Homoleptic, σ-bonded octahedral [M(CO)6]2+ cations of iron(II), ruthenium(II), and osmium(II): Part 1: Syntheses, thermochemical and vibrational characterizations, and molecular structures as [Sb2F11]- and [SbF6]- salts. A comprehensive, comparative study

Article abstract of DOI:10.1021/ic040115u

Homoleptic octahedral, superelectrophilic σ-bonded metal carbonyl cations of the type [M(CO)₆]²⁺ (M = Ru, Os) are generated in the Bronsted-Lewis conjugate superacid HF/SbF₅ by reductive carbonylation of M(SO₃F)₃ (M = Ru, Os) or OsF₆. Thermally stable salts form with either [Sb₂F ₁₁]^- or [SbF₆]^- as anion, just as for the previously reported [Fe(CO)₆]²⁺ cation. The latter salts are generated by oxidative (XeF₂) carbonylation of Fe(CO)₅ in HF/SbF₅. A rationale for the two diverging synthetic approaches is provided. The thermal stabilities of [M(CO) ₆][SbF₆]₂ salts, studied by DSC, range from 180 °C for M = Fe to 350 °C for M = Os before decarbonylation occurs. The two triads [M(CO)₆][SbF₆]₂ and [M(CO) ₆][Sb₂F₁₁]₂ (M = Fe, Ru, Os) are extensively characterized by single-crystal X-ray diffraction and vibrational and ¹³C NMR spectroscopy, aided by computational studies of the cations. The three [M(CO)₆][SbF₆]₂ salts (M = Fe, Ru, Os) crystallize in the tetragonal space group P4/mnc (No. 128), whereas the corresponding [Sb₂F₁₁]^- salts are monoclinic, crystallizing in space group P2₁/n (No. 14). In both triads, the unit cell parameters are nearly invariant of the metal. Bond parameters for the anions [SbF₆]^- and [Sb ₂F₁₁]^- and their vibrational properties in the two triads are completely identical. In all six salts, the structural and vibrational properties of the [M(CO)₆]²⁺ cations (M = Fe, Ru, Os) are independent of the counteranion and for the most part independent of M and nearly identical. Interionic C...F contacts are similarly weak in all six salts. Metal dependency is noted only in the ¹³C NMR spectra, in the skeletal M-C vibrations, and to a much smaller extent in some of the C-O stretching fundamentals (A_1g and T_1u). The findings reported here are unprecedented among metal carbonyl cations and their salts.

Full text of DOI:10.1021/ic040115u

Inorg. Chem. 2005, 44, 4189−4205
+
Homoleptic,
σ
-Bonded Octahedral [M(CO)6]2 Cations of Iron(II),
Ruthenium(II), and Osmium(II): Part 1: Syntheses, Thermochemical and
-
Vibrational Characterizations, and Molecular Structures as [Sb2F11] and
-
[SbF6] Salts. A Comprehensive, Comparative Study
Eduard Bernhardt,†,‡,§ Christian Bach,† Bianca Bley,† Rudolf Wartchow,† Ulrich Westphal,‡
Iona H. T. Sham,| Britta von Ahsen,‡ Chanqing Wang,| Helge Willner,*,†,‡,§ Robert C. Thompson,| and
Friedhelm Aubke*,|
Institut fu¨r Anorganische Chemie der UniVersita¨t HannoVer, Callinstrasse 9, D-30167 HannoVer,
Germany, Fakulta¨t 4, Anorganische Chemie, Gerhard Mercator UniVersita¨t Duisburg,
Lotharstrasse 1, D-47048 Duisburg, Germany, Department of Chemistry, The UniVersity of British
Columbia, 2036 Main Mall, VancouVer B.C., V6T 1Z1 Canada, and FB C, Anorganische Chemie,
Bergische UniVersita¨t Wuppertal, Gaussstrasse 20, D-42097 Wuppertal, Germany
Received October 14, 2004
+
Homoleptic octahedral, superelectrophilic
σ
-bonded metal carbonyl cations of the type [M(CO)6]2 (M
)
Ru, Os)
are generated in the Brønsted
−
Lewis conjugate superacid HF/SbF5 by reductive carbonylation of M(SO3F)3 (M
)
-
Ru, Os) or OsF6. Thermally stable salts form with either [Sb2F11
]
or [SbF6]- as anion, just as for the previously
+
reported [Fe(CO)6]2 cation. The latter salts are generated by oxidative (XeF2) carbonylation of Fe(CO)5 in HF/
SbF5. A rationale for the two diverging synthetic approaches is provided. The thermal stabilities of [M(CO)6][SbF6]2
salts, studied by DSC, range from 180
two triads [M(CO)6][SbF6]2 and [M(CO)6][Sb2F11]2 (M
X-ray diffraction and vibrational and 13C NMR spectroscopy, aided by computational studies of the cations. The
°
C for M
)
Fe to 350
°C for M ) Os before decarbonylation occurs. The
)
Fe, Ru, Os) are extensively characterized by single-crystal
three [M(CO)6][SbF6]2 salts (M
) Fe, Ru, Os) crystallize in the tetragonal space group P4/mnc (No. 128), whereas
-
the corresponding [Sb2F11
]
salts are monoclinic, crystallizing in space group P21/n (No. 14). In both triads, the unit
cell parameters are nearly invariant of the metal. Bond parameters for the anions [SbF6]- and [Sb2F11
]
and their
-
vibrational properties in the two triads are completely identical. In all six salts, the structural and vibrational properties
of the [M(CO)6]2+ cations (M
Fe, Ru, Os) are independent of the counteranion and for the most part independent
of M and nearly identical. Interionic C‚‚‚F contacts are similarly weak in all six salts. Metal dependency is noted
only in the 13C NMR spectra, in the skeletal M
C vibrations, and to a much smaller extent in some of the C
)
−
−O
stretching fundamentals (A1g and T1u). The findings reported here are unprecedented among metal carbonyl cations
and their salts.
Introduction
thermally stable complexes with carbon monoxide. This large
isostructural group of the general composition [M(CO)6]m
extends, as shown in Figure 1 together with their dates of
discovery, from metal carbonyl anions (m ) 1-, 2-) in
groups 4 and 51,2 over the neutral, molecular metal carbonyls
in group 63-5 and unipositive cations (m ) 1+) in group
Mononuclear, homoleptic metal carbonyl complexes with
regular octahedral coordination geometries, d6 electron
configurations, and diamagnetic 1A1g electronic ground states
are known for 16 of the 24 transition metals that form
* To whom correspondence should be addressed. addressed. E-mail:
willner@uni-wuppertal.de (H.W.).
(1) Ellis, J. E. AdV. Organomet. Chem. 1990, 31, 1.
(2) Ellis, J. E. Organometallics 2003, 22, 3322.
(3) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.;
Wiley: New York, 1988.
(4) Elschenbroich, C.; Salzer, A. Organometallics, 2nd ed.; VCH:
Weinheim, Germany, 1992.
† Institut fu¨r Anorganische Chemie der Universita¨t Hannover.
‡ Gerhard Mercator Universita¨t Duisburg.
§ Bergischen Universita¨t Wuppertal.
| The University of British Columbia.
10.1021/ic040115u CCC: $30.25
Published on Web 05/10/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 12, 2005 4189

Bernhardt et al.
vibrational fundamentals of [Fe(CO)6]2+ are observed.20 Band
positions and vibrational assignments together with structural
parameters for [Fe(CO)6]2+ are confirmed by DFT calcula-
tions.21
There are four objectives of this study: (i) the syntheses
and crystal growth of two sets of [M(CO)6]2+ salts (M )
Ru, Os) with [Sb2F11]- and [SbF6]- as the counter anions
and subsequent molecular structure determination by single-
crystal X-ray diffraction; (ii) a complete vibrational assign-
ment of the [M(CO)6]2+ cations (M ) Ru, Os) in analogy to
the earlier study of the corresponding [Fe(CO)6]2+ salts,20
supplemented by 13C NMR data9 and DFT calculations for
[M(CO)6]2+(g) (M ) Fe, Ru, Os);21,23 (iii) a thermal
decomposition study supported by differential scanning
calorimetry (DSC) measurements for [M(CO)6]2+ salts
(M ) Fe,20 Ru, Os) with [Sb2F11]- or [SbF6]-; and (iv) the
improvement and optimization of the reported20 generation
of [Fe(CO)6]2+(solv) by oxidative carbonylation of Fe(CO)5
in HF/SbF5.
The completion of this study and the inclusion of the
results for the iron carbonyl fluoroantimonates20 will permit
an informed detailed “vertical” correlation of two triads of
salts with [M(CO)6]2+ cations (M ) Fe,20 Ru, Os) and
[Sb2F11]- or [SbF6]- counteranions.
There have so far been a number of “horizontal” correla-
tions, involving [M(CO)6]m species (see Figure 1) mainly
from the third (5d) transition series, based on both experi-
mental9,10,13,15 and computational21-24 studies. Some results,
relevant to this work,9,10,13,15,21,23 are discussed below.
Figure 1. Transition metals that form octahedral homoleptic hexacarbonyls
[M(CO)6]m. Shading indicates structurally characterized compounds.
*[Rh(CO)5Cl]2+
.
76,7 to the superelectrophilic,8 predominantly σ-bonded metal
carbonly cations9-12 in groups 8 and 9 (m ) 2+, 3+).
Whereas for groups 4-8 all members of a given triad are
known (see Figure 1), for group 9 in [Ir(CO)6][SbF6]3‚4HF,13
a regular octahedral hexacarbonyl iridium(III) cation, is
found. Rhodium(III) and iridium(III) form derivatives of the
composition [M(CO)5Cl][Sb2F11]2 (M ) Ir,14,15 Rh14,16). The
only homoleptic Co(I) carbonyl cation in [Co(CO)5][(CF3)3-
BF]17 has a trigonal-bipyramidal structure.
A larger number of homoleptic, octahedral metal carbonyl
cations are found in group 8.9-12 The reductive carbonylation
of M(SO3F)3 (M ) Ru, Os) in SbF5 or HF/SbF5 allows the
isolation of [M(CO)6][Sb2F11]2 (M ) Ru, Os).18 Reductive
carbonylation of OsF6 in HF/SbF5 provides a facile alterna-
tive route to [Os(CO)6][Sb2F11]2.19 The cation [Fe(CO)6]2+-
(solv) is best obtained by oxidative carbonylation of Fe(CO)5
in HF/SbF5 with XeF2 as an external oxidizer.20 Single
crystals of [Fe(CO)6][Sb2F11]2 are converted readily to [Fe-
(CO)6][SbF6]2 by repeated washing with anhydrous HF. The
molecular structures of both salts are reported, and all 13
Experimental Section
General Procedures and Reagents. (a) Apparatus. Volatile
materials were manipulated in a glass or stainless steel vacuum
line of known volume equipped with a capacitance pressure gauge
(type 280E, Setra Instruments, Acton, MA) and valves with PTFE
stems (Young, London) or stainless steel needle valves (3762H46Y
Hoke, Creskill, NJ), respectively. To remove moisture as [H3O]-
[Sb2F11] in anhydrous HF, a small amount of SbF5 was added, and
the solutions were stored in a stainless steel cylinder or in PFA
tubes (12 mm o.d., 300 mm long), heat gun sealed at the bottom
and connected on top to a stainless steel needle valve (3762H46Y
Hoke, Cresskil, NJ). All other volatile compounds were stored in
glass containers equipped with a valve with a PTFE stem (Young,
London). In the case of 13CO, the storage vessel contained a
molecular sieve (5 Å, Merck) to recover the excess of 13CO after
use by cooling with liquid nitrogen. For synthetic reactions in HF/
SbF5 solutions, a reactor was used consisting of a 120-mL PFA
bulb with an NS 29 socket standard taper (Bohlender, Lauda,
Germany) in connection with a PFA NS 29 cone standard taper
and a PFA needle valve (Bohlender, Lauda, Germany). The two
parts were pressed together with a metal flange, and the reactor
was leak-tight (<10-5 mbar L s-l) without use of grease. A two-
part V-shaped PFA vessel with valve was used to wash the product
with anhydrous HF. Solid materials were manipulated inside an
(5) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 2nd ed.; Wiley: New York, 1994.
(6) Abel, E. W.; Tyfield, S. P. AdV. Organomet. Chem. 1970, 8, 117.
(7) Beck, W.; Su¨nkel, K. Chem. ReV. 1988, 88, 1405.
(8) Olah, G. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 167.
(9) Willner, H.; Aubke, F. Angew. Chem., Int. Ed. Engl. 1997, 36, 2402.
(10) Willner, H.; Aubke, F. Organometallics 2003, 22, 3612.
(11) Willner, H.; Aubke, F. In Inorganic Chemistry Highlights; Meyer, G.
W., Naumann, D., Wesemann, L., Eds.; Wiley-VCH: Weinheim,
Germany, 2002; p 195.
(12) Willner, H.; Aubke, F. Chem. Eur. J. 2003, 9, 1668.
(13) von Ahsen, B.; Berkei, M.; Henkel, G.; Willner, H.; Aubke, F. J. Am.
Chem. Soc. 2002, 124, 8371.
(14) Willner, H.; Bach, C.; Wartchow, R.; Wang, C.; Rettig, S. J.; Trotter,
J.; Jonas, V.; Thiel, W.; Aubke, F. Inorg. Chem. 2000, 39, 1933.
(15) Bach, C.; Willner, H.; Wang, C., Rettig, S. J.; Trotter, J.; Aubke, F.
Angew. Chem., Int. Ed. Engl. 1996, 35, 1974.
(16) von Ahsen, B.; Berkei, M.; Ko¨ckerling, M.; Willner, H.; Ha¨gele, G.;
Aubke, F. Inorg. Chem. 2003, 42, 3801.
(17) Bernhardt, E.; Finze, M.; Willner, H.; Lehmann, C. W.; Aubke, F.
Angew. Chem., Int. Ed. Engl. 2003, 42, 2077.
(18) Wang, C.; Bley, B.; Blazer-Jo¨llenbeck, G.; Lewis, A. R.; Siu, S. C.;
Willner, H.; Aubke, F. J. Chem. Soc., Chem. Commun. 1995, 2071.
(19) von Ahsen, B.; Bach, C.; Pernice, H.; Willner, H.; Aubke, F. J.
Fluorine Chem. 2000, 102, 243.
(20) Bernhardt, E.; Bley, B.; Wartchow, R.; Willner, H.; Bill, E.; Kuhn,
P.; Sham, I. H. T.; Bodenbinder, M.; Bro¨chler, R.; Aubke, F. J. Am.
Chem. Soc. 1999, 121, 7188 and references therein.
(21) Jonas, V.; Thiel, W. Organometallics 1998, 17, 353.
(22) Szillagy, R. Frenking, G. Organometallics 1997, 16, 4807.
(23) Ehlers, A. W.; Ruiz-Morales, Y.; Baerends, F. J.; Ziegler, T. Inorg.
Chem. 1997, 36, 5031.
(24) Diefenbach, A.; Bickelhaupt, F. M.; Frenking, G. J. Am. Chem. Soc.
2000, 122, 6449.
4190 Inorganic Chemistry, Vol. 44, No. 12, 2005

