Synthesis of novel mixed metal cluster complexes of osmium and mercury: Formation of the wheel-shaped cluster complexes Os6(μ-6-Hg)(μ-PR2)2(CO)20 (R = Ph, i-Bu)

Article abstract of DOI:10.1021/om0110585

The reaction of [PPN] [Os₃(μ-PR₂)(CO)₁₀] (R = Ph 1a, i-Bu 1b) with HgCl₂ in THF at -90 °C under exclusion of light gave the spirocyclic cluster μ₄-Hg[Os₃(μ-PR₂)(μ-CO)(CO)₉ ]₂ (R = Ph 2a, i-Bu 2b). In these compounds the μ₄-Hg bridges one Os-Os edge of each Os₃ subunit. The remaining four edges are bridged by the μ-CO and μ-PPh₂ ligands, respectively. ¹³C NMR spectra of ¹³CO-enriched 2a recorded at room temperature indicate an intramolecular rotation of the Os₃ subunits around the axis running through the mercury atom and the midpoints of the Os-Os vectors it bridges. When 2a is heated in toluene or 1,4-dioxane, a rearrangement of the metal skeleton takes place. The μ4-Hg moiety is shifted to another edge of one of the Os₃ rings, giving the isomer (CO)₁₀(μ-PPh₂)Os₃(μ₄-Hg)Os ₃(μ-PPh₂)(μ-CO)-(CO)₉, 3a. The kinetics of this process were measured by UV/vis spectroscopy at four different temperatures in toluene and 1,4-dioxane, respectively. The reaction is of first order with a rate constant of 6.66(2) × 10^-5 s^-1 in toluene at 331.6(1) K. The thermodynamic parameters ΔH? = 97(5) kJ mol^-1, ΔS? = -33(13) J K^-1 mol^-1, and ΔG? = 107(6) kJ mol^-1 were derived from an Eyring plot. The analogous cluster complex 3b with μ-P(i-Bu)₂ bridges has been obtained, too. Its structure has been confirmed by single-crystal X-ray analysis. 2a and 2b are highly photosensitive in solution. Upon exposure to daylight, they rearrange to give the novel wheel-shaped cluster complexes OS₆(μ₆-Hg)(μ-PR₂)₂(CO)₂₀ (R = Ph 4a, i-Bu 4b). The structure of 4a was confirmed by single-crystal X-ray analysis. In 4a the Hg atom is located at the center of a Os₆ ring. The ring is corrugated with the mercury atom in bonding distance to all osmium atoms. The course of the reaction was monitored for the conversion of 2a by UV/vis spectra exhibiting two isosbestic points indicating an intramolecular reaction.

Full text of DOI:10.1021/om0110585

Organometallics 2002, 21, 1925-1932
1925
Syn th esis of Novel Mixed Meta l Clu ster Com p lexes of
Osm iu m a n d Mer cu r y: F or m a tion of th e Wh eel-Sh a p ed
Clu ster Com p lexes Os6(µ6-Hg)(µ-P R2)2(CO)20 (R ) P h ,
i-Bu )
Hans Egold,* Markus Schraa, Ulrich Flo¨rke, and J anusch Partyka
Anorganische und Analytische Chemie der Universita¨t Paderborn, Fachbereich 13,
Chemie und Chemietechnik, Warburgerstr. 100, D-33098 Paderborn, Germany
Received December 14, 2001
The reaction of [PPN][Os3(µ-PR2)(CO)10] (R ) Ph 1a , i-Bu 1b) with HgCl2 in THF at -90
°C under exclusion of light gave the spirocyclic cluster µ4-Hg[Os3(µ-PR2)(µ-CO)(CO)9]2 (R )
Ph 2a , i-Bu 2b). In these compounds the µ4-Hg bridges one Os-Os edge of each Os3 subunit.
The remaining four edges are bridged by the µ-CO and µ-PPh2 ligands, respectively. 13C
NMR spectra of 13CO-enriched 2a recorded at room temperature indicate an intramolecular
rotation of the Os3 subunits around the axis running through the mercury atom and the
midpoints of the Os-Os vectors it bridges. When 2a is heated in toluene or 1,4-dioxane, a
rearrangement of the metal skeleton takes place. The µ4-Hg moiety is shifted to another
edge of one of the Os3 rings, giving the isomer (CO)10(µ-PPh2)Os3(µ4-Hg)Os3(µ-PPh2)(µ-CO)-
(CO)9, 3a . The kinetics of this process were measured by UV/vis spectroscopy at four different
temperatures in toluene and 1,4-dioxane, respectively. The reaction is of first order with a
rate constant of 6.66(2) × 10-5 s-1 in toluene at 331.6(1) K. The thermodynamic parameters
∆Hq ) 97(5) kJ mol-1, ∆Sq ) -33(13) J K-1 mol-1, and ∆Gq ) 107(6) kJ mol-1 were derived
from an Eyring plot. The analogous cluster complex 3b with µ-P(i-Bu)2 bridges has been
obtained, too. Its structure has been confirmed by single-crystal X-ray analysis. 2a and 2b
are highly photosensitive in solution. Upon exposure to daylight, they rearrange to give the
novel wheel-shaped cluster complexes Os6(µ6-Hg)(µ-PR2)2(CO)20 (R ) Ph 4a , i-Bu 4b). The
structure of 4a was confirmed by single-crystal X-ray analysis. In 4a the Hg atom is located
at the center of a Os6 ring. The ring is corrugated with the mercury atom in bonding distance
to all osmium atoms. The course of the reaction was monitored for the conversion of 2a by
UV/vis spectra exhibiting two isosbestic points indicating an intramolecular reaction.
In tr od u ction
to the flexible bonding properties of the mercury atom
which binds to the M3 cluster subunits via a linear spd5
manifold.11-13 The dynamic properties are related to two
different processes, the first one being an intramolecular
rotation about an axis running through the mercury
atom and the midpoints of the two M-M bonds it
bridges.3,14,15 The second process is based on an in-
tramolecular exchange process in which motion of the
µ4-Hg moiety averages two or all M-M vectors of the
M3 subunits.3-4,14,15 So far an isomerization of the above-
described class of clusters giving µ6-bound mercury has
not been observed, but Rosenberg et al. propose a
sandwich-like intermediate Ru3(µ6-Hg)Ru3 for the rear-
rangement of µ4-Hg[Ru3(µ3-η2-C2(t-Bu))(CO)9]2 in solu-
tion.3 In general a µ6-bonding mode is rare for mercury
and usually found in clusters dominantly built of Hg
and Pt or Pd atoms, respectively.16-25 In these closed
In heptanuclear mixed metal cluster complexes with
a spirocyclic M3(µ4-Hg)M3 (M ) Ru,1-6 Os4,7-10) frame-
work a mercury atom links two M3 cluster subunits.
