Tungsten to tungsten quadruple bonds: Over 30 years of research, 50 structurally characterized compounds and 100 known compounds

Article abstract of DOI:10.1016/S0020-1693(03)00123-3

Since the first reported structure of a quadruply bonded ditungsten complex in 1977, the field of W₂(II, II) chemistry has expanded as new synthetic methodologies have been developed. Based on a recently developed synthetic route in our laboratory, the synthesis and structures of two new W₂(II, II) complexes with substituted formamidinate ligands are reported. With the addition of these two new structures, the number of structurally characterized quadruply bonded W₂(II, II) complexes exceeds 50 and over 100 W₂(II, II) complexes have been synthesized. A compilation of the quadruply bonded W₂(II, II) compounds, W-W bond distances, and synthetic methodologies is presented.

Full text of DOI:10.1016/S0020-1693(03)00123-3

Inorganica Chimica Acta 351 (2003) 217ꢀ224
/
www.elsevier.com/locate/ica
Tungsten to tungsten quadruple bonds: over 30 years of research, 50
structurally characterized compounds and 100 known compounds
Judith L. Eglin a,*, Laura T. Smith a,c, Richard J. Staples b
a Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA
b Harvard University, Cambridge, MA 02138, USA
c DIAL, 205 Research Boulevard, Starkville, MS 39759, USA
Received 28 January 2003; received in revised form 28 January 2003; accepted 22 February 2003
Abstract
Since the first reported structure of a quadruply bonded ditungsten complex in 1977, the field of W2(II, II) chemistry has
expanded as new synthetic methodologies have been developed. Based on a recently developed synthetic route in our laboratory, the
synthesis and structures of two new W2(II, II) complexes with substituted formamidinate ligands are reported. With the addition of
these two new structures, the number of structurally characterized quadruply bonded W2(II, II) complexes exceeds 50 and over 100
W2(II, II) complexes have been synthesized. A compilation of the quadruply bonded W2(II, II) compounds, WꢀW bond distances,
/
and synthetic methodologies is presented.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Tungsten; Quadruple bond; Formamidinate ligand
1. Introduction
gories with exception of W2(m-O2Cꢀ
/
t-Bu)3(O2Cꢀt-
/
Bu)(PMePh2)2 [6]: classic paddlewheel complexes with
four bridging anionic ligands both with and without
While the earliest synthesis of a ditungsten tetracar-
boxylate was performed in 1971 [1], the first structure of
the W2(II, II) core was not reported until 1977 for the
axially coordinated neutral ligands [4,6ꢀ25], compounds
/
coordinated by only anionic ligands [2,3,26,27], and
compounds coordinated by four anionic ligands and
compounds
Li4W2(CH3)xCl8ꢁx
×
/
4THF
[2]
and
W2(C8H8)3 [3]. However, the structure of the first
four neutral ligands [28ꢀ43].
/
ditungsten tetracarboxylate was determined 10 years
2/3(diglyme) where TFA
Of the 50 previously structurally characterized com-
pounds, two paddlewheel compounds have been synthe-
sized with formamidinate ligands as bridging bidentate
ligands; W2(m-DFM)4 where DFM is [(p-CH3C6H3)-
NC(H)N(p-CH3C6H3)ꢁ] [17] and W2(m-DCPF)4 where
DCPF is [(3,5-Cl2C6H3)NC(H)N(3,5-Cl2C6H3)ꢁ] [19].
While first developed for W2(m-DCPF)4 [19], a simpli-
fied synthetic route to multiply bonded ditungsten
complexes using NaBEt3H as the reducing agent has
been expanded to the formamidinate ligands, [((p-
ClC6H4)N)2CH]ꢁ and [((m-MeOC6H4)N)2CH]ꢁ, to
result in the synthesis of the paddlewheel complexes
later in 1981 for W2(TFA)4×
/
represents the anion of trifluoracetic acid [4]. The
progress in the synthesis and structural characterization
of W2(II, II) compounds is closely related to new
developments in synthetic methodologies and ligand
types, but in comparison to the analogous dimolybde-
num chemistry the scope of the ditungsten chemistry is
still rather limited [5]. Of the more than 50 structurally
characterized quadruply bonded W2(II, II) compounds,
the types of compounds fall into three primary cate-
˚
W2((p-ClC6H4N)2CH)4 [2.1924(2) A] and W2((m-
˚
MeOC6H4N)2CH)4 [2.1957(6) A] and bring the total of
* Corresponding author. Present address: P.O. Box 1663, Mail Stop
G740, Los Alamos National Laboratory, Los Alamos, NM 87545,
structurally characterized ditungsten compounds to over
50.
USA. Tel.: ꢂ
E-mail address: eglin@lanl.gov (J.L. Eglin).
/
1-505-667-4087; fax: ꢂ1-505-665-4737.
/
0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0020-1693(03)00123-3

218
J.L. Eglin et al. / Inorganica Chimica Acta 351 (2003) 217ꢀ224
/
2. Experimental
the methanol solution was cooled in an ice bath, a fine
orange powder precipitated from the brown solution.
The methanol was removed by cannula and the orange
powder washed two times with 30 ml aliquots of
methanol and dried under dynamic vacuum (0.503 g,
2.1. General synthetic procedures
Standard vacuum line, drybox, and Schlenk tech-
niques were employed under an argon atmosphere.
Commercial grade tetrahydrofuran (THF) and hexanes
were dried over potassium/sodium benzophenone ketyl
and freshly distilled under argon prior to use. Commer-
cial grade methanol was dried over magnesium turnings
with freshly sublimed iodine and distilled prior to use.
The 1.0 M solution of sodium triethylborohydride in
toluene and 1.4 M solution of methyllithium in diethyl-
ether purchased from Aldrich Chemical Co. were
packaged in sure-seal bottles and used without further
purification. Triethylorthoformate, p-chloroaniline, and
m-anisidine were purchased from ACROS Organics and
used as received. The starting materials WCl4 and the
formamidine ligands, (p-ClC6H4)NHC(H)N(p-ClC6H4)
and (m-CH3OC6H4)NHC(H)N(m-CH3OC6H4), were
prepared according to literature methods, washed under
an argon atmosphere with freshly distilled hexanes, and
stored in the drybox [44,45].
32.1%). UVꢀ
Vis (THF): 410 nm (sh), 1H NMR spectro-
/
scopy (CDCl3) d 8.469 (s, 4H), d 6.894 (t, 8H), d 6.506,
6.486 (d, 8H) 6.096, 6.076 (d, 8H), d 5.913 (s, 8H), d
3.719 (s, 24H).
