Hexakis(carbonyl)iron(II) undecafluorodiantimonate(V), [Fe(CO)6][Sb2F11]2, and -hexafluoroantimonate(V), [Fe(CO)6][SbF6]2, their syntheses, and spectroscopic and structural characterization by single crystal X-ray diffraction and normal coordinate analysis

Article abstract of DOI:10.1021/ja990958y

Hexakis(carbonyl)iron(II) undecafluorodiantimonate(V), [Fe(CO)₆][Sb₂F₁₁]₂, is conveniently prepared by the oxidative carbonylation of Fe(CO)₅ with XeF₂ as external oxidizer in the conjugate Bronsted-Lewis superacid HF-SbF₅. The colorless compound crystallizes from the reaction medium in high purity. The molecular structure is obtained by single-crystal X-ray diffraction. The cation is a regular octahedron, while the vertex-shared di-octahedral [Sb₂F¹¹]^- anion is distorted from D₄(h) symmetry by bending and rotational processes, due to significant interionic interactions, primarily of the F · · · C type. Washing of [Fe(CO)₆][Sb₂F₁₁]₂ with anhydrous HF results in an unusual elution of SbF₅ and the quantitative conversion to hexakis(carbonyl)iron(II) hexafluoroantimonate(V) [Fe(CO)₆][SbF₆]₂. The molecular structure of the salt shows octahedral ions with slight tetragonal distortions for the cation (elongation) and the anion (compression). Both salts are thermally stable up to 150 °C. The averaged bond distances and the vibrational wavenumbers of [Fe(CO)₆]²⁺ are nearly identical in both compounds. The [Fe(CO)₆]²⁺ cation, the first and so far only isolated and structurally characterized dipositive, superelectrophilic carbonyl cation formed by a 3d-metal, is further characterized by a normal coordinate analysis (NCA). The obtained force constants are compared to those of the isoelectronic molecule Cr(CO)₆. Changes in π-back-bonding affect the F(CO/CO) and F(CO/MC) interaction force constants more strongly than the stretching force constants F(CO) and F(MC). All 13 fundamentals of [Fe(CO)₆]²⁺ are detected and assigned with the help of the data obtained from the normal coordinate analysis and density functional calculations published previously. The electronic ground state ¹A₁(g) of the [Fe(CO)₆]²⁺ cation is established by magnetic susceptibility measurements of polycrystalline [Fe(CO)₆][SbF₆]₂ and [Fe(CO)₆][Sb₂F₁₁]₂ between 2 and 300 K. The magnetic impurity formed during synthesis is identified as Fe[SbF₆]₂ which has iron(II) in high spin (⁵T₂(g)) ground state. Consistent with a diamagnetic ground state are the single line in the ⁵⁷Fe Mossbauer spectrum (i.s. = -0.003(8) mm s^-1 relative to α-Fe), obtained on polycrystalline samples and the single sharp line in the ¹³C NMR spectrum in DF solution at 178 ppm with J (⁵⁷Fe- ¹³C) of 19.2 Hz.

Full text of DOI:10.1021/ja990958y

7188
J. Am. Chem. Soc. 1999, 121, 7188-7200
Hexakis(carbonyl)iron(II) Undecafluorodiantimonate(V),
[Fe(CO)6][Sb2F11]2, and -Hexafluoroantimonate(V), [Fe(CO)6][SbF6]2,
Their Syntheses, and Spectroscopic and Structural Characterization by
Single Crystal X-ray Diffraction and Normal Coordinate Analysis
Eduard Bernhardt,†,‡ Bianca Bley,† Rudolf Wartchow,† Helge Willner,*,†,‡ Eckhard Bill,§
Peter Kuhn,§ Iona H. T. Sham,| Matthias Bodenbinder,| Raimund Bro1chler,| and
Friedhelm Aubke*,|
Contribution from the Institut fu¨r Anorganische Chemie der UniVersita¨t, Callinstr. 9,
D-30167 HannoVer, Germany, Max Planck Institut fu¨r Strahlenchemie, Stiftstrasse 34-36,
D-45470 Mu¨lheim, Germany, and Department of Chemistry, UniVersity of British Columbia,
VancouVer, B. C., V6T 1Z1, Canada
ReceiVed March 23, 1999
Abstract: Hexakis(carbonyl)iron(II) undecafluorodiantimonate(V), [Fe(CO)6][Sb2F11]2, is conveniently prepared
by the oxidative carbonylation of Fe(CO)5 with XeF2 as external oxidizer in the conjugate Brønsted-Lewis
superacid HF-SbF5. The colorless compound crystallizes from the reaction medium in high purity. The
molecular structure is obtained by single-crystal X-ray diffraction. The cation is a regular octahedron, while
the vertex-shared di-octahedral [Sb2F11]- anion is distorted from D4h symmetry by bending and rotational
processes, due to significant interionic interactions, primarily of the F‚‚‚C type. Washing of [Fe(CO)6][Sb2F11]2
with anhydrous HF results in an unusual elution of SbF5 and the quantitative conversion to hexakis(carbonyl)-
iron(II) hexafluoroantimonate(V) [Fe(CO)6][SbF6]2. The molecular structure of the salt shows octahedral ions
with slight tetragonal distortions for the cation (elongation) and the anion (compression). Both salts are thermally
stable up to 150 °C. The averaged bond distances and the vibrational wavenumbers of [Fe(CO)6]2+ are nearly
identical in both compounds. The [Fe(CO)6]2+ cation, the first and so far only isolated and structurally
characterized dipositive, superelectrophilic carbonyl cation formed by a 3d-metal, is further characterized by
a normal coordinate analysis (NCA). The obtained force constants are compared to those of the isoelectronic
molecule Cr(CO)6. Changes in π-back-bonding affect the FCO/CO and FCO/MC interaction force constants more
strongly than the stretching force constants FCO and FMC. All 13 fundamentals of [Fe(CO)6]2+ are detected
and assigned with the help of the data obtained from the normal coordinate analysis and density functional
calculations published previously. The electronic ground state 1A1g of the [Fe(CO)6]2+ cation is established by
magnetic susceptibility measurements of polycrystalline [Fe(CO)6][SbF6]2 and [Fe(CO)6][Sb2F11]2 between 2
and 300 K. The magnetic impurity formed during synthesis is identified as Fe[SbF6]2 which has iron(II) in
high spin (5T2g) ground state. Consistent with a diamagnetic ground state are the single line in the 57Fe Mo¨ssbauer
spectrum (i.s. ) -0.003(8) mm s-1 relative to R-Fe), obtained on polycrystalline samples and the single sharp
line in the 13C NMR spectrum in DF solution at 178 ppm with J (57Fe-13C) of 19.2 Hz.
Introduction
polyhedra in metal carbonyl chemistry.1-3 In the absence of
any highly reduced iron carbonylates,4 the three species have
remained the only thermodynamically stable, mononuclear,
homoleptic iron carbonyl complexes. The oxidation state of Fe
ranges from +2 to -2. The history of their discoveries spans
more than a century.
Iron forms three homoleptic, mononuclear 18-electron car-
bonyl species,1-3 which are formally connected by 2-electron
reduction-oxidation and CO-addition-elimination reactions as
illustrated below:
-
[Fe(CO)6]
7
Fe(CO)5
-8 [Fe(CO)4]2- (1)
-2e
+2e
2+
The oldest member of the triad, Fe(CO)5, was first reported
in 1891 by Mond, Langer, and Quincke.5 Its synthesis followed
+CO -CO
(1) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.;
Wiley: New York, 1988; pp 58, 1021.
(2) Pruchnik, F. P. Organometallic Chemistry of Transition Elements;
Plenum: New York, 1990; and references therein.
(3) Crabtree, R. H. The Organometallic Chemistry of Transition Metals,
2nd ed.; Wiley: New York, 1994.
They represent with octahedral, trigonal bipyramidal, and
tetrahedral geometries the three most common coordination
* To whom correspondence may be addressed. E-mail: aubke@
chem.ubc.ca, willner@uni-duisburg.de. Tel: (604) 822-3817.
† Institut fu¨r Anorganische Chemie der Universita¨t.
‡ New address: Anorganische Chemie, Gerhard Mercator Universita¨t,
Lotharstr. 1, D-47048 Duisburg, Germany.
(4) Ellis, J. E. AdV. Organomet. Chem. 1990, 31, 1.
(5) (a) Mond, L.; Quincke, F. Ber. Dtsch. Chem. Ges. 1891, 24, 2248.
(b) Mond, L.; Quincke, F. J. Chem. Soc. 1891, 59, 604. (c) Mond, L.;
Langer, C. J. Chem. Soc. 1891, 59, 1090.
§ Max Planck Institut fu¨r Strahlenchemie.
| University of British Columbia.
10.1021/ja990958y CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/23/1999

