Os5(CO)18(CNBut)2(Cl)2 : A complex with a linear chain of five osmium atoms

Article abstract of DOI:10.1021/om010948h

Various aspects of Os₅(CO)₁₈(CNBu^t)₂(Cl)₂ , a complex with a linear chain of five osmium atoms, were discussed. It was found that the OsOs lengths in 2Cl were somewhat shorter than in compounds with Os₃ chains. The analysis suggested that the presence of the halide ligands in 1X and 2X allows the synthesis of compounds with still longer Os chains.

Full text of DOI:10.1021/om010948h

Organometallics 2002, 21, 773-776
773
Os5(CO)18(CNBu t)2(Cl)2: A Com p lex w ith a Lin ea r Ch a in
of F ive Osm iu m Atom s
Faming J iang,† Glenn P. A. Yap,‡ and Roland K. Pomeroy*,†
The Departments of Chemistry, Simon Fraser University, Burnaby,
British Columbia V5A 1S6, Canada, and the University of Ottawa, 10 Marie Curie Avenue,
Ottawa, Ontario K1N 6N5, Canada
Received November 1, 2001
Sch em e 1
Summary: Complexes of formula (X)Os3(CO)12Os(CO)3-
(L)(X) and (X)(L)(OC)3OsOs3(CO)12Os(CO)3(L)(X) (L )
CNBut; X ) Cl, Br, I) have been prepared by the addition
of Os(CO)4(L) to Os3(CO)12(X)2 in solution at 60 °C. The
structure of Os5(CO)18(CNBut)2(Cl)2 reveals it has an
almost linear chain of five Os atoms.
In tr od u ction
chains of four and five unbridged Os atoms using this
strategy. Organometallic complexes with linear chains
of unbridged metal-metal bonds between heavy transi-
tion metals might be expected to show interesting
electrical conduction and redox properties. This is
because such chains should possess extended molecular
orbitals with a small energy gap between the HOMO
and LUMO.
Much of the work from this laboratory has described
complexes in which an 18-electron organometallic com-
pound acts as a two-electron donor ligand to a 16-
electron organometallic fragment in much the same way
as a conventional ligand such as PPh3. Typical examples
of molecules of this type are (OC)3(R3P)2OsW(CO)5 and
(R3P)(OC)4MRu(CO)3(SiCl3)2 (M ) Ru, Os, but not Fe).1
These complexes contain an unbridged dative bond
between two transition metal atoms.
Exp er im en ta l Section
In some of our earliest studies of complexes of this
type it was observed that isomerization often occurred
for complexes with unbridged dative metal-metal bonds
that also contained a halide or (especially) a hydride
ligand coordinated to the acceptor metal atom.2,3 The
isomerization involved migration of the halide or hy-
dride ligand to the donor atom with concomitant migra-
tion of a carbonyl ligand to the acceptor atom (Scheme
1; in this and the other scheme presented here the
carbonyl ligands not involved in the isomerization have
been omitted). If the halogen ligand is considered as a
one-electron donor ligand, then the isomerization can
be viewed as resulting in the conversion of the dative
metal-metal bond to a nondative bond. In an interest-
ing extension of this reaction we have recently reported
the successive addition of 3 mol of Os(CO)4(L) (L )
CNBut) to Mn(CO)5(I) to give (I)[Os(CO)3(L)]3Mn(CO)5.
The structure of the complex reveals it to contain a
linear chain of four unbridged metal atoms.4
Gen er a l P r oced u r es. Unless otherwise stated, manipula-
tions of starting materials and products were carried out under
a nitrogen atmosphere with the use of standard Schlenk
techniques. Hydrocarbon solvents were refluxed over potas-
sium, distilled, and stored over molecular sieves before use.
