Dicarbonylcyclopentadienyliridium, (η-C5H5)Ir(CO)2, as a ligand

Article abstract of DOI:10.1139/v99-131

The following complexes, in which CpIr(CO)₂ acts as a 2e donor ligand, have been prepared: Cp(OC)₂IrW(CO)₅ (1), Cp(OC)₂IrRu(CO)₃(SiCl₃)₂, Cp(OC)₂IrOs(CO)₃(GeCl₃)(Cl) (2), Cp(OC)₂IrOs(CO)₃(X)₂ (X = Cl, Br (3), I). The characterization of the complexes included the crystal structures of 1, 2, and 3, which reveal that all contain an unbridged metal-metal bond. The carbonyl ligands of 1 in solution undergo exchange on the NMR time scale above -40°C. With the exception of 2, all the complexes dissociate in solution at room temperature, some rapidly so. Only in the case of 3 is an equilibrium with the dissociation products established. The results indicate that CpIr(CO)₂ is a weak ligand.

Full text of DOI:10.1139/v99-131

1327
Dicarbonylcyclopentadienyliridium,
(␩-C5H5)Ir(CO)2, as a ligand
Faming Jiang, Kumar Biradha, Weng Kee Leong, Roland K. Pomeroy,
and Michael J. Zaworotko
Abstract: The following complexes, in which CpIr(CO)2 acts as a 2e donor ligand, have been prepared:
Cp(OC)2IrW(CO)5 (1), Cp(OC)2IrRu(CO)3(SiCl3)2, Cp(OC)2IrOs(CO)3(GeCl3)(Cl) (2), Cp(OC)2IrOs(CO)3(X)2 (X = Cl,
Br (3), I). The characterization of the complexes included the crystal structures of 1, 2, and 3, which reveal that all
contain an unbridged metal–metal bond. The carbonyl ligands of 1 in solution undergo exchange on the NMR time
scale above –40°C. With the exception of 2, all the complexes dissociate in solution at room temperature, some rapidly
so. Only in the case of 3 is an equilibrium with the dissociation products established. The results indicate that
CpIr(CO)2 is a weak ligand.
Key words: tungsten, osmium, iridium, binuclear.
Résumé : On a préparé les complexes suivants dans lesquels le CpIr(CO)2 agit comme ligand donneur de 2e-:
Cp(OC)2IrW(CO)5 (1), Cp(OC)2IrRu(CO)3(SiCl3)2, Cp(OC)2IrOs(CO)3(GeCl3)(Cl) (2), Cp(OC)2IrOs(CO)3(X)2 (X = Cl,
Br (3), I). La caractérisation des complexes inclus la détermination des structures cristallines des composés 1, 2 et 3
qui révèlent la présence dans chacun d’une liaison métal–métal non pontée. En solution, au-dessus de –40°C, les
ligands carbonyles du composé 1 subissent un échange à l’échelle de temps de la RMN. À l’exception du produit 2,
tous les complexes se dissocient en solution, à la température ambiante; dans certains cas, la dissociation est rapide.
On n’a pu établir l’existence d’un équilibre avec les produits de dissociation que dans le cas du composé 3. Les
résultats indiquent que le CpIr(CO)2 est un ligand faible.
Mots clés : tungstène, osmium, iridium, binucléaire.
[Traduit par la Rédaction] Jiang et al. 1335
Introduction
by bridging ligands are more common. Some recent exam-
ples are [RhIr(µ-dppm)2(Me)(CO)3][CF3SO3] (dppm
=
Work from this laboratory over the past 15 years has
described complexes in which an 18-electron organometallic
compound acts as a 2-electron donor ligand to a 16-electron
organometallic fragment in much the same way as a conven-
tional ligand such as PPh3. Typical examples of molecules of
this type are (OC)5OsOs(CO)3(GeCl3)(Cl) (1), (L)(OC)4OsM′-
(CO)5 (M′ = Cr, Mo, W; L = PR3 (2), CNBut (3)) and
(R3P)(OC)4MRu(CO)3(SiCl3)2 (M = Ru, Os, but not Fe) (4).
These complexes contain an unbridged dative bond between
two transition metal atoms, which are rare. Only two com-
plexes in this class that are relevant to this study have been
reported by others; these are (η5-C5Me5)[(PriO)3P](OC)-
IrRh(CO)[P(OPri)3](Cl) (5a) and (η-C5H5)(OC)2RhPt(CO)(C6F5)2
(5b). Complexes with dative metal–metal bonds supported
Ph2PCH2PPh2), [Pt2{Rh(Cl)(CNXyl)}(µ-dpmp)2(CNXyl)][PF6]2
(dpmp = Ph2PCH2P(Ph)CH2PPh2), (OC)3(XylNC)Fe(µ-dppm)-
Re(CO)3(Br), Ni(µ-S2fc)(PMe2Ph) (S2fc = -(η-SC5H4)2Fe), and
Mo2Pd2(µ3-pyphos)4 (pyphos
= 6-diphenylphosphino-2-
pyridonate) (6).