[Fe(CO)6][Sb2F11]2 and [Fe(CO)6][SbF6]2
J. Am. Chem. Soc., Vol. 121, No. 31, 1999 7189
one year after the accidental discovery of Ni(CO)4 by the same
authors.6 Since that time, Fe(CO)5 has been found to have a
rich chemistry1-3,7 with many industrial applications. The readily
obtainable Fe(CO)5 1-3 provides a convenient starting material
for the generation of salts with either [Fe(CO)4]2- or [Fe-
(CO)6]2+ as constituent ions, as will be discussed below.
The anion [Fe(CO)4]2- appears to have been first obtained
in 1929 by Hock and Stuhlmann8 as reported two years later.
Subsequent reports9,10 establish the reductive degradation of Fe-
(CO)5 in aqueous alkaline solution as a convenient route to
[Fe(CO)4]2-. The reduction of Fe(CO)5 by alkaline metals in
liquid ammonia11 has provided subsequently a convenient
alternative. One of the alkali metal salts, Na2[Fe(CO)4], is known
as “Collman’s reagent”.2,3 It finds extensive use as a “super
nucleophile” in organic chemistry for a wide range of oxidative
addition reactions.1-3,12
The existence of the cation [Fe(CO)6]2+ has long been
anticipated.13,14 However, early claims of its formation in a
halide abstraction reaction from Fe(CO)4X2, X ) Cl, Br, by
AlX3 at high temperatures and CO pressures15 had to be
retracted.16 Nevertheless the synthetic approach used has
produced the salts [M(CO)6][AlX4], M ) Mn, Tc, Re, X ) Br,
with the first homoleptic metal carbonyl cations.17-19 Equally
erroneous has been the reported formation of [Fe(CO)6]2+ during
the amine-catalyzed disproportionation of Fe(CO)5.20 Thus even
though related cations of the type [Fe(CO)5X]+, X ) Cl, Br,
NO, H, are known,21 [Fe(CO)6]2+ as well as other multiply
charged or “super electrophilic”22 homoleptic metal carbonyl
cations has remained elusive until recently.
Initial attempts by us to synthesize salts with octahedral
[M(CO)6]2+, M ) Fe, Ru, Os, cations have only been partly
successful. The reductive carbonylations of MX3, M ) Fe, Ru,
Os and X ) Cl or SO3F, in liquid SbF5 have succeeded for
ruthenium and osmium, where [M(CO)6][Sb2F11]2,27 M ) Ru,
Os, form cleanly and in quantitative yield. When either FeCl3
or Fe(SO3F)3 is used as a starting material, the previously known
compound Fe[SbF6]228 forms in quantitative yield. The oxidative
carbonylation of Fe(CO)5 in liquid SbF5 with either AsF5 or
Cl2 as external oxidizing agents produces [Fe(CO)6][Sb2F11]2.29
However, as will be discussed below, neither method is capable
of giving either pure or crystalline material suitable for a
complete characterization. We have subsequently developed a
new synthetic approach where the Lewis superacid SbF5 is
23,24
replaced by the conjugate superacid HF-SbF5
as reaction
medium. In addition, in the oxidative carbonylation of Fe(CO)5,
xenon difluoride is used as oxidizing agent.
As a consequence [Fe(CO)6][Sb2F11]2 is obtained in high yield
and in the form of single crystals. This permits a molecular
structure determination by X-ray diffraction. The use of
anhydrous hydrogen fluoride as solvent allows the facile
conversion to [Fe(CO)6][SbF6]2 according to the following:
[Fe(CO)6][Sb2F11]2 + 2HF anh HF8
[Fe(CO)6][SbF6]2 + 2(HF‚SbF5) (2)
After repeated washings with HF, [Fe(CO)6][SbF6]2 is formed
as single crystals and its molecular structure is readily obtained.
The stabilization of [Fe(CO)6]2+ by two different counter-
anions, [Sb2F11]- and [SbF6]-, the latter an octahedral species
with a simple vibrational spectrum, permits a complete vibra-
tional analysis of the cation, including a normal coordinate
A new synthetic methodology and the extensive use of
superacids23,24 as reaction media, recently reviewed by Weber25a
and two of us,25b has allowed the generation of a wide range of
homoleptic carbonyl cations of metals in the groups 8-12,
analysis (NCA), assisted by recent ab initio calculations.30,31
32,33
among them [Fe(CO)6]2+. Of the superacidic media the Lewis
Comparisons are possible with isoelectronic Cr(CO)6
and
[Mn(CO)6]+.34 For [Re(CO)6]+ the molecular structure35,36 and
a detailed vibrational study37 are reported. There are also recent
23,24
superacid SbF5
has found exclusive use and the superacid
anion [Sb2F11]-,26 formed by self-assembly in liquid SbF5, has
permitted the isolation of thermally stable salts.25b
theoretical calculations on hypothetical gaseous [M(CO)6]n+, n
g
) 1, 2, or 3 species.31,38,39
The high purity, in which both [Fe(CO)6]2+ salts are obtained,
allows temperature-dependent magnetic susceptibility measure-
ments, 57Fe Mo¨ssbauer studies, and the recording of a UV-vis
spectrum, which in turn permits a comparison to related
precedents.40 Finally a comparison of structural and spectro-
scopic parameters for the other two mononuclear iron carbonyl
species Fe(CO)541-44 and [Fe(CO)4]2- 45 to those of [Fe(CO)6]2+
will be undertaken.
(6) Mond, L.; Langer, C.; Qincke, F. J. Chem. Soc. 1890, 58, 749.
(7) Hieber, W.; Beck, W.; Braun, G. Angew. Chem. 1960, 72, 795.
(8) Hock, H.; Stuhlmann, H. Chem.-Ztg. 1931, 91, 374.
(9) Hieber, W. Naturwissenschaften 1931, 19, 1.
(10) Feigl, F.; Krumholz, P. Monatsh. Chem. 1932, 59, 314.
(11) Behrens, H. Z. Naturforsch., B: Chem. Sci. 1952, 71, 322.
(12) Collman, J. P.; Fincke, R. G.; Cawse, J. N.; Brauman, J. L. J. Am.
Chem. Soc. 1977, 99, 2515 and references therein.
(13) Abel, E. W.; Tyfield, S. P. AdV. Organomet. Chem. 1970, 8, 117.
(14) Beck, W.; Su¨nkel, K. Chem. ReV. 1988, 88, 1405.
(15) Hieber, W.; Kruck, T. Angew. Chem. 1961, 73, 580.
(16) Hieber, W.; Frey, V.; John, P. Chem. Ber. 1967, 100, 1961.
(17) (a) Fischer, E. O.; Fichtel, K. Chem. Ber. 1961, 94, 1200. (b) Fischer,
E. O.; Fichtel, K.; O¨ fele, K. Chem. Ber. 1962, 95, 249. (c) Fischer, E. O.;
O¨ fele, K. Angew. Chem. 1961, 73, 581.
(18) Hieber, W.; Lux, F.; Herget, C. Z. Naturforsch., B: Chem. Sci. 1965,
20, 1159.
(19) Hieber, W.; Kruck, T. Z. Naturforsch., B: Chem. Sci. 1961, 16,
709.
(20) Sternberg, H. W.; Friedel, A. R.; Shufler, S. L.; Wender, I.. J. Am.
Chem. Soc. 1955, 77, 2675.
(21) Iqbal, Z.; Waddington, T. C. J. Chem. Soc. 1968, 2958.
(22) The term superelectrophile, first introduced by G. A. Olah, Angew.
Chem., Int. Ed. Engl. 1993, 32, 767, is chosen to designate homoleptic
metal carbonyl cations with charges of +2 or +3.
(23) Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids; J. Wiley:
New York, 1985; and references therein.
(27) Wang, C.; Bley, B.; Balzer-Jo¨llenbeck, G.; Lewis, A. R.; Siu, S.
C.; Willner, H.; Aubke, F. J. Chem. Soc., Chem. Commun. 1995, 2071.
(28) Gantar, D.; Leban, I.; Frlec, B.; Holloway, J. H. J. Chem. Soc.,
Dalton Trans. 1987, 2379.
(29) Bley, B.; Willner, H.; Aubke F. Inorg. Chem. 1997, 36, 158.
(30) Aubke, F.; Wang, C. Coord. Chem. ReV. 1994, 137, 483.
(31) Jonas, V.; Thiel, W. Organometallics 1998, 17, 353.
(32) Jones, L. H.; McDowell, R. S.; Goldblatt, M. Inorg. Chem. 1969,
8, 2349.
(33) Jones, L. H. Inorganic Vibrational Spectroscopy; Marcel Dekker:
New York, 1971; Vol. 1.
(34) McLean, R. A. N. Can. J. Chem. 1974, 52, 213.
(35) Bruce, D. M.; Holloway, J. H.; Russel, D. R. J. Chem. Soc., Dalton
Trans. 1978, 64.
(36) Bruce, D. M.; Holloway, J. H.; Russel, D. R. J. Chem. Soc., Dalton
Trans. 1978, 1627.
(24) O’Donnell, T. A. Superacids and Acidic Melts as Inorganic
Chemical Reaction Media; V. C. H.: Weinheim, Germany, 1993; and
references therein.
(25) (a) Weber, L. Angew. Chem., Int. Ed. Engl. 1994, 33, 1077. (b)
Willner, H.; Aubke, F. Angew. Chem., Int. Ed. Engl. 1997, 36, 2402.
(26) Zhang, D.; Rettig, S. J.; Trotter, J.; Aubke, F. Inorg. Chem. 1996,
35, 6113.
(37) 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.
(38) Ehlers, A. W.; Ruiz-Morales, Y.; Baerends, E. J.; Ziegler, T. Inorg.
Chem. 1997, 36, 5031.
(39) Szilagy, R. K.; Frenking, G. Organometallics 1997, 16, 4807.
(40) Gray, H. B.; Beach, N. A. J. Am. Chem. Soc. 1963, 85, 2922.
(41) Davis, M. I.; Hanson, H. P. J. Phys. Chem. 1965, 69, 466.

7190 J. Am. Chem. Soc., Vol. 121, No. 31, 1999
Bernhardt et al.
room temperature with a Bruker FRA-106 FT Raman accessory
mounted on an optical bench of the IFS-66v instrument using the 1064
nm exciting line (∼500 mW) of a Nd:YAG laser (Adlas, DPY 301,
Lu¨beck, Germany). Crystalline samples were contained in large melting
point capillaries (Ø 2 mm) for recording spectra in the region 5000-
In summary a detailed and comprehensive study of [Fe-
(CO)6]2+ is presented here, which extends from the development
of a new synthetic approach, the structure determination with
both [Sb2F11]- and [SbF6]- as counterions to the extensive
spectroscopic characterization, including a normal coordinate
analysis.
80 cm-1 with a spectral resolution of 2 cm-1
.
(2) NMR Spectroscopy. 13C NMR spectra were obtained at room
temperature on a Bruker MSL-200 FT spectrometer operating at 50.330
MHz. A 4 mm (o.d.) PFA tube containing a saturated HF solution (with
a few percent SbF5) over solid [Fe(13CO)6][Sb2F11]2 was centered inside
a 10 mm glass NMR tube containing the lock and external reference
CDCl3 (δ ) 77.7 ppm referenced to TMS). With this sample it was
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. Anhydrous HF or DF, both containing a few percent
of SbF5, were stored 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, Cresskill, NJ). All other volatile
compounds were stored in glass containers equipped with a valve with
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 a NS 29 socket standard taper (Bohlender, Lauda, Germany)
in connection with a stainless steel NS 29 cone standard taper and a
needle valve (3762H46Y Hoke, Cresskill, NJ). Both parts were pressed
together with a metal flange and the reactor was leak tight (<10-5 mbar
L s-1) without using grease. A two part V-shaped PFA vessel with
valve was used for washing the product with anhydrous HF. Solid
materials were manipulated inside an inert atmosphere box (Braun,
Munich, Germany) filled with argon, with a residual moisture content
of less than 0.1 ppm. In an evacuable box (Mecaplex, Grenchen,
Switzerland) filled with dry nitrogen, supernatant HF/SbF5 solutions
over crystalline products were pipetted off the reactor.
(b) Chemicals. Anhydrous HF (Solvay, AG Hannover, Germany),
SbF5 (Elf Atochem, North America), Fe(CO)5 (purity not stated, BASF
Ludwigshafen, Germany), CO (99%, Linde Gas), and 13CO (99%
enriched, IC Chemicals) were obtained from commercial sources. Fe-
(CO)5 and SbF5 were purified by trap-to-trap distillation and CO by
condensation at -196 °C followed by partial re-evaporation. DF was
prepared from D2O (99.9%, Merck) and an excess of IF5 (J. T. Baker
Chemical Co.) followed by trap-to-trap distillation. XeF2 was obtained
by the photolysis of equimolar mixtures of Xe (Messer Griesheim,
Du¨sseldorf, Germany) and F2 (Solvay AG, Hannover, Germany)
according to a literature procedure.46 Fe(13CO)5 was synthesized by
repeated CO-exchange between Fe(CO)5 and 13CO in THF/NaBH4
suspension following a published procedure.47 From the volatile THF/
Fe(13CO)5 product mixture, THF was separated by trap-to-trap distil-
lation and selective absorption of THF on AlCl3 at room temperature.
The 13C content of the final product (50% yield) was about 95%. Fe-
[SbF6]2 was prepared by the oxidation of iron powder by SbF5 in SO2
as described previously.48
possible to determine the chemical shift of [Fe(13CO)6]2+
and to
solv
estimate a solubility of 5 mmol L-1 by comparing the signal intensity
with the signal of 13CDCl3 in natural abundance. However, even after
1 day of acquisition time, the S/N ratio was too low for detection of
the 57Fe satellites. For this purpose a saturated [Fe(13CO)6][Sb2F11]2
solution in deuterium fluoride, DF (containing a few percent SbF5),
was investigated in a 9 mm (o.d.) PFA tube placed in a 10 mm (o.d.)
glass NMR tube. Measurement parameters: pulse length 3 µs, time
between pulses 14 s, resolution 0.04 Hz/datapoint.
(3) UV Spectroscopy. UV-Vis spectra were recorded in the region
200-500 nm with a 1024 Diode Array spectrometer system (Spec-
troscopy International) using a CLD 30-W deuterium lamp (Hamamatsu,
Japan) as light source. Hg emission lines were used for wavelength
calibration. Single crystals of [Fe(13CO)6][Sb2F11]2 of suitable sizes were
placed in an evacuable dry quartz cell of 1 mm optical path length
(Hellma, Mu¨lheim, Germany). To reduce scattering losses of light from
the surface of the crystals, we added SbF5 to the cell in vacuo. In front
of the entrance slit of the monochromator, the crystals, immersed in
SbF5, were illuminated by the deuterium lamp via a quartz lens and a
suitable aperture. The reference spectrum was measured with the same
cell and aperture at a spot without any crystals.
(4) Mo1ssbauer Spectroscopy. The 57Fe Mo¨ssbauer spectrum of [Fe-
(CO)6][SbF6]2 was recorded using a 57Co in rhodium source (Amersham-
Buchler UK) with constant acceleration mode. Details about the
apparatus are given elsewhere.49 A polycrystalline sample (137 mg)
was distributed over an area of about 1 cm2 in an airtight polyethylene
container. The spectrum was measured at 80 K sample temperature
within several hours, and the isomer shift δ is referenced to R-Fe at
room temperature.
(5) Magnetic Susceptibility Measurements. Magnetic susceptibility
measurements of [Fe(CO)6][SbF6]2 were performed in the temperature
range 2-400 K using SQUID systems (MPMS 7, or MPMS R2
Quantum Design) with a one Tesla superconducting magnet. Polycrys-
talline samples weighing 40-220 mg were placed either in a quartz
tube (5 mm o.d., 100 mm long) flame sealed under vacuum or in a
PVC container. After the experiment a similar empty container was
measured under the same conditions.
(6) X-ray Diffraction. Under dry argon atmosphere, fragments from
single crystals of [Fe(CO)6][Sb2F11]2 and [Fe(CO)6][SbF6]2 were cleaved
and gently wedged in 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 mono-
chromatized Mo KR radiation. The systematic absences observed for
X-ray data of [Fe(CO)6][Sb2F11]2 uniquely define the probable mono-
clinic space group to be P21/n, and those for [Fe(CO)6][SbF6]2 point to
two probable space groups P4nc and Pr/mnc, of which the latter
centrosymmetric one was chosen on the basis of the successful crystal
structure determination. The structures were solved in P21/n or Pr/
mnc, respectively, by direct methods (SHELXS-86),50 and the final full-
matrix least squares refinements of all atomic parameters (SHELXL-
93)51 were carried out on a DECR Vax computer. The figures were
drawn using the DIAMOND program.52
(c) Instrumentation. (1)Vibrational Spectroscopy. Infrared spectra
were recorded at room temperature on a IFS-66v FT spectrometer
(Bruker, Karlsruhe, Germany). Two different detectors together with a
Ge/KBr or a 6 µm Mylar beam splitter operating in the region 5000-
400 or 550-80 cm-1, respectively, were used. One-hundred twenty-
eight scans were co-added for each spectrum, using an apodized
resolution of 2 or 4 cm-1. The samples were crushed between CaF2,
AgBr (Korth, Kiel, Germany), or polyethylene (Cadillac, Hannover,
Germany) disks inside the drybox. Raman spectra were recorded at
(42) Almenningen, A.; Haaland, A.; Wahl, K. Acta Chem. Scand. 1969,
23, 2245.
(43) Beagley, B.; Cruickshank, D. W. J.; Pinder, P. M.; Robiette, A. G.;
Sheldrick, A. M. Acta Crystallogr., Sect. B 1969, 25, 737.
(44) Beagley, B.; Schmidling, D. G. J. Mol. Struct. 1974, 22, 466.
(45) Teller, R. G.; Finke, R. G.; Collman, J. P.; Chin, H. B.; Bau, R. J.
Am. Chem. Soc. 1977, 99, 1104.
(49) Trautwein, A. X.; Bill, E.; Bominaar, E. L.; Winkler, H. Struct.
Bonding 1991, 78, 1.
(46) Holloway, J. H. Chem. Commun. 1966, 22.
(47) Bricker, J. C.; Payne, M. W.; Shore, S. G. Organometallics 1987,
6, 2545.
(50) Sheldrick, G. M. SHELXS-86, Program for crystal structure solution,
Universita¨t Go¨ttingen, 1986.
(51) Sheldrick, G. M. SHELXL-93, Program for crystal structure
refinement, Universita¨t, Go¨ttingen, 1993.
(48) Dean, P. A. W. J. Fluorine Chem. 1975, 5, 499.

