Coordination complexes of bivalent ansa-ytterbocenes: Synthesis, structure and comparison with related unbridged ytterbocenes and ansa-ferrocenes

Article abstract of DOI:10.1039/b418550a

The dimethylsilyl bridged bis(cyclopentadienide) ligands, {[2,4-(Me ₃C)₂C₅H₂]₂SiMe ₂}^2- and {[(Me₃Si)₂C ₅H₂]₂SiMe₂}^2-, have been prepared and used for the preparation of ansa-metallocene derivatives of iron and bivalent ytterbium metallocene adducts. The structures of two ansa-ferrocenes, as well as that of the free tetraene [2,4-(Me ₃C)₂C₅H₃]₂SiMe ₂, have been determined by X-ray crystallography; the lability of the Me₃Si group leads to a different ring substitution pattern in the structure of ansa-{[3,4-(Me₃Si)₂C₅H ₂]₂SiMe₂}Fe compared with that in ansa-{[2,4-(Me₃C)₂C₅H₂] ₂SiMe₂}Fe. The structures of ansa-{[2,4-(Me ₃C)₂C₅H₂]₂SiMe ₂}Yb(OEt₂) and an analogous ansa-ytterbocene isocyanide complex are also reported, and compared with those of non-bridged ytterbocene diethyl ether complexes. Short distances from the ytterbium atom to the diethyl ether β carbon atoms are observed in several structures, consistent with the tendency of ytterbium to maximise its coordination number. Spectroscopic properties for the ansa-ytterbocene etherates and the isocyanide complex are reported and compared with the corresponding adducts of non-bridged ytterbocenes. The reduced Cp(centroid)-metal-Cp(centroid) angle enforced by the ansa-bridge does not lead to significant changes in the optical spectra compared with unbridged metallocene diethyl ether and tetrahydrofuran adducts, but significant changes are observed in the isocyanide adducts; a model rationalising the red and blue shifts of λ_max is proposed. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2005.

Full text of DOI:10.1039/b418550a

P A P E R
Coordination complexes of bivalent ansa-ytterbocenes: synthesis,
structure and comparison with related unbridged ytterbocenes and
ansa-ferrocenes
Madeleine Schultz, Chadwick D. Sofield, Marc D. Walter and Richard A. Andersen*
Chemistry Department and Chemical Sciences Division, Lawrence Berkeley National
Laboratory, University of California, Berkeley, CA, 94720, USA. E-mail: raandersen@lbl.gov;
Fax: +1 510 486 5596; Tel: +1 510 642 4452
Received (in Montpellier, France) 9th December 2004, Accepted 11th March 2005
First published as an Advance Article on the web 20th May 2005
The dimethylsilyl bridged bis(cyclopentadienide) ligands, {[2,4-(Me3C)2C5H2]2SiMe2}2ꢀ and
{[(Me3Si)2C5H2]2SiMe2}2ꢀ, have been prepared and used for the preparation of ansa-metallocene derivatives
of iron and bivalent ytterbium metallocene adducts. The structures of two ansa-ferrocenes, as well as that of
the free tetraene [2,4-(Me3C)2C5H3]2SiMe2, have been determined by X-ray crystallography; the lability of
the Me3Si group leads to a different ring substitution pattern in the structure of ansa-{[3,4-
(Me3Si)2C5H2]2SiMe2}Fe compared with that in ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Fe. The structures of ansa-
{[2,4-(Me3C)2C5H2]2SiMe2}Yb(OEt2) and an analogous ansa-ytterbocene isocyanide complex are also
reported, and compared with those of non-bridged ytterbocene diethyl ether complexes. Short distances from
the ytterbium atom to the diethyl ether b carbon atoms are observed in several structures, consistent with the
tendency of ytterbium to maximise its coordination number. Spectroscopic properties for the ansa-
ytterbocene etherates and the isocyanide complex are reported and compared with the corresponding adducts
of non-bridged ytterbocenes. The reduced Cp(centroid)–metal–Cp(centroid) angle enforced by the ansa-
bridge does not lead to significant changes in the optical spectra compared with unbridged metallocene
diethyl ether and tetrahydrofuran adducts, but significant changes are observed in the isocyanide adducts; a
model rationalising the red and blue shifts of lmax is proposed.
angle in an ansa-metallocene is smaller than in an unbridged
Introduction
metallocene, an ansa-ytterbocene became a synthetic target. In
this paper we describe synthetic, X-ray crystallographic and
spectroscopic data of several ansa-{[2,4-(Me3C)2C5H2]2
SiMe2}Yb(L) adducts, as well as the ligand synthesis and
related ferrocene complexes, which were prepared in order to
confirm the connectivity of the ansa ligand. The optical spectra
of the diethyl ether and THF adducts of the bridged and some
of the unbridged ytterbocenes were studied and compared with
the molecular structures of the diethyl ether adducts of
ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb, (Me5C5)2Yb and [1,3-
(Me3C)2C5H3]2Yb, determined by X-ray crystallography, in
order to rationalise how the colours change as a function of
bend angle.
The design and use of ‘‘ansa-cyclopentadienyl’’ ligands to
direct and control the stereochemistry at metal centres in
polymerisation and other catalytic reactions is a commercial
development of great importance.1,2 This development has
largely been concentrated on the group 4 metals in general
and on zirconium cations in particular, but the neutral group 3
metals and lanthanides have also been studied in this con-
text.3,4 Although many trivalent lanthanide ansa-metallocenes
have been prepared and their potential as catalysts for several
transformations has been explored,5 only a few ansa-metallo-
cenes have been described for the bivalent lanthanides; several
are based on Sm(II)6 and only two on Yb(II).7,8 The samar-
ium(II) ansa-metallocene, {[2,4-(Me3Si)2C5H2][3,4-(Me3Si)2
C5H2]SiMe2}Sm(THF)2, with C1 symmetry, has been obtained
from the reaction of the dipotassium salt of the ligand,
generated by deprotonation of the tetraene [(Me3Si)2C5H3]2-
Results and discussion
Synthesis and structures
n
SiMe2 with BuLi and KOtBu, with SmI2 in THF.6 When the
Ligands and ferrocenes. The tetraene, [(Me3Si)2C5H3]2SiMe2,
was obtained as a mixture of isomers by reaction of the
magnesocene [(Me3Si)2C5H3]2Mg with SiBr2Me2 (the ‘‘magne-
socene route’’), and the mixture was converted to the ansa-
magnesocene, which was then allowed to react with
FeBr2(THF)2 in THF [eqn. (1)].
bridge is Ph2Si rather than Me2Si, the C2v symmetric samar-
ocene, ansa-{[3,4-(Me3Si)2C5H2]2SiPh2}Sm(THF)2, was ob-
tained;6 thus, the regiochemistry depends upon the barrier
for the silatropic shifts of the silyl substituents.