Dichloromethane was dried in a similar manner except that
CaH2 was employed as the drying agent. The precursory
compounds Os(CO)4(CNBut) and Os3(CO)12(X)2 (X ) Cl, Br, I)
were prepared by literature procedures.5,6 NMR spectra were
recorded on a Bruker AMX400 spectrometer at the appropriate
operating frequencies for 1H and 13C{1H} NMR spectra. With
the exception of 2Cl, samples for 13C{1H} NMR spectra were
enriched to ∼30% 13CO prepared from more highly 13CO-
enriched Os3(CO)12.7 All new compounds gave a parent ion by
MS (FAB).
P r ep a r a tion of Os4(CO)15(CNBu t)(Cl)2 (1Cl). A round-
bottom flask (∼100 mL; fitted with a Teflon valve) was
furnished with Os(CO)4(CNBut) (34 mg, 0.088 mmol), Os3-
(CO)12(Cl)2 (74 mg, 0.075 mmol), and toluene (70 mL). The
solution was degassed with three freeze-pump-thaw cycles
and then heated at 60 °C for 66 h. The solvent was removed
on the vacuum-line and the residue dissolved in a minimum
amount of CH2Cl2 (∼3 mL). The solution was subjected to
chromatography on silica gel (12 × 1 cm). Hexane was used
as eluent to remove traces of Os3(CO)12; hexane/CH2Cl2 (1:1)
to remove unreacted Os3(CO)12(Cl)2; and hexane/CH2Cl2 (1:4)
to elute Os4(CO)15(CNBut)(Cl)2. The third fraction was col-
lected, concentrated, and cooled to -25 °C to give yellow
Here we describe the synthesis of (X)Os3(CO)12Os-
(CO)3(L)(X) and (X)(L)(OC)3OsOs3(CO)12Os(CO)3(L)(X)
(L ) CNBut; X ) Cl, Br, I) complexes containing linear
* Corresponding author. E-mail: pomeroy@sfu.ca.
† Simon Fraser University.
‡ University of Ottawa.
(1) (a) J iang, F.; J enkins, H. A.; Biradha, K.; Davis, H. B.; Pomeroy,
R. K.; Zaworotko, M. J . Organometallics 2000, 19, 5049. (b) J iang, F.;
Male, J . L.; Biradha, K.; Leong, W. K.; Pomeroy, R. K.; Zaworotko, M.
J . Organometallics 1998, 17, 5810.
(2) Fleming, M. M.; Pomeroy, R. K.; Rushman, P. J . Organomet.
Chem. 1984, 273, C33.
(3) Einstein, F. W. B.; J ennings, M. C.; Krentz, R.; Pomeroy, R. K.;
Rushman, P.; Willis, A. C. Inorg. Chem. 1987, 26, 1341.
(4) J iang, F.; J enkins, H. A.; Yap, G. P. A.; Pomeroy, R. K. Inorg.
Chem. Commun. 2000, 3, 690.
(5) Shipley, J . A.; Batchelor, R. J .; Einstein, F. W. B.; Pomeroy, R.
K. Organometallics 1991, 10, 3620.
(6) (a) Sanati, H. K.; Becalska, A.; Ma, A. K.; Pomeroy, R. K. J .
Chem. Soc., Chem. Commun. 1990, 197. (b) J ohnson, B. F. G.; Lewis,
J .; Kilty, P. A. J . Chem. Soc. (A) 1968, 2859.
(7) Alex, R. F.; Pomeroy, R. K. Organometallics 1987, 6, 2437.
10.1021/om010948h CCC: $22.00 © 2002 American Chemical Society
Publication on Web 12/28/2001

774 Organometallics, Vol. 21, No. 4, 2002
Notes
crystals of Os4(CO)15(CNBut)(Cl)2 (26 mg, 32%). The bromo and
the iodo analogues were prepared in a similar way in 32% and
19% yield, respectively. 1Cl: IR (CH2Cl2) ν(CN) 2214 mbr; ν-
(CO) 2137 m, 2103 s, 2081 m, 2061 vwsh, 2053 vs, 2030 wsh,
1
2034 m, 2021 vs, 2009 m, 2000 w cm-1; H NMR (CD2Cl2) δ
1.61; 13C{1H} NMR (CD2Cl2/CH2Cl2, 1:4) δ 190.0 (4C), 189.2
(4C), 182.2 (2C), 181.9 (1C), 174.5 (1C), 172.1 (1C), 166.0 (1C),
164.6 (1C); 31.6 (Me). Anal. Calcd for C20H9Cl2O15Os4N: C,
17.99; H, 0.68; N, 1.05. Found: C, 17.88: H, 0.56; N, 1.23.