[M(CO)6][SbF6]2 and [M(CO)6][Sb2F11]2 (M ) Fe, Ru, Os)
Table 1. Crystallographic Data for [M(CO)6][SbF6]2 and [M(CO)6][Sb2F11]2 at 300 K (M ) Fe,20 Ru, Os)
[Fe(CO)6][SbF6]2
[Ru(CO)6][SbF6]2
[Os(CO)6][SbF6]2
[Fe(CO)6][Sb2F11]2 [Ru(CO)6][Sb2F11]2 [Os(CO)6][Sb2F11]2
emp formula
formula weight
cryst syst
space group
color
a (Å)
b (Å)
C6F22FeO6Sb2
695.4
tetragonal
C6F12O6RuSb2
740.63
tetragonal
C6F12O6OsSb2
829.76
tetragonal
C6F22FeO6Sb4
1128.9
C6F22RuO6Sb4
1174.13
monoclinic
P21/n (No. 14)
colorless
9.798(1)
12.567(2)
12.555(1)
110.78(1)
1215.1(3)
2
C6F22O6OsSb4
1263.26
monoclinic
P21/n (No. 14)
colorless
9.800(1)
12.544(1)
10.536(1)
110.80(1)
1210.8(2)
2
monoclinic
P21/n (No. 14)
colorless
9.751(1)
12.457(1)
10.542(1)
110.63(1)
1198.4(2)
2
P4/mnc (No. 128) P4/mnc (No. 128) P4/mnc (No. 128)
colorless
8.258(1)
8.258
12.471(2)
90
850.5(2)
2
1.03
colorless
8.278(1)
8.278(1)
12.449(2)
90
853.1(2)
2
1.058
0.0219
0.0624
colorless
8.274(1)
8.274(1)
12.421(2)
90
850.3(2)
2
1.001
0.0196
0.0506
c (Å)
â (deg)
V (Å3)
Z
GOF on F2
R1 [I > 2σ(I)]
wR2
0.843
0.0282
0.0532
1.01
0.0330
0.0692
1.15
0.0276
0.0608
0.0259
0.0668
inert atmosphere box (Braun, Munich, Germany) filled with argon,
with a residual moisture content of less than 1 ppm.
(b) Chemicals. Anhydrous HF and F2 (Solvay AG, Hannover,
spectra in the region 3500-80 cm-l with a spectral resolution of 2
cm-l. Two-hundred fifty-six or 128 scans were co-added for each
spectrum.
(ii) NMR Spectroscopy. 13C NMR spectra were obtained at room
temperature on a Bruker DRX-500 FT spectrometer operating at
125.758 MHz. A 4-mm (o.d.) PFA tube, containing a saturated HF
solution over solid [M(13CO)6][SbF6]2 (M ) Ru, Os) was centered
inside a 5-mm glass NMR tube containing the lock and external
reference CDCl3 (δ ) 77.7 ppm referenced to TMS). The line width
was 0.5-1.5 Hz. The 99Ru and 187Os satellites were detected.
(iii) Differential Scanning Calorimetry (DSC). Thermo-
analytical measurements were made with a Netzsch DSC 204
instrument. Temperature and sensitivity calibrations in the temper-
ature range of 20-600 °C were carried out with naphthalene,
benzoic acid, KNO3, AgNO3, LiNO3, and CsCl. The heating rate
employed was 10 K min-l, and the furnace was flushed with dry
nitrogen. For the evaluation, the software Netzsch Proteus 4.0 was
employed. The samples were contained in sealed stainless steel
crucibles of an approximate mass of 1.3 g and a volume of 100
µL, which were coated on the inside with gold or rhodium (courtesy
of Degussa-Hu¨ls).
Germany); Ru and Os (Degussa, Darmstadt, Germany); CO (99%,
13
Linde Gas); HSO3F, SO3, and H2SO4 (Allied Chemicals); and
-
CO (99% enriched, IC Chemicals) were obtained from commercial
sources. Fluorosulfuric acid (technical grade) was purified by double
distillation at room temperature as described previously.25 Anti-
mony(V) fluoride (Atochem North America, formerly Ozark-
Mahoning) was purified by atmospheric pressure distillation,
followed by vacuum distillation.26 Bis(fluorosulfuryl)peroxide,
S2O6F6, was synthesized by catalytic (AgF2) fluorination of SO3
(Dupont) with elemental fluorine (Air Products) as described
previously.27,28
The metal tris(fluorosulfates) Ru(SO3F)329 and Os(SO3F)330 were
27,28
obtained by oxidation of the corresponding metals by S2O6F2
in HSO3F as described previously. OsF6 was made by fluorination
of Os as described earlier,31 as was XeF2 by fluorination of Xe.32
Fe(CO)5 (purity not stated, BASF, Ludwigshafen, Germany) was
purified by trap-to-trap distillation.
(c) Instrumentation. (i) Vibrational Spectroscopy. Infrared
spectra in the region 5000-400 cm-1 were recorded at room
temperature on an IFS-66v FT spectrometer (Bruker, Karlsruhe,
Germany) operating with a DTGS detector and a Ge/KBr beam
splitter. Infrared spectra in the region 650-50 cm-1 were recorded
at room temperature on an IFS-66v/s FT spectrometer with a DTGS
detector and a Ge-coated 6-µm Mylar beam splitter (Bruker,
Karlsruhe, Germany). One-hundred twenty-eight or 256 scans were
co-added for each spectrum, using an apodized resolution of 2 or
4 cm-l. The samples were crushed as Nujol mulls between AgBr
(Korth, Kiel, Germany) or polyethylene (Cadillac, Hannover,
Germany) disks inside the drybox.
Raman spectra were recorded at room temperature with a Bruker
RFS 100/S FT Raman spectrometer using the 1064 nm exciting
line (∼500 mW) of a Nd:YAG laser (Adlas, DPY 301, Lu¨beck,
Germany). For the correction of Raman intensities, a calibration
tungsten lamp (3000 K) was used. Crystalline samples were
contained in large melting point capillaries (2 mm o.d.) for recording
(iv) X-ray Diffraction. Under a dry argon atmosphere, fragments
from single crystals of [M(CO)6][Sb2F11]2 and [M(CO)6][SbF6]2
(M ) Ru, Os) were cleaved and gently wedged into 0.3-mm
capillary tubes with a trace of fluorocarbon grease as an adhesive.
Data were recorded at room temperature with a Stoe IPDS
diffractometer using graphite monochromatized Mo KR radiation.
The systematic absences observed for X-ray data of [M(CO)6]-
[Sb2F11]2 (M ) Ru, Os) uniquely defined the probable monoclinic
space group to be P21/n, and those for [M(CO)6][SbF6]2 (M ) Ru,
Os) point to two probable space groups, P4nc and P4/mnc, of which
the latter centrosymmetric one was chosen on the basis of the
successful crystal structure determinations. The structures were
solved in P21/n and P4/mnc, respectively, by direct methods
(SHELXS-86),33 and the final full-matrix least-squares refinements
of all atomic parameters (SHELXL-93)34 were carried out on a
DECR Vax computer. The crystallographic data for [Ru(CO)6]-
[Sb2F11]2, [Os(CO)6][Sb2F11]2, [Ru(CO)6][SbF6]2, and [Os(CO)6]-
[SbF6]2 are summarized in Table 1.
(25) Barr, J.; Thompson, R. C.; Gillespie, R. J. Inorg. Chem. 1964, 3, 1149.
(26) Zhang, D.; Rettig, S. J.; Trotter, J.; Aubke, F. Inorg. Chem. 1996, 35,
6113.
(27) Dudley, F. B.; Cady, G. H. J. Am. Chem. Soc. 1959, 79, 513.
(28) Zhang, D.; Wang, C.; Mistry, F.; Powell, B.; Aubke, F. J. Fluorine
Chem. 1996, 76, 83.
(d) Synthetic Reactions. (i) Synthesis of [M(CO)6][Sb2F11]2
(M ) Ru, Os). The PFA reactor described above, containing a
PTFE-coated magnetic stirring bar, was charged with 0.40 g of
Ru(SO3F)3 or 0.49 g of Os(SO3F)3 in a glovebox. To the reactor
(29) Leung, P. C.; Aubke, F. Can. J. Chem. 1984, 62, 2892.
(30) Leung, P. C.; Wong, G. B.; Aubke, F. J. Fluorine Chem. 1987, 35,
607.
(31) Weinstock, B.; Malm, J. G. J. Am. Chem. Soc. 1958, 80, 4466.
(32) Holloway, J. H. Chem. Commun. 1966, 22.
(33) Sheldrick, G. M. SHELXS-86: Program for Crystal Structure Solution;
Universita¨t Go¨ttingen: Go¨ttingen, Germany, 1986.
(34) Sheldrick, G. M. SHELXS-93: Program for Crystal Structure Refine-
ment; Universita¨t Go¨ttingen: Go¨ttingen, Germany, 1993.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4191

Bernhardt et al.
were added 4 mL (55 mmol) of SbF5 and 2 mL (100 mmol) of HF
using a stainless steel vacuum line. At -196 °C, 9 mmol of CO
was admitted to the reactor. The content was heated to 50-60 °C
for 1-2 days without stirring, and colorless crystals of [M(CO)6]-
[Sb2F11]2 (M ) Ru, Os) formed. All volatile products were removed
in vacuo. In the glovebox, the crystals were transferred into a
V-shaped reactor. The crystalline product was washed with an SbF5/
HF (v/v 2:1) mixture. After removal of all volatiles in vacuo, 1.1
g (0.94 mmol, 94% yield) of pure [Ru(CO)6][Sb2F11]2 or 1.1 g (0.87
mmol, 87% yield) of pure [Os(CO)6][Sb2F11]2 was obtained.
[M(CO)6][Sb2F11]2 salts (M ) Ru, Os) were colorless moisture-
sensitive crystalline solids. IR and Raman band positions and
intensities attributed to the [Sb2F11]- anion in [M(CO)6][Sb2F11]2
(M ) Ru, Os) were identical to those in [Fe(CO)6] [Sb2F11]2.20 Band
positions and intensities attributed to the [M(CO)6]2+ cations in
[M(CO)6][Sb2F11]2 (M ) Ru, Os) were identical and very close to
those in [M(CO)6][SbF6]2, except for δCMC (ν11) values, which
were ca. 10 cm-1 lower. The isotopomers [M(13CO)6][Sb2F11]2 (M
) Ru, Os) were synthesized in an identical manner by using 13CO.
(ii) Synthesis of [M(CO)6][SbF6]2 (M ) Ru, Os). Onto 1.1 g
of powdered [M(CO)6][Sb2F11]2 (M ) Ru, Os) contained in a
“inverted-V” reactor made from PFA was condensed 2 mL of
anhydrous HF in vacuo. The resulting suspension was stirred briefly,
and the solid was allowed to settle. The supernatant solution was
decanted into the sidearm. HF was recondensed into the main arm,
and the washing process was repeated 7-10 times. After this
washing, 0.66 g of [Ru(CO)6][SbF6]2 and 0.69 g of [Os(CO)6]-
[SbF6]2 were obtained in crystalline form, which implied a yield
of 95%. The [M(CO)6][SbF6]2 salts (M ) Ru, Os) were colorless
moisture-sensitive crystalline solids. The isotopomers [M(13CO)6]-
[SbF6]2 (M ) Ru, Os) were obtained in an identical manner from
[M(13CO)6][Sb2F11]2 (M ) Ru, Os). Band positions and intensities
attributed to the [SbF6]- anion in [M(CO)6][SbF6]2 (M ) Ru, Os)
were identical to those in [Fe(CO)6][SbF6]2.20
(iii) Synthesis of [Os(CO)6][Sb2F11]2 from OsF6. Into a PFA
flask of about 120 mL volume, fitted with a PFA top and a magnetic
stirring bar, were added 2.3 g of SbF5 and then 0.2 g of HF. With
the mixture at liquid N2 temperature, 0.13 g (0.43 mmol) of OsF6
was added in vacuo. The CO pressure was then adjusted to give
1.5 atm at 25 °C. After being warmed to 40 °C, the mixture was
stirred for 30 min. During this time, the solution turned from yellow
to very pale yellow, and colorless crystal needles formed. Removal
of all volatiles produced about 0.6 g of a purplish crystalline mass
identified as [Os(CO)6][Sb2F11]2.
(iv) Preparation of [Fe(CO)6][Sb2F11]2 with XeF2 as the
Oxidizing Agent. In a typical experiment, 15 mL of HF, 1.52 g
(9.1 mmol) of XeF2, and 12 mL (160 mmol) of SbF5 were
condensed into a 250-mL PFA reactor. When a homogeneous
solution had been obtained, 1.00 g (5.1 mmol) Fe(CO)5 was added
in vacuo at -196 °C. Then, 530 mbar (12 mmol) of CO was
introduced into the system. The mixture was stirred at -50 °C in
an ethanol bath for 10 min, and the color changed from white to
yellowish. Stirring was then continued at 50 °C for 2 days. Upon
removal of volatiles in vacuo, a mixture of white crystalline solid
and powder was obtained and characterized by Raman spectroscopy.
The crystalline products (5.62 g, expected 5.6 g) were isolated from
the powder in the drybox. A portion of these crystalline solids was
then washed with a (v/v 1: 1) mixture of HF and SbF5 in a
V-shaped PFA reactor to remove the powder on the surface of the
crystals.
of a V-shaped PFA reactor. Five milliliters of HF was then added
in vacuo. The resulting suspension was stirred briefly, and the
crystals were allowed to settle. The supernatant solution was then
decanted into the sidearm. Subsequently, HF was condensed back
into the main arm. This washing process was repeated four times.
After this washing, 3.905 g (5.6 mmol) of solids consisting of white
powder and a few small white crystals was obtained in the main
arm.
(vi) Improved Syntheses and Purification of [Fe(CO)6]-
[Sb2F11]2 and [Fe(CO)6)[SbF6]2. The “crude” crystals of [Fe(CO)6]-
[Sb2F11]2, prepared as described in (iv), were transferred to a 100-
mL round-bottom PFA vessel, and subsequently 8 mL of HF and
1 bar of F2 were introduced into the reactor. The mixture was stirred
at room temperature for 2 days. Upon removal of volatiles, a white
powder remained. This was identified as pure [Fe(CO)6][Sb2F11]2.
An attempt to purify 3.9 g of the crude [Fe(CO)6][SbF6]2 product
was carried out by introducing 6 mL of HF and 1 bar of F2 onto
the solids in a round-bottom PFA flask. Pure [Fe(CO)6][Sb2F11]2
(2.672 g) in the form of a white powder was obtained upon removal
of all volatiles. Some of the purified [Fe(CO)6][Sb2F11]2 (0.864 g,
0.765 mmol) was then converted back to [Fe(CO)6][SbF6]2 (0.515
g, 0.741 mmol, 96.8% yield) by washing with HF.
Results and Discussion
(a) Synthetic Aspects. (i) General Comments. All
presently known σ-bonded metal carbonyl cations9-12 are
either generated while dissolved in Brønsted or Brønsted-
Lewis superacids35,36 or obtained with the help of Lewis
superacids.37,38 The octahedral cations shown in Figure 1,
found in groups 7-9,6,7,9-12 are no exceptions. To generate
the superelectrophilic8 [M(CO)6]2+ cations (M ) Fe,20 Ru,
Os) in group 8, the Lewis superacid SbF537,38 and its solutions
in HF to give the conjugate Brønsted-Lewis superacid HF/
SbF5,39,40 are both suitable9-12 and exclusively used.18-20 In
particular, HF/SbF5 is a superb reaction medium that allows
carbonlyation reactions to proceed in a homogeneous phase.9-12
There are four reasons for the use of HF/SbF5 in the
syntheses of salts with superelectrophilic8 σ-bonded metal
carbonyl cations: (i) Its excellent ionizing ability produces
various cations by solvolysis, reduction or oxidation as
reactive intermediates, that are easily carbonylated.10-12 (ii)
The final products are obtained in crystalline form, and a
substantial number of accurate molecular structures have
been determined recently by single-crystal X-ray diffrac-
tion.10-12 (iii) Gaseous CO is moderately soluble in HF/SbF5
and other superacids,35,36 so that carbonylation reactions can
proceed efficiently under very mild conditions (25-60 °C,
PCO ∼ 1 atm). (iv) The superacid anions,27 namely, [Sb2F11]-
and [SbF6]-, are, according to 19F NMR studies,41 connected
by equilibria of the type
HF/SbF5
[SbF6]-(solv) + SbF5(solv) {
} [Sb2F11]-(solv) (1)
In carbonylations, both anions will act as counteranions.
(35) O’Donnell, T. A. Superacids and Acidic Melts as Inorganic Reaction
Media; VCH: Weinheim, Germany, 1993.
(36) Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids; Wiley: New
York, 1985.
(v) Preparation of [Fe(CO)6][SbF6]2 Obtained from the
Reaction with XeF2. Powdered [Fe(CO)6][Sb2F11]2 (4.625 g, 4.097
mmol, prepared as described above) was loaded into the main arm
(37) Fabre´, P. L.; Devynk, J.; Tremillon, B. Chem. ReV. 1982, 82, 591.
4192 Inorganic Chemistry, Vol. 44, No. 12, 2005

