Metallation of the μ-phosphido bridging ligand in the reaction of the anion [M2Cp2(CO)4(μ-PH2)]- (M = Mo or W; Cp = η5-C5H5) with organometallic monohalides; synthesis

Article abstract of DOI:10.1039/a702877f

Title full: Metallation of the μ-phosphido bridging ligand in the reaction of the anion [M₂Cp₂(CO)₄(μ-PH₂)]^- (M = Mo or W; Cp = η⁵-C₅H₅) with organometallic monohalide

Full text of DOI:10.1039/a702877f

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Metallation of the ì-phosphido bridging ligand in the reaction of the
anion [M2Cp2(CO)4(ì-PH2)]Ϫ (M ؍ Mo or W; Cp ؍ ç5-C5H5) with
organometallic monohalides; synthesis and characterisation of
[M2Cp2(CO)4(ì-H){ì-PH(MЈLn)}] [MЈLn ؍ Mn(CO)5, FeCp(CO)2,
MoCp(CO)3 or WCp(CO)3]†
John E. Davies, Martin J. Mays,* Elizabeth J. Pook, Paul R. Raithby and Peter K. Tompkin
Department of Chemistry, Lensfield Road, Cambridge, UK CB2 1EW
The anions [M2Cp2(CO)4(µ-PH2)]Ϫ (M = Mo or W; Cp = η5-C5H5) reacted with the organometallic halides
[MЈLnX] [MЈLn = Mo(CO)3Cp, X = Cl; MЈLn = W(CO)3Cp, X = Cl; MЈLn = Fe(CO)2Cp, X = Cl or MЈLn =
Mn(CO)5, X = Br] to afford the new complexes [M2Cp2(CO)4(µ-H){µ-PH(MЈLn)}] together with [LnMЈ᎐MЈLn],
each in low yield. The crystal structure of [W2Cp2(CO)4(µ-H){µ-PH[MoCp(CO)3]}] has been determined.
The chemistry of complexes featuring µ-PR2 bridges (R = alkyl
H2
P
C (not obtained)
or aryl) has been the subject of extensive study in recent years,
principally because such bridges often confer stability on di-
and poly-nuclear complexes with respect to fragmentation.1 By
contrast, the reactivity of complexes containing µ-PH2 ligands
has been the subject of relatively few investigations in the liter-
ature, possibly due to the inconvenience and hazards associated
with the use of phosphine gas, the conventional route to such
species.2 We recently reported, however, a preparation of µ-PH2
complexes avoiding the use of PH3.3 Thus, reaction of
[M2Cp2(CO)4(µ,η2-P2)] (1, M = Mo or 2, M = W; Cp = η5-C5H5)
with the hydroxides of alkali metals afforded in good yield the
phosphido bridged anions [M2Cp2(CO)4(µ-PH2)]Ϫ (3, M = Mo
or 4, M = W) which on treatment with acid afforded the corre-
sponding hydrides [M2Cp2(CO)4(µ-H)(µ-PH2)] (5, M = Mo or
6, M = W).3 The chemistry of species with bonds to hydrogen
often differs significantly from that of analogous species with
bonds to alkyl or aryl groups. Accordingly, it seemed of interest
to examine the reactivity of complexes featuring the µ-PH2
ligand, and in this paper the reactivity of the anions 3 and 4
towards a variety of halide-containing organometallic species is
reported. It is shown that such reactions lead to metallation of
the phosphido ligand and regeneration of a bridging hydride
ligand.
(OC)2CpM
MCp(CO)2
M′Ln
LnM′X
–
H2
P
(OC)2CpM
MCp(CO)2
2
3 M = Mo
4 M = W
2–
H2
P
H
P
+
(OC)2CpM
MCp(CO)2
(OC)2CpM
MCp(CO)2
A
H
5 M = Mo
6 M = W
LnM′X – X –
–
H
M′Ln
P
Discussion
(OC)2CpM
MCp(CO)2
B
The air-sensitive anions 3 and 4 are conveniently prepared from
tetrahydrofuran (thf) solutions of complexes 5 and 6 respect-
ively by addition of 1 equivalent of LiBut. The reaction of the
anions 3 and 4 with a range of organometallic monohalides
[MЈLnX] {[MoCp(CO)3Cl] 7, [WCp(CO)3Cl] 8, [FeCp(CO)2Cl]
9 or [Mn(CO)5Br] 10} leads to metallation of the phosphido
ligand and regeneration of a bridging hydride between the two
bonded metal centres giving the new complexes 11–16 (Scheme
1). A similar reaction, in which two Os3 triangles were linked by
utilising the reactivity of a µ-PH2 ligand co-ordinated to one of
them has previously been reported by Lewis and co-workers,4a
other metallated phosphido complexes have been prepared by
deprotonation of complexes of the µ-PHPh ligand followed by
reaction with organometallic halides.4b The spectroscopic data
for the new complexes 11–16 are shown in Table 1 and are
in accord with the proposed structures, shown in Scheme 1.