1Br : IR (CH2Cl2) ν(CN) 2213 mbr; ν(CO) 2136 m, 2102 s, 2080
m, 2061 vwsh, 2052 vs, 2031 m, 2019 vs, 2009 s, 2003 w cm-1
;
1H NMR (CD2Cl2) δ 1.59; 13C{1H} NMR (C2D2Cl4/C2H2Cl4, 1:4)
δ 189.3 (4C), 188.4 (4C), 179.1 (1C), 178.8 (2C), 173.2 (1C),
170.8 (1C), 164.1 (1C) 162.3 (1C). Anal. Calcd for C20H9Br2O15
-
Os4N: C, 16.87; H, 0.64; N, 0.98. Found: C, 16.88; H, 0.68; N,
0.91. 1I: IR (CH2Cl2) ν(CN) 2209 mbr; ν(CO) 2132 m, 2099 s,
2078 m, 2059 vwsh, 2048 s, 2040wsh, 2030 m, 2021vs, 2002
F igu r e 1. Molecular structure of Os5(CO)18(CNBut)2(Cl)2
(2Cl).
vwshw cm-1 1H NMR (CD2Cl2) δ 1.58. Anal. Calcd for
;
Sch em e 2
C
20H9I2O15Os4N: C, 15.82; H, 0.60; N, 0.92. Found: C, 16.14;
H, 0.70; N, 0.96.
P r ep a r a tion of Os5(CO)18(CNBu t)2(I)2 (2I). A round-
bottom flask (∼50 mL; fitted with a Teflon valve) was charged
with Os(CO)4(CNBut) (39 mg, 0.10 mmol), Os3(CO)12(I)2 (58 mg,
0.050 mmol), and toluene (15 mL). The solution was degassed
as described above and then heated at 60 °C for 19 h. The
solvent was removed on the vacuum-line. The residue was
washed with hexane and then dissolved in CH2Cl2 (30 mL).
The same amount of hexane was added. The resulting solution
was concentrated until solids just appeared, and it was then
stored in the freezer (-25 °C) to give Os5(CO)18(CNBut)2(I)2
as a yellow crystalline solid (58 mg, 62%). The chloro and
bromo analogues were prepared in a similar way in a yield of
35% and 72%, respectively. 2Cl: IR (CH2Cl2) ν(CN) 2210w;
ν(CO) 2120 w, 2092 m, 2078 w, 2066 w, 2030 m, 2009 s cm-1
;
2-fold axis. Non-hydrogen atoms were refined with anisotropic
displacement parameters; hydrogen atoms were treated as
idealized contributions. Scattering factors and anomalous
dispersion factors are contained in the SHELXTL v. 5.10
program library.9
1H NMR (CD2Cl2) δ 1.62; 13C{1H} NMR (CD2Cl2/CH2Cl2, 1:4)
δ 191.5 (4C), 190.9 (8C), 182.6 (2C), 175.0 (2C), 166.2 (2C),
62.0 (t-C); 31.7 (Me). Anal. Calcd for C28H18Cl2O18Os5N2: C,
19.87; H, 1.07; N, 1.66; for C29H20Cl4O18Os5N2 (i.e., 2Cl‚CH2-
Cl2): C, 19.60; H, 1.13; N, 1.58. Found: C, 19.86; H, 1.19; N,
1.55. 2Br : IR (CH2Cl2) ν(CN) 2211 w; ν(CO) 2119 w, 2091 m,
Resu lts a n d Discu ssion
2077 w, 2066 w, 2029 m, 2010 s cm-1 1H NMR (CD2Cl2) δ
;
The (X)Os3(CO)12Os(CO)3(L)(X) (L ) CNBut; X ) Cl,
Br, I; 1X) and (X)(L)(OC)3OsOs3(CO)12Os(CO)3(L)(X)
(2X) complexes were prepared by the reaction of Os-
(CO)4(CNBut) with (X)Os3(CO)12(X) in the correct molar
ratio in toluene at 55-60 °C (Scheme 2). The compounds
are yellow microcrystalline solids that appeared stable
in air. Compound 2Br showed no sign of decomposition
after 16 h in toluene at 90 °C (under nitrogen). The
corresponding reaction with (Br)Ru3(CO)12(Br)10 in place
of the Os analogue gave a product that was too unstable
to characterize.