[M(CO)6][SbF6]2 and [M(CO)6][Sb2F11]2 (M ) Fe, Ru, Os)
Depending on the nature of the precursors used, three
different synthetic routes are generally available,9-12 with
two of these used in the generation of [M(CO)6]2+ cations
(M ) Fe,20 Ru, Os) in HF/SbF5: (i) reductive carbonylation,
where a high-valent metal compound of the type MXm (X
) F, Cl, SO3F, etc.; m ) 3-6) is reduced and carbonylated,
with CO functioning as a reducing agent and ligand; (ii)
oxidative methods, where low-valent metal carbonyls [Fe-
(CO)5,22 M(CO)6;41,42 M ) Mo, W] or carbonyl derivatives
{[Rh(CO)2Cl]2, [Ir(CO)3Cl]x}14 are oxidized by either SbF5
or an external oxidizer (AsF5, Cl2, XeF2)20 and carbonylated.
The syntheses of the [M(CO)6]2+ salts (M ) Fe,20 Ru, Os)
discussed below serve as illustrations for both synthetic
pathways.
There is a precedent for the observed anion ambiguity:
The reductive carbonylation of IrF6 in SbF5 or in concentrated
solutions of SbF5 in HF is reported to produce [Ir(CO)6]-
[Sb2F11]3,15,19 whereas in dilute HF/SbF5 (6:1 by volume),
[Ir(CO)6][SbF6]3‚4HF is formed and structurally character-
ized.13 For both salts and for [Ir(CO)6]3+(solv),15 band
positions in the CO stretching region are all identical. In
sharp contrast, formation of [M(CO)4][Sb2F11]2 (M ) Pd,
Pt),43 [Hg(CO)2][Sb2F11]2,44 and [Au(CO)2][Sb2F11]45 by
reductive43,45 or solvolytic44,45 carbonylation occurs in quan-
titative yields. In all instances, only [Sb2F11]- salts are
known,43-45 and only for gold is the structure of a salt
formulated as [Au(CO)2]2[Sb2F11][SbF6] reported.46
19
The reductive carbonylation of OsF6 in HF/SbF5 is a
(ii) Generation and Isolation of Metal Carbonyl Fluo-
roantimonates (M ) Ru, Os) by Reductive Carbonyla-
tion.18 The synthesis of [M(CO)6][Sb2F11]2 (M ) Ru, Os)
by reductive carbonylation of M(SO3F)3 (M ) Ru,29 Os30)
in HF/SbF5 is formulated as:
facile and fast alternative route to [Os(CO)6][Sb2F11]2. The
four-electron reduction proceeds according to
OsF6 + 8CO + 4SbF5 40 °C, 1.5 atm CO8
HF/SbF5, 1 h
[Os(CO)6][Sb2F11]2 + 2COF2 (4)
HF/SbF5, 1 atm CO
2M(SO3F)3 + 16SbF5 + 13CO
8
50-60 °C, 1-2 days
However, despite the low reaction temperature and short
reaction times, some purple color is noted, indicative of a
further reduction of OsF6. In addition, some [SbF6]- appears
to be present in the final product.
2[M(CO)6][Sb2F11]2 + CO2 + S2O5F2 + 4Sb2F9(SO3F)
(M ) Ru, Os) (2)
The reactions differ from the previous report,18 where liquid
SbF5 was used as the reaction medium, on three accounts:
(i) the reaction temperatures are lower by 10-30 °C and
(ii) the reaction times are shorter by 1-2 days; however (iii)
the isolated yields are no longer quantitative, being 94% for
[Ru(CO)6][Sb2F11]2 and 79-87% for [Os(CO)6][Sb2F11]2.
Selected single crystals allow structure determinations and
complete vibrational analyses for the [M(CO)6]2+ cations (M
) Ru, Os). With no evidence of any side reactions or
nonvolatile byproducts, the only cogent explanation is the
presence of some [SbF6]- as a counteranion, given that this
A useful rationale for the formation of fluoroantimonate
salts of σ-bonded metal carbonyl cations9-12 such as
[M(CO)6]2+ (M ) Ru, Os) in the strongly ionizing, conjugate
Brønsted-Lewis superacid35,36 HF/SbF539,40 was introduced
by us12 recently: weakly solvated or “naked” metal cations
such as Ru2+ or Os2+ (as well as Ir3+;13 M2+, M ) Pd,43 Pt,
Hg,44 or Au+ 45) are generated by reduction and/or solvolysis
in the superacid matrix. They are viewed as soft or borderline
Lewis acids, according to Pearsons soft and hard acid and
base (SHAB) concept,47 or as class b metal ions, according
to an earlier classification by Ahrland, Chatt, and Davies,48
and will react preferentially with the soft or b-class base CO-
(solv),47,48 to form σ-bonded diamagnetic carbonyl cations
with octahedral [M ) Ir(III),13 Ru(II), Os(II)], d6 square-
planar [M ) Pd(II),43 Pt(II)], d8 linear [M ) Hg(II),44
Au(I)45], or d10 geometries. Isolable salts form with the
superacid anions [Sb2F11]- 26 and [SbF6]- in the case of
octrahedral d6 cations.12
anion and [Sb2F11]- are both present in HF/SbF5 (see eq
41
1) and are both capable of stabilizing the [M(CO)6]2+ cations
(M ) Ru, Os). There is some evidence for this explanation:
(a) A careful examination of the Raman spectra allows
identification of the A1g [SbF6]- vibration at ∼650 cm-1,
and (b) the subsequent extrusion of SbF5 by repeated washing
of [M(CO)6][Sb2F11]2 (M ) Ru, Os) with HF according to
The observed high thermal stabilities (vide infra) of the
salts are attributed to three interrelated factors: (i) relativistic
effects,49,50 resulting in strong M-C bonds; (ii) extensive
polarization of the C-O bond by the soft47 or class b48 Mm+
HF, 1 atm CO
[M(CO)6][Sb2F11]2 + 2HF 50-60 °C, 1-2 days8
[M(CO)6][SbF6]2 + 2HF/SbF5
(M ) Ru, Os) (3)
produces single crystals of pure [M(CO)6][SbF6]2 (M ) Ru,
Os), but the reduction in bulk weight is only about 95% of
that expected.
(43) Willner, H.; Bodenbinder, M.; Bro¨chler, R.; Hwang, G.; Rettig, S. J.;
Trotter, J.; v. Ahsen, B.; Westphal, U.; Jonas, V.; Thiel, W.; Aubke,
F. J. Am. Chem. Soc. 2001, 123, 588.
(44) Bodenbinder, M.; Balzer-Jo¨llenbeck, G., Willner, H.; Batchelor, B.
J.; Einstein, F. W. B.; Wang, C.; Aubke, F. Inorg. Chem. 1996, 35,
82.
(45) Willner, H.; Schaebs, J.; Hwang, G.; Mistry, F.; Jones, R.; Trotter, J.;
Aubke, F. J. Am. Chem. Soc. 1992, 114, 8972.
(46) Ku¨ster, R.; Seppelt, K. Z. Anorg. Allg. Chem. 2000, 626, 236.
(47) Pearson, R. J. J. Am. Chem. Soc. 1963, 85, 3533.
(48) Ahrland, S.; Chatt, J.; Davies, N. R. Q. ReV. Chem. Soc. 1958, 12,
265.
(38) Christe, K. O.; Dixon, D. A.; McLemore, D.; Wilson, W. W.; Sheehy,
J. A.; Boatz, J. A. J. Fluorine Chem. 2000, 101, 151.
(39) Hyman, H. M.; Quaterman, L.; Kirkpatrick, M.; Katz, J. J. Phys. Chem.
1961, 65, 123.
(40) Gillespie, R. J.; Moss, K. C. J. Chem. Soc. A 1966, 1170.
(41) Culman, J.-C.; Fauconet, M., Jost, R.; Sommer, J. New J. Chem. 1999,
23, 863 and references therein.
(42) Bro¨chler, R., Sham, I. H. T.; Bodenbinder, M.; Schmitz, V.; Rettig,
S. J.; Trotter, J.; Willner, H.; Aubke, F. Inorg. Chem. 2000, 39, 2172.
(49) Pyykko¨, P.; Desclaux, P. J. Acc. Chem. Res. 1979, 12, 276.
(50) Pyykko¨, P. Chem. ReV. 1988, 88, 563.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4193

Bernhardt et al.
acids, first discussed by Goldman and Krogh-Jesperson,51
to explain the unusually high ν(CO) and fCO values,9-12
resulting in polar contributions to the M-C bonds; and (iii)
the formation of extended molecular structures via significant
interionic C‚‚‚F interactions with the strongly electrophilic
carbon atoms of the CO ligands in salts of superelectrophilic8
σ-bonded metal carbonyl cations and the fluoroantimonate-
(V) anions.10-12
However, Fe2+(solv) or other M2+(solv) cations, formed
mainly by metals in the 3d series, when generated in HF/
SbF539,40 or other superacids35,36 in a similar manner, function
as hard47 or class a48 acceptors toward [SbF6]-, resulting in
the formation of layered materials of the type M[SbF6]2 with
M ) Cr, Mn, Fe, Co, Ni, Cu, Zn, Pd, Ag, Cd.52 In these
layered materials, the metal ions are octahedrally coordinated,
frequently with paramagnetic ground states (except for the
d10 ions).53-55 Of the ions listed above, only Pd(II) can be
carbonylated to give square-planar [Pd(CO)4]2+.43
In summary, the failure to generate [Fe(CO)6]2+(solv) by
reductive carbonylation in HF/SbF5 appears to be due to the
acceptor characteristics of Fe2+,47,48 and the difference in
magnetic ground states of the MX3 precursor (M ) Fe, Ru,
Os; X ) Cl, SO3F) as has been argued by us previously.20
The synthesis of [Fe(CO)6]2+ salts by oxidation of Fe(CO)5
by XeF2 in HF/SbF5 in the presence of CO(solv) provides
an alternative approach, as discussed next.
the magnetic properties, will be reported elsewhere.56 The
findings are briefly summarized here by listing additional
components of the bulk reaction products.
The list includes the following species: (1) [Fe(CO)6]-
[SbF6]2 is found, which is not surprising in view of the
findings for the corresponding [M(CO)6]2+ salts (M ) Ru,
Os) and the equilibrium (eq 1)41 discussed above. (2) Fe-
52
[SbF6]2 is found in higher concentrations (∼5 mol %) in
the bulk. This material is nearly insoluble in HF/SbF5 and
can be separated by repeated recrystallization. A mechanism
for its formation is proposed below. It is detected by bulk
magnetic susceptibility measurements.20,55,56 (3) 6SbF3‚5SbF5
is detected by Raman spectroscopy.57 Its composition is
established by a crystallographic study.61 This adduct is
observed as reduced byproduct in the reported oxidation of
M(CO)6 (M ) Mo,58 W42) in HF/SbF5 by SbF5. Its presence
in the bulk product mixture implies that SbF5 functions as
an oxidizer toward Fe(CO)5, despite the fact that XeF2 is
used in excess (see Experimental Section). (4) [XeF]-
[Sb2F11]59,60 and similar adducts of XeF2 and SbF5 might
explain why some XeF2 might not act as an oxidizer (see eq
5). Its presence in the product mixture is suspected, but not
clearly proven, because the diagnostic Xe-F stretching
vibration, expected at 621 cm-1,59,60 falls into an extremely
cluttered region of the Raman spectrum.52,55-57
In summary, the complexity of the oxidative carbonylation
of Fe(CO)5 in HF/SbF5 by XeF2 is evident from the list
discussed above, which might not be complete (vide infra).
The list becomes even longer when AsF5 or Cl2 is used,54
with the reduced byproducts far more difficult to remove54
than Xe(g). Although careful recrystallization of the products
avoids nearly all byproducts, the yield is reduced, and a rather
simplistic view of the formation reaction results.
(iii) Oxidative Carbonylation of Fe(CO)5 by XeF2 in
HF/SbF5 as the Reaction Medium. The recently20 reported
synthesis of [Fe(CO)6][Sb2F11]2 according to
50 °C, 1 atm CO
Fe(CO)5 + XeF2 + CO + 4SbF5
8
HF/SbF5, 2 days
[Fe(CO)6][Sb2F11]2 + Xe (5)
The previously reported20 attempts to attack [Fe(CO)6]2+
salts with strong oxidizers (F2, NiF3) forms the basis for a
useful purification method of crude [Fe(CO)6][Sb2F11]2,
appears to be straightforward and is a definite improvement
over the reactions reported in our initial communication,54
where SbF5 had been used as reaction medium and Cl2 or
AsF5 as the oxidizer. The reduced byproduct Xe(g) is now
removed far more easily than found previously.54 Crystalline
[Fe(CO)6][SbF6]2 is obtained by repeated washing with aHF,
and molecular structures of both salts are determined by
single-crystal X-ray diffraction.20 The cation [Fe(CO)6]2+ is
characterized by a complete vibrational analysis, including
DFT calculations21 and 13C NMR and 57Fe Mo¨ssbauer
spectroscopies.20 Detailed magnetic measurements allow an
estimate of <0.1 mol % Fe[SbF6]2 as a paramagnetic
impurity in the sample.
61
which involves the conversion of 6SbF3‚5SbF5 into SbF5
by fluorination with F2 in aHF, according to
25 °C, 1 atm F2
2(6SbF3‚5SbF5) + 12F2
8 22SbF5
(6)
aHF, 2 days
SbF5 formed in this manner will also convert [SbF6]- to
[Sb2F11]- 41 according to
25 °C
2SbF5 + [Fe(CO)6][SbF6]2 aHF, 2 days8 [Fe(CO)6][Sb2F11]2
(7)
However, the final product, crystalline [Fe(CO)6][Sb2F11]2,
is obtained in 50% yield only.20 A recent careful analysis of
the bulk material formed by evaporation of all volatiles55
reveals a far greater complexity of the formation reaction
than is suggested by eq 5. Details of this study,55 in particular
61
and crude [Fe(CO)6][SbF6]2 with a high 6SbF‚5SbF5
content is transformed to pure [Fe(CO)6][Sb2F11]2 by treat-
(56) Sham, I. H. T.; Willner, H.; Thompson, R. C.; Aubke, F., manuscript
in preparation.
(57) Birchall, T.; Dean, P. A. W.; Valle, B. D.; Gillespie, R. J. Can. J.
Chem. 1975, 53, 667.
(58) Bro¨chler, R.; Freidank, D.; Bodenbinder, M.; Sham, I. H. T.; Willner,
H.; Rettig, S. J.; Trotter, J.; Aubke, F. Inorg. Chem. 1999, 38, 3684.
(59) McRae, V. M.; Peacock, R. D.; Russel, D. R. Chem. Commun. 1969,
62.
(60) Sladky, F. Noble Gases; Butterworth: London, 1972; Vol. 3.
(61) Nandana, W. A. S.; Passmore, J.; White, P. S. J. Chem. Soc., Dalton
Trans. 1985, 1623.
(51) Goldman, A. S.; Krogh-Jesperson, K. J. Am. Chem. Soc. 1996, 118,
12159.
(52) Gantar, D.; Leban, I.; Frlec, B.; Holloway, J. H. J. Chem. Soc., Dalton
Trans. 1987, 2379.
(53) Figgis, B. N. Introduction to Ligand Fields; Interscience: New York,
1966.
(54) Bley, B.; Willner, H.; Aubke, F. Inorg. Chem. 1997, 36, 158.
(55) Sham, I. H. T. Ph.D. Thesis, University of British Columbia, 2002.
4194 Inorganic Chemistry, Vol. 44, No. 12, 2005

