.1) and one Mo–CO
resonance (δ 233.2). A weak pair of satellite peaks are resolved
either side of the W–CO signal for complex 8, caused by
coupling of carbonyl carbon atoms to 183W (I = ¹, 15% abund-
Fig. 4 The molecular structure and atom numbering scheme of
[MoW2(µ3-As)Cp3(CO)6] 9 which has some conformational disorder in
the solid state; the 70% component of the disorder is shown.
Mo; M = Mo, MЈ = W 7; M = MЈ = W), collected in 5, 20 and
6% yield respectively. Four green, crystalline, trimetallic
products, [M2MЈ(µ3-As)Cp3(CO)6] (M = MЈ = Mo; M = Mo,
MЈ = W 8; M = W, MЈ = Mo 9; M = MЈ = W) were also
obtained in 1.2, 6, 12 and 4.3% yield respectively. The identities
of the complexes [Mo2(µ-As2)Cp2(CO)4], [W2(µ-As2)Cp2(CO)4],
[Mo3(µ3-As)Cp3(CO)6] and [W3(µ3-As)Cp3(CO)6] were estab-
lished by comparison of their infrared spectra with published
data for these known compounds.2,3 The new complexes 7, 8
and 9 were fully characterized by IR, 1H and 13C NMR spectro-
scopy, mass spectrometry and microanalysis.
The structure of complex 9 was confirmed by X-ray analysis.
Bond lengths and angles are presented in Table 4 and the over-
all molecular structure is shown in Fig. 4. Owing to disorder
(ca. 70:30%) of two conformers in the crystal, the positions of
Cp(CO)2Mo(1) and Cp(CO)2W(1) are randomly interchanged
throughout the crystal (see Experimental section) so that in
Fig. 4 the atom labelled W(1) is at the site predominantly
occupied by W and Mo(1) is at the site predominantly occupied
by Mo; the second tungsten atom W(2) is unaffected by the
disorder. The parameters involving the two disordered sites
should be regarded with some caution, but due to the close
similarity in the covalent radii of Mo and W observed in the
structures of 2 and 3 some discussion of the bond lengths is
possible.
¯
²
ance).
The reaction is presumed initially to generate the triply
bonded intermediates (OC)2CpMAs (M = Mo or W) as pro-
posed by Ziegler. Reaction of two such intermediates gives rise
to a bimetallic product, whereas the trimetallic complexes are
produced by reaction of (OC)2CpMAs with [MCp(CO)2]2
(M = Mo or W). Thus all the possible bimetallic and trimetallic
species depicted respectively in Schemes 2 and 3 are obtained as
reaction products. The higher yield of 9 as compared to 8 may
reflect a thermodynamic preference based on the greater
The core of the cluster consists of a MoW2 triangle capped
by a naked µ3-As ligand. The three M–M bond lengths are in
Scheme 2 Generation of bimetallic complexes in reaction (h).
J. Chem. Soc., Dalton Trans., 1999, 3679–3686
3683

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by slow evaporation at 0 ЊC of an n-hexane–dichloromethane
solution of this complex (Found: C, 23.66; H, 1.30. C10H5As2-
CoMoO5 requires C, 23.56; H, 0.99%). Mass spectrum: m/z
512 (Mϩ) and Mϩ Ϫ nCO (n = 1–5). νmax/cmϪ1 (hexane) 2055s,
1
2027w, 2011s, 1990vs and 1943s. NMR (CDCl3): H, δ 5.37
[s, 5 H, Cp]; 13C (1H composite pulse decoupled), δ 224.1 [s,
Mo–CO] and 86.6 [s, Cp].
(c) Reaction of [(OC)4CoWCp(CO)3] with [Co2(ꢀ-As2)(CO)6]
The complexes [Co2(CO)8] (400 mg, 1.2 mmol) and [W2Cp2-
(CO)6] (780 mg, 1.2 mmol) were photolysed together in THF
(200 ml) for 24 h after which time the resulting solution was
poured into a nitrogen-filled flask; [Co2(µ-As2)(CO)6] (1.05 g,
2.4 mmol) was added and the mixture refluxed with stirring for
30 min. After removal of the reaction solvent under vacuum the
residue was treated as in reaction (a). Elution with hexane–
dichloromethane (9:1) gave a trace of unchanged [Co2(µ-As2)-
(CO)6] and orange crystalline [(OC)3Co(µ-As2)WCp(CO)2] 3
(305 mg, 21%). Crystals of 3 suitable for diffraction were pro-
duced by recrystallization from dichloromethane solution
(Found: C, 20.19; H, 0.85. C10H5As2CoO5W requires C, 20.09;
H, 0.84%). Mass spectrum: m/z 598 (Mϩ) and Mϩ Ϫ nCO
(n = 1–5). νmax/cmϪ1 (CH2Cl2) 2051s, 2001s, 1985vs and 1924m.