2.2.2. Preparation of W2((p-ClC6H4N)2CH)4 (2)
A slurry of 1.0 g of WCl4 (3.06 mmol) in 20 ml of
THF was cooled to ꢁ
NaBEt3H was added dropwise. The reaction mixture
was allowed to warm to ꢁ20 8C to result in a green
/
70 8C and 7.0 ml (7.0 mmol) of
/
solution. In a separate flask, 1.8 g (6.8 mmol) of (p-
ClC6H4)NHC(H)N(p-ClC6H4) in 10 ml of THF was
cooled in an ice bath and 5.0 ml (7.0 mmol) of
methyllithium added. After the solution warmed to
r.t., the lithium formamidinate salt was transferred via
cannula to the flask containing the ꢁ20 8C reduced
/
ditungsten solution. After the reaction mixture warmed
to r.t., the mixture was refluxed for 6 h. The viscous
dark orangeꢀbrown solution was filtered through
/
2.2. Physical measurements
Celite, all solvents removed under vacuum, and the
residual oil was gently heated under vacuum for 1 h to
The UVꢀ/Vis spectra for W2((m-MeOC6H4N)2CH)4
remove any remaining solvent. To the deep brownꢀ
/
and W2((p-ClC6H4N)2CH)4 were recorded on a Hewlett
Packard model 8453 diode array spectrophotometer
orange oil, 30 ml of methanol was added and the
solution refluxed for 2 h. After cooling the methanol
solution, a fine orange powder precipitated from the
brown solution. Excess methanol was removed by
cannula and the orange powder washed two times with
30 ml aliquots of methanol, and dried under dynamic
1
from 190 to 1100 nm. H NMR spectra were recorded
in 5 mm NMR tubes on a General Electric Omega 400
MHz NMR spectrometer with a 10 mm variable
temperature broadband probe. The proton chemical
shifts (d) of W2((m-MeOC6H4N)2CH)4 and W2((p-
ClC6H4N)2CH)4 were referenced to the residual CHCl3
vacuum. (0.766 g, 35%). UVꢀVis (THF): 400 nm (sh),
/
1H NMR spectroscopy (CDCl3) d 8.178 (s, 4H), d
7.083, 7.060 (d, 162H) d 6.965, 6.944 (d, 16H).
(dꢃ7.27) in the CDCl3 solvent.
/
2.2.1. Preparation of W2((m-MeOC6H4N)2CH)4 (1)
To a slurry of 1.0 g of WCl4 (3.06 mmol) in 20 ml of
2.3. X-ray crystallography
THF cooled to ꢁ
was added dropwise. The reaction mixture was allowed
to warm to ꢁ20 8C to result in a green solution. In a
separate flask, 1.8 (7.0 mmol) of (m-
MeOC6H4)NHC(H)N(m-MeOC6H4) in 10 ml of THF
was cooled in an ice bath and 5.0 ml (7.0 mmol) of
methyllithium added. The lithium formamidinate salt
that formed upon warming the solution to room
temperature (r.t.) was transferred via cannula to the
/
70 8C, 7.0 ml (7.0 mmol) of NaBEt3H
2.3.1. Ditungsten complexes
Orange crystals of W2((m-MeOC6H4N)2CH)4 (1) and
W2((p-ClC6H4N)2CH)4 (2) were grown from THF
solutions of the compounds layered with hexanes.
Data for the ditungsten compounds, W2((m-
MeOC6H4N)2CH)4 (1) and W2((p-ClC6H4N)2CH)4 (2),
were collected using a Siemens SMART CCD (charge
coupled device) based diffractometer equipped with an
LT-2 low-temperature apparatus operating at 213 K.
Suitable crystals were chosen and mounted on glass
fibers using grease. Data for each compound were
measured using omega scans of 0.38 per frame for 30 s
so that a full hemisphere was collected. A total of 1271
/
g
flask containing the ꢁ
tion. After the reaction mixture warmed to r.t., the
mixture was refluxed for 6 h. The viscous dark orangeꢀ
/
20 8C reduced ditungsten solu-
/
brown solution was filtered through Celite, all solvents
removed under vacuum, and the residual oil was gently
heated under vacuum for 1 h to remove any remaining
˚
frames was collected with a final resolution of 0.75 A.
The first 50 frames were recollected at the end of the
data collection to test for decay. However, all crystals
used for the diffraction study showed no decomposition
solvent. To the deep brownꢀ/orange oil, methanol (30
ml) was added and the solution refluxed for 2 h. After

J.L. Eglin et al. / Inorganica Chimica Acta 351 (2003) 217ꢀ
/224
219
during the data collection. Cell parameters were re-
trieved using SMART [46] software and refined using
SAINT on all observed reflections. Data reduction was
performed using the SAINT software [47], which corrects
for Lorentz and polarization effects and decay. Absorp-
tion corrections were applied using SADABS [48] supplied
by George Sheldrick. The structures were solved by
direct method using the SHELXS-90 [49] program and
refined by least-squares methods on F2, SHELX-93 [50]
incorporated in SHELXTL-PC V 5.03 [51].
The structures of 1 and 2 shown in Figs. 1 and 2,
respectively, were solved in the space groups P21/n and
¯
P1 by analysis of systematic absences for structure 1 and
successful solution and refinement of structure 2. All
non-hydrogen atoms are refined anisotropically. Hydro-
gen atoms for both structures were calculated by
geometrical methods and refined as a riding model.
Table 1 provides data collection information for 1 and 2.
Tables 2 and 3 include selected bond distances and
angles for 1 and 2, respectively.
Fig. 2. A thermal ellipsoid plot of W2((p-ClC6H4N)2CH)4 (2) viewed
at approximately 908 to the WꢀW axis. Thermal ellipsoids for W, C
and N atoms are shown at 30% probability. H atoms are omitted for
/
clarity.
W2Cl4(dppp)2 [37], and W2(DCPF)4 [19]. Although the
first ditungsten compound synthesized with the form-
amidinate ligand, W2(m-DFM)4 [17], employed sodium
amalgam as the reducing agent, the synthesis of W2(m-
DCPF)4 [19] was achieved using NaBEt3H with only a
slight decrease in yield from 45 to 34%.