One of the first complexes of this type that we reported
was Cp*(OC)2IrW(CO)5 (Cp* = η5-C5Me5), which was of
marginal stability (7). No attempt was made to prepare the
analogous complex in which (η-C5H5)Ir(CO)2 acted as a
ligand. It was expected that this molecule would be a much
weaker donor than its η5-C5Me5 analogue. It was known that
Os(CO)4(PR3) (R = Me, OMe, Ph etc.) were far superior lig-
ands than Os(CO)5 (2). It is presumed that replacing a car-
bonyl ligand in Os(CO)5 with a stronger σ donor PR3 ligand
increases the electron density on the Os atom and hence in-
creases the dative metal–metal bond strength. More recent
studies have, however, revealed that Os(CO)3(PR3)2 com-
pounds are weaker ligands than the monosubstituted com-
pounds. For example, although it was possible to isolate
(Me3P)2(OC)3OsW(CO)5, the chromium compound could
not be prepared (8), whereas (Me3P)(OC)4OsCr(CO)5 is an
air-stable crystalline solid (2).
Received October 15, 1998.
F. Jiang and R.K. Pomeroy.1 Department of Chemistry,
Simon Fraser University, Burnaby, BC V5A 1S6, Canada.
K. Biradha and M.J. Zaworotko. Department of Chemistry,
Saint Mary’s University, Halifax, NS B3H 3C3, Canada.
W.K. Leong. Department of Chemistry, National University
of Singapore, Lower Kent Ridge Rd., Singapore 119260,
Republic of Singapore.
The failure to prepare (Me3P)2(OC)3OsCr(CO)5 was at-
tributed to excessive steric interactions between the PMe3
ligand, which would necessarily be cis to the OsCr bond and
the radial carbonyls on the Cr atom. This suggested that
1Author to whom correspondence may be addressed.
Telephone: (604) 291-3347. Fax: (604) 291-3765.
e-mail: pomeroy@sfu.ca
Can. J. Chem. 77: 1327–1335 (1999)
© 1999 NRC Canada

1328
Can. J. Chem. Vol. 77, 1999
Table 1. Analytical and infrared dataa for the CpIr(CO)2 derivatives.
Compound
(L = CpIr(CO)2)
%C (calcd.)
22.88(22.62)
%H (calcd.)
0.94(0.79)
(CO) cm–1
(L)W(CO)5 (1)
2077.5(m), 2047.5(m), 2014.5(m), 1931.0 (s), 1924.0(s),
1911.0(m)
(L)Os(CO)3(GeCl3)(Cl) (2)
(L)Ru(CO)3(SiCl3)2
(L)Os(CO)3(Cl)2
15.10(14.98)
15.76(15.65)
18.13(18.24)
0.67(0.63)
0.72(0.66)
0.77(0.77)
0.69(0.67)
—
2122.0(w), 2113.0(vw, sh), 2080.5(m), 2055.5(s),
2047.0(vw, sh) 1999.5(m)
2138.0(w), 2106.0(m), 2064.5(s), 2055.5(w, sh),
2031.0(vs)b,c
2112.0(s), 2090.5(s), 2077.0(vw, sh), 2048.5(s), 2037.5 (vw,
sh), 2021.5(m), 1999.0(m)
2109.0(s), 2088.5(s), 2076.5(vw, sh), 2047.5(vs), 2037.5
(vw, sh), 2021.0(m), 1998.5(m)
(L)Os(CO)3(Br)2 (3)
(L)Os(CO)3(I)2
16.21(16.07)
c
2101.5(s), 2084.0(s) 2073.0(vw, sh) 2045.5 (vs), 2017(m)
1996.5(m)c
aIn CH2Cl2.
bCold solution.
cUnstable.
Table 2. NMR data for complexes of the (Cp)Ir(CO)2 derivatives.
Compound
1H NMRa
13C NMRb
(L = CpIr(CO)2)
CO
Cp
(L)W(CO)5 (1)
(L)Os(CO)3(GeCl3)(Cl) (2)
(L)Os(CO)3(Cl)2
5.84
6.10
5.98
6.00
202.6(1C), 199.3(4C)c 167.4(2C)d
180.1(2C), 172.4(1C), 165.0(2C)e
Not determined
87.9
91.6
(L)Os(CO)3(Br)2 (3)
172.1(2C), 165.9(1C), 164.6(2C)d
92.1
aIn CD2Cl2.
bIn CH2Cl2/CD2Cl2 (4:1).
cJWC = 127.4Hz.
dSample 13CO enriched.
eC-13 in natural abundance.
complexes with the sterically undemanding (η-C5H5)Ir(CO)2
molecule acting as a ligand might be more stable than origi-
nally thought. Herein, we report the results of an investiga-
tion into this possibility that shows that CpIr(CO)2 can
indeed act as ligand, but only with strong acceptor fragments
can the resulting complexes be isolated.
frared data and elemental analyses for the new compounds
1
are given in Table 1; H and 13C{1H} NMR data are given
in Table 2. Because they are unstable, no attempt was made
to obtain mass spectra of the new derivatives.
