Dialkyl and trialkyl heterobinuclear complexes of rhodium and iridium: Models for adjacent-metal involvement in bimetallic catalysts

Article abstract of DOI:10.1021/om058013e

The heterobinuclear dialkyl complexes [RhIr(R)₂(μ-CO)(dppm) ₂] (dppm = μ-Ph₂PCH₂PPh₂; R = CH₃ (2), CH₂Ph (3)) have been prepared. Both A-frame-like compounds have one alkyl group terminally bound to each metal and a bridging carbonyl ligand. Some subsequent reactivity studies of 2 are reported. Reaction of 2 with CO yields [RhIr(CO)₃(dppm)₂] and acetone. If this reaction is monitored at low temperature by NMR spectroscopy, the dicarbonyl species [RhIr(CH₃)₂(CO)₂(dppm)2] (4) is first observed, followed by [RhIr(CH₃)₂(CO) ₃(dppm)₂] (5). In both products, both methyl groups are bound to Ir. Warming to ambient temperature under CO yields acetone and [RhIr(CO)₃(dppm)₂]. Crossover experiments suggest that acetone arises primarily from an intramolecular process. We propose that migratory insertion of a CO and a methyl group occurs on Ir; presumably reductive elimination also occurs from this metal. Compound 2 reacts with H ₂ at -78°C to yield [RhIrH(CH₃)₂(μ-H) (μ-CO)(dppm)₂] (6), in which one methyl group is bound to Rh while the other, together with a hydride ligand, is terminally bound to Ir. This latter species reacts with CO at 0°C to yield [RhIrH-(C(O)CH ₃)(CH₃)(μ-H)(μ-CO)(dppm)₂] (7), in which migratory insertion involving CO and the Rh-bound methyl group has yielded a Rh-bound acetyl group. At ambient temperature, under an atmosphere of CO, [RhIr(CO)₃(dppm)₂] is formed, together with acetaldehyde and methane. Crossover experiments support a predominantly intramolecular process for acetaldehyde formation but are equivocal on the formation of methane. Compound 2 oxidatively adds CH₃I or n-C₄H ₉I, yielding [RhIr(CH₃)₂(R)(μ-I)(μ-CO) (dppm)₂] (R = CH₃, N-C₄H₉), in which the added alkyl group in each case is bound to Ir. The N-butyl product reacts with CO to yield [RhIr(n-C₄H₉)(CH₃) ₂(μ-CO)₂(dppm)₂][I], in which the n-butyl group has migrated to Rh. A similar product, [RhIr(CH₃) ₃(μ-CO)₂(dppm)₂][CF₃SO ₃], is obtained in the reaction of 2 with methyl triflate in the presence of CO.

Full text of DOI:10.1021/om058013e

Organometallics 2005, 24, 4393-4405
4393
Dialkyl and Trialkyl Heterobinuclear Complexes of
Rhodium and Iridium: Models for Adjacent-Metal
Involvement in Bimetallic Catalysts
Robert W. Hilts,† Okemona Oke, Michael J. Ferguson,‡ Robert McDonald,‡ and
Martin Cowie*
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received March 16, 2005
The heterobinuclear dialkyl complexes [RhIr(R)2(µ-CO)(dppm)2] (dppm ) µ-Ph2PCH2PPh2;
R ) CH3 (2), CH2Ph (3)) have been prepared. Both A-frame-like compounds have one alkyl
group terminally bound to each metal and a bridging carbonyl ligand. Some subsequent
reactivity studies of 2 are reported. Reaction of 2 with CO yields [RhIr(CO)3(dppm)2] and
acetone. If this reaction is monitored at low temperature by NMR spectroscopy, the dicarbonyl
species [RhIr(CH3)2(CO)2(dppm)2] (4) is first observed, followed by [RhIr(CH3)2(CO)3(dppm)2]
(5). In both products, both methyl groups are bound to Ir. Warming to ambient temperature
under CO yields acetone and [RhIr(CO)3(dppm)2]. Crossover experiments suggest that acetone
arises primarily from an intramolecular process. We propose that migratory insertion of a
CO and a methyl group occurs on Ir; presumably reductive elimination also occurs from
this metal. Compound 2 reacts with H2 at -78 °C to yield [RhIrH(CH3)2(µ-H)(µ-CO)(dppm)2]
(6), in which one methyl group is bound to Rh while the other, together with a hydride
ligand, is terminally bound to Ir. This latter species reacts with CO at 0 °C to yield [RhIrH-
(C(O)CH3)(CH3)(µ-H)(µ-CO)(dppm)2] (7), in which migratory insertion involving CO and the
Rh-bound methyl group has yielded a Rh-bound acetyl group. At ambient temperature, under
an atmosphere of CO, [RhIr(CO)3(dppm)2] is formed, together with acetaldehyde and
methane. Crossover experiments support a predominantly intramolecular process for
acetaldehyde formation but are equivocal on the formation of methane. Compound 2
oxidatively adds CH3I or n-C4H9I, yielding [RhIr(CH3)2(R)(µ-I)(µ-CO)(dppm)2] (R ) CH3,
n-C4H9), in which the added alkyl group in each case is bound to Ir. The n-butyl product
reacts with CO to yield [RhIr(n-C4H9)(CH3)2(µ-CO)2(dppm)2][I], in which the n-butyl group
has migrated to Rh. A similar product, [RhIr(CH3)3(µ-CO)2(dppm)2][CF3SO3], is obtained in
the reaction of 2 with methyl triflate in the presence of CO.
Introduction
on bimetallic catalysts,4 in which two different metals
are involved, an obvious extension of the modeling
studies is to mixed-metal clusters containing two dif-
ferent metal types.5 Although small, well-defined cluster
complexes cannot be expected to effectively model all
aspects of surface reactivity, they can yield valuable
information relating to substrate binding and the
Small cluster complexes, containing two or more
adjacent metals, have attracted considerable attention,
both as models for processes occurring at adjacent metal
sites on the surfaces of heterogeneous catalysts1 and
because such complexes may display novel reactivity in
their own right, on the expectation that adjacent metals
can act in a cooperative manner in the activation of
substrate molecules.2 The utility of discrete clusters as
models for surface reactivity is based on the premise
that processes occurring on metal surfaces do not differ
in any fundamental way from those taking place in
simple homogeneous systems,3 yet these latter systems
are much easier to study. With the increasing emphasis
(2) See for example: (a) Esteruelas, M. A.; Garcia, M. P.; Lo´pez, A.
M.; Oro, L. A. Organometallics 1991, 10, 127. (b) Broussard, M. E.;
Juma, B.; Train, S. G.; Peng, W.-J.; Laneman, S. A.; Stanley, G. G.
Science 1993, 260, 1784. (c) Su¨ss-Fink, G. Angew. Chem., Int. Ed. Engl.
1994, 33, 67. (d) Baranger, A. M.; Bergman, R. G. J. Am. Chem. Soc.
1994, 116, 3822. (e) Matthews, R. C.; Howell, D. K.; Peng, W.-J.; Train,
S. G.; Treleaven, W. D.; Stanley, G. G. Angew. Chem., Int. Ed. Engl.
1996, 35, 2253. (f) Ishii, Y.; Miyashita, K.; Kamita, K.; Hidai, M. J.
Am. Chem. Soc. 1997, 119, 6448. (g) Sola, E.; Bakhmutov, V. I.; Torres,
F.; Elduque, A.; Lo´pez, J. A.; Lahoz, F. J.; Werner, H.; Oro, L. A.
Organometallics 1998, 17, 683. (h) Komiya, S.; Yasuda, T.; Fukuoka,
A.; Hirano, M. J. Mol. Catal. A: Chem. 2000, 159, 63. (i) Ishii, Y.; Hidai,
M. Catal. Today 2001, 66, 53. (j) Sola, E.; Torres, F.; Jime´nez, M. V.;
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Chem. Rev. 1992, 92, 1709. (f) Schmid, G., Ed. Clusters and Colloids;
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Chem. Soc. 1999, 121, 529.
* Corresponding author. E-mail: martin.cowie@ualberta.ca.
† Present address: Grant McEwan College, Edmonton, AB, Canada.
‡ X-ray Crystallography Laboratory.
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(b) Brady, R. C., III; Pettit, R. J. Am. Chem. Soc. 1980, 102, 6181. (c)
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Johnson, B. F. G., Ed.; John Wiley & Sons Ltd.: New York, 1980;
Chapter 7. (d) Knox, S. A. R. J. Organomet. Chem. 1990, 400, 255. (e)
Sundararajan, G. Organometallics 1991, 10, 1377. (f) Reynolds, G. G.;
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10.1021/om058013e CCC: $30.25 © 2005 American Chemical Society
Publication on Web 07/27/2005