The synthesis of W2((m-MeOC6H4N)2CH)4 and
W2((p-ClC6H4N)2CH)4 is an extension of the series of
ditungsten tetraformamidinate compounds in order to
perform comparisons based upon changes in the Ham-
mett constant of substituents on the formamidinate
ligand [52]. With the synthesis of these two new
W2(form)4 compounds, where form represents a sub-
stituted formamidinate ligand, the range of electron
withdrawing or donating groups on the phenyl group of
the formamidinate ligand is extended to include p-CH3
3. Results and discussion
3.1. Synthesis
While W2(II, II) paddlewheel complexes can be
synthesized by reductive-elimination from 1,2-diethyl
triply bonded compounds [18] or sodium amalgam
reduction of WCl4 with subsequent addition of the
ligand [4], a new synthetic method using NaBEt3H
instead of sodium amalgam as the reducing agent has
been developed and used in the synthesis of compounds
with a variety of ligand types such as W2(O2C-
C6H5)2I2(m-dppm)2 [40], b-W2Br4(dppm)2 [32], a-
[17] (s ꢁ0.17), m-OCH3 (s 0.12), p-Cl (s 0.23), and
/
3,5-Cl2 (s 0.74) [17,19,52], and confirms the viability of
this synthetic method with yields of 32.1 and 35% for 1
and 2, respectively.
3.2. Molecular structures
The Wꢀ
/
W bond lengths of W2((m-MeOC6H4N)2-
CH)4 (1) and W2((p-ClC6H4N)2CH)4 (2) are 2.1957(6)
˚
and 2.1924(2) A, respectively and fall into the bond
length range for quadruply bonded ditungsten com-
pounds coordinated by four bridging anionic ligands
˚
such as W2(O2CEt)4 [2.189(1) A] [18], W2(O2C(CH2)2-
˚
CH3)4 [2.194(3) A] [20], and W2(O2CC6H5)4×
/2THF
˚
[2.196(1) A] [14,21]. The tungstenꢀtungsten bond
lengths in both 1 and 2 are slightly longer than in the
/
Fig. 1. A thermal ellipsoid plot of W2((m-MeOC6H4N)2CH)4 (1)
viewed at approximately 908 to the WꢀW axis. Thermal ellipsoids for
W, C and N atoms are shown at 30% probability. H atoms are omitted
for clarity.
/
previously structurally characterized tetraformamidi-
˚
C7H8 [2.187(1) A] [17]
nate compounds W2(DFM)4×
/

220
J.L. Eglin et al. / Inorganica Chimica Acta 351 (2003) 217ꢀ
/224
Table 1
Crystal data and structure refinement for W2((m-MeOC6H4N)2CH)4
(1) and W2((p-ClC6H4N)2CH)4 (2)
Table 2
˚
Selected bond lengths (A) and bond angles (8) for W2(m-(m-
MeO)form)4
Compound
1
2
Bond lengths
Empirical formula
Formula weight
Temperature (K)
C
60H60N8O8W2
C52H36Cl8N8W2
1424.19
213(2) K
0.71073
triclinic
W(1)ꢀ
W(1)ꢀ
W(1)ꢀ
W(1)ꢀ
W(1)ꢀ
/
W(1)#1
N(1)
N(2)
2.1957(6) N(1)ꢀ
/
C(1)
C(2)
1.326(9)
1.345(9)
1.336(9)
1.326(8)
1388.86
213(2)
/
2.126(6)
2.139(6)
2.137(6)
2.141(6)
N(2)ꢀ
N(3)ꢀ
C(2)ꢀ
/
/
/
C(1)#1
˚
Wavelength (A)
0.71073
monoclinic
P21/n
/
N(3)
N(4)
/N(4)#1
Crystal system
Space group
/
¯
P1
Bond angles
N(1)ꢀW(1)ꢀ
N(1)ꢀW(1)ꢀ
N(2)ꢀW(1)ꢀ
N(1)ꢀW(1)ꢀ
N(2)ꢀW(1)ꢀ
N(3)ꢀW(1)ꢀ
N(1)ꢀW(1)ꢀ
N(2)ꢀW(1)ꢀ
N(3)ꢀW(1)ꢀ
N(4)ꢀW(1)ꢀ
Unit cell dimensions
˚
/
/
N(2)
N(3)
88.7(2)
176.5(2)
91.1(2)
90.7(2)
176.5(2)
89.2(2)
91.4(2)
92.3(2)
92.1(2)
91.2(2)
C(1)ꢀ
C(1)ꢀ
C(10)ꢀ
C(2)ꢀN(2)ꢀ
C(2)ꢀN(2)ꢀ
C(20)ꢀ
N(1)ꢀC(1)ꢀ
N(4)#1ꢀC(2)ꢀ
C(12)ꢀO(1)ꢀ
C(22)ꢀO(2)ꢀ
/
N(1)ꢀ
/
C(10)
116.2(6)
118.0(5)
124.1(4)
115.7(6)
117.0(4)
127.3(5)
121.8(7)
121.0(7)
116.3(6)
118.1(7)
a (A)
˚
15.9004(4)
9.7796(3)
18.2106(5)
90
10.3311(1)
11.4255(1)
12.5851(1)
106.731(1)
92.002(1)
114.150(1)
1278.14(2)
1
/
/
/N(1)ꢀ
/
W(1)
b (A)
˚
c (A)
/
/
N(3)
N(4)
/N(1)ꢀW(1)
/
/
/
/
/
C(20)
a (8)
b (8)
/
/
N(4)
/
/
W(1)
106.376(1)
90
/
/
N(4)
/
N(2)ꢀ
N(3)#1
N(2)
/W(1)
g (8)
/
/
W(1)#1
W(1)#1
W(1)#1
W(1)#1
/
/
3
V (A )
˚
2716.9(1)
2
1.698
/
/
/
/
Z
/
/
/
/C(16)
Dcalc (mg mꢁ3
Absorption coeffi-
)
1.850
4.960
/
/
/
/C(26)
4.295
cient (mmꢁ1
)
F(000)
1376
688
Crystal size (mm3)
u Range for data
collection (8)
0.05ꢄ
/
0.1ꢄ
/0.1
0.15ꢄ
/
0.15ꢄ0.1
/
Table 3
1.50ꢀ24.76
/
1.72ꢀ24.68
/
˚
Selected bond lengths (A) and bond angles (8) for W2(m-(p-Cl)form)4
Limiting indices
ꢁ
/
185
k 511, ꢁ
13581
/
h 5
/
18, ꢁ
/
115
/
ꢁ
/
125
/
h 5
/
12, ꢁ
/
95
/
Bond lengths
/
/215
/l 5
/17 k 5
/
13, ꢁ
/
145
/
l 5
/14
W(1)ꢀ
W(1)ꢀ
W(1)ꢀ
W(1)ꢀ
W(1)ꢀ
/
W(1)#1
N(1)
N(2)
2.1924(2)
2.162(2)
2.143(3)
2.122(2)
2.140(3))
N(1)ꢀ
N(2)ꢀ
N(3)ꢀ
N(4)ꢀ
Cl(1)ꢀ
/
C(1)
1.340(4)
1.330(4)
1.327(4)
1.331(4)
1.747(3)
Reflections col-
lected
10543
/
/C(2)
/
/C(1)#1
Independent reflec- 4650 [Rint
tions
ꢃ
/0.0680]
4235 [Rint
ꢃ
/0.0208]
/
N(3)
N(4)
/C(2)#1
/
/C(13)
Absorption correc- Semi-empirical from
tion
Semi-empirical from
psi-scans
0.914470 and 0.626279
Bond angles
N(3)ꢀW(1)ꢀ
N(3)ꢀW(1)ꢀ
N(2)ꢀW(1)ꢀ
N(3)ꢀW(1)ꢀ
N(2)ꢀW(1)ꢀ
N(4)ꢀW(1)ꢀ
N(3)ꢀW(1)ꢀ
N(2)ꢀW(1)ꢀ
psi-scans
0.9265 and 0.7042
/
/
N(2)
N(4)
86.91(10) N(4)ꢀ
/
W(1)ꢀ
W(1)ꢀ
N(1)ꢀ
N(2)ꢀ
C(1)#1ꢀ
C(2)#1ꢀ
C(12)ꢀC(13)ꢀ
/
W(1)#1
90.42(6)
90.97(6)
117.4(2)
117.1(2)
118.4(2)
119.3(2)
119.3(2)
Max. and min.