The Lewis acidity of base-free ytterbocenes, specifically
(Me5C5)2Yb, has been studied in order to develop a bond
model for weak metal-to-ligand bonding when the Lewis base
is, for example, CO.9–12 A feature of the model is that as the
Cp(centroid)–M–Cp(centroid) angle gets smaller the develop-
ing dipole moment, with the positive end located on the metal
centre, gets larger, increasing the electrostatic interaction be-
tween the metal fragment and the ligand dipole, so the bond
becomes stronger. Since the Cp(centroid)–M–Cp(centroid)
ð1Þ
A single isomer of the extremely soluble ferrocene, ansa-{[3,4-
(Me3Si)2C5H2]2SiMe2}Fe, was isolated in low yield by crystal-
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 1 9 – 9 2 7
919

Table 1 Selected bond distances (A) and bond angles (1) of ansa-
ferrocenes
ansa-{[2,4-
(Me3C)2
C5H2]2SiMe2}Fe
ansa-{[3,4-
(Me3Si)2C5H2]2SiMe2}Fe
Fe–C (mean)
Fe–C (range)
2.07
2.027(3)–2.129(3)
2.06
2.015(2)–2.097(2)
Fe–centroid
Centroid–Fe–
1.67
160.5
1.66
165.5
Fig. 1 ORTEP diagram of ansa-{[3,4-(Me3Si)2C5H2]2SiMe2}Fe (50%
probability ellipsoids).
centroid
C(ring)–Si—
C(ring)
92.8(1)
96.7(1)
a
b
g
26.3(2)
56.7(1)
92.8(1)
160.5
20.3(1)
52.1(1)
96.7(1)
165.5
lisation from hexane as large red-purple crystals. This sequence
of synthetic reactions that lead to the synthesis and structure of
the ansa-ferrocene was developed as a structural proof of the
synthetic methodology: to confirm that the magnesocene func-
tions as a transfer agent, although the connectivity of the
magnesocene is not established, and to ascertain what arrange-
ment of Me3Si groups is observed in the product. The simulta-
neous formation of another product with different
connectivity, which does not crystallise, cannot be ruled out,
although no evidence for a mixture was observed. An ORTEP
diagram is shown in Fig. 1 and important bond distances and
angles are in Table 1; data collection and structure solution
parameters are in Table 2.
Inspection of the structure shows that the idealised symme-
try is Cs and therefore the meso isomer crystallised. The 1H
NMR spectrum in C6D6 is consistent with the crystallographic
result. As can be seen from Fig. 1, the SiMe2 bridge enforces an
eclipsed geometry of the two cyclopentadienyl rings, and the
meso configuration forces the substituents to be eclipsed as
well, in what appears to be a sterically demanding arrange-
ment.
d
The ferrocene crystallises from hexane as red crystals whose
solid state structure is shown in Fig. 3; bond lengths and angles
are listed in Table 1 and data collection and structure solution
parameters are included in Table 2.
The structure has idealised C2 symmetry and therefore the
rac isomer crystallised; in contrast to the Me3Si derivative, the
enforced eclipsed conformation of the Cp rings now results in
the ring substituents lying between each other, in what appears
1
a less sterically demanding configuration. The H NMR spec-
trum is consistent with C2 symmetry in C6D6 solution. Isola-
tion of the meso-ferrocene isomer when the substituents are
Me3Si and the rac-ferrocene isomer when the substituents are
Me3C presumably is related to the lower barrier for Me3Si
fluxional motion relative to Me3C shifts in neutral and anionic
cyclopentadienyl compounds.15
The Me3C analogue of the Me3Si derivative has been used to
prepare the zirconocene dichloride and dicarbonyl derivatives,
but no synthetic details have been published.13,14 We have
prepared the tetraene, [2,4-(Me3C)2C5H3]2SiMe2, by the mag-
nesocene route. This tetraene is obtained as colourless crystals
from hexane and the structure was determined by X-ray
crystallography; an ORTEP diagram is shown in Fig. 2 with
important structural parameters listed in the figure caption,
while data collection and structure solution parameters are
given in Table 2.
Ytterbocenes. The ansa-magnesocenes are not useful as syn-
thons for the ytterbocenes because they do not react with YbI2
in diethyl ether, or under any other conditions examined.
However, the disodium salt of the Me3C tetraene, obtained
by reaction of the tetraene with NaNH2, is a useful synthon.
Thus, the diethyl ether complex ansa-{[2,4-(Me3C)2C5H2]2-
SiMe2}Yb(OEt2) can be synthesised by reaction of the sodium
salt of the substituted cyclopentadienide with ytterbous iodide
in diethyl ether, followed by filtration and crystallisation from
the deep green mother liquor [eqn. (3)]:
1
The H NMR spectrum of this molecule in C6D6 at 25 1C
shows only one isomer to be present in solution and the
structure shows it to be the isomer in which the Me3C groups
are in the 2- and 4-positions in both rings. The tetraene was
converted to the magnesocene and then to the highly soluble
ferrocene, ansa-{[2,4-(Me3C)2C5H3]2SiMe2}Fe, as a structural
proof [eqn. (2)].
Na2Cp02 + YbI2 - Cp02Yb(OEt2) + 2NaI
(3)
where Cp02 = {[2,4-(Me3C)2C5H2]2SiMe2}2ꢀ. It is important
that THF-free reagent is used, otherwise a mixture of the
diethyl ether and THF complexes will result. The latter is
due to residual THF bound to the sodium salt of the ligand,
which can be removed by prolonged exposure of the powdered
sodium salt to dynamic vacuum. Preparation of the pure THF
complex results from the reaction described in eqn. (3), when
THF is used as solvent.
The etherate ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(OEt2)
cannot be desolvated by any of the published methods used
to prepare base-free ytterbocenes.16 Attempted sublimation
leads to an insoluble brown solid, as does the toluene reflux
method. The tetraene does not react with Yb[N(SiMe3)2]2 in
toluene.