4394 Organometallics, Vol. 24, No. 18, 2005
Hilts et al.
subsequent transformations of the bound substrates,
since these clusters can support both terminal and
bridging coordination modes that also appear on metal
surfaces.1g
In considering the effects of metal-metal cooperat-
ivity in substrate activation, the incorporation of dif-
ferent adjacent metals clearly expands the scope of
metal-promoted activation through the wide range of
metal combinations that can be used. Furthermore,
when studying the reactivity of such systems, it is of
interest to establish the metal at which different
processes arise and the roles of the different metals in
promoting the transformations of interest.
We maintain that the prototype clustersthe binuclear
complex, containing only two adjacent metalssis a key
species in establishing the roles of adjacent metals in
substrate activation. Not only do binuclear systems have
the ability to support substrate binding in either bridg-
ing or terminal coordination modes, but the lack of
unnecessary complexity in these two-metal systems
makes them ideal for establishing the natures of metal-
metal and metal-substrate interactions and in estab-
lishing the intimate details of how the substrate trans-
formations occur.6
In this study we extend our previous investigations
on homo-7 and heterobinuclear complexes7a-c,8 contain-
ing a single methyl ligand to complexes involving the
Rh/Ir combination of metals which include two and
three alkyl groups. Alkyl complexes of the late transition
metals are of relevance in a variety of catalytic processes
such as olefin hydrogenation, hydrosilation, and hydro-
formylation,9 Fischer-Tropsch chemistry,10 and metha-
nol carbonylation.11 Complexes containing two or more
alkyl groups can yield additional information about C-C
bond-formation processes. In our heterobinuclear com-
plexes we are attempting to establish the preference of
molecular fragments that are relevant in catalytic
transformations (e.g., CH3, H, CO) to occupy the differ-
ent metal sites and to establish the tendencies of the
different metals toward important processes such as
oxidative addition, migratory insertion, and reductive
elimination. Although a number of related dialkyl
homobinuclear, late-metal complexes have been re-
ported for a number of metals.12-18 similar reports
involving heterobinuclear species are much fewer.5a,19
Our ultimate goal is to obtain an improved understand-
ing about the roles of the different metals in bimetallic
systems.
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Rh and Ir Heterobinuclear Complexes
Organometallics, Vol. 24, No. 18, 2005 4395
because of its extreme air sensitivity (the brick-red solid turned
yellow instantly upon exposure to air). Attempts to obtain
high-resolution mass spectra also failed. 31P{1H} and 1H NMR
spectra of 2 are shown in Figures S1 and S2 in the Supporting
Information. Samples of 2 containing 13CO were prepared as
described above starting with [RhIrCl2(13CO)2(dppm)2].
(b) [RhIr(13CH3)2(µ-13CO)(dppm)2] (2a). Neat 13CH3I (0.500
mL, 1.14 g, 7.98 mmol) was added by syringe to a rapidly
stirring 5.00 mL sample of 1.6 M n-BuLi in hexanes inside a
centrifuge tube that had been cooled to 0 °C in a water/ice
bath. The water/ice bath was removed, and the mixture was
allowed to achieve ambient temperature. About 5 min after
ambient temperature had been reached, a white solid began
to precipitate from solution. The mixture was centrifuged for
20 min, and the colorless supernatant above the compacted
white solid in the bottom of the tube was removed by syringe.
Pentane (5 mL, freshly distilled) was added and the tube was
shaken, producing a milky-white suspension. The tube was
centrifuged a second time for 20 min, and the pentane
supernatant was discarded. Diethyl ether (10 mL) was added,
affording a white suspension, which was transferred through
a cannula to a rapidly stirring orange solution of 0.100 g (0.084
mmol) of [RhIrCl2(13CO)2(dppm)2] in 100 mL of freshly distilled
THF at -78 °C. The dry ice/acetone bath was removed, and
the mixture was allowed to gradually warm to room temper-
ature. Once the solution reached about 0 °C, it changed color
from yellow to orange-red. The solution was stirred for 3 h,
and then the mixture was transferred to a flask containing
ca. 50 g of thoroughly dried and deoxygenated Florisil. The
mixture was stirred over the Florisil for 30 min and then
filtered through a layer of Celite into another flask. The solvent
volume was reduced to ca. 5 mL under vacuum, and 40 mL of
pentane was added, causing the immediate precipitation of a
red solid. The solid was washed twice with 10 mL of pentane,
then dried overnight under vacuum. Mass of red solid iso-
lated: 0.045 g (48% yield).
Experimental Section
General Comments. All solvents were appropriately deoxy-
genated and dried by distillation over the appropriate drying
agents prior to use and were stored under dinitrogen. Deu-
terated solvents used for NMR experiments were degassed and
stored under dinitrogen over molecular sieves. Reactions were
carried out routinely at room temperature (unless otherwise
stated) and under standard Schlenk conditions; compounds
that were isolated as solids were purified by recrystallization.
Hydrated rhodium trichloride was purchased from Engelhard
Scientific, whereas methyl triflate (CH3OSO2CF3), benzylmag-
nesium chloride (2 M in THF), methyllithium (1 M solution
in THF/cumene), methyllithium-d3‚lithium iodide complex (0.5
M solution in diethyl ether), and methyl iodide were obtained
from Aldrich and were used as received. Hydrogen was
purchased from Linde, and carbon monoxide was purchased
from Matheson. 13CO was supplied by Isotec Inc. All gases were
used as received. The compound trans-[RhIr(Cl)2(CO)2(dppm)2]20
(dppm ) µ-Ph2PCH2PPh2; 1) was prepared as previously
reported.
All routine NMR experiments were conducted on a Bruker
AM-400 spectrometer, whereas the 13C{31P} experiments were
conducted on a Bruker AM-200 spectrometer operating at 50.3
MHz, or the AM-400 spectrometer operating at 100.6 MHz.
1H-1H COSY experiments were performed on the AM-400
spectrometer. Infrared spectra were obtained on a Nicolet 7199
Fourier transform or a Perkin-Elmer 883 IR spectrometer,
either as Nujol mulls on KBr plates or as solutions in KCl cells
with 0.5 mm window path lengths. Elemental analyses were
performed by the microanalytical service within our depart-
ment. Spectroscopic data for all compounds are given in Table
1.
Preparation of Compounds. (a) [RhIr(CH3)2(µ-CO)-
(dppm)2] (2). [RhIrCl2(CO)2(dppm)2] (1) (200 mg, 0.17 mmol)
was dissolved in 150 mL of freshly distilled THF, and the
resulting yellow solution was cooled to -78 °C in a dry ice/
acetone bath. Ten molar equivalents of methyllithium (1.68
mL, 1.0 M solution in THF) was added via a gastight syringe,
causing the solution to darken slightly. After removing the dry
ice/acetone bath to allow the reaction mixture to warm to
ambient temperature, the mixture was stirred vigorously for
3 h, during which time the solution changed from yellow to
red. The mixture was then transferred through a cannula to
a second vessel containing ca. 75 g of thoroughly deoxygenated
Florisil under argon, and the resulting red slurry was trans-
ferred through a cannula to a filter frit tube containing a 5
cm layer of deoxygenated Celite. The supernatant was drawn
through the Celite into a 250 mL collection flask filled with
Ar by applying a partial vacuum. The solution volume was
reduced to ca. 5 mL under dynamic vacuum, the flask was
backfilled with Ar, and 40 mL of freshly distilled, scrupulously
deoxygenated pentane was added, causing the precipitation
of a brick-red solid. The solid was washed with 2 × 20 mL of
the aforementioned pentane and then dried overnight in vacuo.
Mass of compound 2 isolated: 0.119 g (yield 62%). A satisfac-
tory elemental analysis could not be obtained for compound 2
(c) [RhIr(CH2Ph)2(µ-CO)(dppm)2] (3). The procedure is
the same as described above except that 100 mg (0.084 mmol)
of [RhIrCl2(CO)2(dppm)2], benzylmagnesium chloride (420 µL,
2 M solution in THF), and 50 mL of dry THF were used and
the mixture was stirred for 4 h at room temperature. The
volume was reduced by one-half and the solution was filtered
through a frit followed by the addition of 50 mL of n-pentane,
causing the precipitation of a dull yellow solid. The extremely
air-sensitive solid was recrystallized from benzene/n-pentane
and dried under N2. Satisfactory microanalyses could not be
obtained due to its extreme air and moisture sensitivity.
(d) [RhIr(CH3)2(CO)2(dppm)2] (4). Carbon monoxide gas
(1.0 mL, 0.041 mmol) was injected by gastight syringe into a
red solution of 0.033 g (0.030 mmol) of compound 2 in 1.0 mL
of CD2Cl2 that had been cooled to -78 °C in a dry ice/acetone
bath. The mixture was warmed to -40 °C and then stirred
vigorously for 15 min. During this time the solution changed
1
1
color from red to orange. The P{1H}, H, and 13C{1H} NMR
spectra for the orange solution showed that the starting
organometallic compound 2 had been essentially completely
converted to [RhIr(CH3)2(CO)2(dppm)2] (4). However, the prod-
uct could not be isolated pure, decomposing within hours, so
was characterized by spectroscopic techniques.
(17) (a) Fallis, K. A.; Xu, C.; Anderson, G. K. Organometallics 1993,
12, 2243. (b) Ladipo, F. T.; Anderson, G. K. Organometallics 1994, 13,
303. (c) Fallis, K. A.; Anderson, G. K.; Lin, M.; Rath, N. P. Organo-
metallics 1994, 13, 478. (d) Xu, C.; Anderson, G. K. Organometallics
1994, 13, 3891. (e) Ladipo, F. T.; Anderson, G. K.; Rath, N. P.
Organometallics 1997, 16, 5096.
(18) (a) Tejel, C.; Ciriano, M. A.; Edwards, A. J.; Lohoz, F. J.; Oro,
L. A. Organometallics 1997, 16, 45. (b) Tejel, C.; Ciriano, M. A.;
Edwards, A. J.; Lohoz, F. J.; Oro, L. A. Organometallics 2000, 19, 4968.
(c) Tejel, C.; Ciriano, M. A.; Lo´pez, J. A.; Lahoz, F. J.; Oro, L. A.
Organometallics 2000, 19, 4977.
(19) See for example: (a) Swartz, D. J.; Ball, G. E.; Andersen, R. A.
J. Am. Chem. Soc. 1995, 117, 6027. (b) Proulx, G.; Bergman, R. G. J.
Am. Chem. Soc. 1996, 118, 1981.
(20) Hutton, A. T.; Pringle, P. G.; Shaw, B. L. Organometallics 1983,
2, 1889.
(e) [RhIr(CH3)2(CO)3(dppm)2] (5). A sample of 4, prepared
as described in part (d), was maintained at -40 °C, and carbon
monoxide was passed through the solution to replace the
dinitrogen atmosphere above the solution. 31P{1H} NMR
spectroscopy established that compound 5 was generated in
approximately 65% yield together with 4 (25%) and several
unidentified species. Removal of the CO atmosphere resulted
in conversion of 5 to 4, while the unidentified products
remained. Warming the temperature to ambient under the CO
atmosphere resulted in the slow conversion to the known
species [RhIr(CO)3(dppm)2]21 in approximately 30% yield of the
phosphorus-containing products together with unidentified