[M(CO)6][SbF6]2 and [M(CO)6][Sb2F11]2 (M ) Fe, Ru, Os)
ment with F2 in aHF at 25 °C. The purity of the iron carbonyl
fluoroantimonate(V) salts is conveniently ascertained by
Raman spectroscopy in the ν(Sb-F) region and comparison
to published data.20
In summary, the complexity of the oxidative carbonylation
of Fe(CO)5, discussed here, and the simplicity of the
reductive carbonylation of M(SO3F)3 (M ) Ru,29 Os30) or
of OsF619 with HF/SbF5 as the reaction medium is apparent
from two observations: (a) Even though Ru(CO)562 and Os-
63
(CO)5 have been known for 60 years63 and longer62 and
more recent synthetic routes have been published,64,65 their
use in oxidations and carbonylations is impractical and
unnecessary. Their molecular structures,66,67 however, ob-
tained by electron diffraction, allow useful comparisons to
the [M(CO)6]2+ cation (M ) Ru, Os) reported here. (b) [Fe-
(CO)6]2+, reported here and elsewhere,20,54 is thus far the only
known homoleptic metal carbonyl cation obtained by oxida-
tive carbonylation.9-12 Early claims of its synthesis by halide
abstraction from M(CO)4X2 (M ) Fe, Os) with M′X3 (M′
68
) Al, Fe; X ) Cl, Br) at high T and pCO or by amine-
69
catalyzed disproportionation of Fe(CO)5 have been either
retracted70 or discredited.6,7
There is another interesting, albeit erroneous, connection
between [Fe(CO)6]2+ 54 and [Os(CO)6]2+ 18 based on pre-
liminary findings: observations of weak IR bands in the CO
stretching region, at 2256 cm-1 for the [Fe(CO)6]2+ ion and
at 2252 cm-1 for [Os(CO)6]2+, had suggested the presence
of small amounts of [M(CO)6]3+ (M ) Fe, Os), given that
the IR-active fundamental ν6 (T1u) for [Ir(CO)6]3+ is found
at 2254 cm-1.13,15,19 Upon reinvestigation, the two bands in
question are now attributed to the cations [ClCO]+ 71 and
trans-[O2Os(CO)4]2+,72 respectively.
(iv) Thermal Stabilities of Metal Carbonyl Fluoro-
antimonates (M ) Fe, Ru, Os). Thermal stabilities and
decomposition pathways of [Fe(CO)6][Sb2F11]2 and [M(CO)6]-
[SbF6]2 (M ) Fe, Ru, Os) were studied by DSC (differential
scanning calorimetry). The corrosive nature of the decom-
position products limits the studies to the determination of
the onset temperatures and a description of peak shapes of
thermal events. To allow the interpretation of these events,
DSC measurements were preceded by heating small samples
in glass vials. Gaseous products were identified by IR
spectroscopy, and solid residues were studied by Raman
spectroscopy. A similar DSC study of the related salts
[M(CO)4][Sb2F11]2 (M ) Pd, Pt)43 has been published.
Figure 2. DSC plots in the temperature range of 25-300 °C of (A) [Fe-
(CO)6][Sb2F11]2 and (B) [Fe(CO)6][SbF6]2.
The Fe(II) salts [Fe(CO)6][Sb2F11]2 and [Fe(CO)6]-
20,54,55
[SbF6]2
are the main focus of this study for two
reasons: (i) The principal decarbonylation products, para-
magnetic Fe[SbF6]252 and FeF2, obtained at T > 400 °C, are
more clearly defined than those of [M(CO)6][SbF6]2 (M )
Ru, Os). (ii) In HF/SbF5, only mixed [Sb2F11]- and [SbF6]-
salts of [M(CO)6]2+ (M ) Ru, Os) are obtained, which are
converted to pure [M(CO)6][SbF6]2 (M ) Ru, Os) as
described and studied here by DSC.
The DSC plot of [Fe(CO)6][Sb2F11]2 and [Fe(CO)6][SbF6]2
in the temperature range of 25-300 °C is shown in Figure
2. There are four endothermic events: (i) A small, irrevers-
ible peak at ∼100 °C in both salts appears to be due to the
evolution of small amounts of HF, presumably present in
samples prepared in HF. (ii) A reversible event, attributed
to a phase transition, is found for all three [M(CO)6][SbF6]2
salts, at 151 °C for the Fe compound, at 163 °C for the Ru
compound, and at 173 °C for the Os compound. A possible
explanation is a change in orientation of the octahedral ions
in the lattice upon heating and possibly even a transition
from tetragonal to cubic symmetry. (iii) Elimination of SbF5
from [Fe(CO)6][Sb2F11]2 gives rise to an endothermic shoul-
der at 170 °C. SbF5 and [Fe(CO)6][SbF6]2 as residues are
identified by their Raman spectra. For [M(CO)6][Sb2F11]2 (M
) Ru, Os), sample shrinking at ∼170 °C was reported in a
preliminary communication18 and is probably due to the loss
of SbF5. (iv) Broad, strongly endothermic peaks due to the
simultaneous, irreversible loss of all six CO ligands are found
for the four salts with the follwing onset temperatures:
[Fe(CO)6][Sb2F11]2, 185 °C; [Fe(CO)6][SbF6]2, 180 °C;
[Ru(CO)6][SbF6]2, 280 °C; and [Os(CO)6][SbF6]2, 350 °C.
Volatile CO(g) and CO2(g) are detected by IR spectros-
(62) Manchot, W.; Manchot, W. J. Z. Anorg. Allg. Chem. 1936, 236, 385.
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(69) Sternberg, H. W.; Friedel, A. R.; Shufler, S. L.; Wender, I. J. J. Am.
Chem. Soc. 1955, 77, 2675.
(70) Hieber, W.; Frey, V.; John, P. Chem. Ber. 1967, 100, 1961.
(71) Bernhardt, E.; Willner, H.; Aubke, F. Angew. Chem., Int. Ed. 1999,
36, 823.
(72) Bernhardt, E.; Willner, H.; Jonas, V.; Thiel, W.; Aubke, F. Angew.
Chem., Int. Ed. 2000, 39, 168.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4195

Bernhardt et al.
Scheme 1
copy. In addition, a black coating is observed at this stage,
which is explained by the equilibrium55,56
2CO a CO2 + C
(8)
The results of a previous DSC study of [M(CO)4][Sb2F11]2
(M ) Pd, Pt43) differ on two accounts: (i) Evolution of SbF5
does not occur, and no evidence for the existence of
[M(CO)4][SbF6]2 is obtained. (ii) Complete loss of CO is
evident from broad, single peaks with onsets at 180 (Pd)
and 230 °C (Pt), but only COF2 is found in the gas phase,
together with CO2 and SiF4, indicative of some partial
hydrolysis. The demonstrated involvement of [Sb2F11]- in
extended structure formation with significant C‚‚‚F contacts
provides an explanation for both observations.
The complete, irreversible loss of all CO ligands upon
heating, with decomposition well above 150 °C, is a
characteristic feature of fluoroantimonate salts with σ-bonded
metal carbonyl cations.9-12 Thus far, no evidence has been
found for the existence of metal carbonyl cations with a low
CO content, formed by the controlled pyrolysis of existing,
thermodynamically stable metal carbonyl cations.9-12
Stepwise reversible dissociation of CO at room tempera-
ture is found only for thermally unstable polycarbonyl
complexes of Ag(I) and Cu(I),73-75 called “non-classical
carbonyls”,75 which are formed by stepwise, reversible
addition of up to four CO molecules to silver(I) and
copper(I) salts with weakly coordinating anions.76-78 This
contrasting thermal behavior reflects a weak, dative OC f
M bond (M ) Ag, Cu), the absence of relativistic contribu-
tions49,50 to the M-CO bond, and ineffective bond polariza-
tion51 by Ag(I) and Cu(I), as discussed recently.10,12
carbonylation of Fe(CO)5.20,54-56 (b) Formation of Fe[SbF6]2
appears to be strongly dependent on the reaction medium
and the oxidizing agents employed. A summary of the
proposed oxidation, carbonylation, and salt formation reac-
tions is shown for both SbF5 and HF/SbF5 in Scheme 1.
The initial process in the oxidative carbonylation is
formally a two-electron oxidative addition of two halogens
to Fe(CO)5, dissolved or suspended in HF/SbF5 or SbF5. The
halogens are provided by the oxidizing agent Cl2;54 XeF2;20,55
or EF5,54 E ) As, Sb.55
Early synthetic studies by W. Hieber79 and students80-82
discovered the partial oxidatiVe substitution reactions
Fe(CO)5 + X2 f Fe(CO)4X2 + CO
X ) I, Br, Cl
(9)
(v) Oxidative Carbonylation of Fe(CO)5 in Super-
acids: Mechanism and Rationale. Two observations mark
the outset of our discussion on mechanistic aspects. (i) From
detailed magnetic susceptibility studies,20,55,56 it is clear that
all preparations of [Fe(CO)6][Sb2F11]2 will contain variable
amounts of a paramagnetic impurity, identified as Fe-
[SbF6]2.52 Its actual content can be as high as 38-32 mol %
when Cl2 or AsF5 is used as the oxidizer in SbF5 or as low
as 0.4% with XeF2 in HF/SbF5.20 Conversion to crystalline
[Fe(CO)6][SbF6]2 by repeated washing with aHF reduces the
Fe[SbF6] content to about 0.1 mol %.20,55 (ii) From the DSC
study discussed above (Figure 2), it is apparent that tem-
peratures well in excess of 170 °C are required to thermolyse
The thermal stabilities of the products decrease with
increasing electronegativity of X and it is not surprising that
Fe(CO)4F2 has not yet been isolated.83,84 Thermal decomposi-
tion involves complete or partial loss of CO [i.e., the
existence of Fe(CO)2Cl2 is claimed79,85].
In addition, at low temperature80 or in liquid HCl,86 an
intermediate of the type Fe(CO)5Cl2 is obtained by oxidatiVe
addition and stabilized by addition of BCl3 as [Fe(CO)5Cl]-
[BCl4], which decomposes at 25 °C under evolution of
BCl3.86
On the basis of these precedents, the existence of two
species in HF/SbF5 or SbF5 is anticipated: the cation [Fe-
(CO)5X]+(solv) or the neutral species Fe(CO)4X2 (X ) Cl,
F), with the fluoro species as reactive transients. Two
competing reactions are possible: (i) Solvolytic carbonyla-
tion10-12 is expected to convert either [Fe(CO)5X]+ or Fe-
(CO)4X2 (X ) Cl, F) into [Fe(CO)6]2+(solv). This step
20
[Fe(CO)6][Sb2F11]2 to Fe[SbF6]2.52 As discussed in the
section on synthesis, the temperatures required for reaction
and product isolation are 50 °C or lower (see eq 5). These
observations allow two conclusions to be drawn: (a)
Fe[SbF6]2,52 found as an impurity, is very likely not a
decomposition product of [Fe(CO)6][Sb2F11]2 or [Fe(CO)6]-
[SbF6]2 but rather a byproduct, formed during the reductive
(79) Hieber, W. AdVances in Organometallic Chemistry; Academic Press:
New York, 1970; Vol. 8.
(80) Hieber, W.; Bader, C. Chem. Ber. 1928, 61, 1717.
(81) Hieber, W.; Lagally, H. Z. Anorg. Allg. Chem. 1940, 245, 295.
(82) Hieber, W.; Wirsching, A. Z. Anorg. Allg. Chem. 1940, 245, 35.
(83) Doherty, N. N.; Hoffman, N. W. Chem. ReV. 1991, 91, 553.
(84) Murphy, E. F.; Murgavel, R.; Roesky, H. W. Chem. ReV. 1997, 97,
3425.
(73) Hurlburt, P. K.; Rack, J. J.; Luck, J. S.; Dec, S. F.; Webb, J. D.;
Anderson, O. P.; Strauss, S. H. J. Am. Chem. Soc. 1994, 116, 10003.
(74) Strauss, S. H. J. Chem. Soc., Dalton Trans. 2000, 1.
(75) Lupinetti, A. J.; Frenking, G.; Strauss, S. H. Prog. Inorg. Chem. 2001,
49, 1.
(76) Krossing, I. Chem. Eur. J. 2001, 7, 490.
(77) Reed, C. Acc. Chem. Res. 1998, 31, 133.
(85) Hieber, W.; Lagally, H.; Wirsching, A. Z. Anorg. Allg. Chem. 1940,
245, 3005.
(78) Strauss, S. H. Chem. ReV. 1993, 93, 927.
(86) Iqbal, Z.; Waddington, T. C. J. Chem. Soc. A 1968, 1238.
4196 Inorganic Chemistry, Vol. 44, No. 12, 2005