ð2Þ
920
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 1 9 – 9 2 7

Table 2 Selected data collection and structure solution parameters for ansa-ferrocenes and [2,4-(Me3C)2C5H3]SiMe2
Compound
ansa-{[3,4-(Me3Si)2C5H2]2SiMe2}Fe
[2,4-(Me3C)2C5H3]2SiMe2
ansa-{[2,4-(Me3C)2C5H3]2SiMe2}Fe
Formula
FW
C24H46FeSi5
530.91
C28H48Si
412.75
C28H46FeSi
466.59
Space group
a/A
C2/c (#15)
15.507(1)
12.067(1)
32.668(1)
96.810(1)
6070.09(9)
8
P21/c (#14)
9.123(1)
16.806(1)
18.725(1)
76.758(1)
2794.68(7)
4
C2/c (#15)
31.016(1)
9.907(1)
19.618(1)
117.403(1)
5351.4(1)
8
b/A
c/A
b/1
U/A3
Z
dcalc/g cmꢀ3
m(Mo-Ka)calc/mmꢀ1
Size/mm3
1.162
0.70
0.981
0.09
1.158
0.62
0.32 ꢂ 0.28 ꢂ 0.18
159
4–45
0.39 ꢂ 0.33 ꢂ 0.13
176
6–46.5
0.33 ꢂ 0.28 ꢂ 0.28
98
4–52.2
T/K
Scan range/1
Reflections integrated
Unique reflections
Rint
12365
4353
11363
3985
12202
4722
0.040
3812
0.031
3501
0.030
4180
Obs. reflections
[F2o 4 2s(F2o)]
Variables
285
276
285
Transmission range
R
Rw
0.884–0.801
0.042
0.098
0.052
1.169
0.34
0.988–0.820
0.0464
0.1139
0.0540
1.089
0.8454–0.8214
0.042
0.105
Rall
GOF
0.049
1.02
0.76
Max peak in final difference
map/e Aꢀ3
0.30
Min peak in final difference
map/e Aꢀ3
ꢀ0.23
ꢀ0.22
ꢀ0.45
The structure of the diethyl ether complex, ansa-{[2,4-
(Me3C)2C5H2]2SiMe2}Yb(OEt2), was determined for compar-
ison with the known structures of (Me5C5)2Yb(OEt2)17 and
[1,3-(Me3C)2C5H3]2Yb(OEt2).18 A room temperature structure
of the latter has been reported, but the low temperature data
set collected in this work will be used for comparison as it was
obtained under identical conditions to the other structures
described here. The published structure is essentially the same
as that determined in this work; the space group is the same
and the important bond distances and angles are within 1% of
those described here. Table 3 contains important bond dis-
tances and angles for the three structures and Table 4 contains
important data collection and structure solution parameters,
while Figs. 4 and 5 show ORTEP diagrams of the diethyl ether
complexes.
coordination.19 The geometries about the metal centre are not
surprising, and the most interesting feature in these structures
is the disposition of the diethyl ether ligand. In (Me5C5)2Yb
(OEt2), one of the g-Me groups of the ether ligand is bent back
towards the metal centre,17 with an ytterbiumꢁ ꢁ ꢁcarbon dis-
tance of 3.23(1) A, which is significantly shorter than the sum
of the van der Waals radii of a methyl group (2.00 A) and the
metallic radius of Yb(II) (1.70 A).20 This type of interaction is
typical of lanthanide complexes, which generally display the
highest possible coordination number without causing undue
steric congestion.16 The complex ansa-{[2,4-(Me3C)2C5H2]2Si-
Me2}Yb(OEt2) crystallises with three unique molecules in the
asymmetric unit; one of which is shown in Fig. 4. Each of these
molecules has one short Ybꢁ ꢁ ꢁg-Me distance; in molecule 1,
Yb(1)ꢁ ꢁ ꢁC(29) is 3.14 A; in molecules 2 and 3 the equivalent
distances are 3.15 and 3.00 A, respectively. These distances are
only about 10% longer than the longest ring carbon–ytterbium
distances observed in typical Yb(II) metallocenes.16,21–26 The
The structures of the diethyl ether complexes involve typical
Yb(II) bond distances to the cyclopentadienyl ligands in 7-
Fig. 2 ORTEP diagram of [2,4-(Me3C)2C5H3]2SiMe2 (50% probabil-
ity ellipsoids). Selected distances (A) and angles (1): C1–Si1 1.928(2);
C14–Si1 1.925(2); C27–Si1 1.871(2); C28–Si1 1.878(2); C1–Si1–C14
105.24(8); C27–Si1–C28 113.0(1); C1–Si1–C27 113.4(1).
Fig. 3 ORTEP diagram of ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Fe (50%
probability ellipsoids).
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 1 9 – 9 2 7
921

Table 3 Selected bond distances (A) and bond angles (1) of the diethyl ether adducts Cp02Yb(OEt2) and the ansa-ytterbocene isocyanide complex
ansa-{[2,4-(Me3C)2
C5H2]2SiMe2}Yb(OEt2)a
[1,3-(Me3C)2C5H3]2
Yb(OEt2)
ansa-{[2,4-(Me3C)2 C5H2]2
SiMe2}Yb(CNC8H9)2
(Me5C5)2Yb(OEt2)b
M–C (mean)
M–C (range)
2.71
2.61(1)–2.80(1)
2.43
2.71
2.651(6)–2.765(6)
2.44
2.69
2.66(1)–2.729(7)
2.41
2.71
2.63(1)–2.81(1)
2.43
2.57
M–centroid
M–O (CN)
2.44(1); 2.411(9); 2.36(1)
123.8
103.2(7)
2.442(4)
131.8
2.467(7)
140.6
Cp(centroid)–Yb–Cp(centroid)
C(ring)–Si–C(ring)
Intramolecular contact
a
123.1
103.4(5)
3.14(1); 3.15(1); 3.00(1)
None
3.23(1)
b
Data for unique molecules in the asymmetric unit presented as Yb(1); Yb(2); Yb(3). From ref. 17.
ytterbium atom is exposed to this interaction because of the
ansa-bridge, which holds the metallocene wedge open with a
Cp(centroid)–Yb–Cp(centroid) angle of 1241, and allows the
close approach of the g-Me groups of the ether ligand without
undue steric congestion between the ligands.