4396 Organometallics, Vol. 24, No. 18, 2005
Hilts et al.

Rh and Ir Heterobinuclear Complexes
Organometallics, Vol. 24, No. 18, 2005 4397
products. Characterization of 5 was by 31P{1H}, 13C{1H}, and
1H NMR spectroscopy at -40 °C.
situ via the reaction of a sample of compound 2 with a
stoichiometric amount of CO gas.) The dry ice/acetone bath
was removed, ambient temperature was achieved after 15 min,
and stirring was continued for 3 h. Gradually, the solution
changed from orange to yellow. The solvent was removed
under vacuum, affording a yellow-brown residue. Recrystal-
lization of the solid from CH2Cl2/hexane (1:1) afforded yellow-
brown crystals of [RhIr(CH3)2(n-Bu)(µ-CO)2(dppm)2][I]‚2CH2Cl2
(10) (0.027 g, 20%). Anal. Calcd for C60H63Cl4IP4O2RhIr: C,
47.92; H, 4.22. Found: C, 47.80; H, 4.59. Compound 10 was
also prepared inadvertently as the product from the reaction
of n-BuI-contaminated 13CH3Li with [RhIrCl2(13CO)2(dppm)2],
during an early attempt to prepare [RhIr(13CH3)2(µ-13CO)-
(dppm)2].
(k) [RhIr(CH3)3(CO)2(dppm)2][CF3SO3] (11). Compound
2 (50 mg, 0.045 mmol) was dissolved in 2 mL of CH2Cl2 at
-78 °C. One equivalent of methyl triflate (5 µL, 0.044 mmol)
was added via a syringe. This was stirred for 5 min, and an
atmosphere of CO was then placed over the solution. The
reaction mixture was stirred for 0.5 h and then taken to
ambient temperature over a 15 min period. The orange product
was precipitated and washed twice with 5 mL of ether and
dried under dinitrogen gas stream and then in vacuo. Yield:
35 mg, 61%. Anal. Calcd for IrRhSP4O5F3C56H53: C, 51.18; H,
4.07. Found: C, 51.41; H, 4.08.
(f) [RhIr(H)(CH3)2(µ-H)(µ-CO)(dppm)2] (6). Compound 2
(90 mg, 0.080 mmol) was dissolved in 15 mL of THF, and the
resulting red solution was cooled to -78 °C in a dry ice/acetone
bath. Hydrogen gas was slowly passed through the solution
at ca. 0.1 mL s-1 for a period of 5 min. The solution was then
slowly warmed to room temperature under an H2 purge and
stirred until the deep red color turned pale yellow (∼10 min).
Dichloromethane (20 mL) was added, the small quantity of
white suspended particles (assumed to be residual LiCl) was
allowed to settle, the clear yellow supernatant was transferred
to another vessel, and the solvent volume was reduced to ca.
5 mL under vacuum. Carefully degassed pentane (40 mL) was
added, affording a yellow solid. The supernatant was dis-
carded, and the solid was washed with 2 × 10 mL of pentane
and then dried under dynamic vacuum for 3 h. Mass of
compound 6 isolated: 0.043 g (48% yield). A satisfactory
elemental analysis for this compound could not be obtained
owing to decomposition of the solid over a several-hour period,
presumably initiated by methane loss. The isotopomer [RhIr-
(
13CH3)2(D)(µ-D)(µ-13CO)(dppm)2] (6a) was prepared in an
analogous fashion starting with [RhIr(13CH3)2(µ-13CO)(dppm)2]
and using D2 in place of H2. The 31P{1H} and 1H NMR spectra
of a 13CO-enriched sample of 6 are given in Figures S3 and S4
in the Supporting Information.
Low-Temperature Intermediates in the Reaction of
Compound 2 with CO. The procedure for preparing com-
pound 4 was repeated except that 1 mL of CO gas was added
at -78 °C and the NMR analysis was carried out at -40 °C.
The major species in solution, in approximately 65% yield, was
[RhIr(CH3)2(CO)3(dppm)2] (5) together with several unidenti-
fied minor species. Compound 5 can also be obtained by cooling
a solution of 4 to -40 °C and adding 1 mL of CO gas at this
temperature.
(g) [RhIr(H)(COCH3)(CH3)(µ-H)(µ-CO)(dppm)2] (7). The
procedure described above for 6 was repeated. Once the pale
yellow solution was obtained, the sample was transferred via
a syringe to an NMR tube. The tube was then immersed in a
dry ice/acetone bath to bring the sample temperature to -78
°C, and CO gas was passed through the solution briefly. NMR
data (1H, 13C{1H}, 31P{1H}) collected at 0 °C after allowing the
solution to stand at this temperature for 2 h showed 7 as the
major product in addition to variable amounts of 6 and the
known tricarbonyl species [RhIr(CO)3(dppm)2].21 Upon warm-
ing to room temperature, compound 7 reacted further with CO
to form [RhIr(CO)3(dppm)2], between 40 and 45% yield together
with equivalent amounts of acetaldehyde and methane. The
presence of methane and acetaldehyde was confirmed by their
Crossover Experiments Involving Compound 6. A
solution containing 0.061 g of compound 6 in 0.50 mL of C6D6
was combined with a solution containing approximately 0.02
g of compound 6a in 0.50 mL of C6D6 in a 5 mm NMR tube.
The mixture was cooled to 0 °C in an ice bath, and 5.0 mL of
13CO gas was injected into the solution via a gastight syringe.
The mixture was warmed to room temperature and then
stirred for 20 min. The NMR spectra for the sample showed
that the expected 13CO-insertion products, [RhIr(H)(13C(O)-
CH3)(CH3)(13CO)(µ-H)(dppm)2] and [RhIr(D)(13C(O)13CH3)-
(13CH3)(13CO)(µ-D)(dppm)2], had been formed and that some
free 13CO was present in solution. The NMR tube containing
the insertion products and free 13CO was kept under Ar
overnight at 23 °C. An NMR spectroscopic analysis of the
reaction mixture the following morning revealed the presence
of CH313C(O)H, 13CH313C(O)D, 13CH313C(O)H, and CH313C(O)D
in an approximate 16.6:4.2:1.0:0.75 ratio based on integration
of the 13C-carbonyl signals in the 13C{1H} NMR spectrum. The
1H NMR spectrum showed the presence of 13CH4, 13CH3D, CH4,
and CH3D in an approximate 1:1.6:6.0:1.1 ratio. Integration
of the resonances for the methane isotopomers was not
accurate owing to substantial overlap of the signals. The 31P-
{1H} and 1H NMR spectra revealed the presence of the known
compound [RhIr(CO)3(dppm)3]21 as approximately 45% of the
products observed. Three other unidentified RhIr(dppm)2-
containing products, in comparable proportions, accounted for
the remaining phosphorus-containing species. No effort was
made to identify these unknowns beyond attempted compari-
sons of their spectra with those of known species.
1
resonances in the H NMR spectrum.
(h) [RhIr(CH3)3(µ-I)(µ-CO)(dppm)2] (8). Neat CH3I (11
µL, 0.18 mmol) was added by microliter syringe to a solution
of compound 2 (0.200 g, 0.178 mmol) in 8 mL of THF at room
temperature. The solution changed immediately from cherry-
red to orange-brown. The mixture was stirred for 1 h, and the
solvent was stripped off in vacuo, affording an orange solid.
Recrystallization of the solid from CH2Cl2/Et2O at 22 °C gave
[RhIr(CH3)3(µ-I)(µ-CO)(dppm)2] (8) (0.081 g, 36%) as yellow
crystals. Anal. Calcd for C54H53P4ORhIrI: C, 51.23; H, 4.22.
Found: C, 51.32; H, 4.14.
(i) [RhIr(CH3)2(n-Bu)(µ-I)(µ-CO)(dppm)2] (9). Neat n-
BuI (10 µL, 0.088 mmol) was added by microliter syringe to a
solution of compound 2 (0.100 g, 0.089 mmol) in 20 mL of THF
at 22 °C. The solution was stirred for 6 h, during which time
the solution gradually changed from red to orange-brown. The
solvent volume was reduced to ca. 2 mL in vacuo, and 15 mL
of diethyl ether was added, causing the immediate formation
of a yellow precipitate. The supernatant was discarded and
the solid washed with 10 mL of diethyl ether. Recrystallization
from CH2Cl2/Et2O (1:1) at ambient temperature afforded [RhIr-
(CH3)2(n-Bu)(µ-I)(µ-CO)(dppm)2]‚2CH2Cl2 (9) (0.037 g, 0.025
mmol, 29%) as yellow crystals. Anal. Calcd for C59H63Cl4IP4-
ORhIr: C, 48.01; H, 4.30. Found: C, 48.18; H, 3.85.
T1 Measurements. To determine the possible influence of
deuterium substitution at the acyl carbon on the integration
of this carbon in the 13C NMR experiments, a determination
of the T1 values for acetaldehyde and acetaldehyde-d1 was
carried out. T1 measurements were carried out on a Varian
Unity 500 MHz instrument using the inversion recovery
method (180-τ-90-aq). Owing to the low boiling point of the
compounds (21 °C), the T1 measurements were carried out at
(j) [RhIr(CH3)2(n-Bu)(µ-CO)2(dppm)2][I] (10). A colorless
solution of n-BuI (10 µL, 0.088 mmol) in 20 mL of CH2Cl2 was
added through a cannula to a rapidly stirring solution of [RhIr-
(CH3)2(CO)2(dppm)2] (4) (ca. 0.105 g, ca. 0.091 mmol) in 20 mL
of CH2Cl2 at -78 °C. (NOTE: compound 4 was prepared in
(21) McDonald, R.; Cowie, M. Inorg. Chem. 1990, 29, 1564.