transmission
Refinement method full-matrix least-
/
/
89.47(10) N(1)ꢀ
/
/W(1)#1
/
/
N(4)
N(1)
175.39(8)
176.11(8)
90.92(9)
92.53(9)
92.35(6)
92.58(6)
C(1)ꢀ
/
/
W(1)
full-matrix least-
squares on F2
4235/0/316
/
/
C(2)ꢀ
/
/
W(1)
squares on F2
/
/
N(1)
N(1)
/
N(3)ꢀ
N(4)ꢀ
Cl(1)
/W(1)
Data/restraints/
parameters
Goodness-of-fit on 1.075
4650/0/352
/
/
/
/W(1)
/
/
W(1)#1
W(1)#1
/
/
1.002
/
/
F2
Final R indices [I ꢀ
2s(I)]
R indices (all data) R1 aꢃ
0.0881
0.614 and ꢁ
/
R1 aꢃ
0.0767
/
0.0404, wR2ꢃ
/
R1 aꢃ
0.0431
/
0.0177, wR2ꢃ
/
3.3. Structurally characterized compounds
3.3.1. Synthetic methodologies
/
0.0679, wR2ꢃ
/
R1 aꢃ
0.0436
/
0.0196, wR2ꢃ
/
Largest difference
peak and hole (e
/0.705
0.829 and ꢁ1.269
/
ꢁ3
˚
A
)
Shown in Fig. 3, a histogram of the number of crystal
structures reported for W2(II, II) compounds (Table 4)
versus the year reported provides insight into the
a
R1ꢃ
/
SjjFojꢁ
/
jFcjj/SjFoj. wR2ꢃ
/
â([(Fo2ꢁFc2)2]/S[wFo4]).
/
˚
and W2(DCPF)4 [2.1920(3) A] [19]. As in the case of the
Re2(form)4Cl2 [53] compounds, there is no correlation
between the metalꢀ
mett constant of the substituents on the ligand. The Wꢀ
N bond distances for the series 1, 2, W2(DFM)4×C7H8
/metal bond distance and the Ham-
/
/
[17] and W2(DCPF)4 [19] are in general longer than the
dimolybdenum compounds and perpetuate the trend of
shorter metalꢀ
/
metal bond distances and longer metalꢀ
/
ligand bond distances for the dimolybdenum compound
in comparison to the ditungsten compounds
[17,19,54,55].
Fig. 3. Histogram of the number of W2(II, II) complexes structurally
characterized (frequency) versus the year the structure was published.

J.L. Eglin et al. / Inorganica Chimica Acta 351 (2003) 217ꢀ
/224
221
Table 4
Structurally characterized compounds with a quadruply bonded W2(II, II) core listed by increasing Wꢀ
/W bond length
˚
2.155(2) A [7]
˚
2.1608(5) A [8]
˚
2.161(1) A [9]
˚
2.1617(4) A [58]
1. W2(dmhp)4×
2. W2(hpp)4×2NaHBEt3
3. W2(mhp)4×CH2Cl2
4. W2(hpp)4
5. W2(dmhp)4×
6. W2(map)4×2THF
7. W2(PhNPy)4×2/3THF
/
0.5(diglyme)
[dmhp represents the anion of 2,4-dimethyl-6-hydroxypyrimidine]
[hpp represents the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine]
[mhp represents the anion of 2-hydroxy-6-methylpyridine]
/
/
˚
/
(diglyme)
2.163(1) A [7]
˚
/
2.164(1) A [10]
[map represents the anion of 2-amino-6-methylpyridine]
[PhNPy represents the anion of 2-anilinopyridine]
˚
/
2.164(2) A [11]
˚
2.168(2) A [11]
˚
2.164(2) A [11]
˚
8. W2(dmhp)2[(PhN)2N]2×
9. W2(dmhp)2[(PhN)2CCH3]2×
10. W2[O2CC6H2(2,4,6-(CH3)3]4×
11. W2(chp)4
12. W2(fhp)4×
13. W2(DFM)4×
/
2THF
2.169(1) A [12]
˚
/2THF
2.174(1) A [13]
˚
/
2CH3C6H5 2.176(1) A [14]
˚
2.177(1) A [15]
[chp represents the anion of 6-chloro-2-hydroxypyridine]
[fhp represents the anion of 6-fluoro-2-hydroxypyridine]
[DFM represents the anion of di-p-tolylformamidine]
˚
/
THF
C7H8
2.185(2) A [16]
˚
/
2.187(1) A [17]
˚
14. W2(O2CEt)4
2.189(1) A [18]
2.1920(3) A [19] [DCPF represents the anion [(3,5-Cl2C6H3)2N)2CH]ꢁ
˚
]
[((p-Cl)form) represents the anion [((p-ClC6H4)N)2CH]
˚
15. W2(DCPF)4
16. W2((p-Cl)form)4
2.1924(2) A
˚
2.194(3) A [20]
˚
2.1957(6) A
17. W2(O2C(CH2)2CH3)4
18. W2((m-MeO)form)4
[((m-MeO)form) represents the anion [((m-MeOC6H4)N)2CH]
˚
19. W2(O2CC6H5)4×
20. W2[O2CC6H4(4-OCH3)]4×
21. W2(TFA)4×2/3(diglyme)
/
2THF
2.196(1) A [14,21]
˚
/2THF
2.203(1) A [14]
˚
/
2.211(2) A [4]
[TFA represents the anion of trifluoracetic acid]
˚
2.207(2) A [4]
˚
22. W2(O2Cꢀ