NMR (CDCl3): 1H, δ 5.41 [s, 5 H, Cp]; 13C (1H composite pulse
decoupled), δ 210.6 [s, W–CO] and 84.4 [s, Cp].
Scheme 3 Generation of trimetallic complexes in reaction (h).
strength of metal–metal bonds involving tungsten as compared
to molybdenum.
Experimental
All reactions were carried out under an atmosphere of dry,
oxygen-free nitrogen, using solvents that were freshly distilled
from the appropriate drying agent. Ultraviolet irradiation
experiments were performed using a 125 W Hanovia medium
pressure mercury vapour lamp. Infrared spectra were recorded
in n-hexane or dichloromethane solution in 0.5 mm NaCl cells,
using a Perkin-Elmer Paragon 1000 Fourier-Transform spec-
trometer or a Perkin-Elmer 1600 series spectrometer. Fast atom
bombardment mass spectra were obtained on a Kratos MS890
instrument, fast ion bombardment mass spectra on a Kratos
MS50 instrument. 3-Nitrobenzyl alcohol was used as a matrix.
The 1H and 13C NMR spectra were recorded on Bruker AM400
or WM250 spectrometers using the solvent resonance as an
internal standard, 31P NMR spectra on a Bruker WM250 spec-
trometer with P(OMe)3 used as a standard. Microanalyses were
performed by the Microanalytical Department, University of
Cambridge.
(d) Reaction of [(OC)3Co(ꢀ-As2)MoCp(CO)2] 2 and PPh3
The complex [(OC)3Co(µ-As2)MoCp(CO)2] 2 (100 mg, 0.20
mmol) and PPh3 (0.52 g, 2.0 mmol) were refluxed in benzene (60
ml) with stirring for 20 h. After removal of the solvent under
reduced pressure, the residue was treated as in reaction (a).
Elution with hexane–dichloromethane (4:1) gave unchanged
starting material (25 mg) and orange-red crystalline [(Ph3P)-
(OC)2Co(µ-As2)MoCp(CO)2] 4 (83 mg, 58%) as the only prod-
uct (Found: C, 43.55; H, 2.69. C27H20As2CoMoO4P requires C,
43.58; H, 2.71%). Mass spectrum: m/z 746 (Mϩ) and Mϩ Ϫ
nCO (n = 1–4). νmax/cmϪ1 (CH2Cl2) 1997m, 1965vs, 1946m and
1913w. NMR (CDCl3): δ 7.5–7.3 [m, 15 H, Ph] and 5.32 [s, 5 H,
Cp]; 13C (1H composite pulse decoupled), δ 226.3 [s, Mo–CO],
208.4 [s, Co–CO], 137.5–128.1 [Ph] and 85.9 [s, Cp].
Preparative thin layer chromatography was carried out on
1 mm plates prepared at the University Chemical Laboratory,
Cambridge. Column chromatography was performed on
Kieselgel 60 (70–230 mesh). Products are given in order of
decreasing Rf values.
Unless otherwise stated, all reagents were obtained from
commercial suppliers and used without further purification.
The complex [Co2(µ-As2)(CO)6] was prepared as described.4
(e) Reaction of [(OC)3Co(ꢀ-As2)WCp(CO)2] 3 and PPh3
The complex [(OC)3Co(µ-As2)WCp(CO)2] 3 (272 mg, 0.46
mmol) and PPh3 (1.20 g, 4.6 mmol) were refluxed in benzene (60
ml) with stirring for 20 h. After removal of the solvent under
reduced pressure, the residue was treated as in reaction (a).
Elution with hexane–dichloromethane (4:1) gave unchanged
starting material (75 mg) and red crystalline [(Ph3P)(OC)2-
Co(µ-As2)WCp(CO)2] 5 (206 mg, 54%) as the only product.