This type of compound is usually easy to prepare by
metathesis or redistribution reactions, respectively. For
example the cluster µ4-Hg[Os3(µ-H)(µ3-S)(CO)10]2 was
synthesized in 80% yield by salt metathesis of HgI2 with
[PPN][Os3(µ-H)(µ3-S)(CO)8)].4 The resulting heptanu-
clear cluster complexes are very dynamic molecules due
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Chem. Soc. 1982, 104, 7672.
10.1021/om0110585 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 04/02/2002

1926 Organometallics, Vol. 21, No. 9, 2002
Egold et al.
Sch em e 1
F igu r e 1. Molecular structure of 2a with 50% probability
ellipsoids. Hydrogen atoms are omitted.
report the synthesis of another cluster of the latter kind
by photochemical conversion of a heptanuclear mixed
Os-Hg cluster complex with a spirocyclic Os3(µ4-Hg)-
Os3 core. Furthermore the thermal isomerization of the
latter compound has been investigated by UV spectros-
copy, and the kinetic data obtained are discussed with
respect to the mechanism of the reaction.
Resu lts a n d Discu ssion
On dropwise addition of 2 equiv of yellow [PPN][Os3-
(µ-PR2)(CO)10] (R ) Ph 1a , i-Bu 1b) dissolved in THF
to a solution of HgCl2 in THF at -90 °C, a deep red
reaction mixture was obtained. After heating to room
temperature and working up in both cases a mixture of
three mixed Os/Hg cluster complexes was obtained. The
compounds could not be separated by TLC, but pure
samples of these clusters were obtained by fractional
crystallization. Comparison of the 31P NMR data of the
obtained pure cluster complexes with 31P NMR data of
the mixtures prior to fractional crystallization proves
that pure samples of all compounds present in the
mixtures were isolated. Four of these cluster complexes
were characterized by single-crystal X-ray analysis.
cluster compounds the Hg atom occupies the center of
a trigonal prism built of Pt or Pd atoms, respectively.
So far there are only five cluster complexes known in
which a µ6-Hg atom is linked to metal atoms other than
Pt or Pd, respectively.26-30 In this paper we wish to
1. P r im a r y Sp ir ocyclic Con d en sa tion P r od u cts
2a a n d 2b. The first type of cluster complex isolated
from the above-mentioned reaction mixtures is the novel
spirocyclic heptanuclear cluster complex µ4-Hg[Os3(µ-
PR2)(µ-CO)(CO)9]2 (R ) Ph 2a , i-Bu 2b) (Scheme 1, step
i). These compounds exhibit a single 31P resonance at δ
) 216.6 for R ) Ph and δ ) 189.1 for R ) i-Bu,
respectively. The presence of µ-CO ligands in solution
is confirmed by ν(CO) absorption spectra showing weak
absorptions at 1803 cm-1 for 2a and 1813 cm-1 for 2b,
respectively. 31P NMR spectra of the reaction mixture
of 1a with HgCl2 recorded at -60 °C show only reso-
nances of 2a , indicating that cluster complexes 2a and
2b, respectively, are the primary condensation products.
(18) Mednikov, E. G.; Eremenko, N. K.; Bashilov, V. V.; Sokolov, V.
I. Inorg. Chim. Acta 1983, 76, L31.
(19) Yamamoto, Y.; Takahashi, K.; Yamazaki, H. Chem. Lett. 1985,
201.
(20) Yamamoto, Y.; Yamazaki, H. J . Chem. Soc., Dalton Trans. 1989,
2161.
(21) Tanase, T.; Horiuchi, T.; Yamamoto, Y.; Kobayashi, K. J .
Organomet. Chem. 1992, 440, 1.
(22) Yamamoto, Y.; Yamazaki, H. Organometallics 1993, 12, 933.
(23) Yamamoto, Y.; Yamazaki, H Inorg. Chim. Acta 1994, 217, 121.
(24) Hao, L.; Vittal, J . J .; Puddephatt, R. J . Inorg. Chem. 1996, 35,
269.
(25) Hao, L.; Vittal, J . J .; Puddephatt, R. J . Organometallics 1996,
15, 3115.
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Hursthouse, M. B. J . Chem. Soc., Dalton Trans. 1981, 126.
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M. Angew. Chem. 1985, 97, 761.
(28) Braunstein, P.; Rose, J .; Tiripicchio, A.; Tiripicchio-Camellini,
M. J . Chem. Soc., Dalton Trans. 1992, 911.
Molecu la r Str u ctu r es of 2a a n d 2b. The molecular
structures of 2a (Figure 1) and 2b (not depicted) were
determined by single-crystal X-ray analysis. The mol-
ecules both exhibit an Os3(µ4-Hg)Os3 metal core. They
are largely isostructural but differ in the substitution
(29) Gade, L. H.; J ohnson, B. F. G.; Lewis, J .; McPartlin, M.; Scowen,
I. J . J . Chem. Soc., Dalton Trans. 1996, 597.
(30) Charadonna, G.; Ingrosso, G.; Marchetti, F. Angew. Chem., Int.
Ed. 2000, 39, 3872.

Cluster Complexes of Osmium and Mercury
Organometallics, Vol. 21, No. 9, 2002 1927
of the µ-P bridge atoms with phenyl residues for 2a and
isobutyl residues for 2b.
2a shows a central spirocyclic µ4-Hg atom which
connects two Os3 ring moieties, Os1-Os3 and Os4-Os6,
both with equal ligand arrangements. Each osmium
atom is attached to three terminal CO ligands, of which
two are in axial and one in equatorial position. The three
Os-Os bonds of each ring fragment are bridged by the
different groups µ-CO, µ-PCy2, and µ-Hg. All these
bridging atoms lie in the Os3 planes; the deviations from
the planes range from -0.08(2) to 0.10(1) Å. With
respect to the metal framework the two µ-PCy2 ligands
are in syn position to each other, as are the µ-CO groups.