4398 Organometallics, Vol. 24, No. 18, 2005
Table 2. Crystallographic Data for Compounds 8 and 10
Hilts et al.
[RhIr(CH3)3(µ-I)(µ-CO)(dppm)2] (8)‚2CH2Cl2
[RhIr(CH3)(n-C4H9)(µ-CO)2(dppm)2][I] (10)‚2CH2Cl2
formula
C56H57Cl4IIrOP4Rh
C60H63Cl4IIrO2P4Rh
1503.79
fw
1433.71
cryst dimens, mm
0.36 × 0.10 × 0.09
0.41 × 0.19 × 0.14
monoclinic
C2/c (No. 15)
48.012(3)b
11.0171(7)
23.225(2)
102.734(1)
11983(1)
cryst syst
space group
a, Å
monoclinic
P21/n (alternate setting of P21/c (No. 14))
20.098(2)a
12.289(1)
22.645(2)
90.942(2)
5592.1(9)
4
b, Å
c, Å
â, deg
V, Å3
Z
8
F
calcd, g cm-3
1.703
3.568
1.667
µ, mm
3.336
radiation (λ, Å)
T, °C
scan type
graphite-monochromated Mo KR (0.71073)
-80
-80
ω scans (0.2°) (25 s exposure)
ω scans (0.2°) (20 s exposure)
2θ(max), deg
total data collected
no. of unique rflns
no. of observns
range of transmn factors
52.80
52.76
27 226 (-25 e h e 20, -14 e k e 15, -25 e l e 28) 32 913 (-58 e h e 60, -13 e k e 13, -21 e l e 29)
11423 (Rint ) 0.0622)
7786 (Fo2 g 2σ(Fo2))
0.7395-0.3599
12250 (Rint ) 0.0360)
9996 (Fo2 g 2σ(Fo2))
0.6524-0.3417
no. of data/restraints/params 11 423 (Fo2 g 3σ(Fo2))/12c/512
12 250 (Fo2 g 3σ(Fo2))/0/658
1.765 to -1.005
0.0356
residual density, e/Å3
R1 (Fo2 g 2σ(Fo2))d
wR2 (Fo2 g -3σ(Fo2))
GOF (S)e
8.313 to -1.530
0.0694
0.1958
0.0949
1.022 (Fo2 g -3σ(Fo2))
1.038 (Fo2 g -3σ(Fo2))
a Cell parameters obtained from least-squares refinement of 6253 centered reflections. b Cell parameters obtained from least-squares
refinement of 6637 centered reflections. c Distances within the disordered solvent molecule (CH2Cl2) were assigned fixed idealized values
2
2
2
2
(d(Cl-C) ) 1.80 Å; d(Cl‚‚‚Cl) ) 2.95 Å). d R1 ) ∑||Fo| - |Fc||/|Fo|; wR2 ) ∑w(Fo - Fc )2/∑w(Fo4)]1/2
.
e S ) [∑w(Fo - Fc )2/(n - p)]1/2 (n )
number of data, p ) number of parameters varied; w ) [σ2(Fo2) + (aoP)2 + a1P]-1, where P ) [max (Fo2, 0) ) 2Fc ]/3. For 8, ao ) 0.1183
2
and a1 ) 7.9278. For 10, ao ) 0.0498 and a1 ) 28.9800.
4.5 °C. The temperature inside the probe was calibrated
against a standard methanol sample provided by Varian and
maintained at ( 0.1 °C during the experiments. Prior to all
T1 measurements the pulse width was calibrated at 4.5 °C.
During the inversion recovery experiments, composite pulse
1H decoupling (Waltz) was applied at all times using a
minimum of decoupling power to minimize sample heating
during the measurements. In addition, to ensure that reso-
nance offset effects were not a problem, the transmitter was
positioned adjacent to the resonance of the carbonyl carbon.
The T1 for this carbon was initially assumed to be as long as
90 s for the deuterated compound, and the relaxation delay
for all measurements was set to be 5 times this value. A total
of nine different τ values were used (1, 2, 4, 10, 25, 50, 100,
200, and 500 s), and a total of eight scans were acquired for
each value of τ. All spectra were processed using 0.5 Hz line
broadening. The data were then fit to an exponential equation
using VNMR 6.1C. An excellent fit was obtained showing spin
lattice relaxation times for the carbonyl carbons of acetalde-
hyde and acetaldehyde-d1 were 14 and 18 s, respectively. The
closeness of these T1 values indicated that under the conditions
of our 13C{1H} NMR data acquisition to determine the con-
centrations of the acetaldehyde isotopomers, only minor dif-
ferences (ca. 3%) in the integrations would result from these
T1 differences.
X-ray Data Collection, Structure Solution, and Re-
finement. (a) Orange crystals of [RhIr(CH3)3(µ-I)(µ-CO)-
(dppm)2] (8)‚2CH2Cl2 were obtained via slow evaporation of a
dichloromethane/chloroform solution of the compound. Data
were collected on a Bruker PLATFORM/SMART 1000 CCD
diffractometer22 using Mo KR radiation at -80 °C. Unit cell
parameters were obtained from a least-squares refinement of
the setting angles of 6253 reflections from the data collection.
The space group was determined to be P21/n (an alternate
setting of P21/c [No. 14]). The data were corrected for absorp-
tion through use of the SADABS procedure. See Table 2 for a
summary of crystal data and X-ray data collection information.
The structure of 8 was solved using the Patterson location of
heavy atoms and structure expansion routines as implemented
in the DIRDIF-9923 program system. Refinement was com-
pleted using the program SHELXL-93.24 Hydrogen atoms were
assigned positions based on the geometries of their attached
carbon atoms and were given thermal parameters 20% greater
than those of the attached carbons. The dppm phenyl rings
attached to P(2) were found to be disordered; each was refined
as two sets of regular hexagons (d(C-C) ) 1.39Å), each with
an occupancy factor of 50% and with a common isotropic
displacement parameter. Both solvent dichloromethane mol-
ecules were found to be disordered; each was split into two
sets of two chlorine and one carbon (and two hydrogen) atom,
which were refined with an occupancy factor of 50%, a common
isotropic displacement parameter, and fixed idealized Cl-C
(1.80 Å) and Cl‚‚‚Cl (2.95 Å) distances. The final model for 8
was refined to values of R1(F) ) 0.0694 (for 7786 data with
2
Fo2 g 2σ(Fo )) and wR2(F2) ) 0.1958 (for all 11 423 independent
data). The high residuals in the final electron density maps
were located in the vicinities of the phenyl rings that did not
show obvious disorder in earlier maps. Attempts to obtain
better models by incorporating crystal twinning were unsuc-
cessful.
(b) Orange crystals of [RhIr(CH3)2(C4H9)(µ-CO)2(dppm)2][I]
(10)‚2CH2Cl2 were obtained via slow diffusion of diethyl ether
into a dichloromethane solution of the compound. Data were
collected and corrected for absorption as for compound 8 above
(see Table 2). Unit cell parameters were obtained from a least-
squares refinement of the setting angles of 6637 reflections
from the data collection, and the space group was determined
to be C2/c (No. 15). The structure of 10 was solved using the
direct-methods program SIR-97,25 and refinement was com-
(23) Beurskens, P. T.; Beurskens, G.; de Gelder, R.; Garcia-Granda,
S.; Israel, R.; Gould, R. O.; Smits, J. M. M. The DIRDIF-99 program
system; Crystallography Laboratory, University of Nijmegen: The
Netherlands, 1999.
(24) Sheldrick G. M. SHELXL-93, Program for crystal structure
determination; University of Go¨ttingen: Germany, 1993.
(22) Programs for diffractometer operation, data collection, data
reduction, and absorption correction were those supplied by Bruker.

