Reactivity of the mixed-metal cluster [Os6(CO)17(Au2dppm)] towards gold electrophiles (dppm=Ph2PCH2PPh2)

Article abstract of DOI:10.1016/S0022-328X(00)00590-8

Reduction of the mixed-metal octanuclear cluster [Os₆(CO)₁₇(Au₂dppm)] (1) with Na-Hg amalgam and subsequent reaction with one equivalent of [Au₂dppm][NO₃] affords two isomers of the neutral decanuclear cluster [Os₆(CO)₁₇(Au₂dppm)₂] (2a, 2b). In a similar reaction between the reduced form of 1 with two equivalents of [AuPPh₃][NO₃] gives two isomers of the related cluster [Os₆(CO)₁₇(Au₂dppm₂)(AuPPh ₃)₂] (3a, 3b). All the new clusters have been characterised by IR and NMR spectroscopy and FAB mass spectrometry. The molecular and crystal structure of 2a has been established by a single-crystal X-ray structure. In 2a the six Os atoms define an octahedron, in contrast to the bicapped tetrahedral osmium core previously observed for 1. Two of the Au atoms in 2a cap adjacent triangular faces of the Os₆ octahedron while the third spans an open OsAu₂ triangle, and the fourth caps a closed OsAu₂ triangle. Overall the decametal core may be viewed as an octahedron fused with a capped square based pyramid.

Full text of DOI:10.1016/S0022-328X(00)00590-8

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 614–615 (2000) 231–237
Reactivity of the mixed-metal cluster [Os6(CO)17(Au2dppm)]
towards gold electrophiles (dppm=Ph2PCH2PPh2)
Zareen Akhter, John F. Gallagher 1, Jack Lewis, Paul R. Raithby *,
Gregory P. Shields
Department of Chemistry, Uni6ersity of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK
Received 26 April 2000
Dedicated to Professor Sheldon G. Shore in recognition of his outstanding contribution to organometallic chemistry.
Abstract
Reduction of the mixed-metal octanuclear cluster [Os6(CO)17(Au2dppm)] (1) with Na–Hg amalgam and subsequent reaction
with one equivalent of [Au2dppm][NO3] affords two isomers of the neutral decanuclear cluster [Os6(CO)17(Au2dppm)2] (2a, 2b).
In a similar reaction between the reduced form of 1 with two equivalents of [AuPPh3][NO3] gives two isomers of the related cluster
[Os6(CO)17(Au2dppm2)(AuPPh3)2] (3a, 3b). All the new clusters have been characterised by IR and NMR spectroscopy and FAB
mass spectrometry. The molecular and crystal structure of 2a has been established by a single-crystal X-ray structure. In 2a the
six Os atoms define an octahedron, in contrast to the bicapped tetrahedral osmium core previously observed for 1. Two of the
Au atoms in 2a cap adjacent triangular faces of the Os6 octahedron while the third spans an open OsAu2 triangle, and the fourth
caps a closed OsAu2 triangle. Overall the decametal core may be viewed as an octahedron fused with a capped square based
pyramid. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Carbonyl cluster; Osmium; Gold; X-ray structure; Ionic coupling
gam can be used to reduce neutral clusters to form
anions with the loss of carbonyl groups [4]. We have
used this methodology to prepare a series of higher
nuclearity mixed-metal clusters by treating reduced os-
mium cluster anions with a range of cationic fragments
including [Ru(h6-C6H6)]2+ [5,6], [Ru(h5-C5H5)]+ [7],
[Rh(h5-C5Me5)]2+ [8], [AuPR3]+ (R=Ph, Cy) [9], and
[Au2dppm]2+ [9,10]. In this paper we report the forma-
tion of the novel decanuclear clusters [Os6(CO)17-
(Au2dppm)2] and [Os6(CO)17(Au2dppm2)(AuPPh3)2]
that have been prepared by the reduction of
[Os6(CO)17(Au2dppm)], presumably to form the dianion
‘[Os6(CO)17(Au2dppm)]2−’, and the subsequent reaction
with [Au2dppm]2+ and [AuPPh3]+, respectively.