The structure of 2Cl, as 2Cl‚CH2Cl2, was determined
by X-ray crystallography (Figure 1). There was disorder
between the Cl and CO(3) sites. The molecule possesses
an inversion center, and therefore the Os(2)-Os(3)-Os-
(2A) angle is 180°. The independent Os3 angle (Os(1)-
Os(2)Os(3)) is 177.01(2)°. In other words, 2Cl contains
an almost linear chain of five unbridged Os atoms. The
radial ligands on each Os atom are staggered with
respect to the radial ligands on the adjacent Os atoms.
The OsOs lengths are 2.8709(7) (Os(1)Os(2)) and 2.8907-
(5) Å (Os(2)Os(3)). These lengths may be compared to
the average OsOs length in Os3(CO)12 of 2.877 Å.11
1.60; 13C{1H} NMR (C2D2Cl4/C2H2Cl4, 1:4) δ 190.5 (4C), 190.3
(8C), 180.1 (2C), 173.9 (2C), 164.6 (2C), 61.3 (t-C); 31.4 (Me).
Anal. Calcd for C28H18Br2O18Os5N2: C, 18.88; H, 1.02; N, 1.57.
Found: C, 18.88; H, 1.05; N, 1.49. 2I: IR (CH2Cl2) ν(CN):
2207w; ν(CO): 2119w, 2089m, 2074w, 2064w, 2027m, 2009s
1
cm-1; H NMR (CD2Cl2) δ 1.58. Anal. Calcd for C28H18I2O18
-
Os5N2: C, 17.93; H, 0.97; N, 1.49. Found: C, 17.82: H, 0.94;
N, 1.45.
Str u ctu r e Deter m in a tion of 2Cl‚CH2Cl2. A suitable
crystal was selected, mounted on thin, glass fiber using
paraffin oil, and cooled to the data collection temperature. Data
were collected on a Bruker AX SMART 1k CCD diffractometer
using 0.3° ω-scans at 0°, 90°, and 180° in φ. Unit-cell
parameters were determined from 60 data frames collected
at different sections of the Ewald sphere. Semiempirical
absorption corrections based on equivalent reflections were
applied.8 Systematic absences in the diffraction data and unit-
cell parameters were consistent with space groups C2/c and
Cc. Initial exploration of the noncentrosymmetric space group
Cc yielded solutions with inversion symmetry. The solution
in the centrosymmetric space group option C2/c gave chemi-
cally reasonable and computationally stable results of refine-
ment. The structure was solved by direct methods, completed
with difference Fourier syntheses, and refined with full-matrix
least-squares procedures based on F2. The molecule is located
at an inversion center with a disordered chloride/carbonyl
ligand having a 50/50 site occupancy distribution. A molecule
of cocrystallized dichloromethane solvent was located at a
(9) Sheldrick, G. M. SHELXTL v. 5.10; Bruker AXS: Madison, WI,
1997.
(10) Nagra, H. K.; Batchelor, R. J .; Bennet, A. J .; Einstein, F. W.
B.; Lathioor, E. C.; Pomeroy, R. K.; Wang, W. J . Am. Chem. Soc. 1996,
118, 1207.
(8) Blessing, R. Acta Crystallogr. 1995, A51, 33.