of the wedge, one on each cyclopentadienide ring. Similar
intramolecular interactions are observed in the solid state
structure of the base-free analogue [1,3-(Me3C)2C5H3]2Yb, in
contrast to other base-free ytterbocenes, which crystallise with
short intermolecular Yb–C distances.16
In the structure of [1,3-(Me3C)2C5H3]2Yb(OEt2), the ether
ligand is twisted so that both g-Me groups of the ligand point
away from the metal (Fig. 5). The Cp(centroid)–Yb–Cp(cen-
troid) angle of 1321 in [1,3-(Me3C)2C5H3]2Yb(OEt2) is 81
smaller than in (Me5C5)2Yb(OEt2), suggesting that the Cp
rings can rotate out of each others’ way in order to bend more
than in the pentamethylcyclopentadienide analogue, which is
less flexible. However, the bulky Me3C groups at the front of
the wedge presumably prevent the close approach of the g-Me
groups of the ether ligand. While no non-bonded me-
talꢁ ꢁ ꢁcarbon distance of less than 3.5 A is observed in this
structure, two of the methyl carbon atoms of the Me3C groups
approach the metal within the sum of the van der Waals radii;
the Ybꢁ ꢁ ꢁC(13) distance is 3.67 A and the Ybꢁ ꢁ ꢁC(24) distance
is 3.59 A. These two carbon atoms are located at the same side
The diethyl ether structures provide evidence that any avail-
able interaction will occur to reduce coordinative unsaturation
in a bivalent ytterbocene. There is not enough room within the
coordination sphere for a second diethyl ether ligand, so the
Ybꢁ ꢁ ꢁg-Me interaction occurs. The various conformations of
the ethyl groups in the diethyl ether adducts in the solid state
are also consistent with the idea that the specific conformations
are energetically similar and the observed conformation is the
one that minimises all repulsive interactions while maximising
the attractive ones.16
The xylyl isocyanide complex of ansa-{[2,4-(Me3C)2C5H2]2-
SiMe2}Yb can be prepared from either the diethyl ether adduct
or the THF adduct by stirring the ytterbocene with the
isocyanide in toluene. The resulting 1 : 2 adduct is relatively
insoluble in aromatic and aliphatic solvents and can be crystal-
Table 4 Selected data collection and structure solution parameters for diethyl ether adducts Cp0 Yb(OEt2) and ansa-{[2,4-(Me3C)2C5H2]2
SiMe2}Yb(2,6-Me2C6H3NC)2
2
ansa-{[2,4-(Me3C)2C5H2]2
SiMe2}Yb(OEt2)
[1,3-(Me3C)2C5H3]2
Yb(OEt2)
ansa-{[2,4-(Me3C)2C5H2]2SiMe2}
Yb(2,6-Me2C6H3NC)2
Compound
Formula
FW
YbSiC32H56
657.92
O
YbC30H52
601.78
O
YbSiN2C46H64
846.15
Space group
a/A
b/A
P212121 (#19)
29.0586(5)
15.9398(2)
24.5206(4)
P21/n (#14)
11.1771(2)
19.1123(1)
14.0739(2)
90.160(1)
3006.47(8)
4
P21/n (#14)
11.8426(6)
16.4380(9)
22.338(1)
99.068(1)
4294.1(3)
4
c/A
b(1)
U/A3
Z
11357.7(3)
12
1.154
d
calc/g cmꢀ3
1.329
31.28
1.309
22.37
m(Mo-Ka)calc/cmꢀ1
25.19
0.60 ꢂ 0.10 ꢂ 0.08
156
Size/mm3
T/K
0.26 ꢂ 0.25 ꢂ 0.15
131
0.36 ꢂ 0.11 ꢂ 0.08
163
4–52.3
Scan range/1
Reflections integrated
Unique reflections
Rint
Obs. reflections [F2o 4 3s(Fo2)]
Variables
4–46.6
47621
16331
0.070
4–52.2
14404
5451
17261
6369
0.045
3667
289
0.083
3243
444
9671
1017
Transmission range
R
Rw
0.928–0.787
0.046
0.047
0.989–0.796
0.033
0.039
0.977–0.626
0.045
0.051
Rall
GOF
0.088
1.24
2.12
0.045
1.29
1.52
0.102
1.18
1.20
Max peak in final
difference map/e Aꢀ3
Min peak in final
difference map/e Aꢀ3
ꢀ1.23
ꢀ0.71
ꢀ1.42
922
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 1 9 – 9 2 7

Fig.
4
ORTEP diagram of ansa-{[2,4-(Me3C)2C5H2]2SiMe2}
Yb(OEt2) (50% probability ellipsoids; only one of three unique mole-
cules shown).
lised from toluene as dark red blocks. The infrared spectrum of
the isocyanide complex contains a band at 2127 cmꢀ1 for the
CN stretch, which is 9 cmꢀ1 higher than the free isocyanide.
The single, sharp CN stretch in both the solid state and toluene
solution is in accord with those previously described for 1 : 2
complexes of unbridged ytterbocenes.9 The structure of the
ansa-ytterbocene isocyanide complex was determined for com-
parison with the published ytterbocene isocyanide complexes.9
Fig. 6 shows an ORTEP diagram, and important bond dis-
tances and angles are presented in Table 3 and the figure
caption, while data collection and structure solution para-
meters are included in Table 4. The structure exhibits the
expected reduction in Cp(centroid)–Yb–Cp(centroid) angle
due to the ansa-bridge. The arrangement of the xylyl isocya-
nide ligands is not affected, however; the averaged Yb–C
(isocyanide) distance of 2.57 A is slightly shorter than that of
2.61 A in [1,3-(Me3C)2C5H3]2Yb(CN-2,6-Me2C6H3), while the
angle between the isocyanide ligands is identical.
Fig. 6 ORTEP diagram of ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(2,6-
Me2C6H3NC)2 (only one of the disordered Me3C groups shown; 50%
probability ellipsoids). Selected bond distances (A) and angles (1) not
given in Table 3: CCNR–N 1.15; N–CR 1.44; CCNR–Yb–CCNR 78.5(4);
Yb–CCNR–N 172.5; CCNR–N–CR 178.0.