23. W2(TFA)4×
24. W2(N2C7H5)4×
25. W2(m-O2Cꢀt-Bu)3(O2Cꢀ
MePh2)2
26. W2(TFA)4×
/
t-Bu)4×
2Pꢀn-Bu3
2THF
/
2PPh3
2.218(1) A [22]
˚
/
/
2.224(1) A [23]
˚
2.227(2) A [24]
˚
2.2345(9) A [6]
/
[N2C7H5 is the anion of 7-azaindole]
/
/
t-Bu)(P-
˚
/2PPh3
2.240(1) A [25]
˚
2.243(1) A
˚
2.2455(5) A [28]
27. W2Cl4(NH2Cy)4
28. W2(TFA)4×3PMe3
29. Na4(TMEDA)4[W2Cl8]
˚
/
2.246(1) A [23]
˚
2.259(1) A [27]
[TMEDA is tetramethylethylenediamine]
˚
2.254(1) A [27]
˚
2.259(1) A [29]
30. W2Cl4(4ꢀ
31. W2Cl4(4ꢀ
32. W2Cl4(4ꢀ
/
t-Bu-py)4×
t-Bu-py)4×
t-Bu-py)4×
/
4ꢀ
/
t-Bu-py
˚
/
/C7H8
2.2602(8) A [29]
˚
2.2605(6) A [29]
/
/(CH3)2CO
˚
33. W2Cl4(PMe3)4
2.262(1) A [30,31]
˚
2.263(2) A [2]
34. Li4W2(CH3)x Cl8ꢁx
×
/
4THF
˚
35.W2Cl4(4ꢀ
36. b-W2Br4(dppm)2
37. Li4W2(CH3)8×4Et2O
38. W2Cl4(Pꢀn-Bu3)4×C7H8
/
t-Bu-py)4
2.2631(6) A [29]
˚
2.2632(1) A [32]
˚
/
2.264(1) A [2]
˚
/
/
2.267(1) A [33]
˚
39. W2(CCMe)2Cl2(PMe3)4
2.268(1) A [34]
˚
2.269(1) A [35]
˚
2.2728(7) A [36]
˚
2.274(2) A [37]
˚
2.2742(9) A [38]
40. b-W2Cl4(dppm)2
[dppm represents Ph2PCH2Ph2]
41. W2Cl4(PMePh2)4×
42. a-W2Cl4(dppp)2
43. W2(CCMe)4(PMe3)4
44. a-W2Cl4(dppe)2×0.5H2O
45. a-W2Cl4(dmpe)2×(toluene)
/C6H6
[dppp represents Ph2PCH2CH2CH2PPh2]
˚
/
2.281(1) A [31,39] [dppe represents Ph2PCH2CH2PPh2]
˚
/
2.287(1) A [30,31] [dmpe represents Me2PCH2CH2PMe2]
˚
2.292(1) A [26]
46. [W2{p-t-Bu-calix[4]-(O)4}2(m-Na)4]
47. W2(O2CC6H5)2I2(m-dppm)2
48. a-W2Cl4(depe)2
49. b-W2Cl4(i-Pr2PCH2CH2CH2P-i-Pr2)2
50. W2(O2CC6H5)2Br2(m-dppa)2×
˚
2.2925(6) A [40]
˚
2.2950(7) A [41] [depe represents CH3CH2PCH2CH2PCH2CH3]
˚
2.297(1) A [42]
˚
2.2986(11) A [43] [dppa represents Ph2PNHPh2]
/2THF
˚
51. b-W2Cl4(dppe)2
52. W2(C8H8)3
2.314(1) A [31,39]
˚
2.375(1) A [3]
development of this field of chemistry. With the first
structurally characterized compound in 1977,
panded towards the more classical paddlewheel com-
pounds coordinated by four anionic ligands such as the
anion of 2-hydroxy-6-methylpyridine, W2(mhp)4 [9],
Li4W2(CH3)xCl8ꢁx 4THF [2], the area quickly ex-
×
/

222
J.L. Eglin et al. / Inorganica Chimica Acta 351 (2003) 217ꢀ224
/
and similar compounds [7,10]. The paddlewheel com-
pounds were synthesized by refluxing W(CO)6 and with
the appropriate ligand [15] such as 2-hydroxy-6-methyl-
pyridine or 2,4-dimethyl-6-hydroxypyrimidine in di-
glyme to form W2(mhp)4 [9] or W2(dmhp)4 [7] with
subsequent addition of the lithiated salt of the chelating
ligand to form other derivatives [7,10,12].
In 1980, Schrock et al. developed a synthetic route to
W2Cl4P4 and W2Cl4(PP)2 complexes where P is a
monodentate phosphine ligand and PP is a bidentate
phosphine ligand [56]. Unlike the earlier preparative
methods using WCl4 as the tungsten starting material
and the addition of LiCH3 in the synthesis of
Table 5
Compounds synthesized with a W2(II, II) core
1. W2(TPP)2 [59]
[tpp represents the tetraphenylporphyrin dianion]
2. [W(OEP)]2 [60]
[OEP represents the dianion 2,3,7,8,12,13,17,18-octaethylporphyrin]
3. [W(TOEP)]2 [61,62]
[TOEP represents the dianion meso-(4?-tolyl)octaethylporphyrin]]
4. W2(mhp)2(TFA)2 [4]
5. W2(TFA)4 [25]
6. W2(TFA)4×
7. W2(TFA)4×
/
2PMe3 [23]
/2PEt3 [23]
8. W2(O2CC6H5)4 [1]
9. W2(O2CC6H4CH3)4 [1]
10. W2(O2CC6F5)4 [1]
11. W2(O2CC3F7)4 [1]
12. W2(O2CCH3)4 [18,22]
Li4W2(CH3)xCl8ꢁx
×
/
4THF [2] and Li4W2(CH3)8×
/
4Et2O
[2], or K2C8H8 for the synthesis of W2(C8H8)3 [3], WCl4
was used as the starting material and subsequently
reduced with sodium amalgam at low temperature.