Rh and Ir Heterobinuclear Complexes
Organometallics, Vol. 24, No. 18, 2005 4399
cm-1, similar to the values reported for the related
compounds [Rh2X2(µ-CO)(dppm)2] (X ) CH3, Cl, Br);15a,26
compound 2 is presumed to have a structure similar to
these analogues.
Scheme 1
The dibenzyl compound [RhIr(CH2Ph)2(µ-CO)(dppm)2]
(3) is prepared by the reaction of 1 with benzylmagne-
sium chloride and has spectral parameters very similar
to those of 2. In particular, the 1H NMR spectrum
displays the benzyl methylene resonances as a triplet
of doublets at δ 1.82, in which 1.5 Hz coupling to Rh is
observed, and as a triplet at δ 2.57. The carbonyl ligand
is clearly bridging, as seen by the IR stretch at 1713
cm-1 and the 13CO resonance in the NMR spectrum at
δ 230.1, displaying 30.5 Hz coupling to Rh. Compound
2 can also be prepared by the reaction of 1 with
MeMgCl, parallel to the preparation of 3; however this
route proved to be more troublesome, giving a number
of unidentified impurities. Although the successful
syntheses of compounds 2 and 3 suggest the possibility
of generating a series of related dialkyl or possibly diaryl
compounds, no other analogues have been prepared at
this time. In addition, the subsequent chemistry of 3,
similar to that described below for 2, has not been
investigated.
pleted using the program SHELXL-93, during which the
hydrogen atoms were treated as for 8. The final model for 10
was refined to values of R1(F) ) 0.0356 (for 9996 data with
Fo2 g 2σ(Fo )) and wR2(F2) ) 0.0949 (for all 12 250 independent
2
data).
Results and Compound Characterization
(a) Dimethyl and Related Species. Reaction of
[RhIrCl2(CO)2(dppm)2] (1, dppm ) µ-Ph2PCH2PPh2)
with an excess of MeLi at -78 °C yields the dimethyl
monocarbonyl product [RhIr(CH3)2(µ-CO)(dppm)2] (2) as
outlined in Scheme 1. This species is exceedingly air
sensitive, decomposing instantly in solution upon ex-
posure to air and within seconds as a solid, so charac-
terization is based upon its spectroscopy and upon the
characterization of a number of its subsequent reaction
products. The 31P{1H} NMR spectrum of 2 displays a
pattern that is typical of an AA′BB′X spin system (X )
103Rh) in such A-frame species in which the Ir-bound
phosphorus nuclei appear as a multiplet at δ 21.5 (AA′
portion of the spectrum) and the Rh-bound nuclei
appear as a doublet of multiplets at δ 31.9 (BB′ portion
of the spectrum), having additional coupling to Rh of
154 Hz (see Figure S1 in the Supporting Information).
All subsequent compounds display a qualitatively simi-
lar pattern, so these spectral features will be discussed
At ambient temperature compound 2 reacts with
carbon monoxide to yield the known tricarbonyl complex
[RhIr(CO)3(dppm)2]21 and acetone as the major products,
each in about 30% yield, together with unidentified
decomposition products (the relative amounts of species
present were established by integration of the 31P{1H}
1
and H NMR spectra; in particular integration of the
dppm methylene signal and that of acetone gave the
relative amounts of the two products). If 13CO is used
in the reaction with unenriched 2, an additional doublet
(2JCH ) 9 Hz) flanking the acetone resonance at δ 2.07
1
1
appears in the H NMR spectrum, due to coupling of
only if they are unusual. In the H NMR spectrum the
the acetone protons with the 13C label. Acetone was also
observed in the carbonylation of the dirhodium analogue
[Rh2(CH3)2(µ-CO)(dppm)2].15a
dppm methylene protons appear as two multiplets at δ
3.86 and 3.53, consistent with the “front/back” asym-
metry of the compound, and the methyl resonances
appear as a triplet of doublets at δ -0.35 and a triplet
at δ 0.22. In both cases the triplet pattern results from
coupling to a pair of adjacent 31P nuclei, whereas the
doublet in the high-field signal results from coupling to
Rh, suggesting that this methyl group is bound to this
metal. This assignment is further supported by selective
31P-decoupling experiments, which clearly establish the
phosphorus nuclei to which the different protons are
coupled, and is confirmed by the 13C{1H} NMR spectrum
of a 13CH3- and 13CO-enriched sample of 2 in which the
Rh-bound methyl group appears as a doublet of doublets
of triplets at δ 2.78 displaying 22.8 Hz coupling to Rh
as well as coupling to a pair of phosphorus atoms and
to the sole carbonyl ligand, while the Ir-bound methyl
group, at δ 4.81, displays coupling to only a pair of 31P
nuclei and the carbonyl group. The carbonyl resonance
in the same sample appears as a complex multiplet at
δ 230.8, but on broad-band 31P decoupling, simplifies
to a doublet of doublets of doublets, retaining 29 Hz
coupling to Rh and coupling to the 13C nuclei in the two
methyl groups. In the IR spectrum the carbonyl stretch
of a nonisotopically enriched sample appears at 1715
In an attempt to identify possible intermediates in
the formation of acetone from 2 the addition of CO was
carried out at low temperatures and the solution was
monitored by multinuclear NMR spectroscopy. At -78
°C compound 2 reacts with carbon monoxide to yield
the dicarbonyl dimethyl product [RhIr(CH3)2(CO)2-
(dppm)2] (4) as shown in Scheme 2. Two carbonyl
resonances of equal intensity appear for 4 in the 13C-
{1H} NMR spectrum, at δ 185.7 and 182.6, and the 68
Hz coupling to Rh of the latter and the absence of Rh
coupling in the former indicate that one carbonyl is
bound to each metal. The IR spectrum confirms the
terminal binding mode of both carbonyls. In the 1H
NMR spectrum the methyl groups appear as triplets at
δ -0.23 and -1.39, and the absence of Rh coupling
indicates that both are bound to Ir. This assignment is
confirmed by selective 31P decoupling experiments,
which establish that the protons of both methyl groups
couple to only the Ir-bound 31P nuclei. The similarities
of the spectral parameters for 4 with those of the methyl
hydrido and methyl cyano analogues [RhIrX(CH3)(CO)2-
(dppm)2] (X ) H, CN) previously described8e support a
(25) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115-119.
(26) (a) Cowie, M.; Dwight, S. K. Inorg. Chem. 1980, 19, 2500. (b)
Cowie, M.; Dwight, S. K. Inorg. Chem. 1980, 19, 2508. (c) Gelmini, L.;
Loeb, S. J.; Stephan, D. W. Inorg. Chim. Acta 1985, 98, L3.

4400 Organometallics, Vol. 24, No. 18, 2005
Hilts et al.
Scheme 2
Scheme 3
by NMR), along with 4 and other unidentified com-
pounds, is the dimethyl tricarbonyl species [RhIr(CH3)2-
(CO)3(dppm)2] (5), shown in Scheme 2. The 13C{1H}
NMR spectrum of this species is consistent with two
carbonyls being terminally bound to Rh (δ 204.8, 190.2)
having coupling to this metal of 77.4 and 60.2 Hz,
respectively, and the third (δ 184.3) bound to Ir. As was
the case for 4, the methyl groups are inequivalent and
both are bound to Ir, as indicated by the lack of Rh
Chart 1
1
coupling in both the H and 13C NMR spectra and by
the 31P-decoupling experiments. Removal of the atmo-
sphere of CO at 0 °C results in facile loss of the carbonyl,
regenerating 4. Warming 5 to ambient temperature
under CO generates acetone, [RhIr(CO)3(dppm)2], and
unidentified decomposition products. Although no acetyl-
containing complex could be unequivocally identified
between 0 and 20 °C, owing to overlapping resonances
similar structural assignment, which has also been ob-
served in the X-ray structural determination of the re-
lated cationic species [RhIr(CH3)(PMe3)(CO)2(dppm)2]+.8e
Warming a sample of 4 to ambient temperature in
the absence of CO does not regenerate 2; compound 4
is stable for several hours under these conditions but
decomposes to unidentified products after this time.
Whereas the ambient-temperature 31P{1H} NMR
spectrum of 4 is characteristic of an AA′BB′X spin
system, as noted for 2, at -80 °C it transforms to a more
complex pattern characteristic of an ABCDX (X ) 103Rh)
spin system, with the four 31P resonances centered at
approximately δ 14.7, 5.9, -14.1, and -24.1, indicating
that all phosphorus nuclei are chemically inequivalent.
As is commonly observed in Rh/Ir systems, the lower-
field resonances correspond to the Rh-bound 31P nuclei,
and this is confirmed by the additional Rh coupling
observed for these signals. The 31P{1H} NMR spectra
of 4 at 0, -40, and -80 °C are given in Figure S5 in the
Supporting Information. In contrast, the 13C{1H} NMR
1
of the other minor products in the H and 31P NMR
spectra, one acyl carbonyl resonance was observed in
the 13C{1H} NMR spectrum (δ 279). This signal dis-
played no coupling to Rh, so was clearly bound to
iridium, suggesting that one of the two Ir-bound methyl
groups had undergone migratory insertion.
To establish whether the generation of acetone from
2 occurred by an intra- or intermolecular pathway,
crossover experiments were carried out using equimolar
amounts of 2 and [RhIr(CD3)2(µ-CO)(dppm)2] (2-CD3),
and the organic products were analyzed by GC/MS.
Carbonylation of this mixture at between 300 and 600
Torr produced only (CH3)2CO and (CD3)2CO, with no
evidence of the crossover product (CH3)C(O)CD3, sup-
porting an intramolecular pathway. If the reaction is
carried out at slightly higher pressures (between 760
and 800 Torr), the crossover product is detected as
approximately 4% of the total acetone produced. Also
at higher pressures, carbonylation in the presence of an
excess of the hydrogen atom source, tris(trimethylsilyl)-
silane (20 µL), produced trace amounts of acetaldehyde
and methane in addition to mainly acetone (by NMR),
suggesting the presence of small amounts of acetyl and
methyl radicals. These observations are consistent with
the onset of a radical pathway at higher pressures,
consistent with earlier findings on the related dirhodium
analogue.15
1
resonances for the carbonyl and the H resonances for
the methyl groups remain invariant over this temper-
ature range, indicating that no exchange involving these
groups is occurring. We suggest that the fluxional
process being observed in the 31P{1H} NMR spectrum
results from twisting of the complex about the Rh-Ir
bond as outlined in Chart 1. In either extreme (A or B)
the phosphorus nuclei on Rh (PA and PB) are inequiva-
lent, being adjacent to either an Ir-bound CH3 or CO
group. At higher temperature rapid libration about the
Rh-Ir bond exchanges these two 31P environments,
yielding an average signal for the two. A similar
fluxional process has been observed for the related
hydrido-alkynyl species [RhIrH(CO)2(µ-C2Ph)(dppm)2].27
In the reaction of 2 or 4 with excess CO at -40 °C
the major species present (in approximately 65% yield
(b) Reaction of 2 with H2. The reaction of 2 with
H2 shown in Scheme 3 proceeds readily at -78 °C,
(27) George, D. S. A.; McDonald, R.; Cowie, M. Organometallics
1998, 17, 2553.