[Fe(CO)6][Sb2F11]2 and [Fe(CO)6][SbF6]2
J. Am. Chem. Soc., Vol. 121, No. 31, 1999 7191
(d) Synthetic Reactions. (1) The synthesis of [Fe(CO)6][Sb2F11]2.
The PFA reactor described above, containing a PTFE coated magnetic
stirring bar, was charged with 0.57 g (3.4 mmol) of XeF2, 7 mL (97
mmol) of SbF5, and 5 mL (250 mmol) of HF using a stainless steel
vacuum line. The mixture was stirred at room temperature to give a
homogeneous solution. At -196 °C, 0.47 g (2.4 mmol) of Fe(CO)5
and 6 mmol of CO were admitted to the reactor. By use of an ethanol
cold bath, the content was stirred at -50 °C for 10 min. During this
time the color of the suspension changed from yellow to orange. After
warming to 0 °C the solution became homogeneous. Subsequently the
content was heated to 50 °C for 2 days without stirring and colorless
single crystals of [Fe(CO)6][Sb2F11]2 several millimeters in length were
formed. In the evacuable drybox the liquid phase was pipetted off and
the crystals were transferred into a V-shaped reactor. The crystalline
product was washed with a SbF5/HF (v/v 1:1) mixture. After removal
of all volatiles in vacuo, 1.35 g (1.2 mmol) of pure [Fe(CO)6][Sb2F11]2
was obtained (50% yield). [Fe(CO)6][Sb2F11]2 is a colorless moisture-
sensitive crystalline solid which is thermally stable to 150°. Heating
beyond this point resulted in the formation of a colorless viscous liquid,
identified as SbF5, before evolution of CO was observed on further
heating. Band positions and intensities attributed to the [Sb2F11]- anion
in [Fe(CO)6][Sb2F11]2 and [Fe(13CO)6][Sb2F11]2 are the following: IR,
νSbF, 723 (w,sh), 712(w,sh), 692(vs), 670(w,sh), 657(vs), 599(w), 492-
(vs); 2δSbF, 430(vw), δSbF; 336(s), 303(s), 277(s), 232(s), 203(w,-
sh), and 127(w); Raman, νSbF, 709.2(w), 694.5(m), 686.8(m), 670.5(s),
647.9(s), 601(vw,sh), 598.1(m), 591.3(w,sh), δSbF; 332.6(vw), 299.3-
(m), 275.5(w), 235.6(w), 226.6(w), and 207.4(vw). The isotopomer [Fe-
(13CO)6][Sb2F11]2 is synthesized in an identical manner by using enriched
Fe(13CO)5 (see ref 47) and 13CO. The 13C contents of the product were
judged to be 95%.
As the vibrational spectra in the CO-stretching region indicate,
[Fe(CO)6][Sb2F11]2 is produced by these routes; however, there
are three serious problems affecting both synthetic approaches:
(i)Bulk magnetic susceptibility measurements on [Fe(CO)6]-
[Sb2F11]2 obtained by either route reveal the presence of
substantial amounts of paramagnetic material. While pure [Fe-
(CO)6][Sb2F11]2 is found in the course of this study to be
essentially diamagnetic, commensurate with an FeII low spin
d6 configuration and a 1A1g octahedral ground state, the observed
paramagnetism is attributed to Fe[SbF6]2,28 whose magnetic
behavior will be discussed below.
(ii) The solid byproducts formulated as SbF4Cl53 and AsF3‚
SbF554 are difficult to remove from the reaction mixture. Heating
of the sample to remove SbF4Cl by sublimation results in further
irreversible decomposition to Fe[SbF6]2.28 The attempted elution
of AsF3 from the adduct AsF3‚SbF554 by repeated washing with
SO2, to produce thermally less stable SO2‚SbF5,55 results in
partial removal of SbF5 from the anion [Sb2F11]- to give
[SbF6]-.
(iii) In samples of [Fe(CO)6][Sb2F11]2 obtained by oxidation
of Fe(CO)5 with an excess of Cl2,29 an additional complication
is encountered. An IR band at 2256 cm-1 is observed, which
has been erroneously attributed to [Fe(CO)6]3+ 29
,
because [Ir-
(CO)6]3+ has the IR active fundamental ν6 (T1u) at 2254 cm-1 56
.
This band is now unambiguously identified as the CO-stretching
vibration of [ClCO]+,57-59 by a complete vibrational analysis
of this novel cation.57
(2) Synthesis of [Fe(CO)6][SbF6]2. To 1.012 g of [Fe(CO)6][Sb2F11]2
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 four times. After washing, 0.610
g of [Fe(CO)6][SbF6]2 was obtained in crystalline form, which implied
a yield of 98%. The isotopomer [Fe(13CO)6][SbF6] was obtained in an
identical manner from [Fe(13CO)6][Sb2F11]2. [Fe(CO)6][SbF6]2 is a
colorless moisture-sensitive crystalline solid which decomposed with
evolution of CO when heated beyond 150 °C. The residue was identified
as Fe[SbF6]2 by its vibrational spectrum.28 Band positions, intensities,
and assignments of the [SbF6]- anion are the following: IR, (ν1 + ν3)
1331(vw), 1310(vw); (ν2 + ν3), 1248(w), 1227(vw,sh); (ν4 + 2ν6), 683-
(w,sh); νSbF(ν3,T1u), 665(vs); δSbF(ν4, T1u), 292(vs); (ν5, T2g), 282-
(w,sh); (ν6, T2u), 196(vw,sh); Raman, νSbF(ν1,A1g) 652.6(vs), (ν2, Eg)
570.7(m), δSbF(ν5, T2g) 281.4(s).
In summary, previously reported synthetic routes to [Fe(CO)6]-
[Sb2F11]2 lead to complex product mixtures and the reported
29
separation and purification procedures25b,29 are inadequate.
Hence a substantially new synthetic method is developed here.
The failure to produce any evidence for [Fe(CO)6][Sb2F11]2 by
reductive carbonylation of either Fe(SO3F)360 or FeCl361 in liquid
SbF5, while Ru(SO3F)362 and Os(SO3F)363 are cleanly converted
to [M(CO)6][Sb2F11]2, M ) Ru, Os27 provides an important clue.
Both M(SO3F)3, M ) Ru,62 or Os,63 are low spin complexes
and have between 300 and 80 K magnetic properties consistent
with a 2T2g octahedral ground state.62,63 On the other hand, both
Fe(SO3F)360 and FeCl361 are high spin complexes with magnetic
6
properties suggestive of a A1g ground state. With both metal
σ-symmetry d-orbitals (dz2 and dx2-y2) singly occupied, reduc-
tion to Fe(II) must bring about a change in spin state before
carbonylation can occur to give octahedral [Fe(CO)6]2+ with a
1A1g ground state. This is clearly not the case, because high
spin Fe[SbF6]228 forms quantitatively. Since other potential Fe-
(II) or Fe(III) precursors such as FeCl2, FeF2, or FeF3 also have
the metal in a high spin configuration,64 it appears that neither
Results and Discussion
(a) Synthesis. As previously reported by us,29 [Fe(CO)6]-
[Sb2F11]2 may be obtained by the oxidative carbonylation
of Fe(CO)5 in liquid antimony(V) fluoride as reaction medium
with either Cl2 or AsF5 as external oxidizing agents according
to the following:
(53) Ruff, O.; Zedner, J.; Knoch, M.; Graf, H. Chem. Ber. 1902, 67,
1700.
(54) Birchall, T.; Dean, P. A. W.; Valle, B. D.; Gillespie, R. J. Can. J.
Chem. 1973, 51, 667.
(55) Moore, J. W.; Baird, H. W.; Miller, H. B. J. Am. Chem. Soc. 1968,
90, 1358.
(56) Bach, C.; Willner, H.; Wang, C.; Rettig, S. J.; Trotter, J.; Aubke,
F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1974.
(57) Bernhardt, E.; Willner, H.; Aubke, F. Angew. Chem., Int. Ed. 1999,
38, 823.
(58) Hoge, B.; Sheehy, J. A.; Prakash, G. K. S.; Olah, G. A.; Christe,
K. O. Abstracts, 12th European Symposium on Fluorine Chemistry; Berlin,
Germany, Aug. 29-Sept. 3, 1998, Vol. B39.
(59) Bausch, J. W.; Prakash, G. K. S.; Olah, G. A. J. Am. Chem. Soc.
1991, 113, 3205.
(60) Goubeau, J.; Milne, J. B. Can. J. Chem. 1967, 45, 2321.
(61) Starr, C.; Bitter, F.; Kaufmann, A. R. Phys. ReV. 1940, 58, 977.
(62) Leung, P. C.; Aubke, F. Can. J. Chem. 1984, 62, 2892.
(63) Leung, P. C.; Wong, G. B.; Aubke, F. J. Fluorine Chem. 1987, 35,
607.
1 atm CO,SbF5
Fe(CO)5 + Cl2 + CO + 6SbF5
8
40-80 °C
[Fe(CO)6][Sb2F11]2 + 2SbF4Cl (3)
or
1 atm CO,SbF5
Fe(CO)5 + AsF5 + CO + 5SbF5
8
60-80 °C
[Fe(CO)6][Sb2F11]2 + AsF3‚SbF5 (4)
(52) Brandenburg, K. DIAMOND, Information system fu¨r Kristallstruk-
turen, Universita¨t Bonn, 1996.
(64) Figgis, B. N.; Lewis, J. Prog. Inorg. Chem. 1964, 6, 37.