[M(CO)6][SbF6]2 and [M(CO)6][Sb2F11]2 (M ) Fe, Ru, Os)
involves formation of the transient 14- or 16-electron cations
[Fe(CO)m]2+(solv) (m ) 4, 5), which act as soft47 or class
b48 acceptors. They are readily converted by reaction with
the soft47 donor CO to the stable 18-electron cation [Fe-
(CO)6]2+(solv). This step is facilitated in an ionizing solvent
such as HF/SbF5,39-41 which provides the superacid anions
[Sb2F11]- and [SbF6]- 26,41 as counteranions. A precedent for
the overall process is the recently reported solvolytic
carbonylation of [Rh(CO)2(µ-Cl)]2 into [Rh(CO)4]+(solv) in
HSO3F,16 (ii) Thermal decarbonylation, as observed for solid
Fe(CO)4Cl2,79,82 is seen as a competing process that provides
a facile pathway to Fe[SbF6]252 via Fe2+(solv) functioning10,12
as hard47 or class b48 acceptor toward the hard base
[SbF6]-.47,48 This alternate pathway is facilitated in SbF5,
where higher reaction temperatures (up to 90 °C),54 needed
to reduce the viscosity of SbF5,9 and longer reaction times54
are conducive to thermolysis as a side reaction.
In summary, although the conditions that favor formation
of [Fe(CO)6]2+ in very high yields20,55 are clearly outlined
in this sectionsHF/SbF539-41 as the reaction medium, XeF2
as the oxidizer,20,55 resulting in low reaction temperatures
and short reaction times and product purification by careful
recrystallization20sit has thus far not been possible to
completely eliminate the formation of Fe[SbF6]2 in competing
side reactions. Our inability to synthesize diamagnetic [Fe-
(CO)6]2+ salts does not rule out an alternative interpretation
that the weak paramagnetism observed is due to TIP
(temperature-independent paramagnetism).53
thermolysis of intermediates to Fe2+(solv). The crystal,
molecular, and extended molecular structures of all six salts
are discussed in the next section.
(b) Structural Aspects. (i) General Comments. As is
evident from the selected crystallographic data collected in
Tables 1 and 2 and the more extensive listings provided as
Supporting Information in Tables S1 and S2, the two
previously characterized [Fe(CO)6]2+ salts20 and the four
metal carbonyl fluoroantimonates (M ) Ru, Os; see Table
1) form two isostructural triads with very similar unit cell
dimensions. Within each triad, the [Sb2F11]- salts crystallize
in the monoclinic space group P21/n (No. 14), whereas the
three [SbF6]- compounds have tetragonal structures with the
space group P4/nmc (No. 128).
Among superelectrophilic,8 structurally characterized
σ-bonded, cationic metal carbonyl compounds,9-11 iso-
structural pairs have previously been reported for octahedral
[M(CO)5Cl][Sb2F11]2 (M ) Rh, Ir)13 and square-planar
[M(CO)4][Sb2F11]2 (M ) Pd, Pt).43 In these monoclinic pairs,
very similar unit cell parameters are found,13,43 with the 5d
complexes (IrIII, PtII) having slightly smaller unit cell volumes
than their 4d (RhIII, PdII) congeners. This is attributed to
relativistic effects.49,50 As a consequence, similar structural
and spectrosopic properties for the two cations of each
pair13,43 are observed. However, the [Sb2F11]- counteranions
in each pair are not symmetry related, and important
diagnostic parameters for the two anions differ markedly.13,43
As a consequence, in each of the two pairs, there are a total
of four crystallographically different [Sb2F11]- anions. This
results in differences in the observed significant interionic
contacts14,43 to form extended structures. For a third iso-
structural pair, the square-planar molecular complexes cis-
M(CO)2(SO3F)288,89 (M ) Pt, Pd), the two sets of structural
and spectroscopic parameters differ measurably.
The two triads of [M(CO)6]2+ complexes (M ) Fe,20 Ru,
Os) discussed here and previously20 include in the compari-
son with [Fe(CO)6]2+ the only known superelectrophilic
σ-bonded metal carbonyl cation9-12 formed by a 3d metal.
In addition, two different symmetry-related anions,20 [Sb2F11]-
and less common [SbF6]-,9-12 permit us to probe both the
metal and anion dependencies of structural and spectroscopic
data. All of these data were obtained in our laboratories using
nearly identical instrumentation and physical methods (see
Experimental Section here and in ref 20).
Therefore, rather than presenting and discussing experi-
mental and relevant computational21-24 data for the metal
carbonyl fluoroantimonates for M ) Ru and Os in isolation
and in a similar manner as reported for isostructural [Fe-
(CO)6][Sb2F11]2 and [Fe(CO)6][SbF6]2,20 we intend to com-
pare all six salts in this study with three basic objectives:
(i) to explore the extent to which the crystal data for the
salts, the bond parameters (internuclear distances and bond
angles) for both the [M(CO)6]2+ cations (M ) Fe,20 Ru, Os)
and the two fluoroantimonate anions [Sb2F11]- and [SbF6]-,
(vi) Synthetic Aspects: Summary and Conclusion. The
29,30
reductive carbonylation of M(SO3F)3
(M ) Ru, Os) or
of OsF631 in HF/SbF539-41 described here allows the genera-
tion of the octahedral cations [M(CO)6]2+(solv) (M ) Ru,
Os)18,19 and the subsequent isolation of two sets of crystalline
salts with either [Sb2F11]- or [SbF6]- as counteranions in
quantitative yields and very high purities. A previously
proposed10,12 rationale for the formation of [M(CO)6]2+
cations (M ) Ru, Os), based on the solubility of CO in SbF587
and HF/SbF510,12,87 and soft47 or class b48 acceptor properties
of the naked M2+(solv) cation (M ) Ru, Os),10,12 is adopted
for both group 8 cations, and the formation of two types of
salts is explained by the well-known solution equilibrium in
HF/SbF5.41
In sharp contrast, the previously reported20,54 salts [Fe-
(CO)6][Sb2F11]2 and [Fe(CO)6][SbF6]2 can be obtained only
by oxidative carbonylation of Fe(CO)5, although the forma-
tion reactions in HF/SbF5 with XeF2 as the oxidizer are much
more complex, as discussed.20,55 Small amounts of paramag-
netic Fe[SbF6]252 remain in samples prepared under optimal
38-41
conditionssoxidation of Fe(CO)5 by XeF2 in HF/SbF5
at 25 °Cseven after careful recrystallization.20,55 A rationale
is proposed, again based on the SHAB concept47 and the
metal ion classification by Ahrland, Chatt, and Davies,48 for
20
52
the formation of both [Fe(CO)6][Sb2F11]2 and Fe[SbF6]2
by two competing pathways: solvolytic carbonylation fol-
lowing oxidation to transient [Fe(CO)m]2+ (m ) 4, 5) and
(88) Wang, C.; Willner, H.; Bodenbinder, M.; Batchelor, R.; Einstein, F.
W. B.; Aubke, F. Inorg. Chem. 1994, 33, 3521.
(89) von Ahsen, B.; Wartchow, R.; Willner, H.; Jonas, V.; Aubke, F. Inorg.
Chem. 2000, 39, 4424.
(87) von Ahsen, B.; Bley, B.; Bodenbinder, M.; Balzer, G.; Willner, H.;
Hwang-Mistry, G.; Wang, C.; Ha¨gele, G.; Aubke, F. Inorg. Chem.,
in preparation.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4197

Bernhardt et al.
Table 2. Crystallographic and Structural Data for Various Structure Determinations of [M(CO)6][SbF6]2 at 300 K (M ) Fe,20 Ru, Os)a
Fe20
Ru(11)
Ru(12)
Ru(5)
Os(10)
Os(6)
Os(9)
(i) Unit Cell Parameters and Dimensions
a ) b (Å)
c (Å)
8.258(1)
12.471(2)
850.5(2)
8.278(1)
12.449(2)
853.1(2)
8.284(1)
12.461(2)
855.1(2)
8.289(1)
12.447(2)
855.2(2)
8.274(1)
12.421(2)
850.3(2)
8.273(1)
12.447(2)
851.9(2)
8.283(1)
12.414(2)
851.7(2)
V (Å3)
(ii) Structure Solution
total data
10466
545
405
39
1.03
12506
555
430
9307
554
395
38
1.004
0.0255
0.0640
9252
547
454
9094
549
362
9327
551
335
6796
555
287
unique data
observed data
parameters
GOF on F2
R1 [I > 2σ(I)]
wR2
39
38
39
39
39
1.058
0.0219
0.0624
1.121
0.0286
0.0794
1.001
0.0196
0.0506
1.270
0.0296
0.0674
0.941
0.0311
0.0538
0.0259
0.0668
(iii) Internuclear Distances (Å)
M-Cl
1.917(7)
1.903(6)
1.097(8)
1.114(7)
1.851(5)
1.832(3)
2.020(6)
2.026(5)
1.100(8)
1.101(6)
1.856(2)
1.843(4)
2.027(6)
2.023(6)
1.099(8)
1.108(7)
1.856(3)
1.841(5)
2.019(7)
2.033(5)
1.106(10)
1.091(6)
1.858(2)
1.839(4)
2.013(8)
2.026(6)
1.109(11)
1.101(7)
1.856(2)
1.840(4)
2.031(10)
2.034(10)
1.094(13)
1.091(11)
1.852(3)
1.836(6)
2.022(11)
2.019(12)
1.090(11)
1.125(12)
1.859(5)
1.843(8)
M-C2
C1-O1
C2-O2
Sb-F1
Sb-F2
(iv) Nonbonding Contacts to M (Å)
M‚‚‚Sb
M‚‚‚F
5.174(7)
3.725(7)
5.179(7)
3.731(6)
5.183(7)
3.735(7)
5.183(7)
3.732(6)
5.173(7)
3.725(6)
5.176(7)
3.732(7)
5.175(7)
3.729(9)
(v) Nonbonding, Interionic Contacts (Å)b
C2‚‚‚F1
O2‚‚‚F1
C2‚‚‚F1′
O2‚‚‚F1′
C1‚‚‚F2
O1‚‚‚F2
2.842(5)
2.853(5)
3.062(5)
3.162(6)
3.339(7)
3.117(6)
2.822(5)
2.867(5)
3.059(5)
3.203(6)
3.307(7)
3.121(6)
2.827(5)
2.876(5)
3.061(5)
3.200(6)
3.308(7)
3.124(6)
2.822(5)
2.868(5)
3.057(5)
3.198(6)
3.313(7)
3.127(6)
2.819(5)
2.868(5)
3.049(5)
3.193(6)
3.306(7)
3.120(6)
2.821(5)
2.873(5)
3.060(5)
3.198(6)
3.303(7)
3.121(6)
2.816(6)
2.880(6)
3.056(6)
3.184(8)
3.305(9)
3.123(8)
a For crystal structure details, see Tables 1 and S1 and ref 20. b ∑rvan der Waals: O‚‚‚F, 2.99 Å; C‚‚‚F, 3.13 Å.
and the interionic bonding in the six salts are similar; (ii) to
subsequently determine whether structural similarities also
extend to relevant spectroscopic (vibrational, 13C NMR)
properties; and (iii) to provide a rationale for any observed
close correspondences or possibly also differences in ex-
perimental data, with assistance from relevant computations,
mainly by Jonas and Thiel.21
(ii) Crystal Data and Unit Cell Parameters. The data
are summarized in Table 1, with a more detailed listing
pertaining to the structural solutions as well, found in Tables
S1 and S2. For the three [M(CO)6][SbF6]2 salts (M ) Fe,20
Ru, Os), the unit cell dimensions are nearly identical.
Differences are very slightly larger than the stated error
limits. The unit cell of [Fe(CO)6][SbF6]2 appears to be more
tetragonally elongated along the c axis by about 0.05 Å
relative to the Os(II) compound and by about 0.02 Å relative
to the Ru(II) compound, whereas a and b are very slightly
shorter. However, the unit cell volumes V for [M(CO)6]-
[SbF6]2 (M ) Fe, Os) are identical within error limits, with
that for [Ru(CO)6][SbF6]2 being slightly larger by about
0.3%.
4), which are both shorter by about 6%. However, this
argument does not appear to apply to [Fe(CO)6][SbF6]2,20
as discussed above.
Additional information pertaining to the crystal data and
the molecular structure determinations of the four metal
carbonyl fluoroantimonates ([Sb2F11]-, [SbF6]-) for M ) Ru
and Os is presented as Supporting Information. The listings
there include atomic coordinates and thermal parameters for
[M(CO)6][Sb2F11]2 (M ) Ru, Os; Tables S3 and S4) and for
[M(CO)6][SbF6]2 (M ) Ru, Os; Tables S5 and S6), the
corresponding data for previously reported20 [Fe(CO)6]-
[Sb2F11]2 and [Fe(CO)6][SbF6]2 (Tables S7a and S7b), and
finally a comparison of atomic coordinates for all three
[M(CO)6][SbF6]2 salts (M ) Fe,20 Ru, Os) in Table S8.
All data in Tables S1-S8 support the strong similarity of
the six salts in the two isostructural triads [M(CO)6][Sb2F11]2
and [M(CO)6][SbF6]2 (M ) Fe,20 Ru, Os) with respect to
crystallographic data (Tables 1 and 2) and atomic coordi-
nates. This similarity is also reflected in the bond parameters,
as discussed in the next section.
(iii) Internal Bond Parameters for [M(CO)6][Sb2F11]2
and [M(CO)6][SbF6]2 (M ) Fe,20 Ru, Os). The internal
bond parameters (internuclear distances and bond angles) for
both cations and anions of the six isostructrual salts of the
compositions [M(CO)6][Sb2F11]2 and [M(CO)6][SbF6]2 (M
) Fe,20 Ru, Os) are presented in the following manner: A
complete listing of internal bond parameters for the new
species [M(CO)6][Sb2F11]2 and [M(CO)6][SbF6]2 (M ) Ru,
Os)18 is found in the Supporting Information as Tables S8
and S9.
The unit cell parameters a, b, c, and â and the volume V
19
are again all very slightly smaller for [Fe(CO)6][Sb2F11]2
than for the Ru (by ∼1.4%) and Os (by ∼1.0%) [Sb2F11]-
salts (see Table 1). In the absence of relativistic effects for
the 3d species,49,50 this is expected, considering the experi-
mental Fe-C distances (see Tables 2 and 3) and the average
Fe-C distances listed in the Cambridge data index90 (Table
(90) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.
G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, Suppl. 1.
4198 Inorganic Chemistry, Vol. 44, No. 12, 2005