–
H
M′Ln
H2
P
P
H
+
(OC)2CpM
MCp(CO)2
(OC)2CpM
MCp(CO)2
11 M′Ln = MoCp(CO)3, M = Mo
12 M′Ln = WCp(CO)3, M = Mo
13 M′Ln = FeCp(CO)2, M = Mo
14 M′Ln = Mn(CO)5, M = Mo
15 M′Ln = MoCp(CO)3, M = W
16 M′Ln = WCp(CO)3, M = W
Scheme 1 Proposed mechanism for the formation of complexes 11–16
In each case, all Cp ligands present in a given complex are
observed to be in distinct environments on the NMR spectro-
† In memory of Sir Geoffrey Wilkinson, who first aroused my interest
in organometallic chemistry.
J. Chem. Soc., Dalton Trans., 1997, Pages 3283–3286
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Table 1 Spectroscopic data for complexes 11–16
Microanalysis* (%)
FAB
mass spectra
Complex νCO/cmϪ1
1H NMR
31P NMR
C
H
P
11
12
13
14
2023s, 1943vs,
1921s, 1858s
5.65 (s, 5 H, Cp), 5.27 (d, 1 H,
1JH P 269, PH), 5.21 (s, 5 H,
Cp), 5.08 (s, 5 H, Cp), Ϫ11.96
(d, 1 H, 2JH P 32, MoH Mo)
5.76 (s, 5 H, Cp), 5.21 (s, 5 H,
Cp), 5.07 (s, 5 H, Cp), 4.87 (d,
1 H, 1JH P 273, PH ), Ϫ11.91 (d,
1 H, 2JH P 32, MoH Mo)
Ϫ74.7 (s)
712 (M ϩ), 626,
598, 570, 542,
514 (M ϩ Ϫ nCO,
n = 3–7)
36.88 (37.08)
2.29 (2.39)
4.22 (4.21)
2019s, 1938vs,
1929vs, 1857s
Ϫ116.8 (s)
800 (M ϩ
)
32.90 (33.00)
2.12 (2.13)
3.88 (3.88)
2021m, 1974m, 5.71 (d, 1 H, 1JH P 263, PH ), 5.19 Ϫ16.34 (s)
1938w, 1918s,
1853s
644 (M ϩ), 588,
532, 476
Not recrystallised due to instability in solution
Not recrystallised due to instability in solution
(s, 5 H, Cp), 5.11 (s, 5 H, Cp),
5.09 (s, 5 H, Cp), Ϫ11.64 (d,
1 H, 2JH P 29, MoHMo)
(M ϩ Ϫ nCO,
n = 2, 4, 6)
2111m, 2023vs, 5.24 (s, 5 H, Cp), 5.14 (s, 5 H,
1940w, 1925s,
1858m
Ϫ75.4 (s)
Ϫ180.0 (s)
Ϫ220.6 (s)
662 (M ϩ), 634,
606, 549, 521,
494, 466, 438,
410 (M ϩ Ϫ nCO,
n = 1, 2, 4–9)
Cp), 4.79 (d, 1 H, 1JH P 266,
PH ), Ϫ11.88 (d, 1 H, 2JH P
30, MoH Mo)
15
16
2023s, 1939vs,
1912vs, 1844s
5.64 (s, 5 H, Cp), 5.32 (s, 5 H,
Cp), 5.22 (s, 5 H, Cp), 5.08 (d,
1 H, 1JH P 282, PH), Ϫ14.26
(d, 1 H, 2JH P 21, 1JH W 42.2,
WH W)
5.76 (s, 5 H, Cp), 5.31 (s, 5 H,
Cp), 5.21 (s, 5 H, Cp), Ϫ14.25
(d, 1 H, 2JH P 22, 1JH W 43,
WH W)
888 (M ϩ
)
30.16 (29.73)
27.10 (27.05)
2.04 (1.91)
1.79 (1.74)
3.54 (3.49)
3.22 (3.18)
2019s, 1935s,
1910s, 1844s
977 (M ϩ ϩ 1),
948, 920, 892,
864, 836
(M ϩ Ϫ nCO,
n = 1–5)
* Calculated values in parentheses.