1. Introduction
There is current interest in developing more ‘rational’
methods for synthesising higher nuclearity transition
metal clusters because of their potential applications in
nanotechnology [1]. One method that has proved suc-
cessful for generating clusters that contain up to 30
metal atoms is that of ionic coupling between lower
nuclearity cluster anions and mono- or dimetallic
cationic fragments [2]. A key step in this process is the
generation of the reactive cluster anion, and Shore was
among the first to develop a systematic method of
preparing these anions from hydrido carbonyl clusters
by stepwise deprotonation using potassium hydride in
tetrahydrofuran [3]. Since that time it has been shown
that potassium/benzophenone or zinc/mercury amal-
2. Results and discussion
* Corresponding author. Fax: +44-1223-336362.
E-mail address: prr1@cam.ac.uk (P.R. Raithby).
1 Present address: School of Chemical Sciences, Dublin City Uni-
versity, Dublin 9, Ireland.
The reduction of the mixed-metal cluster
[Os6(CO)17(Au2dppm)] [11] (1) (dppm=Ph2PCH2PPh2)
0022-328X/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00590-8

232
Z. Akhter et al. / Journal of Organometallic Chemistry 614–615 (2000) 231–237
with an excess of freshly prepared Na–Hg amalgam, in
tetrahydrofuran, gives the dianion [Os6(CO)17-
(Au2dppm)]2−. This anion was not isolated or fully
characterised because of its air-sensitive nature, but was
treated in situ with one equivalent of [Au2dppm][NO3],
in dichloromethane, at room temperature. After work-
up and purification by thin layer chromatography using
dichloromethane:hexane (60:40) as eluent a yellow (2a)
and a pink (2b) product were isolated in 50 and 30%
yields, respectively. The products were characterised
single crystals of 2a were obtained by recrystallisation
from dichloromethane–hexane at −20°C, but the pink
cluster 2b decomposed during all attempts at
recrystallisation.
The overall molecular structure of [Os6(CO)17-
(Au2dppm)2] (2a) is illustrated in Fig. 1 while the metal
core geometry is shown in Fig. 2. Selected bond
parameters are listed in Table 2. The osmium metal
framework can be described as an octahedron two
adjacent faces of which are capped asymmetrically by
two gold atoms, Au(1) and Au(4), one from each of the
two [Au2dppm]2+ units. One of the remaining gold
atoms, Au(3), caps the open triangle, Au(1), Os(1),
Au(4), while the second, Au(2), caps the closed triangle,
Au(1), Os(1), Au(3). Overall, the framework may be
viewed as an octahedron fused with a capped square
based pyramid. The key point to note is perhaps that
there has been a significant structural change in the
osmium core from the bicapped tetrahedral arrange-
ment in the precursor complex [Os6(CO)17(Au2dppm)]
(1) [11] to the octahedral arrangement in 2a. This
change is consistent with the formal addition of two
electrons, with the inclusion of the second
[Au2dppm]2+ unit, as the electron count for 1 is 84
electrons and that for 2a is 86 electrons, with each
digold unit acting as a two-electron donor.
The coordination of the four gold atoms to the
osmium cluster core in 2a is somewhat similar to that
observed for the two [Au2dppm]2+ units in
[Os4H2(CO)11(Au2dppm)2] [12]. In the latter cluster two
Au atoms, one from each [Au2dppm]2+ unit, are m2-
bound to the osmium core rather than m3-bound, and
the other two Au atoms are coordinated to the same Os
atom of the tetrahedron so that with the Au–Au
bonding present a triangle of Au atoms is generated,
similar to the Au(1), Au(2), Au(3) triangle observed in
2a. Within the cluster 2a the Au–Au distances lie in the
1
initially by IR, H- and 31P-NMR spectroscopies and
by negative ion FAB mass spectrometry (Table 1), and
were subsequently formulated as two isomers of the
neutral decametal cluster [Os6(CO)17(Au2dppm)2] (2).