Preparation of cis-Os(CO)4(GeCl3)(Cl)
An autoclave (Parr Instrument Co.; 200 mL) was charged
with a solution of Os(CO)5 (~350 mg, ~0.94 mmol) and
GeCl4 (2.00 mL, 3.75 g, 17.5 mmol) in hexane (75 mL) and
sealed. The autoclave was flushed three times with CO and
then pressurized with CO (ultrapure grade; 90 atm (1 atm =
101.3 kPa)). It was then heated at 90°C for 24 h. The bomb
was cooled and the gases vented; the pale yellow solution
was removed from a white crystalline precipitate, which was
washed with hexane and dried under a stream of nitrogen.
The resulting solid cis-Os(CO)4(GeCl3)(Cl) (0.53 g, ~95%)
was pure by infrared spectroscopy. (The mother solution
yielded 31 mg of yellow crystals that were impure product.)
Experimental section
Unless otherwise stated, manipulations of starting materi-
als and products were carried out under a nitrogen atmo-
sphere with the use of standard Schlenk techniques. Solvents
were thoroughly dried before use. The iridium precursory
complex, (η-C5H5)Ir(CO)2, was prepared by the reaction of
[Ir(CO)3(Cl)]n with Tl[Cp] in thf, a minor modification of
the original preparation (9). The W(CO)5(thf) (2), cis-
Ru(CO)4(SiCl3)2 (10), Os(CO)5 (11), and cis-Os(CO)4(I)2
(12) were synthesized by literature procedures. The prepara-
tions of cis-Os(CO)4(X)2 (X = Cl, Br) (13) and cis-
Os(CO)4(GeCl3)(Cl) (14) are modifications of literature pro-
cedures and are given below for completeness.
Infrared spectra were recorded on a Bomen MB100 spec-
trometer; nuclear magnetic resonance spectra were recorded
on a Bruker WM400 or Bruker SY-100 spectrometer;
microanalyses were determined by M.K. Yang of the
Microanalytical Laboratory of Simon Fraser University. In-
Infrared (warm hexane) νCO
:
2173.5(w), 2106.0(vs),
2083.0(vw), 2061.0(s) cm–1; (CH2Cl2) νCO: 2179.5(m),
2118.5(w,sh), 2110.0(vs), 2069.5(s) cm–1.
Preparation of cis-Os(CO)4(Cl)2
A solution of Os(CO)4(H)2 in hexane (80 mL) was pre-
pared by the reaction of OsO4 (1.0 g, 3.9 mmol) in hexane
© 1999 NRC Canada

Jiang et al.
1329
with CO–H2 (3:1; total pressure 180 atm) at 170°C over-
night (13). After slowly cooling the reaction vessel to room
temperature and venting the gases, the solution was trans-
ferred to a Schlenk flask. The flask was cooled to 0°C and
chlorine gently bubbled through the solution until no more
precipitate formed. The solvent was removed from the white
precipitate, which was washed with hexane and dried on the
vacuum line to give cis-Os(CO)4(Cl)2 in essentially quantita-
tive yield. Infrared (CH2Cl2) νCO: 2189.5(w), 2121.5(vs),
2099.5(ms), 2059.5(s) cm–1. The bromo analogue, cis-
Os(CO)4(Br)2, was synthesized in a similar manner. Infrared
(warm hexane) νCO: 2177.0(vw), 2112.0(vs), 2089.0(ms),
2050.5(s) cm–1; (CH2Cl2) νCO: 2182.5, 2115.5, 2098.0,
2059.5 cm–1. The spectra of both derivatives are in excellent
agreement with those reported in the literature (13).
yellow crystals had formed along with gray decomposition
material. The solvent and excess reagents were removed and
the solids washed with a small volume of hexane and dried
on the vacuum line. A sample of the yellow crystals of
Cp(OC)2IrRu(CO)3(SiCl3)2 were immediately hand picked
for chemical analysis and an infrared spectrum. The com-
pound decomposes both in solution (in a period of minutes)
and in the solid state (after several hours) under nitrogen at
room temperature.
Preparation of Cp(OC)2IrOs(CO)3(Br)2 (3)
A
round-bottom flask with CpIr(CO)2 (~40 mg;
0.13 mmol), cis-Os(CO)4(Br)2 (35 mg, 0.076 mmol), and
hexane (20 mL) was evacuated and the solution degassed as
described above. The reaction vessel was heated at 55–60°C
for ~40 h. The flask was cooled and the solvent removed
from a pale yellow precipitate. An IR spectrum of the pre-
cipitate indicated that it consisted mainly of 3, but also of a
small amount of [Os(µ-Br)(CO)3(Br)]2. The precipitate was
therefore subjected to chromatography on silica (12 × 1 cm)
with CH2Cl2 as the eluant. The pale yellow fraction was col-
lected, concentrated, and stored at –25°C to afford 3 (41 mg;
72%) as pale yellow crystals. The chloro analogue was pre-
pared in a similar manner. Attempts to prepare the iodo
compound gave a red-brown precipitate, an infrared spec-
trum of which (Table 1) indicated that it contained mainly
Cp(OC)2IrOs(CO)3(I)2, but the compound rapidly decom-
posed in solution to give CpIr(CO)2 and [Os(µ-I)(CO)3(I)]2
and could therefore not be further purified.