Crystals of 5 suitable for diffraction were grown by slow evap-
oration at room temperature of a benzene solution of the
complex (Found: C, 38.72; H, 2.32. C27H20As2CoO4PW requires
C, 38.98; H, 2.42%). Mass spectrum: m/z 832 (Mϩ) and
Mϩ Ϫ nCO (n = 1–4). νmax/cmϪ1 (CH2Cl2) 1994m, 1962vs,
1943m and 1905w. NMR (CDCl3): δ 7.5–7.3 [m, 15 H, Ph] and
5.36 [s, 5 H, Cp]; 13C (1H composite pulse decoupled), δ 212.1
[s, W–CO], 137.6–128.1 [Ph] and 83.8 [s, Cp].
(a) Reaction of [Co2(ꢀ-As2)(CO)6] and [Mo2Cp2(CO)4]
The complex [Mo2Cp2(CO)6] (281 mg, 0.57 mmol) was refluxed
in toluene (70 ml) for 18 h, while purging the solution with
nitrogen; [Co2(µ-As2)(CO)6] (500 mg, 1.15 mmol) was then
added and the mixture refluxed with stirring for 6 h. After
removal of reaction solvents under vacuum, the residue was
dissolved in the minimum quantity of dichloromethane and
adsorbed onto silica. The silica was pumped dry and trans-
ferred to the top of a silica chromatography column. Elution
with hexane–dichloromethane (1:1) gave orange [Mo2(µ-As2)-
Cp2(CO)4] 1 (50 mg, 15%) as the only product.
(b) Reaction of [Co2(ꢀ-As2)(CO)6] with [(OC)4CoMoCp(CO)3]
(f) Reaction of [(OC)3Co(ꢀ-As2)MoCp(CO)2] 2 with
[Cr(CO)5]ؒTHF
The complexes [Co2(CO)8] (490 mg, 1.4 mmol) and [Mo2Cp2-
(CO)6] (700 mg, 1.4 mmol) were photolysed together in THF
(200 ml) for 24 h after which time the resulting solution was
poured into a nitrogen-filled flask, [Co2(µ-As2)(CO)6] (1.25 g, 2.9
mmol) added and the mixture refluxed with stirring for 20 min.
After removal of the reaction solvent under vacuum the resi-
due was treated as in reaction (a). Elution with hexane–
dichloromethane (9:1) gave unchanged [Co2(µ-As2)(CO)6] (305
mg) and red-orange crystalline [(OC)3Co(µ-As2)MoCp(CO)2] 2
(514 mg, 36%). Crystals of 2 suitable for diffraction were grown
The complex [Cr(CO)6] (150 mg, 0.68 mmol) in THF (250 ml)
was irradiated for 12 h, [(OC)3Co(µ-As2)MoCp(CO)2] 2 (174
mg, 0.34 mmol) was then added and stirred vigorously for
20 h. After removal of the reaction solvent under vacuum, the
residue was dissolved in the minimum quantity of dichloro-
methane and applied to the base of silica TLC plates. Elution
with hexane–dichloromethane (3:2) gave unchanged [(OC)3-
Co(µ-As2)MoCp(CO)2] 2 (45 mg), followed by red-orange
3684
J. Chem. Soc., Dalton Trans., 1999, 3679–3686

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Table 5 X-Ray crystallographic and data processing parameters for complexes 2, 3, 5, 6 and 9
2
3
5
6
9
Empirical formula
Formula weight
T/K
Crystal system
Space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
C10H5As2CoMoO5
509.85
293(2)
Monoclinic
P21/n
8.899(2)
14.465(2)
11.046(2)
—
91.440(10)
—
1421.4(5)
4
C10H5As2CoO5W
597.76
293(2)
Monoclinic
P21/n
8.899(2)
14.471(3)
11.046(2)
—
91.42(3)
—
1422.0(5)
4
C27H20As2CoO4PW
832.02
293(2)
C20H5As2CoCr2MoO15
C21H15AsMoO6W2
901.89
295(2)
Monoclinic
P21/c
14.042(2)
7.5130(15)
20.877(4)
—
91.762(9)
—
2201.5(7)
4
893.95
293(2)
Monoclinic
P21/n
8.913(4)
19.628(6)
16.694(4)
—
99.77(3)
—
2878.4(16)
4
Triclinic
¯