[M(CO)6][SbF6]2 and [M(CO)6][Sb2F11]2 (M ) Fe, Ru, Os)
Table 3. Experimental and Calculated21 Internuclear Distances for [M(CO)6]2+ in [M(CO)6][Sb2F11]2 (M ) Fe,20 Ru, Os) and Related Species
Fe20
Ru
Os
(a) M-C and C-O (in Brackets) Distances (Å)
(i) [M(CO)6]2+
d(M-C1) [d(C1-O1)]a ×2
d(M-C2) [d(C2-O2)] ×2
d(M-C3) [d(C3-O3)] ×2
1.911(5)
1.912(5)
1.910(5)
1.911(5)
1.908(7)
[1.103(5)]
[1.102(5)]
[1.107(5)]
[1.104(5)]
[1.108(7)]
2.044(9)
2.027(9)
2.045(8)
2.039(8)
2.024(6)
[1.086(9)]
[1.093(10)]
[1.100(8)]
[1.094(10)]
[1.101(6)]
2.032(5)
2.016(5)
2.032(5)
2.027(6)
2.022(7)
[1.097(6)]
[1.109(6)]
[1.097(6)]
[1.102(7)]
[1.104(8)]
d(M-Cavg) [d(C-O)avg
]
b
d(M-C′avg) [d(C′-O)avg
]
(ii) M(CO)5
d(M-Cax) [d(C-O) ×2
d(M-Ceq x 3
1.806(5)
1.833(4)
[1.145(4)]20
1.950(9)
1.969(9)
[1.143(3)]66
1.962(8)
[1.142(4)]67
(b) Statisticalc,d Data from the Cambridge Index90
1.896 (1453)
d(M-C) (n)e
1.782 (2572)
1.902 (1443)
2.045
(c) Calcualted Data20
d(M-C) [d(C-O)]
1.905
[1.130]
2.028
[1.130]
[1.130]
a See Figures 3 and 4 for numbering of atoms. b Data obtained for [M(CO)6][SbF6]2 (M ) Fe,20 Ru, Os) are collected in Tables 2 and 3. c Reference 89.
d d for CO in metal carbonyl compounds is listed as 1.145 Å based on 10022 examples. e Number of examples in ref 89.
Table 4. Vibrational Frequencies (cm-1) for the Fundamentals of [M(CO)6]2+ in [M(CO)6][SbF6]2 (M ) Fe,20 Ru, Os)
Fe
Ru
Os
assignments
(description)
obsdb
2242 s
2219 s
2205 (70)
590 (70)
470 (0.1)
500 w
calcda
2221
2188
2173 (100)
608 (29)
478
508
361
355
380 (3)
338
obsdb
calcda
2235
2192
2172 (100)
572 (23)
490
468
401
377
337 (5)
329
obsdb
2258 s
2214 vs
2189 (105)
560 (50)
509 (0.04)
480 w
calcda
2237
2188
2166 (100)
567 (16)
509
471
428
401
336 (8)
339
ν1, νCO (A1g)
ν3, νCO (Eg)
ν6, νCO (Tlu)
2252 s
2219 s
2198 (80)
556 (46)
483 (0.1)
463 s
ν7, δMCO (T1u)
ν12, δMCO (T2u)
ν10, δMCO (T2g)
ν2, νMC (A1g)
ν4, νMC (Eg)
ν8, νMC (T1u)
ν5, δMCO (T1g)
n9, gCMC (T1u)
ν11, δCMC (T2g)
ν13, δCMC (T2u)
345 m
388 m
379 w, sh
335 (7)
328c
429 m
409 w
357 w, sh
382 (<0.1)
331c
344 (9)
345c
148c
117 (0.3)
98
77
146c
106 (0.1)
91
75
145c
101 (0.0)
93
77
148 vs
139 vs
142 vs
94c
92c
96c
a BP86/ECP2.21 Relative IR intensities in parentheses, where 100 denotes approximately 1035 (Fe), 1260 (Ru), and 1545 (Os) km/mol. b Relative IR
intensities in parentheses normalized to the strongest anion band (νSbF, T1u, denoted as 100). For Raman intensities, s ) strong, m ) medium, w ) weak,
v ) very, sh ) shoulder. c Calculated from combination bands. Estimated uncertainty: ν5, (2; ν9 and ν13, +5 cm-1
.
Selected bond parameters for [M(CO)6][SbF6]2 (M ) Fe,20
Ru, Os) are compiled in Table 2, and in Table 3, experimental
internuclear C-O and M-C distances for the [M(CO)6]2+
cations (M ) Fe,20 Ru, Os) in the triad [M(CO)6][Sb2F11]2
are summarized and compared to the corresponding experi-
mental data for the molecular species M(CO)5, obtained by
electron diffraction,20,66,67 and statistical data for M-CO
complexes (M ) Fe, Ru, Os) from the Cambridge data
index.90
To emphasize the nearly identical internal bond parameters
for the three members in both triads (see Tables 2, 3, S9,
and S10), a generic pictorial presentation is chosen: formula
units of [M(CO)6][Sb2F11]2 and [M(CO)6][SbF6]2 (M ) Fe,20
Ru, Os) are shown in Figures 3 and 4, respectively, to
introduce the numbering used in this study for both cations
and anions. To emphasize their electrophilic nature, the
carbonyl C atoms9-12,21 are highlighted in both figures.
Finally, in Figure 5, important parameters of the diocta-
hedral [Sb2F11]- anions in the three [M(CO)6][Sb2F11]2 salts
(M ) Fe,20 Ru, Os) are shown in a schematic representation
that concentrates on the skeletal bridging moiety and omits
the terminal Feq atoms in both SbF4 groups. As can be seen,
the conformation of all three [Sb2F11]- anions is C1. The
diagnostic criteria, first introduced by Holloway et al.;91 the
bridge angles R; and the dihedral angles ψ in all three
Figure 3. Molecular structure of [M(CO)6][Sb2F11]2: Formula unit; 50%
probability ellipsoids; M ) Fe,20 Ru, Os.
symmetry-related anions are identical within error limits.
Also identical is the asymmetry within the Sb1-Fb-Sb2
bridges and all six Sb-Fax bond lengths, whereas for all three
[M(CO)6][Sb2F11]2 salts (M ) Fe,20 Ru, Os), the observed
Sb-Feq distances for the two planar SbF4 groups fall into
very similar, narrow ranges.
The strong similarities in bond parameters for all three
[Sb2F11]- anions extend also to the [SbF6]- anions for the
second triad of the type [M(CO)6][SbF6]2 (M ) Fe,20 Ru,
(91) Bruce, D. M.; Holloway, J. H.; Russell, D. R. J. Chem. Soc., Dalton
Trans. 1978, 64.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4199

Bernhardt et al.
Table 5. Observed Band Positions and Estimated Intensitiesa for Vibrational Bands (cm-1) Due to [SbF6]- of [M(CO)6][SbF6]2 (M ) Fe,20 Ru, Os)
Fe20
Ru
Os
assignments
and description
IR
Raman
IR
Raman
IR
Raman
ν4 + 2ν6
683 (0.3) sh
665 (100)
680 (0.3) sh
663 (100)
681 (0.3) sh
663 (100)
ν3(T1u) νSbF
ν1(A1g) νSbF
2ν4
652 (30)
590 (1.1) sh
651 (30)
588 (1.0) sh
652 (30)
590 (0.9) sh
ν4 + ν5
581 (0.2)
b
291 (57)
281 sh
584 (0.3)
b
291 (60)
281 sh
ν2(Eg) νSbF
ν4(T1u) δFSbF
ν5(T2g) δFSbF
570 (5.0)
292 (60)
282 sh
570 (4.0)
291 (2.4)
281 (11)
570 (3.9)
292 (2.6) sh
281 (11)
571 (3.2)
292 (3.1) sh
281 (13)
a Relative intensities for anion and cation are bands obtained by setting IR action νSbF(T1u) as 100 and νSbF(A1g) as 30 in the Raman spectra. b Expected
band for ν2(Eg) coincides with ν7 δMCO(T1u) for [M(CO)6]2+ (M ) Fe, Ru, Os).
the M-C distances in the two compounds are identical within
error limits.
When average M-C distances (M ) Fe,20 Ru, Os) are
considered as well (see Table 3), two simple conclusions
are reached: (i) M-CO bonding in octahedral [M(CO)6]2+
(M ) Fe,20 Ru, Os) is of identical strength regardless of
counteranion. (ii) In both triads, [M(CO)6]2+ cations are
completely isostructural for M ) Ru and Os, which is neither
predicted21 nor evident from statistical data, compiled in the
Figure 4. Molecular structure of [M(CO)6][Sb2F11]2: Formula unit; 50%
probability ellipsoids; M ) Fe,20 Ru, Os.
Cambridge data collection, where slightly longer Os-CO
bonds are suggested.21,90 For both [Fe(CO)6]2+ salts,20
experimental as well as calculated21 and statistical90 Fe-
CO bonds are shorter than the corresponding Ru-CO and
Os-CO internuclear distances.
As is commonly found for σ-bonded metal carbonyl
cations,9-20,42-44,58 C-O bond lengths for all six9-17,20,42-44,58
salts in both [M(CO)6]2+ triads (M ) Fe,20 Ru, Os) are,
consistent with calculations,21 among the shortest compiled
in the Cambridge index,90 regardless of metal (Fe, Ru, or
Os) or counteranion ([Sb2F11]- or [SbF6]-). However, at high
C-O bond orders, experimental internuclear C-O distances
are all identical within error limits (see Table 3), and
vibrational data [ν(CO), fCO] provide a much more accurate
estimate of C-O bond strength (Tables 4 and 6, and Figures
6 and 7), as discussed below.
Figure 5. Skeletal moiety of the [Sb2F11]- anion (point group C1; Feq
omitted) and diagnostic structural parameters of the [Sb2F11]- anions in
the three salts [M(CO)6][Sb2F11]- (M ) Fe,20 Ru, Os). In all three salts,
the two [Sb2F11]- units have identical bond parameters.
In addition to the internal bond parameter discussed above,
significant interionic contacts are observed in fluoroanti-
monate salts of superelectrophilic σ-bonded metal
carbonyls8-19,42-45 and their derivatives. Their importance in
the six [M(CO)6]2+ salts (M ) Fe,20 Ru, Os) is now analyzed.
(iv) Inter- and Intraionic Interactions in [M(CO)6]-
[Sb2F11]2 and [M(CO)6][SbF6]2 (M ) Fe,20 Ru, Os). The
strong similarities of the observed internal bond parameters
of the [M(CO)6]2+ cations (M ) Fe,20 Ru, Os; Tables 2 and
3) and the octahedral [SbF6]- anions (Tables 2, 3, and 5)
and the identical conformations as well as all diagnostic bond
parameters of the [Sb2F11]- anions (Figure 5) in all six salts
discussed here allow the conclusion that secondary92 inter-
ionic contacts must be similar in both number and strength
for the three salts in each triad.
Os). The anions are again symmetry-related and depart from
Oh symmetry by a small tetragonal distortion (compression)
as a consequence of the tetragonal space group (vide supra).
This is evident from the Sb-F bond lengths listed in Tables
2 and S9. Approximately octahedral symmetry is also evident
from a vibrational analysis of the [SbF6]- anion in all three
salts, shown in Table 5, as discussed more fully below.
Tetragonal distortion (elongation) is evident also for the
[M(CO)6]2+ cations in the triad [M(CO)6][SbF6]2 (M ) Fe,20
Ru, Os), but as for the [SbF6]- anions, tetragonal distortions
are more pronounced for [Fe(CO)6][SbF6]2,20 just as the unit
cell parameters for the three salts had suggested. For
[M(CO)6][SbF6]2 (M ) Ru, Os), both the M-C distances
(M ) Ru, Os) and the observed tetragonal distortions are
within the stated error limits (see Tables 2, 3, S9, and S10).
Likewise, the M-C and C-O distances, with the latter
less pronounced, in the triad [M(CO)6][Sb2F11]2 (M ) Fe,20
Ru, Os) show trigonal distortions as a consequence of the
monoclinic crystal system (see Table 1). The trigonal
distortions are more noticeable for the Ru and Os salts, but
In particular, the data for the three [Sb2F11]- anions,
summarized in Figure 5, are supportive of this view. We
have previously argued, based on a substantial body of
evidence from structural studies,10-16,20,42-45 that the dioc-
(92) Alcock, N. W. AdV. Inorg. Chem. Radiochem. 1972, 15, 1.
4200 Inorganic Chemistry, Vol. 44, No. 12, 2005