Preparation of Cp(OC)2IrW(CO)5 (1)
A sample of W(CO)5(thf) was prepared from W(CO)6
(40 mg, 0.11 mmol) and the solvent reduced to ~0.5 mL on
the vacuum line. A solution of CpIr(CO)2 (~15 mg,
~0.045 mmol) in cold hexane (8 mL) was immediately
added to the cold W(CO)5(thf)–thf mixture and the resulting
solution stored without stirring at –25°C for 19 h. The sol-
vent was decanted from the solid that had formed, which
was dried on the vacuum line. Unreacted W(CO)6 was re-
moved from the solid by sublimation under high vacuum at
room temperature to a cold finger. The residue was dis-
solved in a minimum amount of CH2Cl2 at –20°C and
quickly filtered through a short column of Florisil (2 ×
1 cm) and the solvent was removed on the vacuum line at
–20°C. As such, 1 was obtained in low yield as a yellow
microcrystalline solid that was analytically pure. In a second
preparation the Schlenk flask containing the reagents was
kept at –25°C for 6.5 d, whereupon yellow crystals of 1
were obtained that were handpicked from black decomposi-
tion material to afford the sample used in the X-ray analysis.
X-ray structure determination of 1, 2, and 3
The X-ray diffraction data for 1–3 were collected on
Siemens SMART/CCD diffractometers. Data for 2 were col-
lected with the sample at 20°C, the data for 1 and 3 were
collected at –70°C with an LT-II low-temperature device.
The diffraction data were corrected for absorption with the
SADABS program. The program SHELXTL was used for
the structure solution and the refinement was based on F2
(15). The non hydrogen atoms in the structures were refined
anisotropically. Hydrogen atoms were fixed in calculated po-
sitions and refined isotropically on the basis of the corre-
Preparation of Cp(OC)2IrOs(CO)3(GeCl3)(Cl) (2)
A round-bottom flask (~100 mL; fitted with a Teflon
valve) was charged with CpIr(CO)2 (~55 mg; 0.18 mmol),
cis-Os(CO)4(GeCl3)(Cl) (100 mg, 0.194 mmol), and hexane
(30 mL). The flask and contents were cooled to –196°C and
the flask was evacuated; the solution was degassed with
three freeze–pump–thaw cycles. The flask was allowed to
warm to room temperature and then heated at 60°C for 3 h.
The flask was cooled to room temperature and the solvent
and excess reagents removed from an off-yellow precipitate.
The precipitate was recrystallized from CH2Cl2–hexane to
yield 2 (83 mg, 59%) as fawn-yellow crystals that appeared
stable in air.
sponding
C atoms (U(H) = 1.2 Ueq(C)). Pertinent
crystallographic data are given in Table 3; selected bond
lengths and angles for 1, 2, and 3 are given in Tables 4–6.
Supplementary tables of crystal structure and refinement
data, atomic coordinates, thermal parameters, and bond
lengths and angles for compounds 1, 2 and 3 have been de-
posited.2
Results and discussion
Preparation of Cp(OC)2IrRu(CO)3(SiCl3)2
Cp(OC)2IrW(CO)5 (1)
A Schlenk tube with cis-Ru(CO)3(SiCl3)2 (31 mg,
0.064 mmol) and CpIr(CO)2 (15 mg; 0.048 mmol) in hexane
(8.0 mL) was stored at –15°C for 6 d. After this period pale
Compound 1 was prepared in low yield by the addition of
CpIr(CO)2 to W(CO)5(thf) in hexane at low temperature
(eq. [1]).
2 Copies of the supplementary material may be purchased from: The Depository of Unpublished Data, Document Delivery, CISTI, National
Research Council of Canada, Ottawa, On K1A 0S2, Canada. Tables of crystal structure and refinement data, atomic coordinates, and bond
lengths and angles have also been deposited with the Cambridge Crystallographic Data Centre and can be obtained on request from: The Di-
rector, Cambridge Crystallographic Data Centre, University Chemical Laboratory, 12 Union Road, Cambridge, CB2 1EZ, U.K.
© 1999 NRC Canada

1330
Can. J. Chem. Vol. 77, 1999
Table 3. Crystal data and experimental details for compounds 1, 2, and 3.
1
2
3
Compound
Formula weight
Crystal system
Space group
Temperature (°C)
a, Å
b, Å
c, Å
β, deg
Cell volume (Å3)
z
C12H5IrO7W
637.21
Monoclinic
P21/na
–70 ± 3
9.5131(6)
12.5714(8)
12.1197(8)
99.4060(10)
1429.9(2)
4
C10H5Cl4GeIrO5Os
801.93
Monoclinic
P21/na
C10H5Br2IrO5Os
747.36
Monoclinic
P21/na
–70 ± 3
8.0754(5)
13.6114(9)
13.7135(9)
102.7510(10)
1470.2(2)
4
20 ± 2
7.9666(1)
13.7214(1)
17.6252(3)
102.6760(10)
1879.70(4)
4
Density, Mg m–3 calcd.
Abs. coeff. (mm–1)
Unique data (I ≥ 2σ)
Parameters
Rb
2.960
17.363
3332
190
0.0338
0.0562
2.834
15.975
4728
199
0.0474
0.0960
3.377
23.121
3410
172
0.0506
0.1114
c
Rw
aNonstandard setting of P21/c.
bRF = S|(|Fo| – |Fc|)|/S|Fo|.