7192 J. Am. Chem. Soc., Vol. 121, No. 31, 1999
Bernhardt et al.
solvolytic nor reductive carbonylation in liquid SbF5, the two
most effective synthetic methods,25b will be successful here.
On the other hand, in the reductive carbonylation of M(SO3F)3,
M) Ru, Os, to [M(CO)6]2+ 27 no change in spin state is
necessary. In addition, compounds of the type M[SbF6]2, M )
Ru, Os, analogous to Fe[SbF6]2 appear to be unknown.65 This
leaves the oxidative carbonylation of diamagnetic Fe(CO)566 as
the only promising alternative. To overcome the above-
mentioned difficulties when AsF5 or Cl2 are used as external
oxidizing agents,29 two drastic departures of previous practices
are implemented; xenon(II) fluoride, XeF2, is used as oxidizing
characterized example, where a homoleptic superelectrophilic
metal carbonyl cation is stabilized by [SbF6]- as counterion
rather than by the ubiquitous [Sb2F11]-.25b The conversion of
[Fe(CO)6][Sb2F11]2 into the corresponding [SbF6]- salt may also
be affected by washing with SO2 or HSO3F and even by
controlled thermal degradation. However, none of these alterna-
tive routes is clean and produces a crystalline material. How
far the novel conversion of [Sb2F11]- into [SbF6]- salts described
here may be applied to other [M(CO)n][Sb2F11]m25b salts remains
to be seen. As far as the substitution of liquid SbF5 by the
23,24
conjugate superacid HF-SbF5
is concerned, preliminary
agent and SbF5 is replaced by the conjugate superacid HF-
indications are that most carbonylation reactions reviewed by
us recently25b are more easily performed in HF-SbF5. While
lower reaction temperatures, shorter reaction times, reduced
separation problems, and the formation of crystalline reaction
products are clear advantages, there is an important disadvan-
tage: glass is no longer tolerable as construction material for
reactors and vacuum lines and its replacement by metal or better
by fluorocarbon polymers is necessary. The improved synthesis
of [Fe(CO)6][Sb2F11]2 reported here is experimentally far more
demanding than the previously described approach.29 It requires
rather specialized and more expensive equipment, as discussed
in the Experimental Section. The technical and experimental
difficulties encountered are similar to those described previously
when using superacids and in particular anhydrous HF73 or HF-
SbF5 in synthetic applications.23,24
23,24
SbF5
as strongly ionizing reaction medium. The overall
reaction proceeds according to the following:
1atm CO,50°C
Fe(CO)5 + XeF2 + CO + 4SbF5
8
HF-SbF5,2d
[Fe(CO)6][Sb2F11]2 + Xe (5)
The removal of the volatile byproduct Xe presents no problem,
and [Fe(CO)6][Sb2F11]2 is obtained in the form of sizable
colorless crystals, suitable for spectroscopic studies and single-
crystal X-ray diffraction.
The exact course of the formation reaction of [Fe(CO)6]-
[Sb2F11]2 is unclear. Initial formation of Fe(CO)4F2 or the ionized
species [Fe(CO)5F]+ and [Fe(CO)4F]+ may be postulated, even
though neither species is isolable.67 Related compounds such
as Fe(CO)4X2, X ) Cl, Br, or I,68-70 or ions such as
[Fe(CO)5X]+, X ) Cl, Br, NO,21 are known. Attempts to obtain
information on the intermediates in HF-SbF5 solution by 13C
or 19F NMR are hindered by line broadening on account of the
presence of paramagnetic Fe2+ species. It appears that, even
with improved synthetic conditions, the formation reaction of
Both [Fe(CO)6][Sb2F11]2 and [Fe(CO)6][SbF6]2 have compa-
rable thermal stabilities to approximately 150 °C. Their modes
of decomposition differ. When [Fe(CO)6][Sb2F11]2 is heated to
150 °C, a clear colorless liquid, identified as SbF5 by its
spectroscopic and physical properties,73 is formed initially. On
further heating, the evolution of CO is noted. The initial
decomposition may be formulated as the following:
28
Fe[SbF6]2 remains in competition with that of [Fe(CO)6]-
[Fe(CO)6][Sb2F11]2 {>140 °C} [Fe(CO)6][SbF6]2 + 2SbF5 (6)
[Sb2F11]2. However, with the use of HF-SbF5 the latter is now
formed in a much higher yield than before,29 judging by the
relative intensities of the bands due to CO-stretching vibrations.
[Fe(CO)6][Sb2F11]2 is isolated in form of single crystals and is
cleanly separated from the byproduct Fe[SbF6]2. The isolation
of single crystals of [Sb2F11]- salts of homoleptic superelec-
trophilic metal carbonyl cations from HF-SbF5, reported here
for the first time, is not restricted to [Fe(CO)6][Sb2F11]2. Two
recent, so far unpublished examples involve the isostructural
pairs [M(CO)4]2+, M ) Pd, Pt,71 and [M(CO)6]2+, M ) Ru,
Os,72 all with [Sb2F11]- as anion. The latter study completes
the structural characterization of the triad [M(CO)6][Sb2F11]2,
M ) Fe, Ru, Os. All three salts are found to be isostructural.72
The use of anhydrous hydrogen fluoride as a protonic solvent
has an additional unexpected advantage: repeated washing of
crystalline [Fe(CO)6][Sb2F11]2 allows its transformation into the
[SbF6]- salt, which is again obtained in the form of single
crystals, suitable for a molecular structure determination (see
eq 2). The salt [Fe(CO)6][SbF6]2 becomes the first fully
25 °C
The subsequent step is irreversible and proceeds according to
the following:
[Fe(CO)6][SbF6]2 >140-150 °C8 Fe[SbF6]2 + 6CO (7)
The second decomposition step is more easily studied when
starting from [Fe(CO)6][SbF6]2, where formation of SbF5 is not
observed. The approximate decomposition rates in mole % per
minute increase from 0.01 at 140-150 °C to 0.06 at 160-170
°C, to 0.2 at 170-180 °C, to 1 at 190-200 °C, and to ∼5 at
200-205 °C. From these data a rough estimate of the Arrhenius
activation energy Ea of 180-210 kJ mol-1 and of the pre-
exponential Arrhenius factor of 1018 s-1 is obtained with ∆Sq
approximately 100 J mol-1 K-1. The solid residue is identified
as Fe[SbF6]2 by its vibrational spectrum.28 No intermediates of
the type [Fe(CO)n][SbF6]2 with n between 1 and 5 are detectable.
Hence, the oxidative carbonylation of Fe(CO)5 in either SbF5
or HF-SbF5 and subsequent thermal decomposition of the
intermediates provide an alternative route to Fe[SbF6]2. How-
ever, this rather involved procedure offers no advantage over
other previously reported methods.28,48,65
(65) Aubke, F.; Cader, M. S. R.; Mistry, F. In Synthetic Fluorine
Chemistry; Olah, G. A., Chambers, R. D., Prakash, G. K. S., Eds.: J.
Wiley: New York, 1992, p 43.
(66) Landolt-Bo¨rnstein. Zahlenwerte und Funklonen in Naturwissen-
schaften und Technik; Springer-Verlag: Berlin, Germany, 1976; Vol. II, p
209.
(67) Doherty, N. M.; Hoffman, N. W. Chem. ReV. 1991, 91, 553.
(68) Hieber, W.; Wirsching, A. Z. Anorg. Allg. Chem. 1940, 245, 35
(69) Hieber, W.; Lagally, H. Z. Anorg. Allg. Chem. 1940, 245, 295.
(70) Hieber, W.; Bader, C. Ber. Dtsch. Chem. Ges. 1928, 61, 1717.
(71) Bodenbinder, M.; Bro¨chler, R.; Rettig, S. J.; Trotter, J.; Willner,
H.; Aubke, F. Inorg. Chem. Manuscript in preparation.
(72) Bach, C.; Bernhardt, F.; Willner, H.; Wartchow, R.; Aubke, F. Inorg.
Chem. Manuscript in preparation.
A brief comment concerns the possible existence of [Fe-
(CO)6]3+. As discussed already, its suggested formation during
29
the oxidation of Fe(CO)5 with Cl2 is clearly in error. All
(73) (a) Dove, M. F. A.; Clifford, A. F. In Chemistry in Non-Aqueous
Ionizing SolVents; Jander, J., Spandau, H., Addison, C. C, Eds.; Viehweg
Publ.: Wiesbaden, Germany, 1971; Vol. 2.1. (b) Kemmitt, R. D. W.; Sharp,
D. W. A. AdVances in Fluorine Chemistry; Stacey, M., Tatlow, J. C., Sharpe,
A. G., Eds.; Butterworth: London, UK, 1965; Vol. 4, p 210.

[Fe(CO)6][Sb2F11]2 and [Fe(CO)6][SbF6]2
J. Am. Chem. Soc., Vol. 121, No. 31, 1999 7193
Table 1. Crystallographic Data for the Structure Determination of
[Fe(CO)6][Sb2F11]2 and [Fe(CO)6][SbF6]2 at 300 K
Table 2. Selected Bond Distances [Å] and Angles [deg] for
[Fe(CO)6][Sb2F11]2 and [Fe(CO)6][SbF6]2
[Fe(CO)6][Sb2F11]2
[Fe(CO)6][SbF6]2
[Fe(CO)6][Sb2F11]2
Fe-C1 (2×)
Fe-C2 (2×)
Fe-C3 (2×)
Sb1-F1
1.911(5) C1-O1 (2×)
1.912(5) C2-O2 (2×)
1.910(5) C3-O3 (2×)
2.053(3) Sb2-F1
1.850(3) Sb2-F7
1.835(3) Sb2-F8
1.842(3) Sb2-F9
1.824(3) Sb2-F10
1.869(3) Sb2-F11
1.103(5)
1.102(5)
1.107(5)
1.998(3)
1.836(4)
1.864(3)
1.847(3)
1.843(3)
1.810(3)
90.8(2)
empirical formula
formula weight
crystal system
space group
color of crystal
a [Å]
C6F22FeO6Sb4
1128.9
C6F12FeO6Sb2
695.4
tetragonal
Nr.128, P 4/mnc
colorless
8.258(1)
8.258
12.471(2)
90
850.5(2)
2
monoclinic
Nr.14, P 21/n
colorless
9.751(1)
12.457(1)
10.542(1)
110.63(1)
1198.4(2)
2
Sb1-F2
Sb1-F3
Sb1-F4
b [Å]
c [Å]
Sb1-F5
Sb1-F6
â [deg]
C1-Fe-C2 (2×)
C2-Fe-C3 (2×)
C3-Fe-C1 (2×)
C1-Fe-C1
C3-Fe-C3
89.2(2)
88.9(2)
89.8(2)
180
180
C1-Fe-C2 (2×)
C2-Fe-C3 (2×)
C3-Fe-C1 (2×)
C2-Fe-C2
V [Å3]
91.1(2)
90.2(2)
180
Z
goodness-of-fit on F2
R1 [I > 2σ(I)]
wR2
0.843
0.0282
0.0532
1.03
0.0259
0.0668
Fe-C1-O1 (2×) 178.1(4)
Fe-C2-O2 (2×) 177.5(4)
Fe-C3-O3 (2×) 179.4(4)
F1-Sb1-F3
F1-Sb1-F4
F1-Sb1-F5
F1-Sb1-F6
F2-Sb1-F3
F2-Sb1-F4
F2-Sb1-F5
F2-Sb1-F6
F3-Sb1-F4
F4-Sb1-F5
F5-Sb1-F6
F6-Sb1-F3
F1-Sb1-F2
F3-Sb1-F5
F4-Sb1-F6
Sb1-F1-Sb2
86.2(2)
86.2(1)
84.3(2)
83.3(1)
96.4(2)
96.6(2)
93.1(2)
94.0(2)
89.7(2)
92.3(2)
89.4(2)
87.0(2)
176.2(1)
170.1(2)
169.2(1)
148.5(2)
F1-Sb2-F8
F1-Sb2-F9
F1-Sb2-F10
F1-Sb2-F11
F7-Sb2-F8
F7-Sb2-F9
F7-Sb2-F10
F7-Sb2-F11
F8-Sb2-F9
F9-Sb2-F10
F10-Sb2-F11
F11-Sb2-F8
F1-Sb2-F7
F8-Sb2-F10
F9-Sb2-F11
84.8(1)
87.2(1)
87.3(2)
85.7(2)
92.0(2)
91.4(2)
95.7(2)
95.8(2)
88.4(2)
86.8(2)
92.4(2)
91.4(2)
176.6(2)
171.0(2)
172.9(2)
subsequent attempts to oxidize [Fe(CO)6]2+ in HF solution with
XeF2, F2, or NiF474 have failed. As discussed below, magnetic
measurements between 4 and 300 K on impure samples of [Fe-
(CO)6][Sb2F11]2 are consistent with the presence of Fe[SbF6]2
as the only paramagnetic species in the reaction mixture. Hence
[Ir(CO)6]3+ 56 remains as the sole homoleptic carbonyl cation
with the metal in a 3+ oxidation state.
The enormous advantage the new synthetic methodology
holds over the previously reported methods (reactions 3 and
4),29 in particular the use of HF-SbF5, is most convincingly
demonstrated by the determination of the molecular structures
of both [Fe(CO)6][Sb2F11]2 and [Fe(CO)6][SbF6]2 to be discussed
next. In summary, the [Fe(CO)6]2+ cation occupies a unique
position among the homoleptic metal carbonyl cations for three
reasons:
[Fe(CO)6][SbF6]2
Fe-C1 (2×)
Fe-C2 (4×)
Sb-F1 (4×)
C1-Fe-C2 (8×)
C1-Fe-C1
1.917(7) C1-O1 (2×)
1.903(6) C2-O2 (4×)
1.852(3) Sb-F2 (2×)
1.096(9)
1.114(8)
1.832(3)
90
180
(i) It is the first isolated superelectrophilic carbonyl cation
formed by a 3d-metal.
90
180
C2-Fe-C2 (4×)
C2-Fe-C2 (2×)
(ii) It is so far the only superelectrophilic carbonyl cation25b
Fe-C1-O1 (2×) 180
Fe-C2-O2 (4×) 178.9(5)
with [SbF6]- as well as [Sb2F11]- as counterions.
F1-Sb-F1 (2×)
F1-Sb-F2 (4×)
F1-Sb-F1 (2×)
88.3(1)
87.2(1)
174.4(2)
F1-Sb-F1 (2×)
F1-Sb-F2 (4×)
F2-Sb-F2
91.9(1)
92.8(1)
180
(iii) It is, as the next sections will show, the most extensively
characterized homoleptic carbonyl cation to date.
The ability of both [Sb2F11]- and [SbF6]- to function as
counterions for [Fe(CO)6]2+ suggests that both anions are
comparably weak nucleophiles. This conclusion has been
reached by us some time ago,75 based on 119Sn Mo¨ssbauer data
for dimethyltin(IV) salts of strong protonic acids and superacids.
In addition, the lattice energy should be larger for [Fe(CO)6]-
[SbF6]2 than for the [Sb2F11]- salt, considering anion sizes only.
Hopefully more [M(CO)n][SbF6]m salts can be obtained by the
novel conversion reaction described here. This will certainly
simplify the vibrational analysis of the salts.
(b) The Crystal and Molecular Structure of [Fe(CO)6]-
[Sb2F11]2 and [Fe(CO)6][SbF6]2. Crystallographic data and
details on the structure determinations are collected in Table 1.
Selected bond distances for both salts are listed in Table 2. The
molecular structures of [Fe(CO)6][Sb2F11]2 and [Fe(CO)6]-
[SbF6]2, showing a formula unit and the numbering system used,
are found in Figures 1 and 2.
[Fe(CO)6][SbF6]2 crystallizes in a tetragonal space group P4/
mnc (no. 128). Not surprisingly both the [SbF6]- anion and the
[Fe(CO)6]2+ cation show very slight distortion from regular
octahedral geometry. The anion (site symmetry 222) is com-
pressed, while the cation, judging by the Fe-C bond distances,
Figure 1. The molecular structure of [Fe(CO)6][Sb2F11]2: formula unit,
50% probability ellipsoids.
is elongated along the c-axis (site symmetry 4/m). The structure
type is related to the CaF2 prototype, with the [SbF6]- occupying
all available tetrahedral holes.
[Fe(CO)6][Sb2F11]2 crystallizes in the monoclinic spacegroup
P21/n (no. 14). All other so far structurally characterized
[Sb2F11]- salts with dipositive homoleptic metal carbonyl cations
belong to this space group as well. The list includes in addition
to [Fe(CO)6][Sb2F11]2 the isostructural pair [M(CO)6][Sb2F11]2,
M ) Ru, Os,72 the group 10 salts [M(CO)4][Sb2F11]2, M ) Pd,
Pt71 and in group 12 [Hg(CO)2][Sb2F11]2.76 In the group 8
compounds [M(CO)6][Sb2F11]2, M ) Fe, Ru, Os, as well as in
(74) (a) Court, T. L.; Dove, M. F. A. J. Chem. Soc., Dalton Trans. 1973,
1995. (b) Zemva, B.; Lutar, K.; Chacon, L.; Allman, J.; Shen, C.; Bartlett,
N. J. Am. Chem. Soc. 1995, 117, 10025.
(75) Mallela, S. P.; Yap, S.; Sams, J. R.; Aubke, F. Inorg. Chem. 1986,
25, 4327.