Notes
Organometallics, Vol. 21, No. 4, 2002 775
It is often found, however, that OsOs lengths in com-
pounds with Os3 chains are somewhat longer than these
lengths.1 For example, in Os3(CO)12(I)2, Os3(CO)10-
[P(OMe3)]2(Br)2, and Os(CO)12(SiCl3)2 the OsOs bonds
are 2.935(2), 2.916(1), and 2.912(1) Å, respectively.12-14
The OsOs bonds in 2Cl from a ground-state view
therefore appear as strong OsOs single bonds. This was
also observed for (I)[Os(CO)3(CNBut)]3Mn(CO)5 (OsOs
bond distances of 2.8894(5) and 2.8928(5) Å).4 We
tentatively ascribed this to stronger OsOs σ bonding
brought about by more extensive delocation over more
metal-metal bonds.15 The other bonding parameters of
2Cl do not show any unusual features.
Compound 2Cl is an 82-electron compound and has
the expected four metal-metal bonds.16 This is the first
complex with a linear chain containing five unbridged
group 8 metal atoms. We have recently reported the
synthesis and structure of Os5(µ-Cl)2(CO)16(L)2, an Os5
cluster with an 82e count, but with the triangular Os3(µ-
Cl)2unit.17 There are also some Ru5 clusters that have
this electron count. Examples are Ru5(µ5-η3-C2PPh2)(µ-
SMe)2(µ-PPh2)(CO)13, Ru5(µ5-C2)(µ-SMe)2(µ-PPh2)2(CO)12,
and Ru5(µ-H)(µ5-η1:η1:η2:η2:η6-C13H7CHC)(µ-CO)(µ3-OH)-
(CO)11.18,19 As indicated by their formulas, these clusters
have a single ligand that bridges all five ruthenium
atoms. Probably as a consequence of this interaction,
the Ru5 chain in each molecule is either doubly or triply
bent.
The pattern of the resonances in the CO region of the
13C{1H} NMR spectra of 2Cl and 2Br indicates that
each molecule has the same structure in solution as
found in the solid state for 2Cl, provided there is the
typical free rotation about the OsOs bonds. The corre-
sponding spectrum of 1Cl (Figure 2) is likewise consis-
tent with the idealized structure shown in Scheme 2
with three OsOs unbridged bonds. The other spectro-
scopic properties of the new complexes are also consis-
tent with the structural assignments. The terminal Os
atoms in 2X are chiral centers. These molecules should
therefore exist as a pair of enantiomers, and a meso
form that is potentially distinguishable from the other
diastereoisomers by NMR spectroscopy. There was
however no evidence for two forms in the NMR spectra
of 2X. It is probable that because the chiral centers are
far removed from each other, the spectral properties of
each disastereoisomer are identical.
F igu r e 2. 13C{1H} NMR spectrum (carbonyl region) of Os4-
(CO)15(CNBut)(Cl)2 (1Cl) in CD2Cl2/CH2Cl2. The tentative
assignment of the signals is given in the inset.
Ta ble 1. Cr ysta l Str u ctu r e Da ta for
Os5(CO)18(CNBu t)2(Cl)2‚CH2Cl2 (2Cl‚CH2Cl2)
empirical formula
fw
C29H20Cl4N2O18Os5
1777.44
color
yellow
temp
293(2) K
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
C2/c
20.020(3)
11.760(2)
19.059(2)
100.171(2)
4416.7(9), 4
2.673
â (deg)
V (Å3), Z
D(calcd) (Mg‚m-3
)
abs coeff (mm-1
)
14.64
indpdt reflns (I>2σ(I))
4520 [R(int) ) 0.090]
a
RF
0.0362
b
RwF
0.0789
RF ) ∑|(|Fo| - |Fc|)|/∑|Fo|. RwF ) [∑(w(|Fo| - |Fc|)2)/∑(wFo2)]1/2
a
b
2
-1
w ) [σ2(Fo)2 + kFo
] .