[M(CO)6][SbF6]2 and [M(CO)6][Sb2F11]2 (M ) Fe, Ru, Os)
Table 6. Summary of Selected Experimental and Calculated Structural and Spectroscopic Parameters of the Superelectrophilic, Octahedral Cations
[M(CO)6]2+ (M ) Fe, Ru, Os)
Fe20
Ru
Os
(a) Solid-State Complex Anion
[SbF6]-
2216
[Sb2F11]-
2215
[SbF6]-
2214
[Sb2F11]-
2216
[SbF6]-
2209
[Sb2F11]-
2211
ν(CO)avg (cm-1
)
fCO × 102 (N‚m-1
d(C-O)avg (Å)
d(M-O)avg (Å)
)
19.83
19.82
19.80
19.83
19.71
19.75
1.108(7)
1.908(7)
368
1.104(5)
1.911(5)
368
1.101(6)
2.024(6)
359
1.094(10)
2.039(8)
362
1.104(8)
2.022(7)
382
1.102(7)
2.027(6)
384
ν(M-C)avg (cm-1
)
(b) 13C NMR data for [M(CO6)]2+(solv)
δ13C (ppm)
178
19.2
168.8
60.8
150.6
91.7
J(13C-M) (Hz)
(c) Calculated Data21 for [M(CO)6)]2+(g)
ν(CO)avg (cm-1
)
2185
19.20
1.130
1.905
1.77
0.63
-0.49
-0.21
2189
19.22
1.130
2.028
1.89
0.63
-0.51
-0.21
2185
19.13
1.130
2.045
2.16
0.60
-0.35
-0.21
FCO × 102 (N‚m-1
)
d(CO) (Å)
d(M-C) (Å)
fM-C × 102 (N‚m-1
)
qc (e-)a
q
q
M (e-)a
O (e-)a
a Partial atomic charges from natural population analysis.20
Figure 7. Raman spectra of [M(CO)6][SbF6]2 (M ) Fe,20 Ru, Os).
Table 7. Selected Bonding and Nonbonding Contactsa (Å) for Cn (n )
1, 2, 3)b in the Structures of [M(CO)6][SbF6]2 and [M(CO)6][Sb2F11]2
(M ) Fe,20 Ru, Os)
Figure 6. Fundamental CO stretching frequencies and 13C chemical shifts
for [M(CO)6]2+ (M ) Fe,20 Ru,18 Os)18.
tahedral [Sb2F11]- anion in salts with superelectrophilic8
metal carbonyl cations10-12 will distort by bending at the F
bridge (expressed in terms of the bridge angle R) and by
rotation of the two SbF4 moieties (resulting in a dihedral
angle Ψ)11,91 from an ideal D4h93 symmetry, to facilitate the
formation of significant and numerous interionic C‚‚‚F
contacts. The strength of these contacts is judged by
comparison to the sums of the corresponding van der Waals
nonbonding radii, tabulated by Bondi.94
bonding
nonbonding
compound
d(M-C1) d(C1-O) d(C1‚‚‚C2/3) d(C1/3‚‚‚F)
[Fe(CO)6][SbF6]2
[Fe(CO)6][Sb2F11]2
[Ru(CO)6][SbF6]2
[Ru(CO)6][Sb2F11]2
[Os(CO)6][SbF6]2
[Os(CO)6][Sb2F11]2
1.917
1.911
2.020
2.044
2.013
2.031
1.097
1.103
1.100
1.086
1.109
1.099
2.701
2.684-2.723 2.718-2.969
2.865 2.822
2.862-2.896 2.724-2.909
2.856 2.819
2.831-2.893 2.741-2.903
2.842
a Uncorrected data listed without estimated standard deviations. b See
Figures 3 and 4 for numbering of atoms.
In salts of σ-bonded metalcarbonyl cations with fluoro-
antimonate anions,9-12 the sums of van der Waals radii of
3.17 and 2.99 Å for C‚‚‚F and O‚‚‚F contacts, respectively,
are viewed as upper limits. For the purpose of illustration,
up to five significant C‚‚‚F contacts for each C atom are
observed with contact distances as short as 2.6 Å43,44
As seen in Figures S1 and S2 in the Supporting Informa-
tion and in Tables 2 and 7, for salts with octahedral
[M(CO)6]2+ cations (M ) Fe,20 Ru, Os), far fewer and
weaker interionic C‚‚‚F contacts are observed: In [M(CO)6]-
[Sb2F11]2 (M ) Fe,20 Ru, Os), only one significant C‚‚‚F
contact per CO ligand is found with the shortest contact
distance of 2.724 Å, whereas in [M(CO)6][SbF6]2, four
44
in [Hg(CO)2][Sb2F11]2 and [M(CO)4][Sb2F11]2 (M ) Pd,
Pt43) with linear44 or square-planar43 metal carbonyl cations,
(93) Sham, I. H. T.; Patrick, B. O.; Thompson, R. C.; Aubke, F.; von Ahsen,
B.; von Ahsen, S.; Willner, H. Solid State Sci. 2002, 4, 1457.
(94) Bondi, A. J. Phys. Chem. 1964, 68, 441.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4201

Bernhardt et al.
C‚‚‚F contacts per [M(CO)6]2+ cation (M ) Fe,20 Ru, Os)
are detected, with distances between 2.818 and 2.842 Å. In
both triads, the strength of these interionic interactions
appears to increase slightly from the Fe to the Os complex.
Slightly shorter contact distances for the [Sb2F11]- salts
reflect the nonrigidity of the dioctahedral anions.11,91
In both [M(CO)6]2+ triads (M ) Fe,20 Ru, Os), the reduced
accessibility of the carbonyl C atoms by F atoms of the
anions in the strictly octahedral cations provides a plausible
explanation for fewer and weaker C‚‚‚F interactions, com-
pared to findings for [M(CO)4][Sb2F11]2 (M ) Pd, Pt43) or
[Hg(CO)2][Sb2F11]2.44 Alternatively, it can be argued that
octahedral [M(CO)6]2+ cations (M ) Fe,20 Ru, Os) are
intrinsically more stable than are square-planar [M(CO)4]2+
(M ) Pd, Pt) or linear [Hg(CO)2]2+ ions, which are stabilized
only by [Sb2F11]- and not by [SbF6]-.43,44 This view is
reflected in the reported thermal stabilities, which, in the 5d
series, follow the order [Hg(CO)2][Sb2F11]2 (160 °C)44 < [Pt-
(CO)4][Sb2F11]2 (230 °C)43 < [Os(CO)6][SbF6]2 (350 °C),
with decomposition points in parentheses.10,11
contacts for the six salts are significantly shorter than the
sum of the van der Waals radii of 3.40 Å94 and reflect the
tight packing of electrophilic C atoms21 in the octahedral
coordination spheres of the [M(CO)6]2+ cations (M ) Fe,20
Ru, Os). For the Ru and Os cations, these C‚‚‚C contacts
are similar in both cations and show a slight dependency on
the counteranion ([SbF6]- or [Sb2F11]-). Shorter C‚‚‚C
contacts suggesting tighter packing of the C atoms is found
for both [Fe(CO)6]2+ salts20 with very slight dependency on
the counteranion. An illustration of the tight packing in
[M(CO)6]2+ (M ) Fe,20 Ru, Os) is found in Figure S7.
The structural data for [M(CO)6][SbF6]2 (M ) Fe,20 Ru,
Os) compiled in Table 2 provide a summary and good
overview of the structure and bonding situation for [Ru(CO)6]-
[SbF6]2 and [Os(CO)6][SbF6]2. Three different crystals of
each salt were chosen in each case. Their structures were
solved identically, with full details of the structure solution
included. The unit cell dimensions and relevant internuclear
distances were obtained for all six crystals, and the results
were compared to the corresponding published data for
[Fe(CO)6][SbF6]2.20 Also included were nonbonding
(M‚‚‚Sb, M‚‚‚F) distances and secondary interionic (C‚‚‚F,
O‚‚‚F) contacts, which were compared to the relevant sums
of the van der Waals radii.94
As seen in Table 2, for the three [M(CO)6][SbF6]2 salts
(M ) Fe,20 Ru, Os), in addition to weak C2‚‚‚F contacts,
there are equally weak O2‚‚‚F interactions, which are slightly
shorter than 2.99 Å, the sum of the van der Waals radii for
O and F.94 This observation is in harmony with a bonding
model for fluoroantimonate salts with σ-bonded metal
carbonyl cations proposed recently.10-12 According to this
model, electron transfer from F atoms of the fluoroanimonate
anions into π* MOs of the CO ligands becomes a competing
process to d(M) f π* back-donation in a classical synergic
bonding view.98,99 With M in oxidation states of 2+ or
3+,10-12 σ donation from CO to Mn+ is augmented by
increased polarization of the CO bond by Mn+, n ) 2,3.51
A vibrational analysis of octahedral homoleptic metal
carbonyls of W0, Re+, Os2+, and Ir3+ 10-13 has provided a
spectroscopic basis for the bonding model. Additional support
for the proposed F f π* interionic back-donation comes
from the observed M-C-O bond angles, which, in most
instances, depart from linearity by ∼2-3° 10-16,42-44 This is
also the case for [Fe(CO)6][Sb2F11]220 and [M(CO)6][Sb2F11]2
(M ) Ru, Os; see Tables S8 and S9) However, in [M(CO)6]-
[SbF6]2 (M ) Fe,20 Ru, Os), the observed M-C1-O angles
(M ) Fe,20 Ru, Os) are strictly linear, whereas for M-C2-O
(M ) Fe,20 Ru, Os), the departures are small, ∼1° or less
(Tables S10 and S11), because the [SbF6]- ion appears to
be less effective in F f π* back-donation than [Sb2F11]-,
as stated above.
As can be seen, the unit cell dimensions, internuclear
distances, and various nonbonding contacts for [Ru(CO)6]-
[SbF6]2 and [Os(CO)6][SbF6]2 are extremely close. Differ-
ences between the two data sets are often smaller than
variations found for the three crystals of the same metal.
20
Although internuclear Fe-C distances in [Fe(CO)6][SbF6]2
are shorter than those for the Ru and Os compounds,
interionic bonding is weaker, reflected in slightly longer C‚
‚‚F contacts than are found for [M(CO)6][SbF6]2 (M ) Ru,
Os). As a consequence, in the triad [M(CO)6][SbF6]2 (M )
Fe,20 Ru, Os), the unit cell dimensions and volumes,
nonbonding M‚‚‚Sb and M‚‚‚F distances (M ) Fe, Ru, Os),
and Sb-F bond lengths are all independent of the metal.
These findings together with other crystallographic and
bonding details, such as the identical conformations and
bonding parameters for the [Sb2F11]- anions, shown in Figure
5, make the six compounds studied here unique and the
results obtained unprecedented among structurally character-
ized metal carbonyl cations and their derivatives.9-17,42-44
(v) Structure and Bonding in the Triads [M(CO)6]-
[Sb2F11]2 and [M(CO)6][SbF6]2 (M ) Fe,20 Ru, Os): A
Summary. Bond parameters for the [Sb2F11]- and [SbF6]-
salts are identical in the two triads, as are the conformations
and diagnostic bond parameters for [Sb2F11]- in [M(CO)6]-
[Sb2F11]2 (M ) Fe,20 Ru, Os). Bond parameters for [M(CO)6]2+
(M ) Ru, Os) are nearly independent of M in both triads on
account of relativistic effects.49,50 Slightly shorter Fe-C
bonds20 are partially compensated by stronger interionic
contacts for the corresponding ruthenium and osmium
compounds, so that unit cell dimensions are independent of
the metal in the triad [M(CO)6][SbF6]2 (M ) Fe,20 Ru, Os)
and very similar for the three [Sb2F11]- salts. It appears that
crystal packing of [SbF6]- ions in the lattice is tighter and
more efficient than that of dioctahedral [Sb2F11]- anions.
In addition to the observed interionic C‚‚‚F and O‚‚‚F
interactions (listed in Tables 2 and 7), there are nonbonding
C‚‚‚C contacts within the three [M(CO)6]2+ cations (M )
Fe,20 Ru, Os), which are also listed in Table 7. The observed
(95) Jones, L. H.; McDowell, R. S.; Goldblatt, M. Inorg. Chem. 1988, 27,
2349.
(96) Abel, E. W.; McLean, R. A. N.; Tyfield, S. P.; Braterman, P. S.;
Walker, A. P.; Hendra, P. J. J. Mol. Spectrosc. 1969, 30, 29.
(97) Cotton, F. A.; Kraihanzel, C. S. J. Am. Chem. Soc. 1962, 84, 4432.
(98) Dewar, J. S. Bull. Soc. Chim. Fr. 1951, 18, C71.
(99) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2939.
4202 Inorganic Chemistry, Vol. 44, No. 12, 2005