Subsequent addition of the phosphine ligand provided
a general route to W2Cl4P4 compounds [56].
13. W2(O2Cꢀ/t-Bu)4 [6,18,22,63]
14. W2(O2Cꢀ
/
t-Bu)4×2PMe2Ph [6]
/
15. W2(O2C(CH2)6CH3)4 [20]
16. W2(O2C(CF2)6CF3)4 [20]
17. W2[O2CC6H4(4-CN)]4×
18. W2(O2CCCo3(CO)3)3(O2CCF3)×
19. W2(O2CCCo3(CO)3)4 [65]
20. W2(m-O2Cꢀt-Bu)4]2×2-t-BuCONMe2 [18]
21. W2(m-O2CMe)4×2MeCONMe2 [18]
2EtCONMe2 [18]
/2THF [14]
Unlike the analogous dimolybdenum chemistry, di-
tungsten chemistry did not nucleate from the prepara-
tion of ditungsten tetracarboxylates [5]. In 1981, the first
structural characterization of a ditungsten tetracarbox-
/2THF [64]
/
/
/
22. W2(m-O2CEt)4×
/
ylate was reported for W2(TFA)4×/2/3(diglyme) [4] where
23. W2(2-THCO2)4 [66]
TFA represents the anion of trifluoracetic acid.
W2(TFA)4 was synthesized by Sattelberger et al. from
the reduction of W2Cl6(THF)4 using two equivalents of
sodium amalgam followed by the subsequent addition of
NaTFA [4]. Based on this new synthetic route to
W2(O2CR)4 and related compounds [11,22], synthesis
of compounds with the W2(II, II) core expanded rapidly
[2-THCO2 represents the 2-thienylcarbonylate ligand]
24. W2(3-THCO2)4 [66]
[3-THCO2 represents the 3-thienylcarbonylate ligand]
25. W2(m-O2Cꢀ
26. W2(m-O2Cꢀ
27. W2(m-O2Cꢀ
28. [W2(m-O2Cꢀ
29. [W2(m-O2Cꢀ
30. [W2(m-O2Cꢀ
31. [W2(m-O2Cꢀ
32. [W2(m-O2Cꢀ
33. [W2(m-O2Cꢀ
/
t-Bu)(O2Cꢀ
t-Bu)(O2Cꢀ
t-Bu)2(O2Cꢀ
t-Bu)3]2(O2CCO2) [67]
/
t-Bu)3(PMe3)2 [6]
/
/t-Bu)3(PMe2Ph)2 [6]
/
/t-Bu)2(PMe3)2 [6]
/
/
t-Bu)3]2(O2C-1,4-C6F4-CO2) [67]
t-Bu)3]2(O2C-1,8-C14H8-CO2) [67]
t-Bu)3]2(O2C-1,4-C14H10-CO2) [67]
t-Bu)3]2(O2C(C5H4)Fe(C5H4)CO2) [67]
t-Bu)3]2(2,5-TH(CO2)2) [66]
/
and included W2(O2CC6H5)4×
THF [16], W2[O2CC6H2(2,4,6-(CH3)3]4×
and W2[O2CC6H4(4-OCH3)]4×2THF [14]. Subsequent
/
2THF [14,21], W2(fhp)4×
/
/
/
2CH3C6H5 [14],
/
/
/
studies included the reactivity of ditungsten tetracar-
boxylates with monodentate phosphine ligands to result
in axial or equatorial coordination of the phosphine as
[2,5-TH(CO2)2 represents 2,5-thienyldicarboxylate]
34. W2(m-O2Cꢀt-Bu)2Cl2(PMe3)2 [6]
35. W2(O2CC6H5)2Cl2(m-dppa)2×2THF [43]
36. W2(O2CC6H5)2I2(m-dppa)2×2THF [43]
37. W2(O2CC6H5)2Cl2(m-dppm)2 [40]
38. W2(O2CC6H5)2Br2(m-dppm)2 [40]
39. Na4(THF)x [W2Cl8] [63]
40. Na4(DME)4[W2Cl8] [63]
41. W2Cl4(PMePh2)4 [56]
/
/
/
in the case of W2(TFA)4×
/
2PPh3 [25] and W2(TFA)4×
/
2Pꢀ
/
n-Bu3 [23], respectively.
Quadruply bonded ditungsten tetracarboxylates may
also be synthesized via an alternative route from
reductive-elimination from 1,2-diethyl compounds with
triple bonds [18]. Specifically, 1,2-W2Et2(NMe2)4 reacts
with acid anhydrides RCOOCOR where R is Me Et,
and t-Bu to form the ditungsten tetracarboxylates in
42. W2Cl4(PMe2Ph)4 [32]
43. W2Cl4(Pꢀ
44. W2Cl4(PEt3)4 [68]
45. W2Cl4(Pꢀn-Pr3)4 [69]
/
n-Bu3)4 [56]
/
46. W2Cl4(PEt3)3(PMe3) [68]
47. W2Cl4(PEt3)2(PMe3)2 [68]
48. W2Cl4(PEt3)3(PMe2Ph) [68]
49. W2Cl4(PEt3)2(PMe2Ph)2 [68]
50. W2Cl4(PEt3)3(PMePh2) [68]
40ꢀ60% yield. The next advance in W2(II, II) chemistry
/
began in 1993 with the discovery that NaBEt3H can be
used to replace sodium amalgam in the synthesis of
W2Cl4P4, W2Cl4(PP)2, and W2(O2CR)4 complexes
[19,32,36,37,41]. In 1993, the first quadruply bonded
compound with a W2Br4 core was prepared using WBr5
as the starting material with NaBEt3H as the reducing
agent to result in a second increase in the number of
crystallographically characterized W2(II, II) com-
pounds. The number of compounds synthesized, Table
5, excluding those crystalographically characterized, has
51. W2Cl4(Pꢀ
52. W2Cl4(Pꢀ
53. W2Cl4(Pꢀ
54. W2Cl4(Pꢀ
/
n-Bu3)3(PMe3) [68]
/
n-Bu3)3(PMe2Ph) [68]
n-Bu3)2(PMe2Ph)2 [68]
n-Bu3)3(PMePh2) [68]
/
/
55. W2Br4(PMe2Ph)4 [32]
56. W2Br4(PMePh2)4 [32]
57. W2(CCꢀ/t-Bu)4(PMe3)4 [70]
58. a-W2Cl4(dppa)2×/2THF [43]

J.L. Eglin et al. / Inorganica Chimica Acta 351 (2003) 217ꢀ
/224
223
59. a-W2Cl4(dmpe)2 [56]
60. a-W2Cl4(dppe)2 [56]
5. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 200147 and 200148. Copies of
this information may be obtained free of charge from
The Director, CCDC, 12 Union Road, Cambridge, CB2
followed the same double bell shaped curve with
significant increases in the late 1970s to early 1980s
and again in the mid 1990s and reflects the development
of new synthetic methodologies in W2(II, II) chemistry.