Table 2 Selected bond lengths (Å) and angles (Њ) for complex 15
W(1)᎐W(2)
W(1)᎐P
W(2)᎐P
P᎐Mo
3.2770(13)
2.466(3)
2.443(3)
2.599(3)
W(1)᎐H(2)
W(2)᎐H(2)
P᎐H(1)
1.85
1.85
1.39(13)
P᎐W(1)᎐W(2)
P᎐W(1)᎐H(2)
P᎐W(2)᎐H(2)
W(1)᎐P᎐Mo
47.82(8)
75.4
76.1
P᎐W(2)᎐W(1)
W(2)᎐P᎐W(1)
W(1)᎐H(2)᎐W(2)
W(2)᎐P᎐Mo
48.43(8)
83.75(10)
124.8
123.90(14)
130.27(14)
consistent with NMR spectroscopic evidence indicating their
magnetic inequivalence; the Cp ligands are similarly trans in the
µ-PH2 complex 3.3 The W᎐W bond length of 3.2770(13) Å is
indicative of a single metal–metal bond, as required for each
tungsten atom to satisfy the 18-electron rule. The WHWP core
of the molecule is essentially planar, as also observed in 3,3 with
the bridging phosphorus occupying a slightly asymmetric pos-
ition [W(1)᎐P(1) 2.466(3), W(2)᎐P(1) 2.443(3) Å]; these bonds
are both rather shorter than the Mo᎐P distance of 2.599(3) Å.
The major residual electron density peaks after the final refine-
ment are in the vicinity of the tungsten atoms and do not corre-
spond to any chemically sensible moiety.
Fig. 1 Molecular structure of complex 15, showing the atom labelling
scheme adopted
scopic time-scale, as compared to 3–6 for which only one signal
is observed. There is a reduction in overall symmetry (loss of C2
axis) as compared to 3–6 on conversion of the µ-PH2 ligand to a
species that can be regarded as a µ-PH(MЈLn) ligand; the trans
geometry of the Cp ligands observed in the solid state in 33 is
presumably retained in solution for the µ-PH(MЈLn) complexes.
This then results in one of the Cp ligands being nearer the
phosphorus-bound H atom and the other being nearer the
newly introduced MЈLn moiety.
The crystal structure of [W2Cp2(CO)4(µ-H){µ-PH[MoCp-
(CO)3]}] 15 has been determined by X-ray diffraction. The
molecular structure is shown in Fig. 1; selected bond lengths
and angles are given in Table 2. The complex shows a trans
orientation of the Cp ligands in the W2Cp2(CO)4 fragment,
Scheme 1 shows a plausible mechanism for the formation of
complexes 11–16. It is suggested that the phosphido anion
[M2Cp2(CO)4(µ-PH2)]Ϫ (3, M = Mo or 4, M = W) is in equi-
librium with [M2Cp2(CO)4(µ-H)(µ-PH2)] (5, M = Mo or 6,
M = W) and the dianion [M2Cp2(CO)4(µ-PH)]2Ϫ A. This
dianion will be more nucleophilic than [M2Cp2(CO)4(µ-PH2)]Ϫ
and will in consequence react more readily with the organo-
metallic halide LnMЈX to displace XϪ. The resulting metallated
anion B could then accept a proton from the neutral hydride
to afford the observed metallated product and regenerate 1
equivalent of [M2Cp2(CO)4(µ-PH2)]Ϫ. The NMR spectroscopic
studies on the anions 3 and 4 did not reveal the presence of any
hydride-containing species in solution, suggesting the pro-
portion of intermediate A present in solution is very small. This
3284
J. Chem. Soc., Dalton Trans., 1997, Pages 3283–3286

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1
is in accord with the fact that the reaction is relatively slow.