2
2
cRwF = [S(w(|Fo| – |Fc|)2)/S(wFo )]1/2·w = [s2(Fo)2+kFo ]–1.
Table 4. Selected bond lengths and angles for 1.
Bond lengths (Å)
W—Ir
3.0418(4)
Ir—C(8)
Ir—C(9)
Ir—C(10)
Ir—C(11)
Ir—C(12)
2.242(7)
2.236(7)
2.258(7)
2.233(7)
2.271(7)
W—C(1)
W—C(2)
W—C(3)
W—C(4)
W—C(5)
Ir—C(6)
2.038(8)
2.034(8)
2.044(8)
2.041(8)
1.945(7)
1.852(9)
1.857(8)
C—O (range)
C—C (range)
1.141(9)–1.162(8)
1.40(1)–1.42(1)
Ir—C(7)
Bond angles (deg)
Ir-W-C(5)
Ir-W-C(1)
Ir-W-C(2)
Ir-W-C(3)
Ir-W-C(4)
C(6)-Ir-W
C(7)-Ir-W
C(6)-Ir-C(7)
169.6(2)
97.3(2)
98.5(2)
85.8(2)
85.8(2)
79.9(2)
80.9(2)
91.3(4)
W-C-O (range)
C(5)-W-C (range)
cis-C-W-C (range)
C(6)-Ir-C(7)
Ir-C(6)-O(6)
Ir-C(7)-O(7)
175.9(7)–178.1(7)
85.6(3)–90.3(3)
86.9(3)–92.0(3)
91.3(4)
175.0(7)
175.7(7)
C-C-C(Cp) (range)
107.2(7)–109.2(7)
gle data are given in Table 4. As can be seen from the fig-
ure, the CpIr(CO)2 molecule is bound to the W(CO)5 unit
via an unbridged IrW bond. Given the 18 and 16 electron
counts of the respective Ir and W moieties, the bond is con-
sidered a dative interaction (Ir →W). The length of the IrW
bond in 1 (3.0418(4) Å) is only marginally shorter than that
in 1* (3.054(1) Å) and, as discussed previously for 1* (7),
this is almost 0.2 Å longer than IrW lengths found in cluster
compounds of these elements. As in other complexes with
dative metal–metal bonds (2–4) and the other structures de-
scribed here, the carbonyls on the donor metal atom lean to-
ward the acceptor atom. In 1 the appropriate W-Ir-C(CO)
angles are 79.9(2)° and 80.9(2)°. (If the Ir moiety had a typi-
cal 3-legged piano stool geometry these angles would be
~90°.) Another interesting feature of the geometry adopted
by 1 is that the carbonyls on the Ir atom are eclipsed with
It is a yellow crystalline solid that can be handled for short
periods in air, but turns black upon prolonged exposure. In
hexane at room temperature 1 decomposed within 20 min to
CpIr(CO)2, W(CO)6, and, presumably, tungsten metal. In this
respect it is somewhat less stable than Cp*(OC)2IrW(CO)5
(1*) (7). Attempts to prepare Cp(OC)2IrCr(CO)5 were un-
successful.
A view of the structure of 1 as determined by X-ray crys-
tallography is shown in Fig. 1; selected bond length and an-
© 1999 NRC Canada

Jiang et al.
1331
Table 5. Selected bond lengths and angles for 2.
Bond lengths (Å)
Os—Ir
Os—Ge
2.8702(5)
2.421(1)
2.430(2)
2.133(4)
2.149(5)
2.151(4)
1.95(1)
Ir—C(4)
Ir—C(5)
Ir—C(6)
Ir—C(7)
Ir—C(8)
Ir—C(9)
Ir—C(10)
1.87(2)
1.84(2)
2.21(1)
2.22(2)
2.22(1)
2.21(1)
2.26(1)
Os—Cl(1)
Ge—Cl(2)
Ge—Cl(3)
Ge—Cl(4)
Os—C(1)
Os—C(2)
Os—C(3)
1.85(1)
1.94(1)
C—O (range)
C—C (range)
1.12(1)–1.16(1)
1.32(2)–1.42(2)
Bond angles (deg)
Ir-Os-Ge
Ir-Os-Cl(1)
Ir-Os-C(1)
Ir-Os-C(2)
Ir-Os-C(3)
173.85(4)
89.91(6)
87.6(4)
93.7(4)
91.2(4)
82.0(4)
85.8(4)
C(4)-Ir-C(5)
92.3(9)
Os-Ge-Cl (range)
Cl-Ge-Cl (range)
Os-C-O (range)
Ir-C(4)-O(4)
Ir-C(5)-O(5)
C-C-C(Cp) (range)
116.2(1)–117.8(2)
100.5(2)–101.3(3)
176(1)–178(1)
174(1)
176(2)
105(2)–109(2)
Os-Ir-C(4)
Os-Ir- C(5)
Table 6. Selected bond lengths and angles for 3.