When a 1 : 1 or 1 : 2 adduct of a metallocene forms, the
resulting Cp(centroid)–M–Cp(centroid) angle of the bent sand-
wich is smaller than in the base-free complex. The donor
ligands interact with the empty a1 (a1 and b2 in a 1 : 2 adduct;
C2v symmetry) orbitals of d-orbital parentage, generating one
or two (in a 1 : 1 or a 1 : 2 adduct, respectively) bonding orbitals
and antibonding ones (Fig. 7). The net result of adduct
formation is therefore to leave the LUMO orbital largely
unaffected. However, the filled orbitals of f-parentage includ-
ing the HOMO, are increased somewhat in energy since adding
electron density raises their energy. This destabilisation can be
thought of as arising from the increased electron-electron
repulsion in the f-orbitals in the closed shell, 4f14 Yb(II). This
effect is not expected to be large, however, as the nephelauxetic
effects for the lanthanides are of similar magnitude as for the
d-transition metals, and cause a decrease in electron-electron
repulsion due to the expanded f-orbitals.28 Thus, the net effect
on the HOMO - LUMO transition as a result of adduct
formation could be either a blue or a red shift, depending on
the extent of bending that increases the HOMO–LUMO gap
and the extent of electron-electron repulsion that decreases it.
The ytterbocene diethyl ether complexes all have very similar
optical spectra, with the long wavelength feature assigned to
the HOMO - LUMO transition varying only slightly from
690 nm for (Me5C5)2Yb(OEt2) to 653 nm for [1,3-(Me3Si)2-
C5H3]2Yb(OEt2) (Fig. 8). Although the solid state structure of
[1,3-(Me3Si)2C5H3]2Yb(OEt2) is not known, it is presumably
similar to that of [1,3-(Me3C)2C5H3]2Yb(OEt2) because their
solution optical spectra are the same. It is interesting that the
complex (Me4C5H)2Yb(OEt2) exhibits a HOMO - LUMO
transition at 683 nm, close to that for the penta-substituted
analogue, implying that the bend angles are similar in solution.
The ansa-bridged complex has a much smaller Cp(centroid)–
Yb–Cp(centroid) angle in the solid state and presumably in
solution also, yet it has a HOMO - LUMO transition that is
similar to those of the other ytterbocene diethyl ether adducts.
Presumably the sterically accessible metal centre leads to great-
er electron-electron repulsion, so the transition is only slightly
red-shifted relative to the unbridged disubstituted ytterbo-
cenes. The two high-energy transitions are the same for all of
the ytterbocene diethyl ether complexes, at 465 and 400 nm,
Optical spectroscopy
The optical spectra of the diethyl ether and THF complexes of
the ansa-ytterbocene, along with the other substituted ytterbo-
cenes described here and previously,16 have been measured and
the data are presented in Table 5. The diethyl ether adducts are
green in colour, while the colours of the THF complexes vary
widely; some are bright green while others are red. These
observations can be rationalised according to the Rosch–Green
¨
model presented previously9 as follows.27
Fig. 5 ORTEP diagram of [1,3-(Me3C)2C5H3]2Yb(OEt2) (50% prob-
ability ellipsoids).
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 1 9 – 9 2 7
923

Table 5 Optical spectra of ytterbocene etherates and the isocyanide complex ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(2,6-Me2C6H3NC)2 (in
methylcyclohexane unless specified)
Compound
l )
max/nm (e/L molꢀ1cmꢀ1
ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(OEt2)
ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(THF)2
ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(THF)2
(Me5C5)2Yb(OEt2)
664 (199), 460 (410), 399 (552)
676 (152), 490 (376), 378 (497)
670 (142), 484 (351), 375 (453)
690 (216), 472 (356), 407 (435)
686 (251), 449 (429), 393 (537)
800 (266), 513 (437), 448 (533)
801 (217), 513 (355), 447 (450)
655 (211), 455 (480), 397 (486)
654 (188), 477 (395), 387 (366)
641 (190), 467 (400), 386 (400)
653 (216), 477 (333), 392 (466)
678 (161), 494 (252), 399 (292)
683 (186), 468 (294), 401 (382)
785 (107), 514 (230), 439 (280)
780 (146), 511 (286), 440 (353)
686 (500), 500 (613)
a
(Me5C5)2Yb(OEt2)a
(Me5C5)2Yb(THF)2
(Me5C5)2Yb(THF)
[1,3-(Me3C)2C5H3]2Yb(OEt2)
[1,3-(Me3C)2C5H3]2Yb(THF)
[1,3-(Me3C)2C5H3]2Yb(THF)b
[1,3-(Me3Si)2C5H3]2Yb(OEt2)
[1,3-(Me3Si)2C5H3]2Yb(THF)
(Me4C5H)2Yb(OEt2)
(Me4C5H)2Yb(THF)2
(Me4C5H)2Yb(THF)
ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(2,6-Me2C6H3NC)2
a
a
b
In toluene. In THF.
which indicates that the f - d and LMCT transitions are
insensitive to the ligand environment, as observed in the
dipyridyl adducts.29
THF per metal centre, (Me5C5)2Yb(THF)2, and a red molecule
1
containing two equivalents of THF, by integration of the H
NMR spectrum, can be crystallised from pentane solvent. The
slightly less substituted analogue (Me4C5H)2Yb(THF)2 crystal-
lises with two equivalents of THF from diethyl ether or THF,30
and is also red. Crystallisation of (Me4C5H)2Yb(THF)2 from
The bond model developed for the diethyl ether adducts may
be extended to the isocyanide adduct. As the Cp(centroid)–
Yb–Cp(centroid) angle gets smaller, the energy of the
LUMO decreases while that of the HOMO increases. Since
the Cp(centroid)–Yb–Cp(centroid) angle in the ansa-ytterbo-
cene isocyanide is 111 smaller than in [1,3-(Me3C)2-
C5H3]2Yb(CNxylyl)2, a red shift is expected and observed;
the lmax of the ansa derivative is observed at 686 nm (Table
5), while that of the non-bridged analogue is at 586 nm.9
The colours and transitions of the THF complexes are less
readily explained (Fig. 9). The adduct (Me5C5)2Yb(THF)
crystallises from toluene as bright red crystals with half an
equivalent of toluene of crystallisation.21 However, in THF
solvent the molecule is thought to have two equivalents of
1
toluene yields a molecule that, by integration of the H NMR
spectrum, contains a single THF molecule per ytterbium atom
and is also bright red. In contrast, the molecule [1,3-(Me3C)2-
C5H3]2Yb(THF) contains a single THF ligand, whether crys-
tallised from diethyl ether or toluene, and is green, with an
optical spectrum that is not dissimilar from the diethyl ether
adduct. The brown Me3Si analogue also has a single THF
ligand, [1,3-(Me3Si)2C5H3]2Yb(THF).19 When dissolved in
THF, this complex changes colour to bright purple, but an
adduct containing more than one equivalent of THF cannot be
obtained by crystallisation. Presumably the steric bulk of the
disubstituted cyclopentadienide rings leads to the 7- rather
than 8-coordinate complexes in the solid state. The sterically
less-encumbered and electronically more exposed ansa-{[2,4-
(Me3C)2C5H2]2SiMe2}Yb(THF)2 crystallises with two equiva-
lents of THF, by integration of the 1H NMR spectrum, as red
needles.