The 31P-NMR spectrum of the major isomer 2a exhib-
ited two equal intensity doublets at l −66.98 and
−112.21 ppm with a coupling constant of JPP=69.8
Hz, that can be attributed to two different phosphorus
environments, indicating that not all the gold atoms
occupy equivalent environments. The signal at l
−112.21 ppm is at quite high field for cluster bound
gold phosphine phosphorus nuclei but such high field
signals have been reported previously in [Os4H4(CO)11-
(Au2dppm)] [10], [Os4H2(CO)11(Au2dppm)2] [12] and
[Os7C(CO)19(Au2dppm)] [13]. No clear signals were ob-
served in the room temperature 31P-NMR spectrum of
2b, a feature that may be consistent with a fluxional
process occurring at this temperature. A low tempera-
ture 31P-NMR investigation was hampered by the low
solubility of the compound. The FAB mass spectra of
2a and 2b did not show the molecular ion in either case,
instead a peak at m/z of 3156 is observed, that corre-
sponds to the loss of one carbonyl group from the
expected formulation. The IR spectrum of 2a displays
six bands in the terminal w(CO) stretching region while
that of 2b shows only four bands, but the band pattern
is somewhat similar.
In order to establish the exact formulation and struc-
ture of the two isomers attempts were made to grow
single crystals of the two products. Suitable yellow,
,
,
range 2.677(3)–3.188(3) A, with a mean of 2.851 A. A
similar range has been found in a variety of homonu-
Table 1
Spectroscopic data for [Os6(CO)17(Au2dppm)2] (2a, 2b) and [Os6(CO)17(Au2dppm)(AuPPh3)2] (3a, 3b)
a
Compound IR w(CO) (cm−1
)
1H-NMR b
(ppm)
l
31P-NMR c l (ppm)
Mass
spectrometry
(m/z) d
2a
2063(m), 2027(vs), 2012(vs), 1973(w), 1946(w, br), 1913(w) 7.32–7.67(m)
−66.98(d), −112.2(d),
3156 (3184)
J
–
–
–
PP=69.8 Hz
2b
3a
3b
2064(s), 2038(vs), 2004(s), 1965(m)
2063(s), 2027(vs), 20139(s), 1974(w, br), 1915(w)
2068(m), 2026(vs), 1992(w)
7.23–7.71(m)
7.27–7.75(m)
7.31–7.78(m)
3156 (3184)
3324 (3324)
3324 (3324)
a In dichloromethane.
b In CDCl3.
c Referenced to trimethylphosphite.
d Based on 192Os, calculated values in parenthesis.

Z. Akhter et al. / Journal of Organometallic Chemistry 614–615 (2000) 231–237
233
Fig. 1. The molecular structure of [Os6(CO)17(Au2dppm)2] (2a) showing the atom numbering scheme adopted.
of the two triangular faces Os(1),Os(2),Os(3) and
Os(1),Os(3),Os(4) that are capped by the two gold
atoms. These five edges have an average length of 3.003
clear poly-gold clusters [14]. The shortest Au–Au dis-
,
tance of 2.677(3) A is between Au(2) and Au(3), two
Au atoms from different [Au2dppm]2+ ligands, a fea-
ture that is consistent with a direct bonding interaction
and is not dependent on the ‘bite’ of the chelating
phosphine ligand. The two Au–Au distances within the
,
A.
All the carbonyl ligands in 2a are terminally coordi-
nated to the Os atoms, three to each metal except for
Os(1) that is linked to only two carbonyls. A number of
short Au···C distances are present {Au(1)···C(32),
,
two bidentate ligands are 2.770(2) and 2.767(3) A are
,
marginally longer than the value of 2.759(1) A found in
,
2.63(4), Au(2)···C(11), 2.46(4) A}, but as in the case of
1 [11]. The Au–Os distances lie in the range 2.744(2)–
,
,
other mixed-metal clusters that contain gold there is
not thought to be a significant bonding interaction
between these atoms [16].
3.095(3) A with an average of 2.869 A. The capping
gold atoms Au(1) and Au(4) make the longer contacts
with Os(1), the metal that has the highest metal–metal
connectivity, with four Os–Os and four Os–Au
The solid-state structure is not completely consistent
with the spectroscopic data for 2a (Table 1). The struc-
ture confirms the presence of 17 carbonyl groups, as in
the parent cluster 1. The structure is not consistent with
the 31P-NMR data that suggests that there are two
,
contacts. The Os–Os edge lengths average 2.932 A,
,
a distance that is ca. 0.07 A longer than that found in
the octahedral cluster [Os6(CO)18]2− [15], but this
increase in length may be attributed to the expansion

234
Z. Akhter et al. / Journal of Organometallic Chemistry 614–615 (2000) 231–237
Fig. 2. The metal core geometry in [Os6(CO)17(Au2dppm)2] (2a).
phosphorus environments, but in the structure the four
phosphorus atoms are inequivalent. A fluxional process
that involves an equilibrium between pairs of phospho-
rus nuclei must be occurring at room temperature.