Bond lengths (Å)
Os—Ir
2.8540(7)
2.585(2)
2.574(2)
1.87(1)
1.91(2)
1.92(2)
1.87(1)
1.83(2)
Ir—C(6)
Ir—C(7)
Ir—C(8)
Ir—C(9)
Ir—C(10)
2.25(2)
2.27(2)
2.24(1)
2.29(1)
2.23(2)
Os—Br(1)
Os—Br(2)
Os—C(1)
Os—C(2)
Os—C(3)
Ir—C(4)
Ir—C(5)
C—O (range)
C—C (range)
1.12(2)–1.17(2)
1.39(2)–1.41(2)
Bond angles (deg)
Ir-Os-C(3)
Ir-Os-Br(1)
Ir-Os-Br(2)
Ir-Os-C(1)
Ir-Os-C(2)
C(4)-Ir-Os
C(5)-Ir-Os
C(4)-Ir-C(5)
Br(1)-Os-Br(2)
176.1(4)
92.45(4)
89.33(4)
89.4(4)
91.3(4)
83.2(5)
83.9(4)
90.8(6)
92.22(5)
C(3)-Os-Br(1)
C(3)-Os-Br(2)
C(1)-Os-C(2)
C(1)-Os-C(3)
C(2)-Os-C(3)
Os-C-O (range)
Ir-C(4)-O(4)
85.0(4)
87.9(5)
94.0(6)
93.3(5)
91.4(6)
176(1)–178(1)
172(1)
175(1)
107(1)–108(1)
Ir-C(5)-O(5)
C-C-C(Cp) (range)
Fig. 1. Molecular structure of Cp(OC)2IrW(CO)5 (1).
two CO ligands on the W atom. The torsion angles between
C(1)—W and Ir—C(7), and between C(2)—W and
Ir—C(6), are 1.1° and 6.1°, respectively. This is also ob-
served in 1* but not in other complexes with dative metal–
metal bonds (e.g., the structures of 2 and 3 described be-
low).
Shown in Fig. 2 are variable temperature 13C{1H} NMR
spectra (carbonyl region) of 1 in CD2Cl2/CH2Cl2. The spec-
trum at –40°C indicates that the solution structure of 1 is the
same as that in the solid state provided there is rapid rotation
about the IrW bond. The sample was prepared from 13CO-
enriched W(CO)5(thf) (2) and it is apparent that CO ex-
change has occurred so that the 13CO label is scrambled over
all carbonyl sites in the molecule. On warming the solution
above –40°C, the carbonyl signals broaden and collapse to
the base line (Fig. 2), consistent with the carbonyl exchange
taking place on the NMR time scale. This exchange also oc-
© 1999 NRC Canada

1332
Can. J. Chem. Vol. 77, 1999
Fig. 2. Variable temperature 13C{1H} NMR spectra (CO region)
of 1 in CH2Cl2/CD2Cl2 (4:1). The peak marked with an asterisk
is due to W(CO)6.
Fig. 3. Molecular structure of Cp(OC)2IrOs(CO)3(GeCl3)(Cl) (2).
0 oC
-20 oC
-40 oC
The infrared spectrum of 2 in CH2Cl2 in the carbonyl
stretching region exhibits shoulders on the main peaks (Ta-
ble 1). We attribute this to the presence of two conformers
of 2, one with the Cp ligand in a cis orientation to the Cl
ligand, the other with a trans configuration of these ligands.
The 13C{1H} NMR spectrum of 2 in CH2Cl2/CD2Cl2 after an
overnight accumulation of spectral data revealed no other
*
200
192
184
ppm
176
168
1
species present (Table 2). The H NMR spectrum of 2 in
curs in 1* such that the carbonyl resonances are virtually
collapsed to the base line in the corresponding 13C NMR
spectrum at –40°C (7). In other words, the barrier to car-
bonyl exchange in 1* is much lower than that in 1. A similar
difference in the barriers to carbonyl exchange is observed
in (η5-C5R5)IrOs2(CO)9 (R = H, Me) (16). For the latter mol-
ecules, it was proposed that the pentamethylcyclopentadienyl
ligand increases the electron density at the Ir atom, which
expands the Ir 5d orbitals due to the increased screening of
the nuclear charge; this in turn results in better overlap with
the empty π* orbitals on the carbonyl ligands on the adjacent
osmium atoms and hence lowers the energy needed to form
the presumed intermediate with bridging carbonyls. The
same arguments can be used to rationalize the difference in
the barriers to CO exchange in 1 and 1*.
CD2Cl2 at –90°C exhibited a singlet for the Cp ligand that
was only slightly broadened from that at ambient tempera-
ture. Barring accidental degeneracy, the spectra indicate that
at –90°C there is still rapid rotation on the NMR time scale
about the IrOs bond in 2.
A view of the structure of 2 is shown in Fig. 3; the struc-
ture was determined at 20°C and consequently the thermal
ellipsoids are much larger than those for 1 and 3 (see be-
low), which were determined at –70°C. The length of the
(unbridged) IrOs bond in 2 is 2.8702(2) Å. We have found
that unbridged dative metal–metal bonds are comparable in
length to corresponding nondative bonds in dinuclear com-
plexes, but longer than corresponding bonds in cluster com-
pounds (2–4, 17). There does not appear to be any report of
a dinuclear complex with an unbridged nondative OsIr bond
length to compare with that in 2 and, as expected, the length
is significantly longer than those in osmium–iridium car-
bonyl cluster compounds. For example, in (η5-
C5Me5)(OC)IrOs2(CO)8 the OsIr distances are 2.7902(5) and
2.8124(5) Å (16).