Direct reaction of the phosphido anions 3 and 4 with mono-
nuclear organometallic halides might be expected to lead to a
trimetallic chain complex of type C (Scheme 1), but no com-
pound with such a formulation was observed. It is probable that
the steric bulk of the organometallic halide and of the Cp and
CO ligands present in the anions both render initial attack of
the metal halide at a metal centre on the anions more diffi-
cult than attack at the phosphorus atom. When the anions are
treated with a small, hard electrophile such as Hϩ, reaction
occurs essentially instantaneously even at 0 ЊC and presumably
takes place at the metal centres to regenerate the neutral
hydride directly.3 Treatment of the neutral hydrides with a large
excess of LiBut affords deep orange solutions, for which a num-
ber of IR bands in the 1600–1700 cmϪ1 region are observed,
suggesting further deprotonation to give A may have occurred,
but these solutions have as yet proved too unstable to character-
ise further.
pink-purple, and was identified by 31P and H NMR spectro-
scopy as containing [W2Cp2(CO)4(µ-PH2)]Ϫ 4.3
Reaction of [Mo2Cp2(CO)4(ì-PH2)]Ϫ 3 with [MoCp(CO)3Cl] 7
To a solution of 3 [prepared from complex 5 (354 mg, 0.756
mmol)] in thf (25 cm3) was added complex 7 (216 mg, 0.770
mmol) and the solution heated to reflux for 2 min. The colour
of the solution became dark orange. The solvent was removed
and the residue redissolved in the minimum quantity of
dichloromethane and applied to the base of TLC plates.
Elution with hexane–dichloromethane (1:1) afforded [Mo2-
Cp2(CO)6] (50 mg, 0.102 mmol, 13%), 3 (trace), [Mo2Cp2-
(CO)4(µ-H){µ-PH[MoCp(CO)3]}] 11 (44 mg, 0.062 mmol, 8%)
and unreacted 7 (trace). Subsequent elution with hexane–
acetone (1:2) afforded green [Mo3Cp3(CO)6(µ3-PO)] 17 (22 mg,
0.032 mmol, 4%).
Reaction of [Mo2Cp2(CO)4(ì-PH2)]Ϫ 3 with [WCp(CO)3Cl] 8
All the new complexes 11–16 are sensitive to air, being more
so in solution. From the reactions leading to complexes 11, 12,
15 and 16 we have also been able to isolate in low yield the green
trimetallic species [Mo3ϪnWnCp3(CO)6(µ3-PO)] (17, n = 0; 18,
n = 1; 19, n = 2 or 20, n = 3) respectively, although analogous
µ3-PO complexes were not isolated during the preparation of
complexes 13 or 14. The phosphorus monoxide complexes were
identified by comparison of their IR and NMR spectra to those
previously reported in the literature.5 A plausible route to the
transformation of the metallated phosphido complexes into the
phosphorus monoxide complexes would involve the initial reac-
tion of O2 with the µ-P(H)(MLn) and hydride ligands to form
a molecule of water and a µ3-PO ligand by oxidation of the
phosphorus atom. Subsequent loss of carbon monoxide and
metal–metal bond formation would then yield the observed
tetrahedrane.
To a solution of 3 [prepared from complex 5 (318 mg, 0.679
mmol)] in thf (25 cm3) was added complex 8 (256 mg, 0.694
mmol) and the solution heated to reflux for 3 min. The colour
of the solution became dark orange. The solvent was removed
and the residue redissolved in the minimum quantity of
dichloromethane and applied to the base of TLC plates.
Elution with hexane–dichloromethane (1:1) afforded [W2Cp2-
(CO)6] (54 mg, 0.081 mmol, 12%), 5 (trace), [Mo2Cp2(CO)4-
(µ-H){µ-PH[WCp(CO)3]}] 12 (60 mg, 0.075 mmol, 11%) and
unreacted 8 (trace). Subsequent elution with hexane–acetone
(1:2) afforded green [Mo2WCp3(CO)6(µ3-PO)] 18 (31 mg, 0.039
mmol, 6%).
Reaction of [Mo2Cp2(CO)4(ì-PH2)]Ϫ 3 with [FeCp(CO)2Cl] 9
To a solution of 3 [prepared from complex 5 (301 mg, 0.643
mmol)] in thf (25 cm3) was added complex 9 (136 mg,
0.640 mmol) and the solution heated to reflux for 3 min. The
colour of the solution became dark orange. The solvent was
removed and the residue redissolved in the minimum quantity
of dichloromethane and applied to the base of TLC plates.