These ligand arrangements lead to slightly distorted
square pyramidal coordination of each Os atom with the
equatorial CO groups in the apical positions. With
respect to the two additional Os-Os bonds all osmium
atoms show a 7-fold coordination pattern which may be
described as distorted pentagonal bipyramidal. All
Os-Os distances differ significantly, at 2.9738(13)
(Os1-Os3), 2.9623(12) (Os5-Os6), 2.9510(13) (Os4-
Os6), 2.9483(15) (Os1-Os2), 2.9252(16) (Os4-Os5), and
2.8937(14) Å (Os2-Os3), respectively. The two shortest
bonds are bridged by the µ-CO groups. The Hg-Os
distances are also different, at 2.8871(13) (Os4), 2.8542-
(15) (Os3), 2.8223(13) (Os1), and 2.8109(15) Å (Os6). The
two shorter bonds are trans to the two Os-P vectors
and the longer ones trans to the Os-µ-CO vectors. The
coordination of the Hg atom deviates strongly from ideal
spirocyclicity with an Os2Hg/HgOs2 angle of 46.29(4)°
instead of 90°. The µ-P bridges are symmetric with
average Os-P bond lengths of 2.321(6) Å, but the µ-CO
groups show one short and one long Os-C bond with
average values of 2.03(2) and 2.29(2) Å, respectively. The
structural unit of a spirocyclic mercury atom linked to
two Os3 rings is known from (µ4-Hg)[Os3(µ-H)(µ3-S)-
(CO)9]2,4 (µ4-Hg)[Os3(µ-Cl)(CO)10]2,9 and (µ4-Hg)[Os3(µ-
SR)(CO)10]2.10 However, in contrast to the planar HgOs3
unit of 2a these molecules show butterfly-type Os3Hg
cores. Nevertheless Hg-Os bond lengths in these struc-
tures range from 2.815(2) to 2.903(3) Å and compare
well with those of 2a .
F igu r e 2. Molecular structure of 3b with 50% probability
ellipsoids. Hydrogen atoms are omitted.
(CO)10 in which the coinage metal bridges the Os-Os
bond bridged by phosphorus.42 We assume that the
formation of 2a and 2b proceeds kinetically controlled.
An analogous cluster complex may be formed in the
initial step of the reaction of 1a with ClMPPh3 too, but
if that is true, it rapidly isomerizes to give the thermo-
dynamically more stable cluster complex with gold
bridging the phosphido-bridged Os-Os bond. This idea
is supported by the example of Os3(AuPEt3)(µ-η3-C3H5)-
(CO)10, in which the allylic moiety bridges one Os-Os
edge and AuPEt3 another.31
2. F or m a tion of 3a a n d 3b by Th er m a l Rea r -
r a n gem en t of 2a a n d 2b, Resp ectively. The above
proposed metal shift from one Os-Os edge to another
can be observed for both 2a and 2b. Both cluster
complexes rearrange on heating under complete exclu-
sion of daylight, giving (CO)10(µ-PR2)Os3(µ4-Hg)Os3(µ-
PR2)(µ-CO)(CO)9 (R ) Ph 3a , i-Bu 3b) (Scheme 1, step
ii). Compared to 2a and 2b in these cluster complexes,
the mercury is shifted to one of the phosphido-bridged
Os-Os edges. This has been confirmed by X-ray analy-
sis of a single crystal of 3b (Figure 2).
Molecu la r Str u ctu r e of 3b. Complex 3b combines
two different Os3 patterns. The Os1-Os3 group is the
same as that known from 2a and 2b with bridging µ-P(i-
Bu2) and µ-CO ligands and the µ4-Hg atom. These lie
in the Os3 plane with deviations from -0.076(3) to
0.055(1) Å. The Os-Os bond lengths are 2.8974(9)
(Os1-Os2), 2.9543(8) (Os1-Os3), and 2.9561(8) Å (Os2-
Os3), respectively, and the shortest one is again bridged
by the carbonyl ligand. The coordination of the Hg atom
with Hg-Os distances of 2.7971(9) (Os3), 2.8996(8) (1),
2.8448(8) (4), and 2.8518(10) Å (5) deviates from ideal
spirocyclicity with an Os2Hg/HgOs2 angle of 68.43(4)°.
The Os4-Os6 group shows a different ligand coordina-
tion pattern. Os4 and Os5 each are attached to three
and Os6 to four terminal CO ligands. The Os4-Os5
bond of 2.9596(11) Å is doubly bridged by the Hg atom
and a µ-P(i-Bu2) group, but the two Os4-Os6 and Os5-
Os6 bonds of 2.8964(12) and 2.8873(9) Å, respectively,
are not bridged. This HgOs3 moiety has a butterfly-type
structure with an Os3/Os2Hg opening angle of 125.54-
(3)°, and the Os2P plane halves this butterfly with
HgOs2/Os2P and Os3/Os2P angles of 62.97(9)° and 62.57-
Though being isostructural to 2a with respect to the
overall ligand arrangement, the compound 2b shows
significant changes of the metal framework concerning
the µ4-Hg atom, which has now almost ideal spirocyclic
coordination with an Os2Hg/HgOs2 angle of 84.93(3)°.
The bridging µ-P and µ-CO groups still lie in the Os3
planes with deviations from these planes between 0.005-
(5) and 0.051(5) Å. Hg-Os bond lengths range from
2.8605(12) to 2.7774(10) Å and the Os-Os distances
from 2.8901(12) to 2.9820(12) Å.
Mech a n istic Asp ects. The formation of this type of
cluster complex is most surprising, as mercury in
heptanuclear cluster complexes with Os3(µ4-Hg)Os3 core
is known to bridge the Os-Os bonds already bridged
by a three-electron-donor ligand only.8-10 Therefore we
initially expected the formation of an analogous cluster
complex with mercury bridging the phosphido-bridged
Os edge (Scheme 1, step iv). However, this type of
product was not observed at all. Interestingly the
condensation of 1a with ClMPPh3 (M ) Cu, Ag, Au)
instead of HgCl2 proceeds as expected, giving the
tetranuclear cluster complexes Os3(MPPh3)(µ-PPh2)-
(31) Housecroft, C. E.; J ohnson, B. F. G.; Lewis, J .; Lunniss, J . A.;
Owen, S. M.; Raithby, P. R. J . Organomet. Chem. 1991, 409, 271.

1928 Organometallics, Vol. 21, No. 9, 2002
Egold et al.
F igu r e 3. Time-resolved UV/vis spectra for the rearrange-
ment 2a f 3a at 58.4 °C in toluene.
F igu r e 4. Eyring plot for the rearrangement 2a f 3a .