Cp(OC)2IrOs(CO)3(GeCl3)(Cl) (2)
Compound 2 was isolated in moderate yield by the route
depicted in eq. [2]. The yellow-fawn crystals of 2 showed no
visible sign of decomposition after 2 d of exposure to air.
We have previously argued that because of the repulsive
dπ – dπ interactions between the two transition metal atoms,
complexes with dative metal–metal bonds will be more stable,
with strong acceptor ligands in both sites trans to the bond if
steric interactions allow (4). In compound 2 we believe that
the GeCl3 unit has excellent acceptor ability, as proposed for
the SiCl3 ligand in (R3P)(OC)4MRu(CO)3(SiCl3)2 complexes
mentioned in the Introduction (4). The structural parameters
found for the Os(CO)3(GeCl3)(Cl) grouping are not signifi-
cantly different from those in (OC)5OsOs(CO)3(GeCl3)(Cl)
(1).
We have found that Ru(CO)3(SiCl3)2 is also superior to
W(CO)5 as an acceptor fragment (4, 18). Addition of
CpIr(CO)2 to cis-Ru(CO)4(SiCl3)2 in hexane at –15°C pro-
duced yellow crystals along with decomposition products.
The handpicked crystals gave a satisfactory chemical analy-
Similarly, using infrared and NMR spectroscopy, 2 showed
no decomposition in CH2Cl2 solution (under nitrogen) after
24 h. We had previously found that the Os(CO)3(GeCl3)(Cl)
fragment is a superior acceptor to the W(CO)5 unit for
Os(CO)4(L) (L = CO, CNBut) ligands. For example, whereas
(OC)5OsOs(CO)3(GeCl3)(Cl) is an air-stable crystalline solid
that is stable in solution to at least 60°C (1), we have been
unable to prepare (OC)5OsW(CO)5 (2).
© 1999 NRC Canada

Jiang et al.
1333
Fig. 4. The 13C{1H} NMR spectrum (CO region) of
Cp(OC)2IrOs(CO)3(Br)2 (13CO in natural abundance) in
CH2Cl2/CD2Cl2 (4:1) after an overnight accumulation of spectral
data. The peaks marked with an asterisk are due to dissociation
products (see eq. [4] and text).
Fig. 5. Molecular structure of 3.
3
3
*
*
3
*
crystals that appeared stable in air. In these reactions the
product precipitates from the (nonpolar) hexane solution,
which prevents the dissociation described below. In the cor-
responding reaction of CpIr(CO)2 with cis-Os(CO)4(I)2
a
precipitate also formed that gave an infrared spectrum con-
taining bands (Table 1) consistent with the presence of
Cp(OC)2IrOs(CO)3(I)2, but also bands due to [Os(µ-I)-
(CO)3(I)]2, identified by analogy to the chloro (19, 20) and
ruthenium analogues (20, 21).3 When the solution was al-
lowed to stand at room temperature there was essentially
complete decomposition to CpIr(CO)2 and [Os(µ-I)-
(CO)3(I)]2 after 1 h.
175
170
165
ppm
sis and an infrared spectrum (Table 1) expected for
Cp(OC)2IrRu(CO)3(SiCl3)2, but the compound was too un-
stable in the solid or in solution (under nitrogen) to charac-
terize further. Because of the smaller covalent radius of Ir
compared to Os, dative metal–metal bonds involving iridium
are shorter than similar bonds where the donor atom is Os.
For example, whereas the IrOs distance in 2 is 2.870 Å, the
OsOs lengths in the independent molecules of
(OC)5OsOs(CO)3(GeCl3)(Cl) are in the range 2.916(2)–
2.931(1) Å (1); in (Me3P)(OC)4OsRu(CO)3(SiCl3)2 the
OsRu lengths are 2.984(1) and 3.014(1) Å (4). For this rea-
son, it may be that the hydrogen atoms of the
cyclopentadienyl ligand in Cp(OC)2IrRu(CO)3(SiCl3)2 make
close contact with the SiCl3 ligand, which is by necessity cis
to the IrOs bond and leads to destabilization of the complex.
When 3 was stirred in CH2Cl2 (or CD2Cl2) there was con-
version over 24 h to an apparent equilibrium mixture con-
taining 3 and CpIr(CO)2 and [Os(µ-Br)(CO)3(Br)]2 (eq. [4]),
1
in an approximate 1:1 ratio, by H NMR spectroscopy.