P1 (no. 2)
12.7720(10)
20.831(2)
10.493(2)
90.430(10)
103.480(10)
90.820(10)
2714.3(6)
4
γ/Њ
U/Å3
Z
µ(Mo-Kα)/mmϪ1
Reflections
collected
Independent
reflections
Final R1, wR2
I > 2σ(I)
all data
6.678
3466
13.876
3443
7.355
5576
4.076
4100
12.512
5104
3263 [R(int) = 0.0365]
3243 [R(int) = 0.0478]
5032 [R(int) = 0.0726]
3071 [R(int) = 0.1017]
3875 [R(int) = 0.0363]
0.0409, 0.0959
0.1101, 0.1831
0.0428, 0.1122
0.0810, 0.1347
0.0539, 0.1457
0.1131, 0.1715
0.0746, 0.1284
0.1815, 0.1560
0.0625, 0.1595
0.0949, 0.1803
[(OC)3Co{µ-As2[Cr(CO)5]2}MoCp(CO)2] 6 (86 mg, 34%). Crys-
tals of 6 suitable for diffraction were grown from a dichloro-
methane–hexane solution of the complex by slow evaporation
at 0 ЊC (Found: C, 25.83; H, 0.68. C20H5As2CoCr2MoO15
requires C, 26.87; H, 0.56%). Mass spectrum: m/z 894 (Mϩ) and
Mϩ Ϫ nCO (n = 5 or 7). νmax/cmϪ1 (CH2Cl2) 2075m, 2054s,
10 H, Mo-Cp]; 13C (1H composite pulse decoupled), δ 229.7 [s,
Mo–CO], 223.2 [s, Mo–CO], 214.0 [d, 1JCW = 184 Hz, W–CO],
89.5 [s, Mo–Cp] and 87.3 [s, W–Cp]. Complex 9 (Found: C,
28.13; H, 1.79. C21H15AsMoO6W2 requires C, 27.97; H, 1.68%):
m/z 902 (Mϩ) and Mϩ Ϫ nCO (n = 1–6); νmax/cmϪ1 (CH2Cl2)
1
1970w, 1935s, 1897m and 1868w; NMR (CDCl3) H, δ 5.09 [s,
1
2024m, 2003s, 1991m, 1967m, 1953vs and 1941m. H NMR
(CDCl3): δ 5.39 [s, 5 H, Cp].
5 H, Mo–Cp] and 5.07 [s, 10 H, W–Cp]: 13C (1H composite
pulse decoupled), δ 233.2 [s, Mo–CO], 216.1 [s, W–CO], 210.8
[W–CO], 88.6 [s, Mo–Cp] and 87.8 [s, W–Cp].
(g) Reaction of [MoWCp2(CO)4] and [Co2(ꢀ-As2)(CO)6]
Crystal structure determinations
The complexes [Mo2Cp2(CO)6] (400 mg, 0.82 mmol) and
[{WCp(CO)3}2] (544 mg, 0.82 mmol) were refluxed in xylene
(100 ml) with stirring for 3 d; [Co2(µ-As2)(CO)6] (790 mg, 1.81
mmol) was added and further refluxed. After removal of the
solvents under reduced pressure, the residue was treated as in
reaction (a). Elution with hexane–dichloromethane (4:1) gave
[(OC)3Co(µ-As2)MoCp(CO)2] 2 (90 mg, 10%) and [(OC)3Co-
(µ-As2)WCp(CO)2] 3 (82 mg, 8%).
X-Ray intensity data were collected on a Rigaku AFC5R
four-circle diffractometer for complexes 2, 3 and 5 and on a
Siemens P4 four-circle diffractometer for 6 and 9. Details of
data collection, refinement and crystal data are listed in Table 5.
Lorentz-polarization corrections were applied to the data of
all compounds. Absorption corrections based on ψ-scans were
applied to the data of 2, 3 and 5. After refinement with isotropic
displacement parameters for all atoms, empirical corrections
were also applied to the data of 6 and 9.35
(h) Reaction of [Mo2Cp2(CO)6] and [W2Cp2(CO)6] with grey
The positions of the metal atoms and most of the non-
hydrogen atoms were located from direct methods,36 and the
remaining non-hydrogen atoms were revealed from subsequent
Fourier-difference syntheses. Refinement was based on F2. In
the crystal of complex 5 there are two independent molecules
and chemically equivalent bond lengths in the two molecules
were “tied” in the refinement. The only crystals that could
be obtained of 6 diffracted relatively poorly at high angle.