Rh and Ir Heterobinuclear Complexes
Organometallics, Vol. 24, No. 18, 2005 4401
Scheme 4
yielding the oxidative-addition product, the dihydride
[RhIrH(CH3)2(µ-H)(µ-CO)(dppm)2] (6). In the H NMR
1
spectrum the Ir-bound methyl group appears at δ -0.86
and shows coupling to only the adjacent pair of phos-
phorus atoms, while the Rh-bound methyl group, at δ
-0.26, shows additional 2.0 Hz coupling to Rh. One
hydride resonance appears as a doublet of multiplets
at δ -10.65 and has coupling of 14 Hz to Rh, while the
other appears as a triplet at -13.36, with coupling to
only the Ir-bound phosphorus atoms. These resonances
correspond to a hydride that bridges both metals and
to one that is terminally bound to Ir, respectively. In
the 13C{1H} NMR spectrum the bridging carbonyl
appears at δ 233.3 and displays 23 Hz coupling to Rh.
The hydride ligands are assumed to have a mutually
cis arrangement as shown, on the basis of an assump-
tion of H2 attack in the A-frame “pocket”, as has been
reported for similar species.27,28 Support for this ar-
rangement is seen in the 1H NMR spectrum of a 13CO-
enriched sample of 6 in which the high-field triplet of
the terminal Ir-bound hydride displays additional trans
coupling to the carbonyl ligand of 36 Hz. In addition, in
a sample that is 13CO and 13CH3 enriched, no coupling
is observed between the carbonyl and the Ir-bound
methyl group, suggesting a mutually cis arrangement
of these groups, as shown in Scheme 3. Compound 6 is
stable for brief periods in solution under an atmosphere
of H2, after which it decomposes. It also slowly decom-
poses in the solid state, either under N2 or vacuum,
presumably by methane loss.
products were analyzed by NMR (1H and 13C). The
major isotopomers of acetaldehyde detected were
CH313C(O)H and 13CH313C(O)D, in an approximate 4:1
ratio, while a small amount of both 13CH313C(O)H and
CH313C(O)D (each approximately 5-6% of the major
isotopomer) was also observed. Because the isotopomer
of 6 containing deuterium was present in about one-
third the initial concentration of that containing hydro-
gen, the above ratio of the two major acetaldehyde
isotopomers is not exactly as expected. However inte-
gration of all signals was difficult owing to overlap of
the resonances. In addition, a small part of the discrep-
ancy in the somewhat lower than expected amount of
13CH313C(O)D detected by 13C{1H} NMR spectroscopy
may result from the differences in 13C relaxation times
of the acyl carbonyl bound to hydrogen or deuterium
(see Experimental Section).29 In the case of methane,
the major products were CH4 and13CH3D; however,
significant amounts of the crossover products 13CH4 and
CH3D were also observed.
(c) Trialkyl Species. Compound 2 reacts with meth-
yl iodide to yield the trimethyl product [RhIr(CH3)3(µ-
I)(µ-CO)(dppm)2] (8), shown in Scheme 4. In the 1H
NMR spectrum the Rh-bound methyl group, at δ -0.23,
displays 2.0 Hz coupling to Rh, as well as coupling to
the Rh-bound phosphorus nuclei, while the two others,
at δ 0.85 and -0.17, are bound to Ir, as shown by their
coupling to the Ir-bound phosphorus nuclei. When
13CH3I is used, exclusive 13CH3 incorporation into one
of the Ir-bound sites is observed, corresponding to the
1H resonance at δ 0.85, which in this case displays
coupling to 13C of 132 Hz; the 13C{1H} resonance for this
group appears at δ -11.6. The resonance for the
bridging carbonyl appears at δ 220.4, with coupling of
Compound 6 reacts with CO at 0 °C to give the acetyl-
methyl-dihydride species [RhIrH(C(O)CH3)(CH3)(µ-H)-
(dppm)2] (7). At temperatures below 0 °C the reaction
is slow, with unreacted starting material being the
1
major species after several hours. The H NMR spec-
trum of 7 shows the Ir-bound methyl group as a triplet
with coupling to the Ir-bound phosphorus nuclei, the
bridging hydride at δ -11.83 with 20 Hz coupling to
Rh and coupling to all phosphorus nuclei, and the
terminal Ir-bound hydride at δ -13.58 with coupling
to only the Ir-bound phosphorus nuclei. The acetyl
methyl protons appear as a singlet at δ 0.97. In the 13C-
{1H} NMR spectrum the bridging carbonyl appears at
δ 227.2 with 24 Hz coupling to Rh and the acyl carbonyl
appears at δ 277.5 with 29 Hz coupling to Rh. The
presence of the bridging and acyl carbonyls is supported
by the IR spectrum, which shows stretches for these
groups at 1742 and 1626 cm-1, respectively. When an
atmosphere of 13CO gas is placed over a sample of
unlabeled 6 at 0 °C, the 13C{1H} NMR spectrum taken
immediately shows 13CO incorporation only in the
metal-bridged position, suggesting that migratory inser-
tion precedes carbonyl addition. Compound 7 could not
be isolated since it decomposes upon workup. Upon
warming to room temperature under an atmosphere of
CO, 7 transforms into [RhIr(CO)3(dppm)2],21 together
with methane and acetaldehyde, as established by
NMR.
To determine whether the formation of acetaldehyde
and methane occurs by intra- or intermolecular pro-
cesses, a mixture of two isotopomers of 6, namely,
[RhIrD(13CH3)2(µ-D)(µ-13CO)(dppm)2] and [RhIrH(CH3)2-
(µ-H)(µ-13CO)(dppm)2], were reacted with 13CO and the
(29) (a) Gust, D.; Pearson, H.; Armitage, I. M.; Roberts, J. D. J. Am.
Chem. Soc. 1976, 98, 2723. (b) Lickfield, G. C.; Savitsky, G. B.;
Beyerlein, A. L.; Spencer, H. G. Macromolecules 1983, 16, 396.
(28) Vaartstra, B. A.; Cowie, M. Inorg. Chem. 1989, 28, 3138.