(The bromo dimer was similarly identified by comparison of
its infrared spectrum3 with those of the chloro and ruthe-
nium analogues (19–21).) The ratio was unchanged, by in-
frared spectroscopy, after standing for a further 24 h. The
13C NMR spectrum of 3 (13C in natural abundance) in
CH2Cl2/CD2Cl2 after an overnight accumulation of spectral
data at room temperature showed additional peaks besides
those due to 3 at 175.0 and 85.8 assigned to CpIr(CO)2 (22)
and signals at 165.0 and 163.9 (in a 1:2 ratio) assigned to
[Os(µ-Br)(CO)3(Br)]2 (Fig. 4). The signals due to the disso-
ciation products were barely visible in the corresponding
spectrum of 3 (13CO enriched)4 after ~1 h of data accumula-
Cp(OC)2IrOs(CO)3(X)2 (X = Cl, Br: 3, I)
The chloro and bromo (3, eq. [3]) derivatives were pre-
pared in good yield by using a procedure similar to that for
synthesizing 2. The compounds were isolated as pale yellow
3 Infrared for [Os(µ-X)(CO)3(X)]2, (CH2Cl2) νCO (X = Cl): 2135(s), 2061(s, br) (20); (X = Br): 2130.5(m), 2057.5(s, br); (X = I): 2119.5(s),
20045.0(s, br) cm–1.
4 The sample of 3 enriched with 13CO was prepared from 13CO-enriched Os(CO)4(Br)2, which was in turn synthesized by heating a hexane
solution of unenriched Os(CO)4(Br)2 under 13CO (99%; 2 atm) overnight. The 13C NMR spectrum indicated all carbonyl sites in 3 were
equally enriched, consistent with CO exchange on the synthetic time scale.
© 1999 NRC Canada

1334
Can. J. Chem. Vol. 77, 1999
tion. The equilibrium depicted in eq. [4] is similar to the
as better donor ligands than the parent CpIr(CO)2. It should
also be pointed out that the acceptor fragments employed in
this investigation were neutral five-coordinate fragments and
it may be that complexes of CpIr(CO)2 with fragments posi-
tively charged or of lower coordination numbers (with fewer
steric interactions) might be more stable. The stable (η5-
C5Me5)[(PriO)3P](OC)IrRh(CO)[P(OPri)3](Cl) (5a) and (η-
C5H5)(OC)2RhPt(CO)(C6F5)2 (5b) complexes mentioned in
the Introduction are consistent with the latter suggestion.
equilibrium
that
we
have
observed
for
(OC)5OsRu(CO)3(SiCl3)(Br) (18).
The chloro analogue of 3, Cp(OC)2IrOs(CO)3(Cl)2, simi-
larly dissociated in CH2Cl2 at room temperature at approxi-
mately the same rate as 3, but there was apparently also
decomposition of [Os(µ-Cl)(CO)3(Cl)]2 so that after 24 h an
infrared spectrum of the reaction solution showed bands due
to CpIr(CO)2, trace amounts of Cp(OC)2IrOs(CO)3(Cl)2, and
[Os(µ-Cl)(CO)3(Cl)]2, plus unidentified products.
These preliminary results indicate that Os(CO)3(Br)2 is a
better acceptor than the iodo analogue, as might be expected
from the electronegativities of the two halogens. That the ac-
ceptor abilities of the Os(CO)3(X)2 (X = Cl, Br) fragments
toward CpIr(CO)2 are similar may be rationalized by invok-
ing more pπ donation from the halogen atoms to the dπ
orbitals of osmium for the chloro fragment compared to the
bromo analogue. This would decrease the Lewis acidity of
Os(CO)3(Cl)2 and offset the superior electron-withdrawing
ability of the Cl atoms. This is of course similar to the argu-
ments that have been used to explain why BF3 is a weaker
Lewis acid than BCl3 or BBr3 (23). Consistent with the view
that Cl pπ donation to Os dπ orbitals is significant is that the
Os—CO length trans to the Cl atom in 2 (1.85(1) Å) is con-
siderably shorter than the Os—C distances (1.94(1),
1.95(1) Å) to the carbonyl ligands that are in a mutually
trans configuration (Table 5).
The structure of 3 reveals (Fig. 5), once again, an
unbridged metal–metal bond. The length of the IrOs bond in
3 is 2.8540(7) Å, which is 0.016 Å shorter than the corre-
sponding length in 2. From a ground state viewpoint, there-
fore, the IrOs bonds in 2 and 3 are of comparable strength. It
has been proposed, however, that transition state stabiliza-
tion effects are important in determining the lability of da-
tive metal–metal bonds (4, 24). Besides the bond lengths
mentioned above, the IrOs distance in 3 may be compared to
Acknowledgments
We thank the Natural Sciences and Engineering Research
Council of Canada, the National University of Singapore,
and St. Mary’s University for financial support.
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Å in the related
(OC)5OsOs(CO)3(Cl)2 recently prepared in this laboratory.
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The results of this study indicate that CpIr(CO)2 is a weak
ligand and only with 16e organometallic fragments that are
strong acceptors can complexes be isolated. Except for
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solution, which results in most cases in decomposition. The
ligating ability of CpIr(CO)2 therefore appears comparable
to that of Os(CO)5 (1, 2). Since Os(CO)4(PR3) compounds
are better ligands than Os(CO)5 (2a, 18), molecules of the
type CpIr(CO)(PR3) with small P ligands will probably act
© 1999 NRC Canada

Jiang et al.
1335
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© 1999 NRC Canada