7194 J. Am. Chem. Soc., Vol. 121, No. 31, 1999
Bernhardt et al.
Figure 2. The molecular structure of [Fe(CO)6][SbF6]2: formula unit,
50% probability.
[Hg(CO)2][Sb2F11]2,76 the cations have crystallographic inversion
symmetry. Hence, both [Sb2F11]- anions are symmetry related
and have identical bond parameters. This is not the case for the
anion pairs in [M(CO)4][Sb2F11]2,71 M ) Pd, Pt, and for [Ir-
(CO)5Cl][Sb2F11]256 where two structurally different [Sb2F11]-
anions are encountered in all three compounds.
In all cases discussed here, the [Sb2F11]- ions are distorted
from ideal D4h symmetry, as, for example, found in [H3F2]-
[Sb2F11].77 There are primarily two distortional processes
involved:
Figure 3. Selected significant interionic contacts in [Fe(CO)6][Sb2F11]2.
The Fe-Cavg bond distances of 1.908(6) and 1.911(5) Å are
unusually long compared to the qu value of 1.801 Å based on
2572 data contained in the Cambridge index.81 Agreement with
bond distances from ab initio calculations is excellent to fair.
In the study by Jonas and Thiel31 Fe-C values of 1.903 and
1.905 Å are obtained for [Fe(CO)6]g2+, depending on the
computational method employed. A DFT study by Ziegler et
al.38 reports an Fe-C bond length of 1.895 Å. Calculated C-O
bond distances fall in both studies between 1.12938 and 1.14031
Å and are in fact longer than in free CO.80 This underscores
the point that, in compounds of this type with very strong C-O
bonds, accurate bond distances are equally difficult to determine
and to calculate.
(i) bending of the two SbF5 moieties about the bridging F
atom, expressed in terms of a Sb-F-Sb bridge angle, and
(ii) rotation of the two SbF4 planar groups relative to each
other, expressed in terms of a dihedral angle.
For [Fe(CO)6][Sb2F11]2, bridge and dihedral angles of 148.5°
and 36.8° are found, respectively. Identical bridge- and dihedral
angles are found for the isostructural salts [M(CO)6][Sb2F11],
M ) Ru, Os. Very similar distortions are found for [H3O]-
[Sb2F11],26 which has three crystallographically different [Sb2F11]-
anions in the unit cell of a rather complex polymeric structure.
Here the cause for the distortions of the [Sb2F11]- anions and
their dissimilarity is seen in various types of interionic asym-
metric O-H‚‚‚F hydrogen bonds.26 There is an interesting
parallel between the cations [H3O]+ and [Fe(CO)6]2+: Both form
thermodynamically stable, structurally characterized salts with
[Sb2F11]- 26 as well as with [SbF6]- 78,79 as counteranions. In
both types of salts the [Sb2F11]- anions are distorted in a similar
manner, while the crystal structures of [Fe(CO)6][SbF6]2 and
[H3O][SbF6]79 indicate tetragonal and cubic symmetry, respec-
tively, and the [SbF6]- anions depart only slightly from Oh
symmetry.79
The octahedral cations in [Fe(CO)6][Sb2F11]2 and in [Fe(CO)6]-
[SbF6]2 are virtually identical. The average bond distances for
the [Sb2F11]- salt, Fe-Cavg of 1.911(5) Å and C-Oavg of 1.104-
(5) Å, compare very closely to and within the esd values of
1.908(6) and 1.108(6) Å, respectively, found for [Fe(CO)6]-
[SbF6]2. The observed C-O bond distances merit little discus-
sion beyond the statement that they appear to be shorter than
that in free CO (1.1281 Å)80 and that they rank at the lower
end of 10 022 data collected in the Cambridge compilation81
where the lower quartile value ql is 1.132 Å. In metal carbonyl
cations25b the strength of the CO bond is better probed by
vibrational spectroscopy as will be discussed subsequently.
There are, for the [Fe(CO)6]2+ cation in both salts, very slight
angular distortions from regular octahedral symmetry. In [Fe-
(CO)6][Sb2F11]2 the C-Fe-C angles depart from 90° by 0.2°
to 1.1° while the Fe-C-O angles range from 179.4(4)° to
177.5(4)°. In [Fe(CO)6][SbF6]2 only four of the six Fe-C-O
angles depart with 178.9(5)° very slightly from linearity. Similar
76
departures from linearity are found for [Hg(CO)2][Sb2F11]2
and [M(CO)4][Sb2F11]271, M ) Pd, Pt, where linear76 and square
planar71 cations are present. As the detailed vibrational analy-
ses71,76 show, the angular distortions and the tetragonal distortion
of the [Fe(CO)6]2+ cation in the [SbF6]- salt are insufficient to
destroy the inversion symmetry and affect the mutual exclusion
rule for IR and Raman active vibrations in all instances.71,72,76
In summary, the [Fe(CO)6]2+ cation is essentially octahedral.
Its relevant bond parameters (bond length and angles) are
virtually identical in both [Fe(CO)6][Sb2F11]2 and [Fe(CO)6]-
[SbF6]2. The observed long Fe-C and short C-O bonds suggest
that CO is primarily σ-bonded to Fe2+, with π-back-bonding
minimal. In agreement with a recent theoretical analysis82 and
our findings for other homoleptic metal carbonyl cations,25b the
C-O bond in [Fe(CO)6]2+ is strongly polarized with a positive
partial charge on the carbon atom.
As in other salts of homoleptic metal carbonyl cations with
[Sb2F11]- as counteranion, there are in [Fe(CO)6][Sb2F11]2, but
also to a lesser extent in [Fe(CO)6][SbF6]2 a number of
secondary83 interionic contacts. These contacts are shown for
the respective formula units of both salts in Figures 3 and 4.
They differ from other such contacts observed previously
between [Sb2F11]- and various homoleptic metal carbonyl
cations25 like linear [Hg(CO)2]2+ 76 and square planar [M-
(76) Bodenbinder, M.; Balzer-Jo¨llenbeck, G.; Willner, H.; Batchelor, R.
J.; Einstein, F. W. B.; Wang, C.; Aubke, F. Inorg. Chem. 1996, 35, 82.
(77) Mootz, D.; Bartmann, K. Angew Chem., Int. Ed. Engl. 1988, 27,
391.
(78) Christe, K. O.; Charpin, P.; Soulie, E.; Bougon, R.; Fawcett, J.;
Russel, D. R. Inorg. Chem. 1984, 23, 3756.
(79) Larson, E. M.; Abney, K. D.; Larson, A. C.; Eller, P. G. Acta
Crystallogr., Sect. B 1991, 47, 206.
(80) Herzberg, G. Spectra of Diatomic Molecules, 2nd ed.; Van Nos-
trand: Toronto, Canada, 1966; p 521.
(CO)4]2+ 71
M ) Pd, Pt, in four significant ways:
,
(i) The contacts are now due mainly to C‚‚‚F and to a lesser
extent to O‚‚‚F interactions.
(82) Goldman, A. S.; Krogh-Jesperson, K. J. Am. Chem. Soc. 1996, 118,
12159.
(83) Alcock, N. W. AdV. Inorg. Chem. Radiochem. 1972, 15, 1.
(81) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.
G.; Taylor, R. J. Chem. Soc., Dalton Trans. Supplement, 1989, S1.