[M(CO)6][SbF6]2 and [M(CO)6][Sb2F11]2 (M ) Fe, Ru, Os)
An additional controlling factor that affects the size of the
[M(CO)6]2+ cations (M ) Fe,20 Ru, Os) is repulsive
interactions between the C atoms of the six carbonyl ligands
(see Figure S7 and Table 7), which, according to a natural
population analysis,21 have nearly identical positive partial
charges (see Table 6 and ref 21).
In harmony with the results of the DSC study reported
here, M-C bond strength appears to increase from Fe to
Ru to Os in the cation [M(CO)6]2+ (M ) Fe, Ru, Os) because
of increased C-O bond polarization and relativistic ef-
fects.49,50
A similar trend of increasing bond strength for the C-O
bonds in the three cations is expected but not found (see
Figure 4), as the subsequent discussion of the vibrational
spectra shows, because F f π* external electron delocaliza-
tion10-12 will follow the order Fe < Ru e Os. In this respect,
[Sb2F11]- is found to be a slightly more effective donor of
electron density than [SbF6]-. This is reflected in the
crystallographic data reported here, as well as in the case of
many more [Sb2F11]- salts with superelectrophilic metal
carbonyl cations.9-17,20,42-45
Although the members of the two triads [M(CO)6][Sb2F11]2
and [M(CO)6][SbF6]2 (M ) Fe, Ru, Os) are isostructural and
the counteranions [Sb2F11]- and [SbF6]- have identical bond
parameters and conformations, a careful analysis brings to
light subtle but fascinating differences as well as similarities
that are unique in metal carbonyl chemistry.1-7,9-12,75
(c) Vibrational Spectra of [M(CO)6][SbF6]2 (M ) Fe,20
Ru, Os). Although the crystal and molecular structures of
[M(CO)6][Sb2F11]2 and [M(CO)6][SbF6]2 (M ) Ru, Os) are
reported here for the first time, the initial report on the
synthesis of [M(CO)6][Sb2F11]2 (M ) Ru, Os18) contains a
preliminary vibrational assignment for the [M(CO)6]2+
cations (M ) Ru, Os). An early review9 compares vibrational
spectra of the isoelectronic octahedral species W(CO)6,95 [Re-
(CO)6]+,96 [Os(CO)6]2+,18 and [Ir(CO)6]3+ 15 in the ν(CO)
range. A second horizontal correlation of the same four
M(CO)6 species (M ) W, Re+, Os2+, Ir3+) contains a
complete vibrational analysis of [Os(CO)6]2+ and [Ir-
(CO)6]3+ 13 (Figure S5) in conjunction with the structural
characterization of [Ir(CO)6][SbF6]3‚4HF. An extensive DFT
study21 of octahedral M(CO)6 species includes a complete
vibrational assignment and calculated band positions and
intensities of IR bands for [M(CO)6]2+(g) (M ) Fe, Ru, Os),
and other theoretical studies22-24 report more limited vibra-
tional frequencies in the ν(CO) range. All of these earlier
experimental studies9,13,15 and the DFT calculations of Jonas
and Thiel21 establish a strong similarity of the vibrational
spectra, at least in the CO stretching range, between
[M(CO)6]2+ (M ) Ru, Os) and [Fe(CO)6]2+. The latter has
been extensively characterized with both [Sb2F11]- and
[SbF6]- as counteranions by 13C substitution, normal coor-
dinate analysis (NCA), and force constant (GVFF) calcula-
tions.20
overlap. In addition, Raman spectra are more informative,
whereas IR spectra of crystalline samples are occasionally
ambiguous, on account of the Christiansen effect, other
artifacts, and interference from combination bands.
Hence, to avoid unnecessary duplication and redundancy,
the following approach is taken here: (i) Rather than
providing a complete vibrational analysis of [M(CO)6]2+
(M ) Ru, Os), the data obtained by us are compared to
experimental results for [Fe(CO)6]2+ 20 and to previously
reported calculated data21 for [M(CO)6]2+(g) (M ) Fe, Ru,
Os). (ii) Earlier studies on [M(CO)6]2+ (M ) Fe, Ru,
Os)9,13,18,20 limited to the ν(CO) range, are summarized in
the form of a diagram, shown in Figure 6. Also listed here
are ν(CO)avg, fCO,
97 and 13C chemical shifts (discussed later).
(iii) A complete list of vibrational fundamentals for [M(CO)6]2+
(M ) Fe,20 Ru, Os) is provided in Table 4. Experimental
frequencies are compared to calculated values.21 (iv) The
Raman spectra of [M(CO)6]2+ (M ) Fe,20 Ru, Os) in the
[SbF6]- salts are shown in Figure 7, and the fundamentals
for [SbF6]- in all three salts are listed in Table 5. (v) A
complete listing of all IR-active vibrations including over-
tones for [M(CO)6][SbF6]2 (M ) Fe,20 Ru, Os) together with
an assignment are available as Supporting Information in
Table S11. (vi) Vibrational spectra of [Ru(CO)6][SbF6]2 and
[Os(CO)6][SbF6]2 are shown as Supporting Information in
Figures S3 and S4.
As reported for [Fe(CO)6]2+,20 the band positions of all
13 fundamentals have been obtained experimentally also for
the [M(CO)6]2+ cations (M ) Ru, Os; see Table 4) with the
band positions of ν5 (T1g), ν9 (T1u), and ν12 (T2u) estimated
from combination bands. The inactive vibration ν12 (T2u) is
observed as a very weak band in the IR spectra.
The assignment of the vibrational spectra for [Ru(CO)6]2+
and [Os(CO)6]2+ is obtained in a similar manner as described
for [Fe(CO)6]2+ in greater detail. Support comes from the
calculated20 (BP86/ECP2) band positions and IR intensities.
As discussed previously,20 calculated band positions for the
CO stretching modes ν1, ν3, and ν6 are underestimated,20 as
is ν[CO(g)] itself,21 pointing to a limitation of the DFT
method. Values for ν9, ν11, and ν13 are also somewhat higher
than calculated frequencies, presumably because of crystal
lattice effects. For the remaining fundamentals, only minor
discrepancies between experimental and calculated band
positions are noted.
Just as reported for both [Fe(CO)6]2+ salts20 and suggested
by the structural data discussed above, vibrational data for
all [M(CO)6]2+ cations (M ) Fe,20 Ru, Os) are independent
of the counteranion [Sb2F11]- or [SbF6]-. This is convinc-
ingly illustrated by a comparison of the CO stretching
18,20
fundamentals ν1, ν3, and ν6 for [M(CO)]6][Sb2F11]2
(M ) Fe, Ru, Os), plotted in Figure 6, to the corresponding
values for the same three fundamentals for [M(CO)6][SbF6]2
(M ) Fe,20 Ru, Os) listed in Table 6. Differences in band
positions are at most 4 cm-1 and fall for the most part within
error limits.
The reasonably close similarities of calculated21 and
experimentally observed fundamentals of [M(CO)6]2+ (M )
Fe, Ru, Os18-20) and the noted independence of the coun-
In the course of this study,20 it was found that [Fe(CO)6]-
[SbF6]2 is more suitable for a vibrational analysis than the
[Sb2F11]- salt, on account of the much simpler vibrational
spectrum of octahedral [SbF6]-, which results in less band
Inorganic Chemistry, Vol. 44, No. 12, 2005 4203

Bernhardt et al.
teranion ([Sb2F11]- or [SbF6]-), best documented in the CO
stretching fundamentals (Figures 6 and 7, Table 4), together
with the limited number of rather weak interionic contacts
in all six [M(CO)6]2+ salts (M ) Fe,20 Ru, Os), are all
indications of the exceptional intrinsic stability of the
octahedrally coordinated, homoleptic, superelectrophilic,8
σ-bonded metal carbonyl cations9-12 in group 8.
The same conclusions can be extended to superelectro-
philic8 [Ir(CO)6]3+, where all fundamentals13,15,19 compare
well to calculated values and where identical band positions
are found in the CO stretching region in [Ir(CO)6]3+(solv),
HF/SbF5,13 [Ir(CO)6][Sb2F11]3,15,19 and [Ir(CO)6][SbF6]3‚
4HF.13
Due to increased π-back donation in the series [M(CO)6]2+
(M ) Fe,20 Ru, Os), there is a slight dependence of
vibrational band positions most noticeable in the C-O and
M-C stretching regions, on the central metal ion M2+ (M
) Fe,20 Ru, Os), which appears to affect the three funda-
mentals ν1 (A1g), ν3 (Eg), and ν6 (T1u) differently. In the CO
stretching region (see Figures 6 and 7 and Table 4), the A1g
mode increases gradually in the triad [M(CO)6]2+ from Fe20
to Ru to Os by 10 and 6 cm-1, respectively. The Eg mode ν3
changes very little, and for Ru and Os, a decrease of 5 cm-1
is noted. Finally, for ν6 (T1u), a gradual decrease by 7 and 9
cm-1 is noticed. As a consequence of these changes in
opposite directions, ν(CO)avg and fCO are nearly identical for
[Fe(CO)6]2+ and [Ru(CO)6]2+ and only slightly smaller for
[Os(CO)6]2+ (see Table 7). For the skeletal M-C stretching
fundamentals, a similar, more pronounced trend emerges.
However, in this far more cluttered spectral region, vibra-
tional assignments can occasionally be ambiguous, and
vibrational coupling or “mixing” is a strong possibility.
Hence, some caution is advised.
As seen in Figure 7, in the Raman spectra, the band
positions for ν(MC) of ν2 (A1g) and ν4 (Eg) shift strongly
from Fe to Os to higher frequencies, and the peak separation
increases. As seen in Table 4, the observed shifts of ν2 are
43 and 41 cm-1 and those of ν4 are 22 and 30 cm-1, with
band separations of 12, 9, and 20 cm-1, respectively, for the
two fundamentals in [M(CO)6]2+ in the order Fe20 < Ru <
Os. There is good agreement between experimental and
calculated21 frequencies in this region as discussed above.
The trend of ν8 (T1u) in the IR spectra is less clear, because
there is both, a dependence of the metal atom mass and force
constant.
of the CO stretching fundamentals versus the nuclear charge
for the four complexes, as shown in Figure S5. The increased
frequencies for all three fundamentals, in particular for ν1
(A1g), are affected by electron delocalization from F into the
π* MOs of the CO ligands,10-13 which increases with
increasing oxidation state of M, M ) Re+, Os2+ Ir3+. In
addition to an increase in ν(CO), the band separation for
the three fundamentals decreases with increasing oxidation
state of M.9,13,95,96
The intensities of the CO stretching fundamentals have
been discussed before.9 In all instances, in the Raman spectra,
ν1 (A1g) is of lower intensity than ν3 (Eg),9-12,18,95,96 and only
for [Ir(CO)6]3+ 9,13,20 the two fundamentals do have identical
intensities.
Finally, consistent with their strong structural similarities
(see Tables 2 and 3), discussed above, the fundamentals of
[SbF6]- in [M(CO)6][SbF6]2 (M ) Fe,20 Ru, Os) have
identical band positions in all three salts as shown in Table
5.
(d) 13C NMR Spectra of [M(CO)6]2+ (M ) Fe, Ru, Os).
The solubilities of the [M(CO)6][SbF6]2 salts (M ) Fe, Ru,
Os) in HF are sufficiently high that their 13C NMR spectra
can be recorded. In addition to the 13C resonance, in all cases,
1
we are able to detect satellite resonances, produced by J
(13C-M) coupling with an NMR-active metal nucleus (see
Table S12).
The rather rare case of coupling to a nucleus with a spin
of 5/2 for 1J (13C-99Ru) is shown in Figure S6. All six satellite
resonances are detected. The line widths of the satellites are,
at 2 Hz, significantly broader than is observed in the case of
1J (13C-57Fe) or 1J (13C-187Os) on account of the quadrupole
5
moment of 99Ru. Coupling with 101Ru (s ) /2, abundance
17.1%) is not observed, on account of its much higher
quadrupole moment (0.44). The same explanation is valid
for 189Os (s ) 3/2, abundance 16.1%, Q ) 0.8 × 10-24 cm2).
The observed 13C NMR parameters are reported in Table 6.
In the triad Fe, Ru, Os, the chemical shifts decrease down
the group, and at the same time, the reduced coupling
constants increase. A similar trend of chemical shifts is
obtained in DFT calculations.23 The larger change in J
between Ru and Os compared to the change between Fe and
Ru is seen to be due mainly to relativistic effects.49,50 A more
detailed analysis will also involve the pairs [M(CO)4]2+
(M ) Pd, Pt),43 [M(CO)5Cl]2+ (M ) Rh, Ir),14 and [Hg-
(CO)2]2+.44
Nevertheless, a simple interpretation is possible: The
strength of the M-C bond and, to a less noticeable degree,
of the C-O bond in the [M(CO)6]2+ cations increases in the
order of Fe20 < Ru < Os. As the covalent character of the
M-C bond increases, the band separations between ν1 and
ν3 and between ν2 and ν4 increase, as seen in Figure 7.
Support comes from the thermal stabilities of [M(CO)6]-
[SbF6]2 (M ) Fe, Ru, Os), studied by DSC, discussed in the
first part of this publication and from DFT calculations of
fM-C, included in Table 6.
Summary and Conclusions
With the conjugate Brønsted-Lewis superacid HF/
35-41
SbF5
as the reaction medium, two divergent synthetic
approaches, reductive carbonylation9-12,18,19 of M(SO3F)3 (M
) Ru, Os)29-31 or OsF6 and oxidative carbonylation of Fe-
54
(CO)5 with XeF2 as the oxidizer,20 allow the syntheses of
two isostructural triads: [M(CO)6][Sb2F11]2 and [M(CO)6]-
[SbF6]2 (M ) Fe,20 Ru, Os).18,19 A previously reported
rationale for reductive carbonylation11,12 is now extended to
20,54
A previously reported horizontal correlation for isoelec-
tronic, octahedral [M(CO)6] complexes with M ) W, Re+,
Os2+, and Ir3+ in the 5d series was used to generate a plot
the oxidative carbonylation of Fe(CO)5
The compounds of the composition [M(CO)6][SbF6]2 show
thermal stabilities that range from 180 °C (M ) Fe) to 350
(Scheme 1).
4204 Inorganic Chemistry, Vol. 44, No. 12, 2005

[M(CO)6][SbF6]2 and [M(CO)6][Sb2F11]2 (M ) Fe, Ru, Os)
°C (M ) Os) before CO evolution occurs, according to a
DSC study (see Figure 2), supplemented by a gas-phase IR
analysis of volatile products.
As seen in Table 1, the members in each triad [M(CO)6]-
[Sb2F11]2 (monoclinic, P21/n; Figure 3) and [M(CO)6][SbF6]2
(tetragonal, P4/mnc; Figure 4) are isostructural; have very
similar unit cell parameters, including unit cell volumes and
identical bond parameters for the anions, [Sb2F11]- (see
Figure 2) and [SbF6]- (Tables 2 and 3), and have vibrational
properties (Table 4) that are invariant of M.
The structural properties of the [M(CO)6]2+ (M ) Fe,20
Ru, Os) are independent of the counteranion. Very similar,
slightly shorter Fe-C internuclear distances (Tables 2 and
3) are partly compensated by stronger interionic M‚‚‚C
contacts for M ) Ru, Os (Table 4), resulting in similar sizes
of the cations, apparent from the listed nonbonding C‚‚‚C
contacts in the octahedral coordination spheres (Table 7).
Dependence of spectroscopic properties on the metal is
found for (i) 13C NMR data, summarized in Table 6 and
illustrated in Figure 6; (ii) M-C skeletal vibrations, listed
in Table 4 and shown in Figure 7; and (iii) small shifts in
the CO stretching range (see Figure 7 and Table 4). Trends
in ν(CO) and ν(MC) suggest increased bond strength of the
M-C bond and covalency in [M(CO)6]2+ (M ) Fe, Ru, Os)
with increased nuclear charge of M.
The results reported here allow the conclusion that the
octahedral [M(CO)6]2+ cations (M ) Fe, Ru, Os) should also
be generated in other Brønsted-Lewis superacids35,36 and
stabilized by counteranions other than the fluoroantimonates,
which have been used exclusively so far.9-16,18-20,42-46 As a
first result in this direction, the synthesis and structural
characterization of [M(CO)6][BF4]2 (M ) Fe, Ru, Os)100 is
reported as part 2 of this study.
Acknowledgment. Financial support by Deutsche For-
schungsgemeinschaft, Fonds der Chemischen Industrie,
Natural Sciences and Engineering Research Council of
Canada, North Atlantic Treaty Organization, and Alexander
v. Humboldt Foundation is gratefully acknowledged. We are
indebted to Ms. M. Litz (Universita¨t Wuppertal) for typing
the manuscript, Ms. S. Harbour (UBC) for editing and Ms.
E. Varty (UBC) for illustrations. Degussa-Hu¨ls is thanked
for the gift of Ru and Os powders.
Supporting Information Available: Detailed crystallographic
data, information on structure solution, atomic coordinates and
thermal parameter, bond distances and bond angles, listings of the
vibrational bands, structural drawings and figures of the vibrational
spectra for [M(CO)6][Sb2F11]2 and [M(CO)6][SbF6]2 (M ) Ru, Os),
and X-ray crystallographic files. This material is available free of
charge via the Internet at http://pubs.acs.org.
The principal experimental and calculated21 structural and
spectroscopic properties of the intrinsically stable, octahedral
[M(CO)6]2+ cations (M ) Fe,20 Ru, Os) are summarized in
Table 6.
IC040115U
(100) Finze, M.; Bernhardt, E.; Willner, H.; Lehmann, C. W.; Aubke, F.
Inorg. Chem. 2005, 44, 4206-4214.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4205