Elution with hexane–dichloromethane (1:1) afforded 5 (trace),
orange [Mo2Cp2(CO)4(µ-H){µ-PH[FeCp(CO)2]}] 13 (46 mg,
0.071 mmol, 11%) and [Fe2Cp2(CO)4] (39 mg, 0.110 mmol,
17%).
Experimental
All reactions were performed under an atmosphere of dry
oxygen-free nitrogen, using solvents which had been freshly dis-
tilled from the appropriate drying agent. Infrared spectra were
recorded in dichloromethane solution in 0.5 mm NaCl cells,
using a Perkin-Elmer 1710 Fourier-transform spectrometer.
Fast atom bombardment (FAB) mass spectra were recorded on
a Kratos MS890 instrument using 3-nitrobenzyl alcohol as a
matrix. Proton and 31P NMR spectra were recorded on either
a Bruker WM250 or AM-400 spectrometer; 31P NMR spec-
troscopy chemical shifts are referenced to P(OMe)3 at δ 0.0 with
upfield shifts negative. Microanalyses were performed by the
microanalytical department, University of Cambridge. Pre-
parative TLC was carried out on 1 mm silica plates prepared
at the University of Cambridge. Products are given in order of
decreasing Rf values. The complexes [M2Cp2(CO)4(µ-H)-
(µ-PH2)] (5, M = Mo or 6, M = W),3 [MoCp(CO)3Cl] 7,6 [WCp-
(CO)3Cl] 86 and [FeCp(CO)2Cl] 97 were prepared by liter-
ature methods; [Mn(CO)5Br] 10 was purchased from Strem
Chemicals Inc. and used without further purification.
Reaction of [Mo2Cp2(CO)4(ì-PH2)]Ϫ 3 with [Mn(CO)5Br] 10
To a solution of 3 [prepared from complex 5 (298 mg, 0.637
mmol)] in thf (25 cm3) was added complex 10 (175 mg, 0.636
mmol) and the mixture stirred at room temperature for 3 d. The
colour of the solution slowly became dark orange. The solvent
was removed and the residue redissolved in the minimum
quantity of dichloromethane and applied to the base of TLC
plates. Elution with hexane–dichloromethane (1:1) afforded
[Mn2(CO)10] (20 mg, 0.051 mmol, 16%), 5 (34 mg, 0.073 mmol,
11%) and orange [Mo2Cp2(CO)4(µ-H){µ-PH[Mn(CO)5]}] 14
(20 mg, 0.030 mmol, 5%).
Reaction of [W2Cp2(CO)4(ì-PH2)]Ϫ 4 with [MoCp(CO)3Cl] 7
Reaction of [Mo2Cp2(CO)4(ì-H)(ì-PH2)] 5 with LiBut
To a solution of 4 [prepared from complex 2 (291 mg, 0.452
mmol)] in thf (25 cm3) was added 7 (129 mg, 0.459 mmol) and
the solution heated to reflux for 3 min. The colour of the solu-
tion became dark orange. The solvent was removed and the
residue redissolved in the minimum quantity of dichloro-
methane and applied to the base of TLC plates. Elution with
hexane–dichloromethane (1:1) afforded [Mo2Cp2(CO)6] (25
mg, 0.051 mmol, 22%), 2 (trace), [W2Cp2(CO)4(µ-H){µ-PH-
[MoCp(CO)3]}] 15 (39 mg, 0.044 mmol, 10%) and unreacted 7
(trace). Subsequent elution with hexane–acetone (1:2) afforded
green-brown [MoW2Cp3(CO)6(µ3-PO)] 19 (20 mg, 0.023 mmol,
5%).
To a solution of complex 5 (306 mg, 0.667 mmol) in thf (25
cm3) was added LiBut (0.41 cm3 as 1.7  solution, 0.697 mmol).