[Fe(CO)6][Sb2F11]2 and [Fe(CO)6][SbF6]2
J. Am. Chem. Soc., Vol. 121, No. 31, 1999 7195
Figure 4. Selected significant interionic contacts in [Fe(CO)6][SbF6]2.
Figure 5. Vibrational spectra of [Fe(CO)6][Sb2F11]2.
(ii) The contact distances are longer and the interactions
presumably weaker.
(iii) There are within the formula unit fewer C‚‚‚F contacts
per CO group (about 1) compared to 5 such contacts for [Hg-
76
(CO)2][Sb2F11]2 and approximately 3 in [M(CO)4][Sb2F11]2-
71, M ) Pd, Pt.
(iv) Multiple F‚‚‚CO contacts involving up to four per F atom
in the [Sb2F11]- salt are observed. Previously only single or
double (bifurcated) contacts have been observed.71,76
The strength of secondary interactions is customarily83 judged
by comparison to the sum of the van der Waals radii84 Σrvdw of
the atoms involved which have values of 3.17 and 2.99 Å for
C‚‚‚F and O‚‚‚F interactions, respectively. In case of [Fe(CO)6]-
[Sb2F11]2, there are two pairs of significant C‚‚‚F contacts at
2.718 and 2.803 Å, respectively, and a third set of marginal
interactions at 2.870 Å. The interactions involve all six C atoms
of the [Fe(CO)6]2+ cation and both symmetry-related F8 atoms
of the anions (see Figure 3).
Figure 6. Vibrational spectra of [Fe(CO)6][SbF6]2.
In [Fe(CO)6][SbF6]2 weaker and fewer interionic C‚‚‚F
contacts of 2.842 and 3.061 Å are observed together with a
pair of O‚‚‚F contacts at 2.853. The contacts involve only four
of the six CO groups. The remaining two CO groups are trans
to each other and have slightly longer Fe-C (1.917(7) Å) and
shorter C-O (1.096(9) Å) bonds. The determined crystal-
lographic site symmetry of 4/m (C4h) for the [Fe(CO)6]2+ cation
in the [SbF6]- salt necessitates that two symmetry-related axial
Fe-C-O fragments be linear. The four symmetry equivalent
Fe-C-O fragments with bond angles of 178.9(5)° deviate only
very slightly from linearity. It hence seems that slight departures
from linearity observed for this M-C-O group both in [Fe-
(CO)6]2+ cations as well as in [Hg(CO)2]2+ 76 and in [M-
used for the fundamentals of [Fe(CO)6]2+ is adopted from a
vibrational study of the neutral metal hexacarbonyls M(CO)6,
M ) Cr, Mo, W.32,33 Of these, Cr(CO)6 is isoelectronic and
isosteric with [Fe(CO)6]2+, which provides help in the subse-
quent band assignment. Comparison to the early study32,33 is
particularly useful because vibrational data are obtained on
gaseous, solid, and dissolved (in CCl4) samples,32 while our
study is restricted to solid-state compounds.
There are four noteworthy features in the vibrational spectra
of the [Fe(CO)6]2+ salts:
(i) The relative intensities of bands in the CO-stretching region
have increased relative to bands in the 700-550 cm-1 region.
There is a marked difference between the spectrum of [Fe(CO)6]-
[Sb2F11]2 published previously29 and the one shown in Figure
5 with respect to band intensities.
(CO)4]2+ 71
M ) Pd, Pt, are caused by significant interionic
,
F‚‚‚C contacts.
It can be concluded that, for the octahedral [Fe(CO)6]2+
cation, stabilization by secondary contacts to the anion is less
important than in case of linear [Hg(CO)2][Sb2F11]276 or square
planar [M(CO)4][Sb2F11]271,M ) Pd, Pt, compounds on account
of a greater intrinsic stability of [Fe(CO)6]2+, which is in
(ii) Band positions and intensities of bands due to [Fe(CO)6]2+
in both salts are nearly identical. Cation bands are easily
identified with the help of 13C substitution.
(iii) Vibrational bands of the [SbF6]- anion are much simpler
and more intense than those of [Sb2F11]-.
agreement with theoretical predictions for gaseous [Fe-
(iv) The IR spectra of both salts show a fair number of
overtones, which are similar in band positions and intensities
to combination bands found and assigned for Cr(CO)6.32,33 These
combination bands are useful in locating inactive fundamentals
(T1g, T2u) and in assigning low-intensity bands of the [Fe-
(CO)6]2+ cation.
2+ 38
(CO)6]g
.
(c) Vibrational Spectra of [Fe(CO)6][Sb2F11]2 and [Fe-
(CO)6][SbF6]2. The IR and Raman spectra of [Fe(CO)6][Sb2F11]2
and [Fe(CO)6][SbF6]2 are depicted in Figures 5 and 6, respec-
tively. The vibrational wavenumbers of the [Fe(CO)6]2+ cation
and its 13C isotopomer are listed in Table 3. The numbering
The assignment of the fundamental vibrations of the [Fe-
(84) Bondi, A. J. Phys. Chem. 1964, 68, 441.
(CO)6]2+ cation is for the most part straightforward. Of the four

7196 J. Am. Chem. Soc., Vol. 121, No. 31, 1999
Bernhardt et al.
Table 3. Vibrational Data for [Fe(CO)6]2+ in [Fe(CO)6][Sb2F11]2, [Fe(13CO)6][Sb2F11]2, [Fe(CO)6][SbF6]2, and [Fe(13CO)6][SbF6]2
[Fe(CO)6][Sb2F11]2
[Fe(CO)6][SbF6]2
[Fe(13CO)6][Sb2F11]2
[Fe(13CO)6][SbF6]2
symm.
species
assignment νa
IR
Ra
IR
Ra
IR
Ra
IR
Ra
ν1 + ν6
4441(vw)
4402(vw)
2828(vw)
4341(vw)
4303(vw)
ν3 + ν6
ν1 + ν7
ν3 + ν7
2835(vw)
2812(vvw)
2703(vvw)
2688(vvw)
2623(vw)
2600(w)
2766(vw)
2745(vvw)
2635(vvw)
2621(vvw)
2564(vw)
2542(w)
2509(w)
2499(w,sh)
2476(vvw)
ν1 + ν12,ν6 + ν10
ν3 + ν12
ν1 + ν8
2707(vvw)
2688(vvw)
2623(vw)
2600(w)
2564(w)
2551(w,sh)
2563(vw)
2543(w)
2509(w)
ν3 + ν8
ν6 + ν4
2562(w)
ν6 + ν2
2551(w,sh)
2535(vvw)
2390(vvw)
2348(vvw)
2336(vvw,sh)
2500(w,sh)
ν6 + ν5
ν3 + ν9
2386(vvw)
2335(vvw)
ν6 + ν11
ν3 + ν13
ν1, νCO
ν3, νCO
ν6, νCO
ν7 + ν10
ν10 + ν12
ν7 + ν5
A1g
Eg
T1u
2241.2(vs)
2219.5(vs)
2241.9(vs)
2219.0(vs)
2189.7(vs)
2169.0(vs)
2190.5(vs)
2168.4(vs)
2204(vs)
2205(vs)
1088(vvw)
967(w)
923(w)
878(w)
2154(vs)
2155(vs)
968(w)
922(w)
881(w)
799(w)
935(w)
897(w)
856(w)
775(w)
933(w)
899(w)
855(w)
774(w)
ν8 + ν10
ν5 + ν12
ν4 + ν8
799(w)
738(vw)
615(vw)
(598(mw,sh)
{590(vs)
(570(mw,sh)
ν11 + ν12
(589(w,sh)
{576(vs)
{566(w,sh)
ν7, δFeCO
T1u
586(vs)
571(vs)
ν10, δFeCO
ν12, δFeCO
ν8, νFeC
T2g
T2u
T1u
Eg
A1g
T1u
T2g
T2u
500.9(vvw)
499.8(vvw)
468(vw)
380(w)
470(vw)
382(w)
454(vw)
374(w)
375(w)
ν4, νFeC
361(w,sh)
347.1(m)
357(vw,sh)
345.3(m)
356(vw,sh)
342.7(m)
353(vw,sh)
341.6(m)
ν2, νFeC
ν9, δCFeC
ν11, δCFeC
ν13, δCFeC
170(vw)
114(vw)
171(vw)
170(vw)
114(vw)
171(vw)
138(s,br)
148(s,br)
138(s,br)
148(s,br)
a Numbering of fundamentals from ref 32.
IR active fundamentals of octahedral [Fe(CO)6]2+, both ν6 (ν
C-O) and ν7 (δ Fe-CO) are very intense. For ν7, pronounced
fine structure is observed in the spectrum of [Fe(CO)6][SbF6]2,
which is less noticeable for [Fe(CO)6][Sb2F11]2, where a shoulder
at 599 cm-1 is assigned to the [Sb2F11]- anion. The remaining
two IR active fundamentals ν8 (νFe-C) and ν9 (δC-Fe-C)
have very low intensities so that the position of ν8 is initially
calculated from its combination bands ν1 + ν8 and ν3 + ν8,
before the band is detected in the far-IR spectrum. Since the
site symmetry of the cation is not strictly Oh, as discussed in
the preceding section, the inactive T2u fundamentals ν12 and
to mixed 12CO-13CO species. In the IR and Raman spectra of
[Fe(CO)6]2+, bands of [Fe(CO)5(13CO)]2+ are seen on account
of the 1.1% natural abundance of 13C. Likewise, in the spectra
of [Fe(13CO)6]2+ with an isotopic enrichment of ∼95%, bands
attributable to [Fe(CO)(13CO)5]2+ and [Fe(CO)2(13CO)4]2+ are
observed. These bands are omitted from Table 3 but may be
found together with a partial assignment in a complete listing
of vibrational bands in the Supporting Information section.
Vibrational bands due to the anions are listed in the
Experimental Section. The [Sb2F11]- anion has no symmetry
(Ci), and hence, an extensive coincidence of IR and Raman
bands is noted. A band observed at 492 cm-1 is assigned as
asymmetric Sb-F-Sb stretch. For [SbF6]- the symmetry in the
crystal lattice departs from Oh and the mutual exclusion rule is
no longer strictly valid. As a consequence the forbidden T2g
and T1u vibrations are observed as weak shoulders in the far-
IR spectrum. In addition to the fundamentals, a number of
combination bands are found for [SbF6]-. A vibrational assign-
ment for the [SbF6]- anion is included in the Experimental
Section.
The vibrational assignments for the [Fe(CO)6]2+ cation in [Fe-
(CO)6][Sb2F11]2 and in [Fe(13CO)6][Sb2F11]2 are summarized in
Table 4. The experimentally obtained wavenumbers for all 13
fundamentals are compared to the results of ab initio calcula-
tions31 and to the data calculated in a normal coordinate analysis
(NCA) to be discussed subsequently. There is excellent agree-
ment between the NCA values and experimentally determined
wavenumbers. Since ab initio calculations31,38 involve the
hypothetical gas-phase species [Fe(CO)6]g2+, some small dis-
ν13 are observed as weak bands in the IR spectrum of [Fe(CO)6]-
[Sb2F11]2. The remaining inactive fundamental ν5 (T1g) is not
seen, but its position can be obtained from combination bands.
Finally for ν6 a distinct manifestation of the Christiansen effect
is noted in the observed band shape in both salts.
Of the six Raman active fundamentals of [Fe(CO)6]2+, ν1,
ν3, and ν11 are highly intense, while ν2 is of medium, and both
ν4 and ν10 are of very low intensities. Assignments of ν2 and
ν4, which occur in a similar spectral region, are made with
reference to the spectra for the isoelectronic species [V(CO)6]-,37
Cr(CO)6,32,33 and [Mn(CO)6]+.34 The exact order is determined
by comparing the order and the ν2/ν4 intensity ratios in these
species32-34,37 and in ν2 + ν6/ν4 + ν6. The assignment is also
supported by ab initio calculations.85
In addition to the bands listed in Table 3 for [Fe(CO)6]2+
and [Fe(13CO)6]2+, a number of medium- to low-intensity bands
are observed in the CO-stretching region which are attributed
(85) Jonas, V. Personal communication.