Note that Table 5 includes all W2(II, II) compounds
reported in the literature and includes redundant core
structures if the coordinating solvent or absence of a
solvent molecule is different than in the structurally
characterized compound.
1EZ, UK (fax: ꢂ44-1223-336-033; e-mail: deposit@
/
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
J.L. Eglin and L.T. Smith would like to acknowledge
the NSF for research support, grant no. NSF-EPS
9874669. NMR measurements were made at the Mis-
sissippi Magnetic Resonance Facility at Mississippi
State University supported by the NSF grant no.
CHE-9214521.
3.4. Structural trends
The list of structurally characterized compounds in
Table 4 includes all quadruply bonded W2(II, II)
compounds without restrictions to ligand types or
structural motifs. Shown in Table 4, the tungstenꢀ
/
References
tungsten bond distances for the structurally character-
ized W2(II, II) compounds fall into several distinct
categories. Compounds with the classic paddlewheel
geometry both with and without axial or equatorial
coordination of neutral ligands represent the category
[1] F.A. Cotton, M. Jeremic, Synth. Inorg. Metalꢀ
/Org. Chem. 1
(1971) 265.
[2] D.M. Collins, F.A. Cotton, S.A. Koch, M. Millar, C.A. Murillo,
Inorg. Chem. 17 (1978) 2017.
[3] F.A. Cotton, S.A. Koch, J. Am. Chem. Soc. 99 (1977) 7371.
[4] A.P. Sattelberger, K.W. McLaughlin, J.C. Huffman, J. Am.
Chem. Soc. 103 (1981) 2880.
with the shortest metalꢀmetal bond distances (2.155(2)
/
˚
[7] to 2.246(1) A [23]). Without bridging ligands,
compounds with either eight anionic ligands or four
anionic ligands and four neutral ligands in the coordi-
[5] F.A. Cotton, R.A. Walton, Multiple Bonds between Metal
Atoms, 2nd ed., Oxford University Press, Oxford, 1993.
[6] P. Mountford, J.A.G. Williams, J. Chem. Soc., Dalton Trans
(1993) 877.
˚
nation sites (2.2455(5) [28] to 2.314(1) A [31,39])
represent the next category with the neutral ligands
composed exclusively of either phosphine or amine
ligands. Within this category, complexes with bidentate
[7] F.A. Cotton, R.H. Niswander, J.C. Sekutowski, Inorg. Chem. 18
(1979) 1152.
[8] F.A. Cotton, P. Huang, C.A. Murillo, D.J. Timmons, Inorg.
Chem. Commun. 5 (2002) 501.
phosphine ligands have longer tungstenꢀ
bonds due a torsion angle providing deviation from the
optimum eclipsed geometry of the s2p4d2 bond [30ꢀ
32,35,37,39,41,42]. The longest WꢀW bond length is
/tungsten metal
[9] F.A. Cotton, P.E. Fanwick, R.H. Niswander, J. Sekutowski, J.
Am. Chem. Soc. 100 (1978) 4725.
/
[10] F.A. Cotton, R.H. Niswander, J.C. Sekutowski, Inorg. Chem. 17
(1978) 3541.
/
˚
[11] A.R. Chakravarty, F.A. Cotton, E.S. Shamshoum, Inorg. Chem.
23 (1984) 4216.
2.375(1) A for the air-stable compound W2(C8H8)3 [3].
[12] F.A. Cotton, W.H. Ilsley, W. Kaim, Inorg. Chem. 19 (1980) 1450.
[13] F.A. Cotton, W.H. Ilsley, W. Kaim, Inorg. Chem. 18 (1979) 3569.
[14] F.A. Cotton, W. Wang, Inorg. Chem. 23 (1984) 1604.
[15] F.A. Cotton, W.H. Ilsley, W. Kaim, Inorg. Chem. 19 (1980) 1453.
[16] F.A. Cotton, L.R. Falvello, S. Han, W. Wang, Inorg. Chem. 22
(1983) 4106.
4. Conclusions
A recent compilation of 467 structures containing a
[17] F.A. Cotton, T. Ren, J. Am. Chem. Soc. 114 (1992) 2237.
[18] M.H. Chisholm, H.T. Chiu, J.C. Huffman, Polyhedron 3 (1984)
759.
Moꢀ
/
Mo quadruple bond indicated distances ranging
˚
from 2.04 to 2.24 A with a ‘normal’ range of 2.06 to 2.17
[19] K.M. Carlson-Day, J.L. Eglin, L.T. Smith, R.J. Staples, Inorg.
Chem. 38 (1999) 2216.
˚
A [57]. While there are only 52 structures of compounds
with a WꢀW quadruple bond, the range in bond lengths
is from 2.155(2) [7] to 2.375(1) A [3]; slightly longer on
average than the analogous MoꢀMo bond distances but
encompassing a surprisingly similar range (0.2 A) in the
metalꢀmetal bond distance.
/
[20] D. Baxter, R. Cayton, M.H. Chisholm, J.C. Huffman, E. Putilina,
S. Tagg, J. Wesemann, J. Zwanziger, F. Darrington, J. Am.
Chem. Soc. 116 (1994) 4551.
˚
/
[21] F.A. Cotton, W. Wang, Inorg. Chem. 21 (1982) 3859.
[22] D.J. Santure, J.C. Huffman, A.P. Sattelberger, Inorg. Chem. 24
(1985) 371.
˚
/

224
J.L. Eglin et al. / Inorganica Chimica Acta 351 (2003) 217ꢀ224
/
[23] D.J. Santure, J.C. Huffman, A.P. Sattelberger, Inorg. Chem. 23
(1984) 938.
[49] G.M. Sheldrick, SHELXS-90, Program for the Solution of Crystal
Structures, University of Gottingen, Gottingen, Germany, 1986.
¨
¨
[24] F.A. Cotton, L.R. Falvello, W.N. Wang, Inorg. Chim. Acta 261
(1997) 77.