Ta ble 1. Ra te Con sta n ts a n d Th er m od yn a m ic Da ta
lyzed by formal integration,32 giving a rate law of first
order and a rate constant for each temperature (Table
1). The temperature dependence of the rate constants
was investigated by an Eyring plot (Figure 4) giving the
thermodynamic data of this reaction (Table 1). As can
be seen from Table 1, the thermodynamic activation
parameters are with respect to the large standard
deviation independent from the solvent used. ∆Gq
amounts 107(6) kJ mol-1 in toluene, explaining the low
yield of 3a and 3b due to their slow formation at room
temperature. The value of ∆Sq is negative or at least
close to zero, strongly indicating an intramolecular
isomerization without dissociative steps. There are two
possible mechanisms for this isomerization. Either the
µ4-Hg is shifted to the phosphido-bridged Os-Os edge
or the phosphido bridge is shifted from one edge to
another. Apparently these data give no clear hint on
the pathway, but comparison with other M3(µ4-Hg)M3
systems supports the idea of mercury shifting from one
edge to another.4,11
of th e Rea r r a gem en t 2a f 3a
temp
[K]
rate constant
[1/s]
solvent
thermodynamic data
331.6(1) toluene
342.9(1) toluene
352.6(1) toluene
363.1(1) toluene
6.66(2) × 10-5
2.501(1) × 10-4 ∆Hq )97(5) kJ /mol
6.63(2) × 10-4
∆Sq ) -34(13) J /(K mol)
1.504(5) × 10-3 ∆Gq ) 107(6) kJ /mol
331.6(1) 1,4-dioxane 3.815(11) × 10-5
342.9(1) 1,4-dioxane 1.646(5) × 10-4 ∆Hq ) 103(6) kJ /mol
352.6(1) 1,4-dioxane 4.391(16) × 10-4 ∆Sq ) -19(16) J /(K mol)
363.1(1) 1,4-dioxane 1.07(2) × 10-3
∆Gq ) 109(7) kJ /mol
(9)°. The structure of this Os3 unit compares well with
the butterfly geometries of the complexes cited above.
The 31P NMR spectra of 3a and 3b prove the existence
of two different phosphido bridges. For example 3a
shows 31P NMR resonances at δ ) 209.7 and δ ) 43.9.
As a result of comparison with the data of 2a , the
chemical shift of the resonance at low field must be
assigned to the phosphorus bridging an Os-Os edge not
bridged by mercury. Consequently the latter resonance
must be assigned to the other phosphorus bridging the
same edge as mercury.
P h otoch em ical For m ation of Wh eel-Sh aped Clu s-
ter Com p lexes 4a a n d 4b. The last type of cluster
complex isolated from the reaction of 1a and 1b with
HgCl2 is the wheel-shaped Os6(µ6-Hg)(µ-PR2)2(CO)20 (R
) Ph 4a , i-Bu 4b).
Kin et ics a n d Th er m od yn a m ic Da t a . 3a and 3b
are the second type of cluster isolated from the mixture
resulting from the reaction of 1a and 1b, respectively,
with HgCl2. Hence the isomerization of 2a and 2b giving
3a and 3b, respectively, already takes place at room
temperature. Nevertheless according to 31P NMR spec-
tra recorded from the reaction mixtures, 3a and 3b are
formed in low yield. This observation is in good agree-
ment with the thermodynamic parameters of isomer-
ization. The latter were determined for the rearrange-
ment of 2a . The course of the reaction was followed by
UV/vis spectroscopic measurements at 58.4, 69.7, 79.4,
and 89.9 °C in toluene and 1,4-dioxane, respectively. In
Figure 3 time-dependent UV/vis spectra of the reaction
in toluene at 58.4 °C are depicted. The course of the
reaction was monitored for 12 h. During this period of
time the absorption maximum of 2a at 500 nm slowly
decreases. Meanwhile the absorption band at 554 nm
belonging to 3a is slowly increasing. At 525 nm an
isosbestic point is observed indicating a unitary course
of reaction. At longer reaction times the isosbestic point
is lost due to decomposition reactions. Therefore the
extinctions at t equals infinity, Eλ∞, could not be
determined. Assuming an intramolecular rearrange-
ment,4,11 the time-dependent extinction data were ana-
Molecu la r Str u ctu r e of 4a . The molecular structure
of 4a (Figure 5) was determined by single-crystal X-ray
analysis. The latter shows a completely new structure
type with a 6-fold coordinated mercury atom in the
center of an Os6 ring. This ring unit is not planar but
rather twisted. The deviations of Os atoms from the best
plane through this metal arrangement with Hg lying
in the plane range from -0.852(1) to 0.839(1) Å. Atoms
Os1 to Os4 are attached to three terminal CO ligands
each, two in axial and one in equatorial position.
Additionally Os1 and Os2 as well as Os3 and Os4 are
bridged by µ-PPh2 groups, and Os5 and Os6 are at-
tached to four terminal carbonyl groups. Each Os atom
thus reaches a 7-fold coordination from four nonmetal
ligands, two neighboring osmium atoms and the central
Hg atom, which can best be described as a distorted
pentagonal bipyramid with the two CO groups at the
apexes. The short Hg-Os1 and Hg-Os4 bond lengths
of 2.8583(12) and 2.8598(13) Å include a nearly linear
Os1-Hg-Os4 angle of 172.80(4)°. This structural fea-
(32) Mauser, H. Experimentelle Methoden der Physik und Chemie
(Band 1): Formale Kinetik; Bertelsmann Universita¨tsverlag, 1974.

Cluster Complexes of Osmium and Mercury
Organometallics, Vol. 21, No. 9, 2002 1929
Sch em e 2
min the solution was irradiated with more intense
polychromatic light for another 10 min in order to
complete the conversion. In the course of the reaction
the intensity of the absorption band of 2a at 489 nm
rapidly decreased. In addition two isosbestic points are
observed at 395 and 315 nm, indicating a unitary course
of reaction32,33 that most likely proceeds by an intramo-
lecular rearrangement of the metal core. The final
spectrum in the sequence recorded is that of pure 4a ,
confirming complete conversion. Such a clean conversion
is obtained only in highly diluted solution. In more
concentrated solutions (c ≈ 1 mmol L-1) decomposition
reactions occur as well, reducing the yield in preparative
reactions to about 50%. To find out more about the
photocyclization process, we decided to take a closer look
at the educts 2a and 2b. Their molecular structures
(compare structure discussions above) show dihedral
angles Os2Hg/HgOs2 of 46.29(4)° and 84.93(3)°, respec-
tively. The different angles indicated that the rings are
able to rotate about an axis running through the
mercury atom and the midpoints of the two Os-Os
bonds it bridges. This hypothesis could be supported
only by 13C NMR spectra showing the resonances of the
CO ligands.4 Due to the low solubility of the compounds
(cmax ≈ 1 mmol L-1), a 13CO-enriched sample had to be
prepared. The enrichment has been achieved by varia-
tion of a well-known reaction. On heating Os3(µ-H)(µ-
PPh2)(CO)10, 5, the orthometalated cluster complex
Os3(µ-H)2(µ3-η3-PPh(C6H4))(CO)9, 6, is formed (Scheme
F igu r e 5. Molecular structure of 4a with 50% probability
ellipsoids. Hydrogen atoms are omitted.