4402 Organometallics, Vol. 24, No. 18, 2005
Hilts et al.
Figure 1. Perspective view of [RhIr(CH3)3(µ-I)(µ-CO)-
(dppm)2] (8) showing the atom-labeling scheme. Non-
hydrogen atoms are represented by Gaussian ellipsoids at
the 20% probability level. Hydrogens on the dppm meth-
ylene groups and the methyl ligands are drawn artificially
small. Only the ipso carbons of the phenyl rings are shown.
Table 3. Selected Distances and Angles for
[RhIr(CH3)3(µ-I)(µ-CO)(dppm)2] (8)
Figure 2. Perspective view of the complex cation of [RhIr-
(n-C4H9)(CH3)2(µ-CO)2(dppm)2][I] (10). Thermal parameters
as described for Figure 1.
Distances (Å)
Ir-Rh
Ir-I
Ir-C(1)
Ir-C(2)
Ir-C(3)
2.9152(8)
2.8008(8)
2.152(9)
2.15(1)
Rh-I
3.092(1)
1.935(9)
2.20(1)
3.049(4)
3.071(3)
Rh-C(1)
Rh-C(4)
P(1)-P(2)a
P(3)-P(4)a
group are bound to Ir, as shown in Scheme 4. In the 1H
NMR spectrum the resonances for the n-Bu group
appear at δ 2.14, 1.01, 0.85, and 0.39 in a 2:2:2:3 ratio
and the Ir-bound methyl group appears at δ -0.22. The
13C resonance of the carbonyl group is very close to that
observed for 8, and its low-field chemical shift (δ 222.6)
together with its 37 Hz coupling to Rh clearly establish
its bridging geometry. Although the location of the iodo
ligand cannot be established spectroscopically, it is
assumed to bridge the metals, as determined for 8, on
the basis of the close similarity of the spectral param-
eters in the two compounds. Isotopic labeling on com-
pound 9 was not carried out, so the relative positions
of the Ir-bound n-butyl and methyl groups were not
established. They are assigned the positions shown on
the basis of analogy with the closely related compound
8.
Under a CO atmosphere compound 9 transforms to
[RhIr(CH3)2(n-C4H9)(µ-CO)2(dppm)2][I] (10), and this
product can also be obtained in the reaction of 4 with
n-BuI. Compound 10 displays only one resonance in the
13C{1H} NMR spectrum at δ 215.4, corresponding to
both bridging carbonyl ligands, with typical coupling to
Rh of 40 Hz. The IR spectrum confirms the bridging
arrangement, displaying a band at 1776 cm-1. In the
1H NMR spectrum only one signal (integrating as six
protons) at δ -0.19 is observed for the pair of methyl
ligands, showing that they are chemically equivalent on
the NMR time scale, and the resulting symmetry on
either side of the RhIrP4 plane is further shown by the
appearance of only one resonance for the four dppm
methylene protons. The methyl ligand resonance dis-
plays coupling to only the Ir-bound phosphines. The
resonances for the n-butyl group appear at δ 0.02 (t),
0.59 (m), 1.78 (m), and 0.88 (m), integrating as 3:2:2:2.
No Rh coupling is evident in any of the methylene
signals; nevertheless the symmetry of the compound,
as indicated by the other spectral data, make it clear
that this group is bound to Rh. This has been confirmed
by the X-ray structure determination of 10, and a
representation of the cationic complex is shown in
Figure 2. Selected bond lengths and angles are given
2.16(1)
Angles (deg)
I-Ir-C(3)
85.9(3)
86.9(4)
Rh-I-Ir
59.06(2)
90.8(4)
C(1)-Ir-C(2)
C(2)-Ir-C(3)
I-Rh-C(4)
C(1)-Rh-C(4)
Rh-C(1)-Ir
Ir-C(1)-O(1)
Rh-C(1)-O(1)
80.2(4)
135.4(7)
133.7(7)
129.5(3)
127.5(4)
a Nonbonded contacts.
38 Hz to Rh. If 13CH3I is added to 13CO-enriched 2, the
13C resonances for the added methyl group and the 13CO
group do not show mutual coupling, suggesting that
they have a mutually cis arrangement. This implies that
the 13CH3 and I fragments have a trans arrangement,
as diagrammed in Scheme 4. The structure of 8 has been
confirmed by an X-ray structure determination, shown
in Figure 1, with selected bond lengths and angles given
in Table 3. Although the crystal used in this determi-
nation was of poor quality (as were all crystals of this
compound investigated), the data are certainly good
enough to unambiguously establish the connectivity
within the molecule. In particular, the iodo group is
shown to be bridging both metals. The Rh-Ir separation
(2.9152(8) Å) is intermediate between a normal single
bond and a nonbonded separation, but is significantly
shorter than the nonbonded, intraligand P-P separa-
tions (3.049(4), 3.071(3) Å), indicating compression along
the Rh-Ir vector, suggesting a weak bonding interac-
tion. The carbonyl ligand is symmetrically bridging, as
shown by the similarity of the Ir-C(1)-O(1) and Rh-
C(1)-O(1) angles. On Ir, the pair of methyl groups
occupy the sites opposite the bridging I and CO groups,
while on Rh the lone methyl group is opposite the Rh-
Ir bond. All metal-methyl bonds appear normal.
Compound 2 also reacts readily with n-BuI to give a
product, [RhIr(CH3)2(n-C4H9)(µ-I)(µ-CO)(dppm)2] (9), that
closely resembles the trimethyl species (8) in all spec-
troscopic parameters, so is assumed to have a similar
1
structure. Only the methyl signal at δ -0.24 in the H
NMR spectrum displays coupling to Rh, so clearly this
group is bound to Rh. The n-Bu and the other methyl

Rh and Ir Heterobinuclear Complexes
Organometallics, Vol. 24, No. 18, 2005 4403
Table 4. Selected Distances and Angles for
[RhIr(n-C4H9)(CH3)2(µ-CO)2(dppm)2][I] (10)
Chart 2
Distances (Å)
Ir-Rh
2.7916(4)
2.031(5)
2.028(4)
2.144(5)
2.155(5)
2.091(5)
2.116(5)
Rh-C(5)
2.122(5)
1.512(7)
1.537(8)
1.525(8)
3.048(2)
3.051(2)
Ir-C(1)
Ir-C(2)
Ir-C(3)
Ir-C(4)
Rh-C(1)
Rh-C(2)
C(5)-C(6)
C(6)-C(7)
C(7)-C(8)
P(1)-P(2)a
P(3)-P(4)a
Angles (deg)
is surprising to us since the methyl group is generally
thought to be a better donor than chloride, being
comparable to an iodo ligand,31 suggesting that the
carbonyls would be more tightly bound than in the
dichloro species. Consistent with this idea, the dicar-
bonyl product (4), once synthesized by CO addition to 2
at -80 °C, does not readily lose CO. The stage at which
carbonyl loss occurs, in the transformation of 1 to 2, is
not known, but the irreversible formation of [RhIr-
(CH3)2(CO)2(dppm)2] (4) makes it clear that it cannot
occur from a species such as 4.
Irreversible addition of CO to 2 yields 4, having the
unexpected structure diagrammed as C in Chart 2. One
might have expected compound 4 to have a structure
D, analogous to the dichlorodicarbonyl precursor (1), a
series of homobinuclear analogues,32 and to the closely
related [RhIrCl2(CO)2(depm)2], containing the bis(di-
ethylphosphino) methane ligand.32c In C both methyl
groups are bound to Ir, giving a Rh(0)/Ir(+2) oxidation-
state formulation, instead of the Rh(+1)/Ir(+1) formula-
tion of D. The observed structure (C) will be favored by
the greater number of stronger bonds associated with
the third-row metal,33 by the additional presence of a
Rh-Ir bond, and by the greater tendency of Ir to assume
a higher oxidation state.31,34 Nevertheless, it remains
puzzling that the closely related species 1 and 4 have
different structures.
At approximately -40 °C under an atmosphere of CO
the dicarbonyl compound 4 is transformed reversibly
into the tricarbonyl product [RhIr(CH3)2(CO)3(dppm)2]
(5). NMR experiments establish that the geometry of 5
is much like that of 4 in which both methyl groups
remain bound to Ir with the additional carbonyl bound
to Rh. The subsequent observation that warming a
mixture of 5 and CO to ambient temperature results in
the formation of acetone and [RhIr(CO)3(dppm)2] raises
some interesting questions about the roles of the dif-
ferent metals in acetone formation. Presumably, forma-
tion of acetone from the dimethyl compound 5 proceeds
via an unobserved acetyl- and methyl-containing inter-
mediate from which reductive elimination of acetone
occurs. On the basis of the well-established tendencies
for migratory insertions involving metals within a given
triad,35 we would have expected acetyl formation to have
occurred preferentially at Rh. However, the geometries
C(1)-Ir-C(3)
C(2)-Ir-C(4)
C(3)-Ir-C(4)
C(1)-Rh-C(5)
C(2)-Rh-C(5)
90.9(2)
91.0(2)
Ir-C(1)-O(1)
Ir-C(2)-O(2)
Rh-C(1)-O(1)
Rh-C(2)-O(2)
Rh-C(5)-C(6)
148.4(4)
150.6(4)
126.1(4)
124.4(4)
114.8(3)
81.2(2)
126.0(2)
141.1(2)
in Table 4. The Rh-Ir separation (2.7916(4)Å) is con-
sistent with a single bond. Although the nature of the
metal-metal bond is equivocal, we prefer to consider
this interaction as a dative bond between a coordina-
tively saturated Ir(+3) center and an unsaturated Rh-
(+1) center; this dative bond is necessary to give Rh a
favorable 16e configuration. Ignoring, for the moment,
the metal-metal bond, the Ir center has an octahedral
geometry in which the mutually cis methyl groups are
opposite the semibridging carbonyls. These carbonyls
are primarily bonded to Ir (Ir-C ) 2.031(5), 2.028(4)
Å) while involved in a somewhat weaker interaction
with Rh (2.091(5), 2.116(5) Å). The angles at these
carbonyl carbons are consistent with this description in
which the Ir-C-O angles (148.4(4)°, 150.6(4)°) are more
linear than the corresponding Rh-C-O angles (126.1-
(4)°, 124.4(4)°). The metal-alkyl distances are normal,
as are the parameters within the n-butyl group.
If the reaction of 2 with methyl triflate is carried out
at -78 °C, a trimethyl monocarbonyl species is formed,
which upon warming rearranges to what appears to be
an isomer. We have not investigated these isomers
further, except to show that when 13CH3OTf is used,
13CH3 incorporation occurs initially at Ir, with eventual
scrambling of the label through all methyl sites, even
at -78 °C. However, if the reaction is carried out under
an atmosphere of CO, the trimethyl dicarbonyl product
[RhIr(CH3)3(µ-CO)2(dppm)2][CF3SO3] (11) is formed,
having spectral parameters that are almost identical to
those of 10 (apart from the additional proton resonances
due to the butyl group), indicating that it has an
analogous structure, as diagrammed in Scheme 4. No
evidence of migratory insertion is seen at the low
pressure of CO investigated.
Discussion
The substitution of the chloride ligands in trans-
[RhIrCl2(CO)2(dppm)2] (1) by a pair of alkyl groups, to
yield the products [RhIr(R)2(µ-CO)(dppm)2] (R ) CH3
(2), CH2Ph (3)), is accompanied by loss of a carbonyl
ligand, much as observed in the analogous dirhodium
chemistry.15 We had anticipated that this mixed-metal
complex, in which one Rh had been replaced by a less
labile Ir,30 might be less prone to CO loss than the Rh2
species, but this appears not to be the case. Loss of a
carbonyl in either system upon replacement of Cl by CH3
(31) Atwood, J. D. Coord. Chem. Rev. 1988, 83, 93.
(32) (a) Cowie, M.; Dwight, S. K. Inorg. Chem. 1980, 19, 2500. (b)
Cowie, M.; Dwight, S. K. Inorg. Chem. 1981, 20, 1534. (c) Ristic-
Petrovic, D.; Anderson, D. J.; Cowie, M. Unpublished data.
(33) (a) Ziegler, T.; Tschinke, V. Bonding Energetics in Organome-
tallic Compounds; Marks, T. J., Ed.; American Chemical Society:
Washington, DC, 1990; Chapter 19. (b) Ziegler, T.; Tschinke, V.;
Ursenbach, B. J. Am. Chem. Soc. 1987, 109, 4825. (c) Armentrout, P.
B. Bonding Energetics in Organometallic Compounds; Marks, T. J.,
Ed.; American Chemical Society: Washington, DC, 1990; Chapter 2.
(34) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M.
Advanced Inorganic Chemistry, 6th ed.; John Wiley & Sons: New York,
1999; Chapter 16.
(30) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry:
Principles of Structure and Reactivity, 4th ed.; Harper Collins: New
York, 1993; Chapter 14.