[50] G.M. Sheldrick, SHELXL-93, Program for the Refinement of
Crystal Structures, University of Gottingen, Gottingen, Germany,
¨
¨
[25] D.J. Santure, K.W. McLaughlin, J.C. Huffman, A.P. Sattelber-
ger, Inorg. Chem. 22 (1983) 1877.
1993.
[51] SHELXTL 5.03 (PC-Version), Program Library for Structure
Solution and Molecular Graphics, Siemens Analytical Instru-
ments Division, Madison, WI, 1995.
[26] L. Giannini, E. Solari, C. Floriani, N. Re, A. Chiesi-Villa, C.
Rizzoli, Inorg. Chem. 38 (1999) 1438.
[27] F.A. Cotton, G.N. Mott, J. Am. Chem. Soc. 104 (1982) 6781.
[28] F.A. Cotton, E.V. Dikarev, S. Herrero, Inorg. Chem. Commun. 2
(1999) 98.
[52] L.P. Hammett, Physical Organic Chemistry, McGraw-Hill, New
York, 1970.
[53] J.L. Eglin, C. Lin, T. Ren, L. Smith, R.J. Staples, D.O. Wipf, Eur.
J. Inorg. Chem. (1999) 2095.
[29] F.A. Cotton, E.V. Dikarev, J.D. Gu, S. Herrero, B. Modec,
Inorg. Chem. 39 (2000) 5407.
[54] C. Lin, J.D. Protasiewicz, E.T. Smith, T. Ren, Inorg. Chem. 35
(1996) 6422.
[30] F.A. Cotton, M.W. Extine, T.R. Felthouse, B.W.S. Kolthammer,
D.G. Lay, J. Am. Chem. Soc. 103 (1981) 4040.
[31] F.A. Cotton, T.R. Felthouse, D.G. Lay, J. Am. Chem. Soc. 102
(1980) 1431.
[55] C. Lin, J.D. Protasiewicz, E.T. Smith, T. Ren, J. Chem. Soc.,
Chem. Commun. (1995) 2257.
[56] P.R. Sharp, R.R. Schrock, J. Am. Chem. Soc. 102 (1980) 1430.
[57] F.A. Cotton, L.M. Daniels, E.A. Hillard, C.A. Murillo, Inorg.
Chem. 41 (2002) 2466.
[32] F.A. Cotton, J.L. Eglin, C.A. James, Inorg. Chem. 32 (1993) 681.
[33] F.A. Cotton, J.G. Jennings, A.C. Price, K. Vidyasagar, Inorg.
Chem. 29 (1990) 4138.
[58] F.A. Cotton, P. Huang, C.A. Murillo, X. Wang, Inorg. Chem.
Commun. 6 (2003) 121.
[34] T.C. Stoner, S.J. Geib, M.D. Hopkins, J. Am. Chem. Soc. 114
(1992) 4201.
[59] J.C. Kim, V.L. Goedken, B.M. Lee, Polyhedron 15 (1995) 57.
[60] J.P. Collman, J.M. Garner, L.K. Woo, J. Am. Chem. Soc. 111
(1989) 8141.
[35] J.M. Canich, F.A. Cotton, Inorg. Chim. Acta 142 (1988) 69.
[36] F.A. Cotton, J.L. Eglin, C.A. James, Acta Crystallogr., C 49
(1993) 893.
[61] J.P. Collman, S.T. Harford, S. Franzen, T.A. Eberspacher, R.K.
Shoemaker, W.H. Woodruff, J. Am. Chem. Soc. 120 (1998) 1456.
[62] J.P. Collman, J.M. Garner, R.T. Hembre, Y. Ha, J. Am. Chem.
Soc. 114 (1992) 1292.
[37] J.L. Eglin, E.J. Valente, K.R. Winfield, J.D. Zubkowski, Inorg.
Chim. Acta 245 (1996) 81.
[38] K.D. John, V.M. Miskowski, M.A. Vance, R.F. Dallinger, L.C.
Wang, S.J. Geib, M.D. Hopkins, Inorg. Chem. 37 (1998) 6858.
[39] F.A. Cotton, T. Felthouse, Inorg. Chem. 20 (1981) 3880.
[40] K.M. Carlson-Day, J.L. Eglin, C. Lin, T. Ren, E.J. Valente, J.D.
Zubkowski, Inorg. Chem. 35 (1996) 4727.
[63] R.R. Schrock, L.G. Sturgeoff, P.R. Sharp, Inorg. Chem. 22
(1983) 2801.
[64] V. Calvo-Perez, T.P. Fehlner, A.L. Rheingold, Inorg. Chem. 35
(1996) 7289.
[41] K.M. Carlson-Day, J.L. Eglin, K.M. Huntington, R.J. Staples,
Inorg. Chim. Acta 271 (1998) 49.
[65] T.P. Fehlner, V. Calvo-Perez, W. Cen, J. Electron Spectrosc.
Related Phenomena 66 (1993) 29.
[42] M.D. Fryzuk, C.G. Kreiter, W.S. Sheldrick, Chem. Ber. 122
(1989) 851.
[66] M.J. Byrnes, M.H. Chisholm, J. Chem. Soc., Chem. Commun.
(2002) 2040.
[43] J.L. Eglin, L.T. Smith, R.J. Staples, E.J. Valente, J.D. Zub-
kowski, J. Organomet. Chem. 596 (2000) 136.
[67] R.H. Cayton, M.H. Chisholm, J.C. Huffman, E.B. Lobkovsky, J.
Am. Chem. Soc. 113 (1991) 8709.
[44] M.A.S. King, R.E. McCarley, Inorg. Chem. 12 (1973) 1972.
[45] W. Bradley, I. Wright, J. Chem. Soc. (1956) 640.
[46] SMART V 4.043, Software for the CCD Detector System, Siemens
Analytical Instruments Division, Madison, WI, 1996.
[47] SAINT V 4.035, Software for the CCD Detector System, Siemens
Analytical Instruments Division, Madison, WI, 1995.
[48] R.H. Blessing, SADABS, Acta Crystallogr., A 51 (1995) 33.
[68] M.H. Chisholm, J.M. McInnes, J. Chem. Soc., Dalton Trans.
(1997) 2735.
[69] P.E. Fanwick, W.S. Harwood, R.A. Walton, Inorg. Chem. 26
(1987) 242.
[70] T. Stoner, W.P. Schaefer, R.E. Marsh, M.D. Hopkins, J. Cluster
Sci. 5 (1994) 107.