NMR spectroscopy to be equally enriched (e.g., Figure
2). This indicates there is CO exchange on the synthetic
time scale within these molecules. The 13C{1H} NMR
spectrum of a mixture of 1Br and 2Br in CD2Cl4/CH2-
Cl4 at 90 °C, however, showed no significant line
broadening indicative of CO-exchange on the NMR time
scale.
In Table 3 the UV-vis spectra in CH2Cl2 of (X)(L)-
(OC)3OsOs(CO)4(X), (X)[Os(CO)4]3(X), (X)(L)(OC)3OsOs-
(CO)4Os(CO)3(L)(X), (X)(L)(OC)3Os[Os(CO)4]2Os(CO)4(X)
(i.e., 1X), and (X)(L)(OC)3Os[Os(CO)4]3Os(CO)3(L)(X)
(i.e., 2X) are reported. The (X)(L)(OC)3OsOs(CO)4(X) and
(X)(L)(OC)3OsOs(CO)4Os(CO)3(L)(X) derivatives were
prepared by the addition of Os(CO)4(L) to Os(CO)4(X)2;
details of their preparation and characterization will be
reported in a future publication.
The samples used for the 13C NMR spectra of 1Cl,
1Br , and 2Br were prepared from the appropriate 13CO-
enriched Os3(CO)12(X)2 and unenriched Os(CO)4(L). All
carbonyl sites in the molecules were found by 13C{1H}
(11) Churchill, M. R.; DeBoer, B. G. Inorg. Chem. 1977, 16, 878.
(12) Cook, N.; Smart, L.; Woodward, P. J . Chem. Soc., Dalton Trans.
1977, 1744.
(13) Chen, Y.-S.; Wang, S.-L.; J acobson, R. A.; Angelici, R. J . Inorg.
Chem. 1986, 25, 1118.
The lowest energy band in the 340-390 nm region
in each spectrum is assigned to transitions that involve
electrons in mainly OsOs σ-bonding MOs being pro-
moted to σ*-antibonding MOs, in accordance with
earlier studies on molecules with metal-metal bonds.20
(14) Willis, A. C.; van Buuren, G. N.; Pomeroy, R. K.; Einstein, F.
W. B. Inorg. Chem. 1983, 22, 1162.
(15) Albright, T. A.; Burdett, J . K.; Whangbo, M. H. Orbital
Interactions in Chemistry; Wiley: New York, 1985; p 247.
(16) Cifuentes, M. P.; Humphrey, M. G. In Comprehensive Organo-
metallic Chemistry II; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;
Pergamon: Oxford, 1995; Vol. 7, p 907.
(17) J iang, F.; Yap, G. P. A.; Pomeroy, R. K. J . Clust. Sci. 2001, 12,
433.
(18) Adams, C. J .; Bruce, M. I.; Skelton, B. W.; White, A. H. J . Chem.
Soc., Dalton Trans. 1997, 2937.
(19) Lau, C. S.-W.; Wong, W.-T. J . Chem. Soc., Dalton Trans. 1999,
607.
(20) (a) Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochem-
istry; Academic: New York, 1979; p 20. (b) Tyler, D. R.; Altobelli, M.;