Again, the insolubility of the cluster precludes a low
temperature investigation. A possible explanation for
the 31P-NMR spectrum of 2a is that the two observed
phosphorus resonances are from two phosphorus atoms
of a single dppm ligand, both dppm ligands being
rendered equivalent by the process, with both the P
atoms of each dppm remaining inequivalent. The cou-
pling presumably arises through the methylene group.
Exchange of the Au(3) and Au(2) positions (Fig. 2),
involving Au(3)–Os(3) bond formation and Au(2)–
Os(1) bond cleavage, while maintaining the Au(2)–
Au(3) bond would achieve this.
decanuclear cluster [Os6(CO)17(Au2dppm)(AuPPh3)2]
(3). The major yellow isomer 3a was obtained in 45%
yield, and the minor pink isomer 3b in 25% yield. Both
clusters showed a low intensity peak in the FAB mass
spectrum at m/z 3324 that corresponds to the formula-
tion, and since, by analogy with 2, it is unlikely that the
clusters have scavenged carbonyl ligands from the reac-
tion the assignment is thought to be correct. The 31P-
NMR spectra for both clusters showed only broad,
weak peaks, consistent with a fluxional process, and
solubility problems precluded a low temperature inves-
tigation. However, the band structure in the terminal
Table 2
Selected metal core dimensions for [Os6(CO)17(Au2dppm)2] (2); bond
,
lengths (A) and angles (°)
From the structure of 2a and the spectroscopic data
for 2b it is not possible to suggest a structure for the
latter. Since the electron count for the two isomers is
the same it is likely that 2b also has an octahedral
osmium core, although isomers of the octahedron such
as the capped square based pyramid cannot be ruled
out (both these frameworks have the same number of
skeletal electron pairs). It is also probable that the
arrangement of the two [Au2dppm]2+ units on the
surface of the osmium core is different.
In a related reaction the reduced cluster anion
[Os6(CO)17(Au2dppm)]2− can be treated with two
equivalents of [AuPPh3][NO3], in dichloromethane, and
after purification by TLC two products are obtained.
The complexes were again characterised by IR and
NMR spectroscopies and FAB mass spectrometry
(Table 1), and were identified as two isomers of the
Os(1)–Os(2)
Os(1)–Os(3)
Os(1)–Os(4)
Os(1)–Os(5)
Os(2)–Os(3)
Os(2)–Os(5)
Os(2)–Os(6)
Os(3)–Os(4)
Os(3)–Os(6)
Os(4)–Os(5)
Os(4)–Os(6)
Os(5)–Os(6)
Au(1)–Os(1)
Au(1)–Os(3)
2.987(3)
3.038(2)
3.006(3)
2.876(2)
3.005(3)
2.844(3)
2.865(3)
2.980(3)
2.896(2)
2.869(3)
2.856(3)
2.958(3)
2.958(3)
2.796(2)
Au(1)–Os(4)
Au(2)–Os(1)
Au(3)–Os(1)
Au(4)–Os(1)
Au(4)–Os(2)
Au(4)–Os(3)
Au(1)–Au(2)
Au(1)–Au(3)
Au(2)–Au(3)
Au(3)–Au(4)
Au(1)–P(1)
Au(2)–P(2)
Au(3)–P(3)
Au(4)–P(4)
2.896(2)
2.791(2)
2.768(2)
3.095(3)
2.744(2)
2.905(3)
2.770(2)
3.188(3)
2.677(3)
2.767(3)
2.327(10)
2.280(10)
2.288(12)
2.295(11)
P(1)–Au(1)–Os(4) 138.1(3)
P(1)–Au(1)–Os(1) 146.0(3)
P(3)–Au(3)–Os(1) 166.4(3)
P(4)–Au(4)–Os(3) 136.9(3)
P(1)–Au(1)–Os(3)
P(2)–Au(2)–Os(1)
P(4)–Au(4)–Os(2)
P(4)–Au(4)–Os(1)
144.5(3)
164.7(3)
151.5(3)
140.6(3)

Z. Akhter et al. / Journal of Organometallic Chemistry 614–615 (2000) 231–237
235
F254, or using laboratory prepared glass plates coated to
1 mm thickness with Merck Kieselgel 60 F254
.