Thus, the ytterbocenes with two equivalents of THF are all
red, while those with a single THF ligand vary from green to
Fig. 7 Schematic energy diagram showing the possible effects of an
additional ligand on the energies of the molecular orbitals and the
HOMO - LUMO transition of a bent metallocene.
Fig. 8 Optical spectra of diethyl ether adducts of ytterbocenes in
methylcyclohexane.
924
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 1 9 – 9 2 7

(Me3C)2C5H3]2Yb are nearly identical, presumably reflecting
the notion that the electron density at the Yb(II) centre in the
metallocenes is nearly identical. The long wavelength absorp-
tions, however, differ by 500 cmꢀ1 (100 nm), which is a
reflection of the smaller bend angle in the bridged metallocene
and therefore the extent to which the HOMO–LUMO gap
decreases.
Experimental
General
All reactions and product manipulations were carried out
under dry nitrogen using standard Schlenk and drybox tech-
niques. Dry, oxygen-free solvents were employed throughout.
The elemental analyses and mass spectra were performed at
the analytical facility at the University of California at Berke-
ley. The following compounds were prepared as previously
described: [1,3-(Me3Si)2C5H3]2Mg,33 [1,3-(Me3Si)2C5H3]22
Mg,34,35 Br2SiMe2,36 NaNH2,37 YbI2,38 Yb[N(SiMe3)2]2
(OEt2)2,38 Yb[N(SiMe3)2]2,39 (Me5C5)2Yb(OEt2),38 (Me5C5)2
Yb(THF),21 [1,3-(Me3Si)2C5H3]2Yb(OEt2),26 (Me4C5H)2Yb
(OEt2)16 and [1,3-(Me3C)2C5H3]2Yb(OEt2).16,18 (Me5C5)2
Yb(THF)2 was prepared by dissolving the mono-THF adduct
Fig. 9 Optical spectra of THF adducts of ytterbocenes in methyl-
cyclohexane.
brown to red, depending on the substituents on the cyclopen-
tadienide rings. In the solid state, the bend angle of an
ytterbocene complex with two ethereal ligands is no different
from that of a complex with only one.19 It is therefore tempting
to suggest that the red colour of the ytterbocenes with two
equivalents of THF results from a red-shift in the HOMO -
LUMO transition due to increased electron-electron repulsion
from the second ligand, which raises the energy of the HOMO,
while the LUMO is not strongly affected as the bend angle is
not changed. However, the optical spectra of the complexes
that can be crystallised with either one or two equivalents of
THF are identical, regardless of the stoichiometry deduced
from the 1H NMR spectra. This suggests that the second THF
is not bound to the metal in solution and exchanges rapidly on
the NMR time scale. No molecular structures have been
determined crystallographically for these complexes, and the
reported structures of ytterbocenes bound to two THF ligands
involve less substituted cyclopentadienide rings.31,32 Therefore,
the differences in colour must result from the balance of
electron-electron repulsion with the bend angle of the ytterbo-
cene.
1
in THF and crystallising from pentane [integration of the H
NMR spectrum indicates that it is a bis(THF) adduct]. The
structure of [1,3-(Me3Si)2C5H3]2Yb(THF) has been reported19
but no further characterisation has been published. The iso-
cyanide 2,6-Me2C6H3NC was purified by sublimation.
Syntheses
[2,4-(Me3C)2C5H3]2SiMe2. The complex [1,3-(Me3C)2
C5H3]2Mg (10.0 g, 26 mmol) was dissolved in THF (50 mL)
and cooled to 0 1C. Dibromodimethylsilane (3.4 mL, 26 mmol)
was added slowly using a syringe (note: using SiCl2Me2, the
reaction does not proceed satisfactorily). The reaction mixture
was allowed to warm to room temperature and was stirred
overnight, producing a yellow solution with a colourless pre-
cipitate. The solvent was removed under reduced pressure and
the residue was extracted with hexane (50 mL). The filtered
extract was concentrated to a volume of 15 mL and cooled to
ꢀ20 1C, which yielded colourless crystals over a period of 3
days (5.4 g, 50%). Anal. calcd for C28H46Si: C, 81.47; H, 11.72;
found: C, 81.50; H, 11.88. 1H NMR (C6D6): d 6.69 (s, 4H,
C5H3), 5.0 (br s, 2H, C5H3), 1.55 (s, 36H, Me3C), 0.11 (s, 6H,
Me2Si).
Conclusions
The preparation of a single isomer of an ansa-bridged bis
(cyclopentadienide) ligand bearing silyl groups is complicated
by the lability of the carbon–silicon bond upon deprotonation
of the cyclopentadienide ring. Thus, the two ansa-ferrocenes
reported here, containing Me3Si or Me3C groups, have 3,4 and
2,4 substitution patterns, respectively, although the synthetic
routes used were analogous. The solution and crystallographic
characterisation of the free Me3C substituted tetraene provides
evidence in that case that the ring substitution pattern is
unaffected by deprotonation and coordination to iron.
The strength of coordination of diethyl ether in the adduct of
the divalent ansa-ytterbocene precludes formation of a base-
free ansa-ytterbocene. As the metallocene bends, the net dipole
moment increases, resulting in a larger electrostatic interaction
between the dipoles on the Lewis acid and Lewis base com-
pared with an unbridged ytterbocene. This interaction com-
pensates the free energy of dissociation, rendering the ether
adduct stable except to exchange with stronger Lewis bases.