Figu r e 6. Time-resolved UV/vis spectra of the photochemi-
cal rearrangement 2a f 4a .
ture is probably indicating that mercury is strongly
bonding along this axis via a linear spd5 manifold.11-13
All other Os-Hg bond lengths are significantly longer,
and their Os-Hg-Os angles are far from linear. Two
medium bonds of 2.8969(13) and 2.9097(13) Å are
directed to Os2 and Os3, respectively, which make the
shortest Os-Os bond of 2.9656(12) Å. All other Os-Os
bond lengths range from 3.0576(14) to 3.1023(15) Å. The
two longest Hg-Os bonds are those to Os5 (3.0683(13)
Å) and Os6 (3.0506(15) Å). The accompanying Os-Hg-
Os angles are Os2-Hg-Os5 152.16(4)° and Os3-Hg-
Os6 152.68(4)°, both being clearly more acute than the
Os1-Hg-Os4 angle. We propose that the interaction
between Os2, Os3, Os5, Os6, respectively, and Hg is
based on OsfHg donor/acceptor bonds such as those
found in [Os3(CO)11Hg]3.36 The latter exhibits short Os-
Hg lengths of 2.717(4) Å due to strong σ-bonding and
longer donor/acceptor bond lengths of 2.859(3) Å be-
tween Os(CO)4 groups and Hg.
In vestiga tion of th e P h otocycliza tion P r ocess.
The fascinating compounds 4a and 4b are formed in a
photoreaction of 2a and 2b, respectively. The latter are
very photosensitive in solution. For example a solution
of 2a in THF is converted by daylight at room temper-
ature to give 4a within 30 min (Scheme 1, step iii). The
conversion of a solution of 2a in THF (c ) 0.043 mmol
L-1) was monitored by time-dependent UV/vis spectra.
(Figure 6). The solution was irradiated by polychromatic
light, and every 60 s a spectrum was recorded. After 6
2). 6 is known to add nucleophiles L to give Os3(µ-H)-
13
(µ-PPh2)(CO)9L in high yield.34 We heated 6 under
-
CO pressure. The resulting equilibrium between 5 and
6 gave after 5 h at 80 °C and subsequent chromatog-
raphy 13CO-enriched 5. Comparison of the 13C NMR
spectra of the latter compound with a sample not
enriched proves enrichment at all CO ligand positions.
13CO-enriched 5 was subsequently deprotonated with
DBU and converted to 13CO-enriched 1a , which was
reacted with HgCl2, giving a pure sample of 13CO-
enriched 2a . A 13C NMR spectrum of this cluster
complex recorded in CDCl3 at room temperature exhib-
ited six resonances for the terminal CO ligands and one
resonance for the bridging CO ligands. The observed
number of resonances is most likely due to the above
proposed intramolecular rotation of the Os3 rings. For
a rigid molecule 2a nine resonances for the terminal CO
ligands would have been expected. Obviously the rota-
tion averages the chemical environment of the CO
ligands perpendicular to the Os3 planes, reducing the
number of resonances to the observed value of six.
Therefore we assume that the photocyclization process
(33) Autorenkollektiv. Photochemie, VEB Deutscher Verlag der
Wissenschaften: Berlin, 1975.
(34) Colbran, S. B.; Irele, P. T.; J ohnson, B. F. G.; Lahoz, F. J .; Lewis,
J .; Raithby, P. R. J . Chem. Soc., Dalton Trans. 1989, 2023.

1930 Organometallics, Vol. 21, No. 9, 2002
Egold et al.
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg)
Sch em e 3
2a
Hg1-Os1
Hg1-Os4
Os1-P1
2.8223(13)
2.8871(13)
2.318(6)
2.9738(13)
2.331(6)
2.07(2)
2.9252(16)
2.28(2)
Hg1-Os3
Hg1-Os6
Os1-Os2
Os2-C7
Os2-Os3
Os4-C14
Os4-Os6
Os5-P2
2.8542(15)
2.8109(15)
2.9483(15)
2.30(2)
2.8937(14)
1.99(2)
2.9510(13)
2.325(5)
2.311(6)
Os1-Os3
Os2-P1
Os3-C7
Os4-Os5
Os5-C14
Os5-Os6
2.9623(12)
Os6-P2
Os6-Hg1-Os1
Os1-Hg1-Os3
Os1-Hg1-Os4
126.17(4)
63.18(3)
155.53(5)
Os6-Hg1-Os3
Os6-Hg1-Os4
Os3-Hg1-Os4
156.69(5)
62.37(3)
119.37(4)
3b
Hg1-Os3
Hg1-Os5
Os1-C4
Os1-Os3
Os2-P1
Os3-P1
Os4-Os6
Os5-P2
2.7971(9)
2.8518(10)
2.005(11)
2.9543(8)
2.339(3)
2.326(3)
2.8964(12)
2.375(3)
Hg1-Os4
Hg1-Os1
Os1-Os2
Os2-C4
2.8448(8)
2.8996(8)
2.8974(9)
2.278(11)
2.9561(8)
2.379(3)
Os2-Os3
Os4-P2
Os4-Os5
Os5-Os6
2.9596(11)
2.8873(9)
Os5-Hg1-Os1
Os1-Hg1-Os3
Os1-Hg1-Os4
124.05(2)
62.45(2)
141.53(3)
Os5-Hg1-Os3
Os5-Hg1-Os4
Os3-Hg1-Os4
149.89(3)
62.60(2)
133.85(3)
4a
Hg1-Os1
Hg1-Os2
Hg1-Os6
Os1-P1
Os1-Os2
Os2-Os3
Os3-Os4
Os4-Os5
2.8583(12)
2.8969(13)
3.0506(15)
2.332(5)
3.1023(15)
2.9656(12)
3.0865(13)
3.0767(14)
Hg1-Os4
Hg1-Os3
Hg1-Os5
Os1-Os6
Os2-P1
2.8598(13)
2.9097(13)
3.0683(13)
3.0874(13)
2.313(5)
2.294(5)
2.320(6)
3.0576(14)
leading to 4a and 4b, respectively, is initiated when both
Os3 rings reach a synperiplanar orientation during the
above-described rotational process since this orientation
is preserved in 4a and 4b (Scheme 3). The photocycliza-
tion process is probably thermodynamically favored, as
the number of metal-metal contacts increases from 10
in 2a and 2b to 12 in 4a and 4b.