4404 Organometallics, Vol. 24, No. 18, 2005
Hilts et al.
of 4 and 5, in which both methyl groups are bound to
Ir, and the observation, in the 13C{1H} NMR spectrum
of the reaction mixture, of an Ir-bound acyl group and
the absence of signals for a Rh-bound acyl suggests that
migratory insertion occurs at Ir. Although migratory
insertion at Ir in preference to Rh is initially surprising,
insertion at the heavier metal will be favored to some
degree by the higher oxidation state of this metal.36 We
cannot rule out that methyl migration back to Rh occurs
with subsequent migratory insertion at this metal, but
no evidence of this is observed. The proposal of migra-
tory insertion occurring at Ir suggests an interesting
possibility that this process could somehow be promoted
by the presence of the adjacent Rh center. Unfortunately
the diiridium analogue has never been investigated, so
we are unable to make this comparison. However,
enhancement of migratory insertion at Pd and Pt by an
adjacent Co center has been noted.2h We assume further
that reductive elimination of acetone from a methyl,
acetyl species also occurs from Ir. As noted earlier,
reductive elimination is predominantly intramolecular,
and we have observed no 2,3-butanedione, as was
reported in the dirhodium system.15a Although the
formation of this diketone was found to be intermolecu-
lar, it presumably results from carbonyl insertion into
both Rh-CH3 bonds under higher CO pressures, to
generate a bis-acetyl intermediate. The absence of such
a species in our Rh/Ir chemistry is consistent with the
lower tendency for migratory insertion on Ir.35 The
appearance of both (CH3)2CO and (CH3)213CO when
13CO is reacted with unenriched 2 indicates that migra-
tory insertion involving both the original and added CO
occurs, indicating that carbonyl scrambling is occurring
at some stage in the reaction.
The failure of either the di- or tricarbonyl products
(4 or 5) to reductively eliminate ethane, even though
both methyl groups are mutually cis on the same metal,
is consistent with previous findings that elimination of
two alkyl groups is unfavorable, owing to the unfavor-
able reorientation energy of the directional alkyl groups.37
Compound 2 is susceptible to oxidative addition,
reacting readily with H2, CH3I, n-C4H9I, and CH3OTf.
Addition of the alkyl groups was shown by labeling
studies to be on Ir. More facile oxidative addition at the
third-row metal in preference to the second-row ana-
logue is not unexpected.38 The observation that addition
of CH3I results in a product in which the two oxidative-
addition fragments are mutually trans and not cis is
also in keeping with previous observations in related
mononuclear species.39 In the case of methyl triflate
addition to 2, addition at low temperature showed that
the methyl group initially attacks Ir, but that subse-
quent scrambling of the labeled CH3 group over the
other methyl sites occurs readily. In the iodo complexes
8 and 9 binding of the iodo ligand in a bridging site
presumably inhibits the “merry-go-round” movement of
equatorial ligands that is presumably responsible for
ligand transfer from metal to metal, whereas for the
triflate species loss of labile triflate anion will allow the
scrambling to occur. Consistent with this proposal,
replacement of the iodo ligand by CO results in migra-
tion of the n-butyl group from Ir to Rh in the transfor-
mation of 9 to 10.
Oxidative addition of H2 to compound 2 yields the
dihydride [RhIrH(CH3)2(µ-H)(µ-CO)(dppm)2] (6), in which
concerted addition in the A-frame “pocket”, probably at
Ir, has occurred, much as has been previously observed
in [RhIr(CO)2(µ-C2Ph)(dppm)2][CF3SO3],27 [RhIr(CO)2-
(µ-S)(dppm)2],28 and [RhIr(CO)2(µ-Cl)(dppm)2][BF4].28
Compound 6, in which there is a pair of mutually cis
hydride and methyl groups (one at Rh and the other at
Ir), is believed to be the first example of its kind, and
its stability for brief periods is surprising; we had
expected reductive elimination of methane with con-
comitant decomposition of the complex, which presum-
ably accounts for its lack of long-term stability (although
we made no attempt to detect the products of decom-
position).
The second step in the tandem addition of H2 and CO
to 2 yields the dihydrido, methyl acetyl product 7
(Scheme 3), in which migratory insertion has occurred
at Rh to yield a product that is structurally analogous
to the dimethyl precursor (6). The observation of migra-
tory insertion at Rh in preference to Ir is consistent with
well-established trends.35 The addition of CO to 7 is in
contrast to that noted earlier for 2, in that migration of
the Rh-bound methyl group to Ir does not occur. In the
earlier observation (vide supra) we had noted that
methyl migration from Rh to Ir resulted in Ir having a
favored higher oxidation state. For 7 we can formulate
a favorable Rh(+1)/Ir(+3) formulation, so methyl migra-
tion is not necessary. As a consequence, carbonyl
addition presumably occurs at Rh followed by migratory
insertion at that metal.
Crossover experiments establish that reductive elimi-
nation of acetaldehyde occurs preferentially by an
intramolecular process. As noted, small amounts of the
crossover products are obtained. The intramolecular
reductive elimination is consistent with the mutually
cis arrangement of the acetyl and hydride ligands at
Rh, from which reductive elimination should be favor-
able. We also note that methane elimination from
compound 7 yields significant amounts of the crossover
products 13CH4 and CH3D in addition to those expected
(13CH3D and CH4) for a concerted reductive-elimination
process. We assume that reductive elimination of ac-
etaldehyde from Rh occurs in preference to methane
elimination from Ir, for which the metal-ligand bonds
are stronger.33 Elimination of acetaldehyde presumably
yields a highly reactive intermediate which must not
undergo concerted methane elimination.
(35) George, R.; Andersen, J.-A. M.; Moss, J. R. J. Organomet. Chem.
1995, 505, 131.
(36) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987; Chapter 6.
(37) (a) Low, J. J.; Goddard, W. A., III. J. Am. Chem. Soc. 1984,
106, 6928. (b) Low, J. J.; Goddard, W. A., III. J. Am. Chem. Soc. 1984,
106, 8321.
Conclusions
(38) (a) Collman, J. P.; Roper, W. R. Adv. Organomet. Chem. 1968,
7, 53. (b) Vaska, L. Inorg. Chim. Acta 1971, 5, 295. (c) Stille, J. K. The
Chemistry of the Metal-Carbon Bond; Hartley, F. R., Patai, S., Eds.;
John Wiley & Sons Ltd.: New York, 1985; Vol. 2, Chapter 9.
(39) (a) Collman, J. P.; Sears, C. T., Jr. Inorg. Chem. 1968, 7, 27.
(b) Chatt, J.; Johnson, N. P.; Shaw, B. L. J. Chem. Soc. A 1967, 604.
(c) Deeming, A. J.; Shaw, B. L. J. Chem. Soc. A 1969, 1128.
Studies of the type described in this paper can be
important in helping us understand the roles of adjacent
metals in processes of relevance to catalysis by bimetal-
lic species. Although much of what is observed for the

Rh and Ir Heterobinuclear Complexes
Organometallics, Vol. 24, No. 18, 2005 4405
Rh/Ir compounds under study is consistent with antici-
pated trends within a triad, some observations are
contrary to expectations. As examples of reactivity
proceeding as anticipated, oxidative addition is observed
to occur at Ir in preference to Rh, while in one compound
migratory insertion occurs preferentially at Rh. How-
ever, the observation, in a closely related compound, of
migratory insertion appearing to occur at Ir in prefer-
ence to Rh demonstrates how metals in close proximity
can give rise to unexpected reactivity. In this case, facile
ligand redistribution over the metals has led to a change
in oxidation states of metals, which has led to concomi-
tant changes in reactivity that would have been less
favored in mononuclear systems.
individual metals, leading to reversals of trends usually
associated with different metals. Such effects seem less
likely on surfaces.
Acknowledgment. We thank the Natural Sciences
and Engineering Research Council of Canada (NSERC)
and the University of Alberta for financial support of
this research, the NSERC for funding the P4/RA/
SMART 1000 CCD diffractometer, Dr. Dusan Ristic-
Petrovic for helpful discussions, and Mark Miskolzie and
Dr. Albin Otter for the T1 determinations.
Supporting Information Available: Tables of X-ray
experimental details, atomic coordinates, interatomic distances
and angles, anisotropic thermal parameters, and hydrogen
parameters for compounds 8 and 10. 31P{1H} and 1H NMR
spectra for compounds 2 and 6 and 31P{1H} NMR spectra at
0, -40, and -80 °C for compound 4. This material is available
free of charge via the Internet at http://pubs.acs.org.
This latter observation also demonstrates that small
clusters have limits concerning their utility as models
for reactivity at metal surfaces, and processes such as
migratory insertion can be dictated by the natures of
the ancillary ligands and the oxidation states of the
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