The additional room available around the metal centre due to
the small Cp(centroid)–Yb–Cp(centroid) angle in the ansa-
ytterbocene is partially occupied in the solid state structure
by g-agostic interactions with the diethyl ether ligand.
[(Me3Si)2C5H3]2SiMe2. A procedure similar to that used to
prepare [2,4-(Me3C)2C5H3]2SiMe2 was followed (above). The
hexane solution was filtered and the solvent was removed
under reduced pressure. The product was distilled at 140 1C
under vacuum (10ꢀ2 torr) to yield a yellow oil containing a
mixture of isomers, which was used without further purifica-
tion (50% yield). A different preparation is described in ref. 6.
ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Mg. The tetraene [2,4-
(Me3C)2C5H3]2SiMe2 (8.2 g, 20 mmol) was treated with dibu-
tylmagnesium (26 mL, 0.76 M, 20 mmol) in heptane. The
mixture was heated to vigorous reflux with stirring for 7 days,
after which time the solvent was removed under dynamic
vacuum, leaving a colourless solid, which was used without
purification because it was clean by 1H NMR spectroscopy
(4.95 g, 54%). Mp 125–126 1C. 1H NMR (C6D6): d 6.58 (s, 4H,
C5H2), 1.25 (s, 36H, Me3C), ꢀ0.01 (s, 6H, Me2Si).
ansa-{[(Me3Si)2C5H2]2SiMe2}Mg. This complex was pre-
pared in a similar manner to that used for the Me3C analogue
and isolated in 51% yield. 1H NMR (C6D6): d 6.83 (s, 4H,
C5H2), 0.24 (s, 6H, Me2Si), ꢀ0.04 (s, 36H, Me3Si).
The nCN in the infrared spectra of the 2,6-Me2C6H3NC
adducts of ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb and [1,3-
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 1 9 – 9 2 7
925

ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Fe. Tetrahydrofuran (200
mL) was added to a mixture of FeBr2(THF)2 (4.09 g, 11.4
mmol) and ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Mg (4.95 g, 11.4
mmol). The solution turned dark reddish-brown immediately
and over the next 48 h became dark red. The solvent was
removed under reduced pressure and the solid was extracted
with hexane (200 mL) and filtered. The dark red filtrate was
concentrated to a volume of 10 mL and cooled slowly to ꢀ40 1C.
Large, very soluble red crystals formed (1.50 g, 28%). The low
yield reflects the high solubility. Mp 140–142 1C. Anal. calcd
for C28H46FeSi: C, 72.08; H, 9.94; found: C, 71.79; H, 9.80. 1H
NMR (C6D6): d 4.31 (s, 2H, C5H2), 4.28 (s, 2H, C5H2), 1.30 (s,
36H, Me3C), 0.67 (s, 6H, Me2Si). The EI mass spectrum
showed a parent ion at m/z = 466 emu; the parent ion isotopic
cluster was simulated (calcd %, obsd %): 464 (6, 6), 465
(2, 3), 466 (100, 100), 467 (39, 36), 468 (11, 10), 469
(2, 2).
ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(2,6-Me2C6H3NC)2.
The isocyanide 2,6-Me2C6H3NC (0.18 g, 14.4 mmol) was
added under a flow of nitrogen to a Schlenk flask containing
ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(OEt2) (0.5 g, 7.7 mmol).
Toluene (25 mL) was added and the dark red solution was
stirred briefly. The volume of solvent was reduced slightly and
the solution was cooled to ꢀ40 1C, resulting in the formation of
dark red crystals in good yield. Mp 245–247 1C. Anal. calcd for
C46H64N2SiYb: C, 65.3; H, 7.62; N, 3.31; found C, 64.3; H,
8.03; N, 3.17. 1H NMR (C6D6): d 6.80 (t, J = 7.5 Hz, 2H,
C6H3), 6.51 (d, J = 2.5 Hz, 2H, C5H2), 6.49 (d, J = 7.5 Hz,
4H, C6H3), 6.36 (d, J = 2.5 Hz, 2H, C5H2), 2.09 (s, 12H,
Me2C6H3), 1.67 (s, 18H, Me3C), 1.56 (s, 18H, Me3C), 1.31
(s, 6H, Me2Si).
Crystallographic studies
A crystal of appropriate dimensions was mounted on a glass
fibre using Paratone N hydrocarbon oil. All measurements
were made on a Bruker SMART 1K CCD diffractometer.40
Cell constants and an orientation matrix were obtained of the
measured positions of reflections with I 4 10s to give the unit
cell. The systematic absences uniquely determined the space
group in each case. An arbitrary hemisphere of data was
collected at low temperature (see Tables 2 and 4 for details)
using the o scan technique with 0.31 scans counted for 10–30 s
per frame. Data were integrated using SAINT41 and corrected
for Lorentz and polarisation effects. The data were analysed
for agreement and absorption using XPREP,42 and an empiri-
cal absorption correction was applied based on comparison of
redundant and equivalent reflections. The structures were
solved by direct methods and expanded using Fourier techni-
ques. Non-hydrogen atoms were refined anisotropically (unless
stated otherwise), while the hydrogen atoms were included in
calculated positions, but not refined. The ytterbium structures
were solved and refined using the software package TeXsan;43
in all other cases SHELXS-97 (structure solution)44 and
SHELXL-97 (refinement)45 were used. The structure solution
refinements were unexceptional, except for the following two
compounds.w
ansa-{[3,4-(Me3Si)2C5H2]2SiMe2}Fe. This complex was pre-
pared in a manner similar to that used for the preparation of
the Me3C analogue. Large, extremely soluble red-violet crys-
tals were isolated by cooling a saturated hexane solution to
ꢀ40 1C (10%). The low yield reflects the high solubility. Mp
203–205 1C. Anal. calcd for C22H46FeSi5: C, 54.3; H, 8.78;
found: C, 52.8; H, 8.66. 1H NMR (C6D6): d 4.42 (m, 4H,
C5H2), 0.35 (s, 36H, Me3Si), 0.47 (s, 6H, Me2Si).