Os3-P2
Os4-P2
Os5-Os6
Os1-Hg1-Os4
Os4-Hg1-Os2
Os4-Hg1-Os3
Os1-Hg1-Os6
Os2-Hg1-Os6
Os1-Hg1-Os5
Os2-Hg1-Os5
Os6-Hg1-Os5
172.80(4)
121.90(4)
64.68(3)
62.90(3)
127.69(4)
110.98(4)
152.16(4)
59.96(3)
Os1-Hg1-Os2
Os1-Hg1-Os3
Os2-Hg1-Os3
Os4-Hg1-Os6
Os3-Hg1-Os6
Os4-Hg1-Os5
Os3-Hg1-Os5
65.23(3)
122.24(4)
61.42(3)
110.11(4)
152.68(4)
62.41(3)
Exp er im en ta l Section
126.47(4)
All reactions were performed in solvents free of oxygen that
were dried according to literature methods, distilled, and
stored under argon atmosphere. TLC was carried out on glass
plates (20 × 20 cm2) coated with a mixture of gypsum and silica
gel (Merck 60 PF254, 1 mm thick). The reaction products were
characterized by ν(CO) FTIR spectroscopy (Nicolet P510, CaF2
optics) and 1H, 13C, and 31P NMR spectroscopy (Bruker AMX
300). UV/vis spectra were recorded with a Lambda 15 (Perkin-
Elmer) spectrometer. The temperature of the cell was kept
constant with a cryostat (MWG Lauda M3, accuracy ( 0.1 °C).
Irradiation with polychromatic light was performed with a KL
1500 electronic (Schott) lamp. The intensity of the light was
chosen to be 20% of the maximum intensity of the lamp for
kinetic investigations and 40% of the maximum intensity for
preparative reactions.
P r ep a r a tion of [P P N][Os3(µ-P R2)(CO)10] (R ) P h 1a ,
i-Bu 1b). In a Schlenk tube Os3(µ-H)(µ-PR2)(CO)10 (R ) Ph,
i-Bu) [100 mg; 0.964 mmol for R ) Ph, 0.100 mmol for R )
i-Bu] was dissolved in THF and treated with 1.1 equiv of DBU.
The reaction mixture was stirred for 1 h, at which point 1 equiv
of [PPN]Cl was added. Next the solvent was removed under
reduced pressure and the orange residue taken up in 5 mL of
methanol. On dropwise addition of water, a yellow precipitate
formed, which was isolated by filtration and successively
washed three times with water and hexane. The resulting
orange solid was dried under reduced pressure, giving pure
1a (133 mg, 87%) and 1b (130 mg, 83%), respectively. Analyti-
cal and spectroscopic data: 1a Anal. Calcd for C58H40NO10
-
Os3P3: C, 44.25; H, 2.56; N, 0.89. Found: C, 44.32; H, 2.43;
N, 0.78. IR (CH2Cl2): νCtO 2069 (sh), 2060 (w), 2004 (s), 1990
(vs), 1971 (m), 1942 (sh), 1928 (m). 1H NMR (CDCl3): 7.1-8.0
(m, Ph). 31P NMR (CDCl3): 21.7 (s, [PPN]+); 76.2 (s, µ-PPh2).
1b Anal. Calcd for C54H48NO10Os3P3: C, 42.27; H, 3.15; N, 0.91.
Found: C, 42.15; H, 3.02; N, 0.85. IR (CH2Cl2): νCtO 2052 (w),
2017 (sh), 1990 (vs), 1961 (m), 1936 (m), 1925 (m).
Rea ction of [P P N][Os3(µ-P R2)(CO)10] (R ) P h 1a , i-Bu
1b) w ith HgCl2. For this reaction daylight has to be entirely
excluded. In a Schlenk tube 13.0 mg (0.048 mmol) of HgCl2
was dissolved in 10 mL of THF and cooled to -90 °C. To the
colorless solution was added an orange solution of 1a (151 mg,
0.096 mmol) in 10 mL of THF. The resulting deep red solution
was stirred for 10 min at -90 °C. Then the mixture was
HgCl2, [PPN]Cl (bis(triphenylphosphane)iminium chloride),
and DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) were purchased
from Fluka and used as received. Os3(µ-H)2(µ3-η3-PPh(C6H4))-
(CO)934,35 and Os3(µ-H)(µ-PPh2)(CO)10 was prepared according
to literature methods.34 Os3(µ-H)(µ-P(i-Bu)2)(CO)10 was pre-
pared analogously to the latter compound from Os3(CO)11(HP-
(i-Bu)2),38,37 but this precursor was deprotonated with MeLi
at -90 °C instead of DBU.
(35) Colbran, S. B.; Irele, P. T.; J ohnson, B. F. G.; Lahoz, F. J .; Lewis,
J .; Raithby, P. R. J . Chem. Soc., Dalton Trans. 1989, 2033.
(36) Fajardo, M.; Holden, H. D.; J ohnson, B. F. G.; Lewis, J .; Raithby,
P. R. J . Chem. Soc., Chem. Commun. 1984, 24.
(37) J ohnson, B. F. G.; Lewis, J .; Pippard, D. A.. J . Chem. Soc.,
Dalton Trans. 1981, 407.