IR spectra were recorded as dichloromethane solu-
tions on a Perkin–Elmer 1710 Fourier Transform spec-
1
trometer. H- and 31P{1H}-NMR spectra were recorded
on a Bruker AM-400 spectrometer and were referenced
to external tetramethylsilane and trimethylphosphite,
respectively. Mass spetral data were obtained by nega-
tive ion FAB mass spectrometry on a Kratos MS902
mass spectrometer.
The compounds [Os6(CO)17(Au2dppm)] [11], [Au2-
dppm][NO3]2 [18], and [AuPPh3][NO3] [18] were pre-
pared by literature methods. All other chemicals were
used as purchased without further purification.
3.1. Preparation of [Os6(CO)17(Au2dppm)2] (2)
[Os6(CO)17(Au2dppm)] [11] (1) (50 mg, 2.1×10−5
mol) was reduced by fresh Na–Hg amalgam by dissolv-
ing the cluster in 20 cm3 of dry deoxygenated THF and
transferring the resulting solution to a flask containing
freshly prepared amalgam, with the help of a canula.
The reaction mixture was stirred for a few minutes. The
stirring was stopped when the reduction was complete,
as monitored by IR spectroscopy. The solution was
passed through a celite pad using the canula–septum
technique to remove the amalgam. The solvent was
removed under vacuum and to the resulting residue
dry, deoxygenated dichloromethane (20 cm3) was
added. One equivalent of [Au2dppm][NO3] (18.8 mg)
was dissolved in 10 cm3 of dichloromethane and added
to the suspension, and the mixture stirred for 30 min.
The solid residue obtained after removal of the solvent
was purified by TLC eluting with a mixture of
dichloromethane:hexane (60:40). A yellow product (2a)
was obtained in 50% yield and a pink product (2b) in
30% yield, and characterised as two isomers of
[Os6(CO)17(Au2dppm)2]. Anal. Calc. for 2a (Os6Au4-
P4O17C67H44): C, 25.35; H, 1.40. Found: C, 25.10; H,
1.36%; Anal. Calc. for 2b (Os6Au4P4O17C67H44): C,
25.35; H, 1.40. Found: C, 25.70; H, 1.20%.
Scheme 1. The reaction scheme for the formation of 2 and 3 from 1.
carbonyl stretching region of the IR spectrum for 3a is
very similar to that of 2a, consistent with a similar
symmetry for the two molecules. On this tentative
evidence, 3a can be assigned a structure similar to that
of 2a, with an octahedral osmium core and, perhaps, a
similar arrangement of the gold phosphine ligands. The
IR spectrum of 3b is somewhat different from that of
2b and no structural assignments can be made. It may
be of significance that both 2b and 3b are pink in
colour. From electron counting rules, both 3a and 3b
might be expected to have octahedral osmium cores
since both are 86 electron clusters.
The overall transformations from 1 to 2 and 3 are
illustrated in Scheme 1, but the positions of the gold
phosphine ligands in the structures of 2b, 3a and 3b
must be viewed as completely speculative.
3.2. Preparation of [Os6(CO)17(Au2dppm)(AuPPh3)2] (3)
3. Experimental
The cluster 1 (50 mg, 2.1×10−5 mol) was reduced in
the same manner as described above, and the redis-
solved residue treated with two equivalents of
[AuPPh3][NO3] (21.9 mg) in dichloromethane (20 cm3).
The resulting solution was worked up as described
above, and two compounds, one yellow (3a) and one
pink (3b), were isolated after purification by TLC, and
identified as isomers of [Os6(CO)17(Au2dppm)2-
(AuPPh3)2]. Anal. Calc. for 3a (Os6Au4P4O17C78H52): C,
28.27; H, 1.60. Found: C, 28.10; H, 1.45%. Anal. Calc.