Relatively high displacement parameters for some carbonyl
atoms and the Cp ring indicated some rotational disorder of the
Cr(CO)5 groups and at Mo(1), but this was only resolved for
three of the terminal oxygen atoms O(5), O(24) and O(25).
Weak restraints were applied to the anisotropic displacement
parameters of C(7), C(9) and C(10) to prevent their becoming
non-positive definite. The structure of 9 consisted of a random
distribution of two conformers in a 70:30 ratio throughout
the crystal, so that in the observed molecular image all the
atoms superimposed within experimental error, but W and Mo
shared the same two sites [with relative occupancies 0.70/0.30 at
the site labelled W(1) and 0.30/0.70 at the site labelled Mo(1)].
The hydrogen atoms of the cyclopentadienyl rings of com-
plexes 6 and 9 were not included in structure factor calculations.
All other hydrogen atoms were placed in calculated positions
with displacement parameters equal 1.2Ueq of the parent
carbon atoms. In the final cycles of full-matrix least-squares
arsenic
The complex [Mo2Cp2(CO)6] (500 mg, 1.02 mmol), [WCp-
(CO)3]2 (677 mg, 1.02 mmol) and grey arsenic (765 mg, 10.2
mmol) were refluxed in xylene (70 ml) with stirring for 24 h.
After removal of the reaction solvent under vacuum, the
residue was treated as in reaction (a). Elution with hexane–
dichloromethane (9:1) gave orange [Mo2(µ-As2)Cp2(CO)4] (60
mg, 5%), red-orange [MoW(µ-As2)Cp2(CO)4] 7 (274 mg, 20%),
orange [W2(µ-As2)Cp2(CO)4] (85 mg, 6%), green [Mo3(µ3-As)-
Cp3(CO)6] (12 mg, 1.2%), green [Mo2W(µ3-As)Cp3(CO)6] 8 (61
mg, 6%), green [MoW2(µ3-As)Cp3(CO)6] 9 (142 mg, 12%) and
green [W3(µ3-As)Cp3(CO)6] (58 mg, 4.3%). Crystals of 9 suit-
able for diffraction were grown from a solution of n-hexane–
dichloromethane at Ϫ20 ЊC.
Complex 7 (Found: C, 25.20; H, 1.59. C14H10As2MoO4W
requires C, 25.03; H, 1.50%): mass spectrum m/z 672 (Mϩ) and
Mϩ Ϫ nCO (n = 1–4); νmax/cmϪ1 (CH2Cl2) 1972w, 1950s and
1898m; NMR (CDCl3) 1H, δ 5.17 [s, 5 H, Mo–Cp], 5.13 [s, 5 H,
W–Cp]; 13C (1H composite pulse decoupled) δ 225.5 [s,
Mo–CO], 211.3 [s, W–CO], 84.29 [s, Mo–Cp] and 82.28 [s,
W–Cp]. Complex 8 (Found: C, 30.88; H, 1.79. C14H10As2-
MoO4W requires C, 30.99; H, 1.86%): m/z 814 (Mϩ) and
Mϩ Ϫ nCO (n = 1–6); νmax/cmϪ1 (CH2Cl2) 1970w, 1936s, 1900m
1
and 1871w; NMR (CDCl3) H, δ 5.06 [s, 5 H, W–Cp], 5.05 [s,
J. Chem. Soc., Dalton Trans., 1999, 3679–3686
3685

View Article Online
17 M. H. Chisholm, K. Foltig, J. C. Huffman and J. J. Koh, Polyhedron,
refinement anisotropic displacement parameters were assigned
to the metal and arsenic atoms in 9 and in the other structures
to all non-hydrogen atoms.
CCDC reference number 186/1663.
See http://www.rsc.org/suppdata/dt/1999/3679/ for crystallo-
graphic files in .cif format.
1985, 4, 893.
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Acknowledgements
We thank the Defence Research Agency (Fort Halstead) for
financial support to J. D. K. and the EPSRC and the Cam-
bridge Crystallographic Data Centre for support to J. E. D.
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