Gray, H. B. J . Am. Chem. Soc. 1980, 102, 3022. (c) Meyer, T. J .; Casper,
J . V. Chem. Rev. 1985, 85, 187. (d) Male, J . L.; Pomeroy, R. K.; Tyler,
D. R. Organometallics 1997, 16, 3431.

776 Organometallics, Vol. 21, No. 4, 2002
Notes
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles (d eg) for 2Cl
Bond Lengths
Os(1)-Os(2)
Os(2)-Os(3)
Os(1)-C(10)
Os(1)-Cl
Os(1)-C(1)
Os(1)-C(2)
Os(1)-C(3)
2.8709(7)
2.8907(5)
2.01(1)
Os(2)-C(4)
Os(2)-C(5)
Os(2)-C(6)
Os(2)-C(7)
Os(3)-C(8)
Os(3)-C(9)
1.94(1)
1.89(1)
1.94(1)
1.96(1)
1.93(1)
1.94(1)
2.495(12), 2.502(14)a
1.92(1)
1.94(1)
1.83(3), 1.84(3)a
Bond Angles
Os(1)-Os(2)-Os(3)
Os(2)-Os(1)-C(1)
Os(2)-Os(1)-Cl
177.01(2)
Os(2)-Os(1)-C(2)
Os(2)-Os(1)-C(10)
Os(3)-Os(2)-C
87.3(3)
86.4(3)
89.5(3)-91.2(3)
177.2(4)
86.1(4), 88.0(4)a
a
Cl and CO(3) are disordered.
Ta ble 3. UV-Vis Sp ectr a (in CH2Cl2) of Com p lexes w ith Lin ea r Ch a in s of Os Atom s
compound
X ) Cl
X ) Bra
X ) I
(X)(L)Os2(CO)7(X)
(X)Os3(CO)12(X)
(X)(L)Os3(CO)10(L)(X)
(X)(L)Os4(CO)15(X), 1X
(X)(L)Os5(CO)18(L)(X), 2X
295 m, 326 w,b 359 wb
256 m, 341 s
301 (0.81), 337 (0.40), 368 (0.25)
259 (1.7), 297 (1.0), 348 (2.6)
251 (1.7),b 334 (95), 365 (13)b
259 (2.6), 293 (2.1), 355 (3.6)
267 (39), 322 (43),b 350 (50)
321 s, 356 vvw, 386 vvw
255 m, 280 w,b 358 s
264 w,b 341 s, 374 mb
276 w, 360 s,br
281 vw,b 332 s, 357 mb
255 w, 286 m, 353 s,br
266 m, 317 vw,b 348 s,br
275 m, 348 s,br
a
b
Approximate ꢀ, × 10-4 L mol-1 cm-1 in parentheses. Shoulder.
tion from the MO that is lowest in energy. The energy
of this MO might be expected to get lower as the metal
chain length increases.
Con clu sion s
The previous absence of reports describing molecules
with more than three Os (or Ru) atoms in a linear chain
suggested to us that such molecules were intrinsically
unstable. (The little-studied polymeric [Ru(CO)4]n is
reported to possess a chain of unbridged RuRu bonds
with lengths comparable to those in Ru3(CO)12.)21 The
synthesis of the 1X and 2X molecules with four and five
unbridged Os atoms bound in a row as robust, air-stable
crystals indicates this is not the case. Indeed, that the
OsOs lengths in 2Cl are somewhat shorter than in
compounds with Os3 chains indicates that chains with
more than three atoms may be more stable than those
with three atoms. The UV-vis spectra of the complexes
are also in agreement with this view. The presence of
the halide ligands in 1X and 2X may allow the synthesis
of compounds with still longer Os chains.
F igu r e 3. UV-vis spectra of approximately equimolar
concentrations of 2Br (upper trace) and 1Br (lower trace)
in CH2Cl2.
As the number of OsOs bonds increases, this absorption
increases in intensity and becomes broader and the
maximum shifts slightly to shorter wavelengths (e.g.,
Figure 3). The shifts are small and smaller than the
shifts that occur when the halide is changed or a CO is
replaced with a ButNC ligand. The shift to lower
frequency of the maxima as the Os chain length
increases is also contrary to what might be expected if
the transition was entirely due to the highest lying
HOMO-LUMO transition associated with the metal-
metal bonds. It may be however that all of the occupied
molecular orbitals associated with the OsOs bonds are
close in energy and the maximum represents the transi-
Ack n ow led gm en t. We thank the Natural Sciences
and Engineering Research Council of Canada for finan-
cial support.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
structure and refinement data, atomic coordinates, thermal
parameters, and bond lengths and angles for 2Cl. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM010948H
(21) Masciocchi, N.; Moret, M.; Cairati, P.; Regaini, F.; Sironi, A. J .
Chem. Soc., Dalton Trans. 1993, 471.