[Fe(CO)6][Sb2F11]2 and [Fe(CO)6][SbF6]2
J. Am. Chem. Soc., Vol. 121, No. 31, 1999 7197
Table 4. Vibrational Assignments of the Observed (obs.) and Calculated (Calc.) Wavenumbers ν [cm-1] of the Fundamentals for [Fe(CO)6]2+
in [Fe(CO)6][Sb2F11]2 and [Fe(13CO)6][Sb2F11]2
[Fe(CO)6][Sb2F11]2
[Fe(13CO)6][Sb2F11]2
ν
sym. species
BP86/ECP2 calcda
obsb
calcdc
obsb
calcdc
ν1, νCO
A1g
Eg
2221(-20)
2188(-32)
2173(-31)
608(22)
508(7)
478(10)
380(0)
355(-6)
361(14)
338(2)
117(-53)
98(-40)
77(-37)
2241.2
2219.5
2204
586
500.9
468
380
361
347.1
[336]
170
2241.5
2219.9
2204
586
501
468
380
361
347.7
336
171
2189.7
2169.0
2154
571
[482]
452
374
355
342.7
[326]
170
2189.3
2168.9
2154
571
482
452
375
355
342.1
326
170
ν3, νCO
ν6, νCO
T1u
T1u
T2g
T2u
T1u
Eg
A1g
T1g
T1u
T2g
T2u
ν7, δFeCO
ν10, δFeCO
ν12, δFeCO
ν8, νFeC
ν4, νFeC
ν2, νFeC
ν5, δFeCO
ν11, δCFeC
ν9, δCFeC
ν13, δCFeC
138
114
138
114
138
114
138
114
a Values in parentheses are calculated ab initio minus observed wavenumbers, ref 31. b Values in square brackets are derived from overtones.
c Using the force field presented in Table 5.
Table 5. Calculated Force Constants (102 N m-1, Normalized on 1
crepancies are expected and found. Crystal lattice effects are
Å Bond Length) of the GVFF for [Fe(CO)6]2+ and Cr(CO)6 (g)
responsible for an increase of about 40 cm-1 for the observed
[Fe(CO)6]2+
Cr(CO)6
a
C-Fe-C deformation modes ν9, ν11, and ν13. The calculated
values for the CO-stretching vibrations ν1, ν3, and ν6 are lower
by about 20-30 cm-1, which is commonly noted for νCO
obtained by ab initio methods.31,38,39,86 Despite these and other
small deviations, the calculated ab inito wavenumbers represent
an excellent assignment aid for experimental data, since, for
IR active vibrations, relative intensities are available as well.31
Because the [Fe(CO)6]2+ cation in the solid state is only
marginally affected by site symmetry perturbations or interionic
interactions as discussed earlier, gaseous [Fe(CO)6]g2+ provides
a very good model and allows an excellent match of calculated
and experimental wavenumbers.
FCO
FMC
Ftrans
Fcis
19.698(55)
1.673(24)
0.063(59)
0.047(34)
0.367(48)
0.008(52)
-0.022(29)
0.462(24)
-0.025(3)
0
16.676(67)
2.029(67)
0.279 (67)
0.204(34)
0.614(64)
-0.028(64)
-0.052(28)
0.486(67)
-0.019(3)
0
CO,CO
CO,CO
FCO,MC
Ftrans
CO,MC
Fcis
CO,MC
Ftrans
MC,MC
Fcis
MC,MC
FCO,γ ) FCO,R’- FCO,R’’
FFeC,γ ) FFeC,R’- FFeC,R’’
FCO,â
0.035(30)
0
-0.150(84)
0
FFeC,â
-0.036(7)
1.202(91)
1.174(51)
0.2
0.413(12)
0.101(12)
0.001(5)
0.029(7)
0.036(7)
0.045(11)
0.022(12)
-0.070(24)
0.571(55)
0.519(64)
0.172(64)
0.511(20)
0.126(20)
0.000(8)
0.015(19)
-0.087(4)
-0.109(27)
-0.033(27)
As can be seen in Table 4, all 13 fundamentals of [Fe(CO)6]2+
are observed and assigned. The assignment is supported by ab
initio31 and NCA calculations. Among the salts with homoleptic
metal carbonyl cations25b so far known, an equally extensive
vibrational characterization has only been possible for [Au(CO)2]-
[Sb2F11].87
FR ) FR + FRR’’- 2FRR’’’’
Fγ ) FR - FRR’’
Fγγ ) 2(FRR’- FRR’’’)
Fâ
Fââ’
Fââ’’
Fââ’’’
(d) Normal Coordinate Analysis (NCA) for [Fe(CO)6]2+
.
FRâ ) FRâ’ - FRâ’’
Fγâ’ ) FRâ’ + FRâ’’
Fγâ’’ ) 2FRâ’’’
The molecular structure determinations of [Fe(CO)6][SbF6]2 and
[Fe(CO)6][Sb2F11]2, described in section b, provide two sets of
nearly identical structural parameters for the cation [Fe(CO)6]2+
(see Table 2). For a normal coordinate analysis we have chosen
the average values of 1.13 Å (d C-O) and 1.91 Å (d Fe-C).
Based on a modified valence force field according to Wilson’s
F. G. matrix,88 the normal coordinate analysis is carried out
with the program MOLVIB.89-91
a Vibrational data are taken from ref 32. Structural data are taken
from ref 92.
manner. The force constants obtained are summarized in Table
5 and compared to those for Cr(CO)6, which are also obtained
here by use of the MOLVIB program. The structural parameters
used for Cr(CO)6 are 1.171 and 1.916 Å for d C-O and d Cr-
C, respectively, taken from an electron diffraction study.92
Vibrational data for solid Cr(CO)6 are found in refs 32 and 33.
As can be seen in Table 5, the decrease in π-back-bonding with
increasing positive charge is reflected in a slight increase of
For the 13 atom molecular cation [Fe(CO)6]2+ in the
symmetry point group Oh, the irreducible representations for
vibrations are the following:
Γvib ) 2 A1g (Ra) + 2 Eg (Ra) + 1 T1g (-) + 4 T1u (IR) +
2 T2g (Ra) + 2 T2u (-)
FCO and a concomitant even smaller decrease of FMC. Far more
pronounced, however, are differences in the interaction force
constants FCO/CO or FCO/MC, which are considerably smaller for
[Fe(CO)6]2+ than for Cr(CO)6, on account of decreased π-back-
donation in the former. Unfortunately the molecular structure
of [Mn(CO)6]+ is unknown and not enough vibrational data are
available. Hence, a normal coordinate analysis cannot be
extended to this species.
Even though a different labeling of individual atoms is found
in refs 32 and 33, the force constants are defined in a similar
(86) Jonas, V.; Thiel, W. J. Chem. Phys. 1995, 102, 8474.
(87) Willner, H.; Schaebs, J.; Hwang, G.; Mistry, F.; Jones, R.; Trotter,
J.; Aubke, F. J. Am. Chem. Soc. 1992, 114, 8972.
(88) Wilson, E. B.; Decius, J..C.; Cross, P. C. Molecular Vibrations;
McGraw-Hill Book Co. Inc.: New York, 1955; p 175.
(89) Sundius, T. QCPE No. 604 MOLV1B, Indiana University, 1991.
(90) Sundius, T. J. Mol. Struct. 1990, 218, 321.
Not surprisingly, a FCO value of 19.82 × 102 N m-1 for [Fe-
(CO)6]2+, obtained by using the Cotton-Kraihanzel93,94 ap-
(91) Bublitz, D. Modification of the program of MOLVIB, University
of Kiel, Germany.
(92) Arnesen, S. P.; Seip, H. M. Acta Chem. Scand. 1966, 20, 2711.
(93) Cotton, F. A.; Kraihanzel, C. S. J. Am. Chem. Soc. 1962, 84, 4432.

7198 J. Am. Chem. Soc., Vol. 121, No. 31, 1999
Bernhardt et al.
proximation, is in good agreement with the NCA value of
19.698(55) × 102 N m-1 for [Fe(CO)6]2+. On the other hand,
approximated93,94 interaction force constants of 0.21 and 0.06
× 102 N m-1 for FtrCO/CO and FcisCO/CO, respectively, agree much
less with the NCA values of 0.063 and 0.047 × 102 N m-1
.
Reasonable agreement is found between the stretching force
constants FCO and FFeC listed in Table 5 and values of 19.20
and 1.77 × 102 N m-1 from ab initio calculations31 for gaseous
[Fe(CO)6]2+(g). As the bond length data (section b) have shown,
the strength of the C-O bond is slightly underestimated while
the strength of the Fe-C bond appears to be overestimated in
these calculations.31
In summary, the force constants for [Fe(CO)6]2+ and Cr(CO)6,
obtained from a normal coordinate analysis with the MO-
LVIB89-91 program and listed in Table 5, provide a more
detailed and sensitive description of changes in π-back-bonding
between both species than a comparison of bond length data or
of CO-stretching frequencies can provide. In addition, vibra-
tional wavenumbers for all 13 fundamentals of [Fe(CO)6]2+
,
recalculated from the force constants, produce an excellent
match for the experimental data as seen in Table 4 and discussed
in the preceding section.
(e) Magnetic Susceptibility Measurements of [Fe(CO)6]-
[Sb2F11]2 and [Fe(CO)6][SbF6]2. The molecular structure
determinations and the analysis of the vibrational spectra have
shown that [Fe(CO)6]2+ is essentially octahedral in [Fe(CO)6]-
[Sb2F11]2 as well as in [Fe(CO)6][SbF6]2. It follows that the
Figure 7. Magnetic moments of Fe[SbF6]2 and various preparations
of [Fe(CO)6][Sb2F11] between 4 and 300 K.
1
eff
300
complex cation must be diamagnetic with a low-spin A1g
essentially diamagnetic. A small µ value is caused by less
ground state. On the other hand, the oxidative carbonylation of
Fe(CO)5 has produced strongly paramagnetic products, when
either Cl2, AsF5, or XeF2 is used as an external oxidizing agent.
The thermal decomposition of [Fe(CO)6][SbF6]2 produces
quantitatively Fe[SbF6]2, which has Fe2+ in a high spin
configuration with a 5T2g octahedral ground state.48 It is hence
likely that variable amounts of Fe[SbF6]2, present in the product
mixtures, are responsible for the observed paramagnetism.
However, an unambiguous identification of Fe[SbF6]2 in the
mixtures is not a trivial matter. The IR and Raman spectra of
Fe[SbF6]2 are known,28 but the reported band positions between
704-540 cm-1 and 285-273 cm-1 fall into extremely cluttered
regions of the vibrational spectrum (see section c).
than 0.1% of Fe[SbF6]2 or may be due to temperature-
independent paramagnetism.95
(f) The 57Fe Mo1ssbauer, 13C NMR, and UV-Vis Spectra
of [Fe(CO)6]2+. The results reported here complete the spec-
troscopic characterization of [Fe(CO)6]2+ by standard techniques
and confirm the 1A1g ground state for iron. The 57Fe Mo¨ssbauer
spectrum of polycrystalline [Fe(CO)6][SbF6]2 features a single
line with an isomer shift δ of -0.003(8) mm s-1 (rel. to R-Fe
at R.T.) and a line width of 0.346(15) mm s-1. The isomer shift
is identical to the previously reported value for [Fe(CO)6]-
29
[Sb2F11]2 and comparable to δ values reported for the single
line spectra of K4[Fe(CN)6]‚3H2O (0.042(5) mms-1) and (NH4)4-
[Fe(CN)6] (-0.059(3) mms-1).97
A more convincing identification of Fe[SbF6]2 is possible with
the help of magnetic susceptibility measurements between 2 and
300 K. In Figure 7 the magnetic moments in this temperature
range are shown for various preparations of “[Fe(CO)6]-
[Sb2F11]2” with Cl2, AsF5, or XeF2 as external oxidizing agents
for Fe(CO)5 and SbF5 as reaction medium. They are compared
to the magnetic moments of Fe[SbF6]2 in the same temperature
range. The temperature dependence of magnetic moments shown
In the 13C NMR spectrum of [Fe(13CO)6]2+ in HF or DF with
small amounts of SbF5 added, a sharp single line at δ ) 178.0
ppm (line width 0.4 Hz) is seen. J(13C-57Fe) is found to be 19.2
Hz. The value is slightly smaller than J(13C-57Fe) found for
Fe(CO)5 (23.4 Hz). Since the coordination number of Fe in [Fe-
(CO)6]2+ is higher than in Fe(CO)5,98 its coupling constant is
smaller and the ratio 23.4/19.2 ≈ 6/5 is reasonable.
5
Substantially higher 1J(M-13C) constants have been observed
previously by us for [Pt(CO)4]2+ (1550 Hz)99 and [Hg(CO)2]2+
(5220 Hz).76 The higher values are explained by the lower
coordination numbers and the substantially higher gyromagnetic
ratios of the metals (Pt or Hg). Again the sharp line for [Fe-
(CO)6]2+ in solution and the observation of 1J(13C-57Fe), which
is experimentally demanding on account of the low natural
abundance of 57Fe (2.2%), are suggestive of a highly sym-
metrical [Fe(CO)6]2+ cation in solution.
in Figure 7 is typical for high spin Fe2+ with a T2g ground
state and in an octahedral environment.64,95 The magnetic
moment of 5.40 µB at 300 K is comparable to a previously
reported moment of 5.42 µB48 and just slightly below the µS+L
moment of 5.48 µB.66 A detailed analysis of the magnetic
behavior of Fe[SbF6]2 and related M[SbF6]2 salts, M ) Cr, Cu,
Pd, or Ag, is planned96 at a later point, but it appears clear that
all preparations of [Fe(CO)6][Sb2F11]2 have variable amounts
of Fe[SbF6]2 as impurities, because a similar temperature
dependence of µ is noted in all samples.
(97) Danon, J. In Chemical Applications of Mo¨ssbauer Spectroscopy;
Goldanskii, V. I., Herber, R. H., Eds.; Academic Press: New York, 1968;
p 252.
A sample of [Fe(CO)6][SbF6]2 obtained from highly pure
crystalline [Fe(CO)6][Sb2F11]2 as described in section a is
(98) Kalinowski, H. O.; Berger, S.; Braun, S. 13C NMR Spectroscopy;
Wiley-Interscience: New York, 1988. (98b) Mann, B. E. AdV. Organomet.
Chem. 1974, 12, 135 and references therein.
(94) Kraihanzel, C. S.; Cotton, F. A. Inorg. Chem. 1963, 2, 533.
(95) Figgis, B. N. Introduction to Ligand Fields; Interscience: New York,
1966, p 272.
(99) Hwang, G.; Wang, C.; Aubke, F.; Willner, H.; Bodenbinder, M.
Can. J. Chem. 1993, 71, 1532.
(96) Sham, I. H. T.; Thompson, R. C.; Aubke, F. In press.