All reactions were performed under an atmosphere of
purified dinitrogen using standard Schlenk and vacuum
line techniques [17]. Subsequent work-up of products
was carried out without precautions to exclude air.
Solvents used were distilled from appropriate drying
agents under dinitrogen. Routine separations of prod-
ucts were performed by thin-layer chromatography
(TLC) using commercially prepared glass plates, pre-
coated to 0.25 mm thickness with Merck Kieselgel 60

236
Z. Akhter et al. / Journal of Organometallic Chemistry 614–615 (2000) 231–237
for 3b (Os6Au4P4O17C78H52): C, 28.27; H, 1.60. Found:
C, 28.82; H, 1.30%.
1/[|2(F2o)+(0.0807P)2+185.648P] where P=(F2o+
2F2c)/3 was introduced. The final converged R factors for
367 refined parameters were R1=0.0678 (for 5474 reflec-
tions with I\2|(I)) and wR2=0.483 (for all data),
goodness-of-fit=1.054.
3.3. Crystal structure determination of
[Os6(CO)17(Au2dppm)2] (2a)
Yellow crystals of [Os6(CO)17(Au2dppm)2] (1a) were
obtained by recrystallisation from dichloromethane–
hexane at −20°C. A suitable crystal was mounted on a
glass fibre with epoxy resin and transferred to a Rigaku
AFC7R diffractometer. Intensity data were recorded
using graphite-monochromated Mo–Ka radiation and
an ꢀ–2q technique in the range 5.0852q550.0°. A
semi-empirical absorption correction based on „ scans
was applied. Crystal data, data collection parameters,
and details of structure solution and refinement are
presented in Table 3. The structure was solved by direct
methods (SHELXTL-PLUS [19]) and refined by full-matrix
least-squares on F2 (SHELXL–97 [20]). The Os, Au and
P atoms were assigned anisotropic displacement parame-
ters while the remaining non-hydrogen atoms were
assigned isotropic displacement parameters. Hydrogen
atoms were placed in idealised positions and allowed to
ride on the relevant carbon atoms. In the final cycles of
refinement a weighting scheme of the form w=
4. Supplementary material
Crystallographic data for the structure reported in
this paper have been deposited with the Cambridge
Crystallographic Data Centre, CCDC no. 142216 for
compound 2a. Copies of this information may be ob-
tained 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).
Acknowledgements
We gratefully acknowledge the Cambridge Common-
wealth Trust and the UK Committee of Vice Chancellors
and Principals (Z.A.), the Engineering and Physical
Sciences Research Council (G.P.S.), the Cambridge
Crystallographic Data Centre (G.P.S.), and the Eu-
ropean Union (J.F.G.) for financial support. We also
thank the EPSRC for funding for the purchase of X-ray
equipment and Johnson Matthey plc for the generous
loan of osmium salts.
Table 3
Crystal data and refinement parameters for [Os6(CO)17(Au2dppm)2]
(2a)
Empirical formula
Formula weight
Temperature (K)
C67H44Au4O17Os6P4
3173.97
293(2)
References
,
Wavelength (A)
0.71073
Triclinic
Crystal system
Space group
(
P1 (no. 2)
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Unit cell dimensions
,
a (A)
14.889(5)
19.863(5)
14.478(6)
101.37(3)
109.74(3)
93.68(3)
3911(2), 2
2.695
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
3
,
Volume (A ), Z
Dcalc (Mg m−3
)
Absorption coefficient (mm−1
F(000)
)
17.313
2828
Crystal size (mm)
q Range for data collection (°)
0.15×0.22×0.25
2.59–24.99
Limiting indices
05h56, −215k521,
−175l514
1.000, 0.425
10 219
Max. and min. transmission
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I\2|(I)]
R indices (all data)
10 219
10 155/0/367
1.054
R1=0.0678, wR2=0.1505
R1=0.1592, wR2=0.4826
2.366 and −2.015
Largest difference peak and hole
−3
,
(e A
)

Z. Akhter et al. / Journal of Organometallic Chemistry 614–615 (2000) 231–237
237
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