Cluster Complexes of Osmium and Mercury
Organometallics, Vol. 21, No. 9, 2002 1931
Ta ble 3. Cr ysta llogr a p h ic Da ta
2b
2a
3b
4a
formula
fw
C
44H20HgO20Os6P2
C36H36HgO20Os6P2
2192.4
C36H36HgO20Os6P2‚0.5pentane
2228.4
C44H20HgO20Os6P2‚CHCl3
2391.7
2272.3
cryst syst
space group
temperature/K
a/Å
b/Å
c/Å
monoclinic
P21/c
293(2)
19.482(4)
15.934(3)
18.154(5)
monoclinic
P21/c
203(2)
22.005(4)
9.169(2)
25.072(3)
triclinic
P1h
triclinic
P1h
203(2)
10.459(4)
16.475(3)
16.676(4)
83.63(1)
87.55(1)
82.58(1)
2830.6(14)
2
2.806
16.38
2136
203(2)
8.797(3)
11.403(2)
28.201(4)
84.76(2)
81.25(2)
71.98(3)
2655.9(11)
2
R/deg
â/deg
108.41(2)
91.45(1)
γ/deg
V/Å3
5347.1(19)
4
2.823
17.19
4040
5057.0(16)
4
2.880
18.17
3912
Z
D
calc/g cm-3
2.787
17.30
1998
µ(Mo KR)/mm-1
F(000)
scan range θ/deg
2.0 to 27.5
13 674/11 765
0.062/0.132
0.881
2.7 to 27.5
14 038/11 596
0.052/0.123
0.936
2.7 to 27.5
14 427/12 144
0.045/0.095
1.015
2.5 to 27.0
13 055/12 344
0.071/0.134
1.002
no. of reflns
measd/unique R1a/wR2b
Goof
min./max. ∆F/e Å-3
-0.84/0.96
-0.73/0.87
-1.21/1.38 near Os
-1.38/1.21 near Os
R1(F > 4σ(F)) ) ∑||Fo| - |Fc||/∑|Fo|. bwR2(F2, all data) ) [∑w(Fo - Fc2)2/∑w(Fo2)2]1/2
.
a
2
warmed to 0 °C and the solvent removed at reduced pressure.
The red residue was subjected to TLC separation using CH2-
Cl2/hexane (2:1) as eluent. Two main bands (violet, higher Rf
value, and red, lower Rf value) were developed, but did not
completely separate. The head of the violet band and the tail
of the red band were worked up. According to 31P NMR spectra
the violet fraction mainly consists of 3a and small amounts of
4a , whereas the red fraction mainly consists of 2a and smaller
amounts of 3a and 4a . From the violet fraction pure 3a in a
yield of about 15 mg (13%) was isolated by fractional crystal-
lization from 1,4-dioxane. From the red fraction pure 2a in a
yield of about 40 mg (37%) and 4a in a yield of about 7 mg
(6%) were isolated by fractional crystallization from n-hexane/
CHCl3. The fraction between head and tail was isolated as well
and consists of a mixture of 2a , 3a , and 4a . Since all of the
isolated cluster complexes have the same molecular mass their
overall yield was easily determined to be 78 mg (71%).
The reaction of 1b (148 mg, 0.096 mmol) with HgCl2 was
performed analogously to the above-described reaction except
pure 3b was obtained by crystallization of the violet fraction
from n-hexane/CHCl3. The overall yield in this case was 80
mg (76%).
NMR (CDCl3): 85.4 (s, µ-P). 4b Anal. Calcd for C36H36HgO20-
Os6P2: C, 19.72; H, 1.66. Found: C, 19.95; H, 1.56. IR (CH2-
Cl2): νCtO 2110 (w), 2069 (s), 2063 (vs), 2019 (s), 2005 (w), 2046
(s), 1990 (m), 1973 (w), 1967 (w). 1H NMR (CDCl3): 1.2-2.8
(m, i-Bu). 31P NMR (CDCl3): 54.5 (s, µ-P).
P r ep a r a tive P h otoch em ica l Con ver sion of µ4-Hg[Os3-
(µ-P P h 2)(µ-CO)(CO)9]2 (2a ) t o Os6(µ-Hg)(µ-P P h 2)2(CO)20
(4a ). In a 150 mL Schlenk tube 92 mg (0.04 mmol) 2a was
dissolved in 40 mL of THF. The solution was stirred and
simultaneously irradiated by polychromatic light at room
temperature. The course of the reaction was followed by
analytical TLC. When almost all of 2a had reacted, the solvent
was removed under reduced pressure and the red residue
subjected to TLC with CH2Cl2/hexane (2:1) as eluent. A tiny
red band of educt 2a (higher Rf value) and a large red band
containing 4a developed. In addition a large gray band of
decomposition products was observed on the starting spot.
From the first band 7 mg (8%) of 2a was isolated. 4a was
isolated from the main band in a yield of 48 mg (53%).
P r ep a r a tion of 13CO-En r ich ed Os3(µ-H)(µ-P P h 2)(CO)10
(5). In a glass autoclave 190 mg (0.188 mmol) of Os3(µ-H)2(µ3-
η3-PPh(C6H4))(CO)9, 6, was dissolved in 25 mL of toluene under
an argon atmosphere. Subsequently the vessel was closed, and
0.5 bar 13CO was added. Now the solution was heated to 80
°C and stirred at this temperature for 5 h. After cooling to
room temperature the overpressure was let off and the orange
solution was transferred into a flask. Reduced pressure was
then applied to remove the solvent. The orange residue
obtained was 182 mg (96% yield) of 13CO-enriched Os3(µ-H)-
(µ-PPh2)(CO)10 (5). The exact degree of enrichment was not
determined, but comparison with 13C NMR spectra of not
enriched 5 exhibited that enrichment with approximately the
same degree had taken place at all CO ligand positions. Anal.
Calcd for C12(13C)10H11O10Os3P: C, 26.17; H, 1.05. Found: C,
25.98; H, 1.22. IR (CH2Cl2): νCtO 2100 (m), 2054 (vs), 2047
(sh), 2017 (s), 1986 (m), 1976 (sh), 1950 (vw). 1H NMR (CDCl3):
An a lytica l a n d Sp ectr oscop ic Da ta . 2a Anal. Calcd for
C44H20HgO20Os6P2: C, 23.26; H, 0.89. Found: C, 22.99; H, 0.83.
IR (CH2Cl2): νCtO 2108 (vw), 2091(m), 2067 (vw), 2056 (m),
2031 (vs), 2009 (m), 1999 (w), 1981 (w), 1973 (m), 1803 (vw).
1H NMR (CDCl3): 7.0-8.0 (m, Ph). 13C NMR (CDCl3) (13CO-
enriched sample, CO region): 177.9 (d, J PC ) 6 Hz), 178.3 (d,
J PC ) 5 Hz), 180.9 (s), 182.7 (s), 184.0 (s), 186.4 (s), 205.7(d,
2J PC ) 15 Hz, µ-CO). 31P NMR (CDCl3): 216.6 (s, µ-P). 2b Anal.