[Fe(CO)6][Sb2F11]2 and [Fe(CO)6][SbF6]2
J. Am. Chem. Soc., Vol. 121, No. 31, 1999 7199
Table 6. Selected Structural and Spectroscopic Data for the Homoleptic Iron Carbonyls [Fe(CO)4]2-, Fe(CO)5, and [Fe(CO)6]2+ a
species
[Fe(CO)4]2-
Fe(CO)5
[Fe(CO)6]2+
(i) structure geometry
bond length: Fe-C [Å]
C-O [Å]
tetrahedral
trigonal bipyramidal
1.833(4)d; 1.806(5)e
1.145(3)d
octahedral
1.762(17)b; 1.746(4)c
1.155(17)b; 1.175(5)c
1.908(7)f; 1.911(5)g
1.108(9)f; 1.104(5)g
(ii) vibrational data
νCOavg [cm-1
]
1815h
2030i
2215
1.673(24)
19.698(55)
fFeC × 102 [N m-1
]
]
ax. 2.57i eq 2.64i
ax. 17.43i eq 16.47i
fCO × 102 [N m-1
∼13.30h
-0.18j
(iii) 57Fe Mo¨ssbauer data
isomer shifts δ [mms-1
rel. to R-Fe at 80 K
(iv) 13C NMR data
δ [ppm]
]
-0.09j
-0.003(6)
209.6k; 211.9k
23.2k,l
178.0
19.2
J(13C-57Fe) [Hz]
24.0k
a Abbreviations: ax. ) axial; eq. ) equatorial. b [Na(crypt)2][Fe(CO)4], ref 45. c K2[Fe(CO)4], ref 45. d Fe-CO eq., ref 43. e Fe-CO ax., ref 43.
f [Fe(CO)6][SbF6]2. g [Fe(CO)6][Sb2F11]2. h Ref 105. i Ref 101. j Ref 102. k Ref 107. l Ref 98.
Unfortunately it has not been possible for us to record the
UV-vis spectrum of [Fe(CO)6]2+ in HF-SbF5 solution on
account of experimental difficulties. The spectrum recorded on
[Fe(13CO)6][Sb2F11]2 crystals suspended in liquid SbF5 features
two broad bands at 31.2 × 103 and 38.3 × 103 cm-1. The band
positions are similar to those found for [Fe(CN)6]4-aq at 31.0
× 103 and 37.04 × 103 cm-1 40or for Mo(CO)6 at 31.35 and
37.68 × 103 cm-1 and assigned to the d-d transitions 1A1g f
allows a comparison to Cr(CO)6.32,33 The force constants, in
particular interaction force constants, provide insights into
bonding in the absence of significant π-back-donation for [Fe-
(CO)6]2+. Magnetic susceptibility measurements confirm the low
1
spin configuration of Fe2+ in [Fe(CO)6]2+ with a A1g ground
state in an octahedral environment and allow identification of
28,48
Fe[SbF6]2
as the sole paramagnetic contaminant, which is
formed in a competing reaction to the formation of [Fe(CO)6]-
[Sb2F11]2. The magnetic measurements are supported by the
results of 57Fe Mo¨ssbauer and 13C NMR spectroscopy. Attempts
to record the UV-vis spectrum of [Fe(CO)6]2+ are inconclusive,
and a satisfactory band assignment is not possible at this time.
The principal structural and spectroscopic data for [Fe-
(CO)6]2+ are summarized in Table 6. They are compared to
those for Fe(CO)5 and [Fe(CO)4]2-, as far as these are available.
The molecular structure of Fe(CO)5 has been studied in the solid
1
1
1T1g and A1g f T2g. A ligand field analysis would produce
values of 10 Dq ) 34.6 × 103 cm-1 and B ∼ 487 cm-1, similar
40
to the corresponding data for [Fe(CN)6]4- of 33.8 × 103 cm-1
and ∼400 cm-1 or Cr(CO)6 at 34.15 × 103 cm-1, respec-
95
tively. There are three reasons which make such an assignment
and the spectral analysis implausible for [Fe(CO)6]2+
:
(i) The band assignment is not supported by reliably
determined extinction coefficients, which should be ∼10 L
mol-1 cm-1 for spin allowed d-d transitions.95
state as well as in the gas phase. An early X-ray diffraction
100
(ii) The spectrum recorded by us looks very different from
study of solid Fe(CO)5
suggested a trigonal bipyramidal
the one depicted in ref 95 for [Fe(CN)6]4-
.
structure with identical Fe-C (1.79(2) Å) and C-O (1.12(2)
Å) bond distances for both the axial and equatorial CO groups.
Electron diffraction data of gaseous Fe(CO)5 listed in Table 6
show stronger Fe-C bonds for axial CO groups than for the
equatorial ones43 with the difference ∆Fe-C 0.0204(55) Å,44
while CO distances are identical in both studies.43,44 As a
consequence of the difference in Fe-C-O bond lengths, there
are two different 13C resonances in the 13C NMR spectrum98
and two distinctly different sets of internal valence stretching
force constants101 for Fe(CO)5. As expected for a trigonal
bipyramidal molecule, the 57Fe Mo¨ssbauer spectrum97,102 shows
a quadrupole splitting of 2.57 mms-1 97 whereas both [Fe-
(CO)4]2- 102 and [Fe(CO)6]2+ have single line 57Fe Mo¨ssbauer
spectra.
(iii) A 10 Dq value as high as 34.6 × 103 cm-1 would be
nearly identical to 10 Dq reported for Cr(CO)6,40 while a B value
of 487 cm-1 compared to a free ion B0 value of 1060 cm-1 95
would suggest extensive electron delocalization.
We hope that it will be possible to record a UV-vis spectrum
of [Fe(CO)6]2+ in HF solution to assign both d-d and charge-
transfer (CT) transitions in the future. However, this is not a
trivial undertaking, due to the corrosive nature of the solvent.24,73a
So far no other solvent has been found which will dissolve any
of the two [Fe(CO)6]2+ salts without decomposition. Even then
the assignment of d-d transitions will be a difficult task.95
Summary and Conclusions
On account of strong interionic interactions the [Fe(CO)4]2-
ion can be distorted from the expected tetrahedral symmetry,
depending on the cation. For example, in Na2[Fe(CO)4]‚1.5
(dioxane)45 and to a lesser extent in K2[Fe(CO)4],45 the observed
C-Fe-C bond angles depart significantly from the expected
109.5°. In [Na(crypt)]2[Fe(CO)4], a regular Fe(CO)4 tetrahedron
is observed.45 As seen in Table 6, there are some rather small
differences in Fe-C and C-O bond lengths between the
potassium and the [Na(crypt)]+ salt.
The oxidative carbonylation of Fe(CO)5 with XeF2 allows
the facile generation of superelectrophilic [Fe(CO)6]2+ in
superacid media in high yield. The switch from liquid SbF5 as
reaction medium for carbonylation reactions25b to HF-SbF5,
reported here for the first time, produces [Fe(CO)6][Sb2F11]2 in
very high purity and in the form of single crystals. The facile
transformation of this salt to [Fe(CO)6][SbF6]2 by washing with
anhydrous HF provides the first thermally stable [SbF6]- salt
of a homoleptic superelectrophilic metal carbonyl cation.25b The
cations in [Fe(CO)6][Sb2F11]2 and in [Fe(CO)6][SbF6]2 have very
similar average values for corresponding bond parameters and
nearly identical band positions in their vibrational spectra. All
13 vibrational fundamentals of [Fe(CO)6]2+ are observed
experimentally. The assignment is confirmed by the results of
a normal coordinate analysis and supported by ab initio
calculations.31 A general valence force field for [Fe(CO)6]2+
The spectroscopic characterization of the [Fe(CO)4]2- anion
is rather cursory and incomplete. A limited number of IR bands
(100) Donohue, J.; Caron, A. Acta Crystallogr. 1964, 17, 663.
(101) Jones, H. L.; McDowell, R. S.; Goldblatt, M.; Swanson, B. I. J.
Chem. Phys. 1972, 57, 2050.
(102) Greatrex, R.; Greenwood, N. N. Discuss. Faraday Soc. 1969, 47,
126.

7200 J. Am. Chem. Soc., Vol. 121, No. 31, 1999
Bernhardt et al.
of aqueous [Fe(CO)4]2- are reported.103 A Raman spectrum of
an aqueous solution of Na2[Fe(CO)4]104 appears to be incomplete
according to a subsequent study on solid Na2[Fe(CO)4]‚
dioxane.105 We have calculated an average CO-stretching
frequency and an approximate stretching force constant on the
basis of the latter study.105 A much better situation is found for
Fe(CO)5, where the complete vibrational spectrum is known33,106
and the potential force constants have been calculated.101
Also the 13C chemical shift of [Fe(CO)4]2- is not reported98
and the coupling constant J(13C-57Fe) listed in Table 6 is
calculated and not experimentally determined.107 Seemingly
strong interionic interactions in the solid state45 and solvent-
dependent ion pair formation in solution108 have limited the
spectroscopic characterization of the supernucleophilic [Fe-
The νjCOavg wavenumbers increase in steps of ∼200 cm-1
from [Fe(CO)4]2- over Fe(CO)5 to [Fe(CO)6]2+, while the force
constants fCO and fFeC101 appear to move in opposite directions.
These observations are readily explained with decreasing π
contributions and increasing σ contributions to Fe-CO bonding
as the oxidation state of Fe moves from -2 to +2.
The data listed in Table 6 together with the results of ab initio
calculations,31,38 magnetic susceptibility measurements, and
NCA and GVFF calculations presented here, indicate that [Fe-
(CO)6]2+, which is known only for a few years and studied only
in the solid state, is equally well characterized as is Fe(CO)5,
which has been known well over a century.5 This is a
consequence of modern, widely available, physical, computa-
tional, spectroscopic, and diffraction techniques.
(CO)4]2- 12
.
For [Fe(CO)6]2+, secondary contacts to either [Sb2F11]- or
[SbF6]- (see Figures 3 and 4) are less numerous and rather weak
compared to O‚‚‚K contacts in K2[Fe(CO)4].45 As a consequence
any departures from octahedral symmetry for [Fe(CO)6]2+ are
slight. Observed Fe-C distances for [Fe(CO)6]2+ are about 0.1
Å longer than the upper quartile value qu of 1.801 Å from the
Cambridge index81 and may in fact be the longest distances
recorded for iron carbonyls. Even for Fe(CO)5,43,44 the average
Fe-C value is with 1.820(10) Å slightly longer than ql,81 while
the average Fe-C distances in K2[Fe(CO)4]45 and [Na(crypt)]2-
[Fe(CO)4]45 approximate ql.81 The reverse trend is found for
C-O bond lengths in the three homoleptic iron carbonyl species.
Acknowledgment. Financial support by Deutsche Fors-
chungsgemeinschaft, DFG (operating funds H.W.; Postdoctoral
Fellowship, E.B., M.B.), Alexander v. Humboldt Foundation
(Research Prize F.A., Postdoctoral Fellowship R.B.), North
Atlantic Treaty Organization, NATO (collaborative research
grant to H.W. and F.A.), the Natural Sciences and Engineering
Research Council of Canada, NSERC (operating grant F.A.,
Postgraduate Scholarship I.H.T.S.), and the Fonds der Chemis-
chen Industrie (H.W. and B.B.) is acknowledged.
Supporting Information Available: Detailed crystallo-
graphic data, atomic coordinates, bond distances and bond
angles, vibrational data, IR spectrum in the spectral range 2200-
2900 cm-1, UV-vis spectrum of [Fe(13CO)6][Sb2F11]2 in
suspended liquid SbF5, structural drawings, internal coordinates
of [Fe(CO)6]2+, and X-ray crystallographic files for [Fe(CO)6]-
[Sb2F11]2 and [Fe(CO)6][SbF6]2. This material is available free
of charge via the Internet at http://pubs.acs.org.
(103) Edgell, W. F.; Huff, J.; Thomas, J.; Lehman, H.; Angell, C.; Asato,
G. J. Am. Chem. Soc. 1960, 82, 1254.
(104) Stammreich, H.; Kawai, K.; Tavares, Y.; Krumholz, P.; Behmoiras,
J.; Bril, S. J. Chem. Phys. 1960, 32, 1482.
(105) Braterman, P. S. J. Mol. Spectrosc. 1977, 68, 334.
(106) Edgell, W. F.; Wilson, W. F.; Summitt, R. Spectrochim. Acta 1963,
19, 863.
(107) Smith, G. W. General Motors Corp. Res. Publ. 1964, GMR 44.
(108) Darrensbourg, M. Y. Prog. Inorg. Chem. 1985, 33, 221.
JA990958Y