The solution changed immediately from orange to purple, and
1
was identified by 31P and H NMR spectroscopy as containing
[Mo2Cp2(CO)4(µ-PH2)]Ϫ 3.3
Reaction of [W2Cp2(CO)4(ì-H)(ì-PH2)] 6 with LiBut
To a solution of complex 6 (355 mg, 0.550 mmol) in thf (25
cm3) was added LiBut (0.35 cm3 as 1.7  solution, 0.585
mmol). The solution changed immediately from orange to
J. Chem. Soc., Dalton Trans., 1997, Pages 3283–3286
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Reaction of [W2Cp2(CO)4(ì-PH2)]Ϫ 4 with [WCp(CO)3Cl] 8
Acknowledgements
To a solution of 4 [prepared from complex 2 (404 mg, 0.627
mmol)] in thf (25 cm3) was added 8 (238 mg, 0.646 mmol) and
the solution heated to reflux for 3 min. The colour of the solu-
tion became dark orange. The solvent was removed and the
residue redissolved in the minimum quantity of dichloro-
methane and applied to the base of TLC plates. Elution with
hexane–dichloromethane (1:1) afforded [W2Cp2(CO)6] (42
We thank the EPSRC for funding the X-ray diffractometer
and for financial support (to J. E. D. and P. K. T.) and the
Cambridge Crystallographic Data Centre for support (to
J. E. D.).
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mg, 0.063 mmol, 20%),
2
(trace), [W2Cp2(CO)4(µ-H)-
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{µ-PH[WCp(CO)3]}] 16 (53 mg, 0.054 mmol, 9%) and unre-
acted 8 (trace). Subsequent elution with hexane–acetone (1:2)
afforded green-brown [W3Cp3(CO)6(µ3-PO)] 20 (40 mg, 0.042
mmol, 7%).
Crystal-structure determination of [W2Cp2(CO)4(ì-H)-
{ì-PH[MoCp(CO)3]}] 15
Single crystals of complex 15 suitable for X-ray diffraction
studies were grown by slow diffusion of hexane into a
dichloromethane solution under a nitrogen atmosphere at 0 ЊC.
Data were collected on an orange crystal, 0.10 × 0.15 × 0.15
mm in dimensions, by the ω–2θ scan method on a Rigaku
AFC7R four-circle diffractometer.
Crystal data. C22H17MoO7PW2 15: M = 887.97, triclinic,
¯
space group P1, a = 10.047(4), b = 15.409(5), c = 7.653(3) Å,
α = 94.22(3), β = 103.73(3), γ = 90.42(3)Њ, U = 1152.0(7) Å3,
T = 150(2) K, graphite-monochromated Mo-Kα radiation,
λ = 0.710 713 Å, Z = 2, Dc = 2.560 Mg mϪ3, F(000) = 820, µ(Mo-
Kα) = 10.608 mmϪ1, relative transmission 1.000–0.845. Data
collection range 5.30 < 2θ < 55.02Њ, 0 р h р 13, Ϫ20 р k р 20,
Ϫ9 р l р 9, 5558 reflections collected of which 5259 were
independent (Rint = 0.0797) used in all calculations. Three
standard reflections were monitored at intervals of 200 reflec-
tions. Cell parameters were obtained by least-squares refine-
ment on diffractometer angles from 25 centred reflections
(15 < 2θ < 20Њ). A semiempirical absorption correction based
on ψ-scan data was applied. The structure was solved by direct
methods (SIR 92)8 and subsequent Fourier-difference syntheses
and refined anisotropically on all non-hydrogen atoms by full-
matrix least-squares on F 2 (SHELXL 93).9 Cyclopentadienyl
hydrogen atoms were placed in idealised positions and refined
using a riding model; the coordinates of H(1) were refined
freely and the bridging hydride located using the program
HYDEX.10 In the final cycles of refinement a weighting scheme
8 A. Altomare, G. Cascarano, C. Giacavazzo, A. Guagliardi,
M. C. Burla, G. Polidori and M. Camalli, J. Appl. Crystallogr., 1994,
27, 435.
9 G. M. Sheldrick, SHELXL 93, University of Göttingen, 1993.
10 A. G. Orpen, J. Chem. Soc., Dalton Trans., 1980, 2509.
2
2
of the form w = 1/[σ2(Fo ) ϩ (xP)2 ϩ yP], P = (Fo2 ϩ 2Fc )/3
was introduced which produced a flat analysis of variance.
Final wR(F 2) on all data 0.134, R1 = 0.053 on 3852 reflections
with I >2σ(I), 301 parameters, goodness of fit 1.047, greatest
peak and hole in final electron density map 3.023 and Ϫ2.403
e Å3.
CCDC reference number 186/596.
Received 28th April 1997; Paper 7/02877F
3286
J. Chem. Soc., Dalton Trans., 1997, Pages 3283–3286