Calcd for C36H36HgO20Os6P2: C, 19.72; H, 1.66. Found: C,
19.83; H, 1.45. IR (CH2Cl2): νCtO 2108 (vw), 2092 (w), 2058
(m), 2046 (m), 2025 (vs), 1981 (m), 1967(sh), 1813 (w). 1H NMR
(CDCl3): 1.2-2.5 (m, i-Bu). 31P NMR (CDCl3): 189.1 (s, µ-P).
3a Anal. Calcd for C44H20HgO20Os6P2: C, 23.26; H, 0.89.
Found: C, 23.41; H, 0.95. IR (CH2Cl2): νCtO 2112 (w), 2096
(vw), 2075 (sh), 2069 (m), 2054 (m), 2027 (vs), 1998 (m), 1975-
2
-18.07 (d, J PH ) 19.7 Hz, 1H, µ-H), 6.72-7.91 (m, 10H, Ph)
1
(m), 1963 (sh), 1813 (vw). H NMR (CDCl3): 6.7-8.0 (m, Ph).
ppm. 13C NMR (CDCl3): C(Ph): 127.5 (d, J CP ) 11 Hz), 128.8
31P NMR (CDCl3): 43.9 (s, µ-P), 209.7 (s, µ-P). 3b Anal. Calcd
for C36H36HgO20Os6P2: C, 19.72; H, 1.66. Found: C, 19.92; H,
1.33. IR (CH2Cl2): νCtO 2112 (w), 2094 (vw), 2070 (m), 2054
(m), 2030 (vs), 1998 (m), 1968(m), 1963 (sh), 1815 (vw). 1H
NMR (CDCl3): 1.2-2.6 (m, i-Bu). 31P NMR (CDCl3): 23.9 (s,
µ-P), 195.7 (s, µ-P). 4a Anal. Calcd for C44H20HgO20Os6P2: C,
23.26; H, 0.89. Found: C, 23.13; H, 0.88. IR (CH2Cl2): νCtO
2114 (w), 2071 (vs), 2054 (m), 2025 (vs), 2011 (m), 1998 (m),
1973 (sh), 1967 (m). 1H NMR (CDCl3): 7.1-8.1 (m, Ph). 31P
1
(s), 129.0 (d, J CP ) 12 Hz), 131.3 (d, J CP ) 53 Hz, C1), 131.4
(d, J CP ) 3 Hz), 132.4 (d, J CP ) 11 Hz), 136.1 (d, J CP ) 10 Hz),
1
2
145.9 (d, J CP ) 43 Hz), C (CO): 172.2 (d, J CP ) 7 Hz), 173.0
2
(d, J CP ) 8 Hz), 173.6 (s (broad)), 177.6 (s), 178.4 (s), 180.2
(d, J CP ) 8 Hz). 31P NMR (CDCl3): 16.1 (s, µ-P).
2
Cr ysta l Str u ctu r e Deter m in a tion s. Crystals suitable for
X-ray analysis for compounds 2a , 2b, 3b, and 4a were obtained
from chloroform/pentane solutions. Despite various attempts

1932 Organometallics, Vol. 21, No. 9, 2002
Egold et al.
for 4a only crystals of minor quality and scattering power could
be gained.
each temperature. The temperature dependence of the rate
constants was evaluated by an Eyring plot,41 leading to the
thermodynamic parameters of activation at T ) 298.2 K (Table
1). The mean deviation of these parameters was determined
according to Gauss’s law of propagation of error.
Pertinent crystallographic data are summarized in Table
3. All data sets were collected on a Bruker AXS P4 diffracto-
meter with graphite-monochromated Mo KR radiation. Stan-
dard reflections monitored after every 400 reflections showed
a decrease of 1-2% for 2a , 2b, and 4a and of 9% for 3b,
respectively. The intensities of the data sets were corrected
accordingly. Intensities of all data sets were corrected for
Lorentz-polarization effects, and absorption corrections via
ψ-scans were applied. The structures were solved by direct and
conventional Fourier methods. Full-matrix least-squares re-
finement was based on F2. All but hydrogen atoms were refined
anisotropically; geometrically placed hydrogen atoms were
refined with a riding model and U(H) ) 1.2 U(Ciso) and
1.5U(Ciso) for methyl-C. Programs used for calculations:
SHELX-97.39
P h otoch em ica l Rea r r a n gem en t 2a f 4a . The reaction
was monitored by UV/vis spectroscopy at 5 °C in THF from
190 to 700 nm with a scan rate for each spectrum of 60 nm
min-1. The concentration of 2a amounted to 0.0426 mmol L-1
.
After the recording of each spectrum the cuvette was irradiated
with polychromatic light for 60 s. Then the next spectrum was
recorded. This procedure was repeated six times. Eventually
in order to complete conversion, the curette was irradiated
with more intense polychromatic light for 10 min. The final
spectrum recorded is identical with a spectrum of a solution
of pure 4a .
UV/Vis Kin etics of th e Rea r r a n gem en t 2a f 3a . All
important data are summarized in Table 1. The scan rate of
all spectra recorded was 240 nm min-1. At higher temperatures
the measuring range was reduced in order to get a sufficient
number of spectra. The measurements were performed under
complete exclusion of daylight and were ended long before
conversion of 2a to 3a was complete, as at longer periods of
time secondary reactions occurred, making the evaluation of
the obtained data impossible. Subsequently periodically oc-
curring errors in measurement were eliminated according to
literature methods.40 The resulting reduced sets of data were
analyzed by formal integration,32 giving the rate constants at
Su p p or t in g In for m a t ion Ava ila b le: Refined atomic
coordinates, bond lengths and angles, anisotropic thermal
parameters, and calculated hydrogen atomic coordinates for
2a , 2b, 3b, and 4a . This material is available free of charge
via the Internet at http://pubs.acs.org.
OM0110585
(39) Sheldrick, G. M. SHELX-97, A Program for Crystal Structure
Solution and Refinement; University of Go¨ttingen, 1998.
(40) Swinbourne, E. S. Auswertung und Analyse kinetische Messun-
gen; Verlag Chemie: Weinheim, 1975.
(41) Wedler, G. Lehrbuch der physikalischen Chemie, 4.Auflage;
VCH-Wiley: Weinheim, 1997.
(38) Drake, S. R.; Khattar, R. Organomet. Synth. 1988, 4, 234.
(42) Egold, H.; Schraa, M.; Flo¨rke, U., unpublished results.