[2,4-(Me3C)2C5H2]2SiMe2Na2. Sodium amide (0.72 g, 18.5
mmol) was transferred to a large Schlenk tube equipped with a
magnetic stirrer and suspended in THF (60 mL). The tetraene
[2,4-(Me3C)2C5H3]2SiMe2 (3.45 g, 8.4 mmol) was weighed into
a Schlenk flask and dissolved in THF (50 mL). The solution of
[2,4-(Me3C)2C5H3]2SiMe2 was added to the suspension of
NaNH2 with stirring. The yellowish suspension was stirred at
room temperature overnight. The contents of the Schlenk tube
were transferred to a centrifuge bottle, which was centrifuged
at 3000 rpm for 30 min. The clear yellow solution was decanted
into a clean flask. The solvent was removed under reduced
pressure and the residue was washed in pentane. The base-free
disodium salt was obtained in 80% yield by prolonged expo-
sure (24 h) of powdered samples to dynamic vacuum at room
temperature. The salt is insoluble in non-coordinating solvents
and was not further characterised.
ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(OEt2). Only Friedel-
equivalent (P222) data were merged. Five of the carbon atoms
were refined isotropically while the remainder of the non-
hydrogen atoms were refined anisotropically. The absolute
configuration analysis performed by teXsan indicated that
the enantiomorph was correct, and it has a lower R factor
than the inverted structure. A diffuse region of electron density
in the structure, which is possibly disordered solvent, has been
modelled as 24 partial occupancy carbon atoms. The initial
positions of these atoms was based on the largest peaks in the
difference Fourier map, and subsequent refinement and exam-
ination of the Fourier and difference Fourier maps indicated
that the electron density is centred around those positions. The
thermal parameters of these carbon atoms was fixed at 8.0 and
their individual occupancy parameters were refined. Refine-
ment led to occupancies of between 0.195 and 0.710. The sum
of the occupancies is 9.855, which is approximately equal to
one and a third diethyl ether molecules per asymmetric unit.
ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(OEt2). The base-free
disodium salt [2,4-(Me3C)2C5H2]2SiMe2Na2 (2.3 g, 4.4 mmol)
was stirred with a suspension of YbI2 (2.0 g, 4.7 mmol) in
diethyl ether overnight. Pale green crystals were isolated after
filtration, concentration, and cooling of the solution to ꢀ40 1C
(2.5 g, 86%). Mp 275–282 1C. Anal. calcd for C32H56OSiYb: C,
58.4; H, 8.58; found: C, 58.4; H, 8.67. 1H NMR (C6D6): d 6.26
(d, J = 2.6 Hz, 2H, C5H2), 5.99 (d, J = 2.6 Hz, 2H, C5H2),
3.02 (br q, 4H, OEt2), 1.51 (s, 18H, Me3C), 1.23 (s, 6H, SiMe2),
1.22 (s, 18H, Me3C), 0.96 (t, J = 7 Hz, 6H, OEt2).
ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(THF)2. The green ethe-
rate ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(OEt2) was dissolved
in THF, resulting in an immediate colour change to deep
purple. The solvent was removed under reduced pressure and
the resulting pink residue was extracted into diethyl ether and
cooled. Red crystals were obtained in 50% yield by cooling to
ꢀ40 1C. The low yield presumably reflects the high solubility of
the product rather than any side reactions. Mp 248–255 1C.
Anal. calcd for C36H62O2SiYb: C, 59.4; H, 8.58; found: C, 59.5;
ansa-{[2,4-(Me3C)2C5H2]2SiMe2}Yb(2,6-Me2C6H3NC)2.
Due to poor data, one cyclopentadienide ring carbon atom was
refined isotropically while the remainder of the non-hydrogen
atoms were refined anisotropically with the following excep-
tion: one of the Me3C groups on one of the cyclopentadienide
1
H, 8.48. H NMR (C6D6): d 6.26 (d, J = 2.4 Hz, 2H, C5H2),
5.97 (d, J = 2.4 Hz, 2H, C5H2), 3.42 (br m, 8H, THF), 1.54
(s, 18H, Me3C), 1.28 (m, 8H, THF), 1.22 (s, 6H, Me2Si), 1.20
(s, 18H, Me3C).
w CCDC reference numbers 245770–245775. See http://www.rsc.org/
suppdata/nj/b4/b418550a/ for crystallographic data in .cif or other
electronic format.
926
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 1 9 – 9 2 7

rings was disordered. It was modelled with two sets of three
methyl groups, each set having their occupancies constrained
to be equal, and the sum of the occupancies of the two sets
constrained to 1.0. Refinement led to occupancies of 0.49 and
0.51 for the sets of methyl carbon atoms. These atoms were
refined isotropically.
orthorhombic, P212121 (#19), T = 173 K, a = 15.100(2), b =
18.289(2), c = 8.540(1) A, U = 2358 A3, Z = 4, dcalc = 1.458 g
cmꢀ3, 2274 reflections, 253 variables, R = 0.028.
18 A. V. Khvostov, A. I. Sizov, B. M. Bulychev, S. Y. Knjazhanski
and V. K. Belsky, J. Organomet. Chem., 1998, 559, 97–105.
19 R. D. Rogers, J. Organomet. Chem., 1996, 512, 97–100.
20 L. Pauling, The Nature of the Chemical Bond, Cornell University
Press, Ithaca, NY, 1960, pp. 260 and 403.
21 T. D. Tilley, R. A. Andersen, B. Spencer, H. Ruben, A. Zalkin and
D. H. Templeton, Inorg. Chem., 1980, 19, 2999–3003.
22 T. D. Tilley, R. A. Andersen, B. Spencer and A. Zalkin, Inorg.
Chem., 1982, 21, 2647–2649.
23 C. J. Burns and R. A. Andersen, J. Am. Chem. Soc., 1987, 109,
915–917.
24 C. J. Burns and R. A. Andersen, J. Am. Chem. Soc., 1987, 109,
941–942.
25 C. J. Burns and R. A. Andersen, J. Am. Chem. Soc., 1987, 109,
5853–5855.
Acknowledgements
This work was supported by the Director, Office of Energy
Research, Office of Basic Energy Sciences, Chemical Sciences
Division of the U.S. Department of Energy under Contract
No. DE-AC03-76SF00098. We thank Dr Fred Hollander (at
CHEXRAY, the U. C. Berkeley X-ray diffraction facility) for
assistance with the crystallography. We are grateful to Ged
Parkin for providing ref. 14 in advance of publication.
26 P. B. Hitchcock, J. A. K. Howard, M. F. Lappert and S. Prashar,
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