Cationic carbonyl complexes of rhodium(I) and rhodium(III): Syntheses, vibrational spectra, NMR studies, and molecular structures of tetrakis(carbonyl)rhodium(I) heptachlorodialuminate and -gallate, [Rh(CO)4][Al2Cl7] and [Rh(CO)4][Ga2Cl7]

Article abstract of DOI:10.1021/ic0206903

Dimeric rhodium(I) bis(carbonyl) chloride, [Rh(CO)₂(μ-Cl)]₂, is found to be a useful and convenient starting material for the syntheses of new cationic carbonyl complexes of both rhodium(I) and rhodium(III). Its reaction with the Lewis acids AlCl₃ or GaCl₃ produces in a CO atmosphere at room temperature the salts [Rh(CO)₄][M₂Cl₇] (M = Al, Ga), which are characterized by Raman spectroscopy and single-crystal X-ray diffraction. Crystal data for [Rh(CO)₄][Al₂Cl₇]: triclinic, space group P1 (No. 2); a = 9.705(3), b = 9.800(2), c = 10.268(2) A; α = 76.52(2), β = 76.05(2), γ = 66.15(2)°; V = 856.7(5) A³; Z = 2; T = 293 K; R₁ [I > 2σ(I)] = 0.0524, wR₂ = 0.1586. Crystal data for [Rh(CO)₄][Ga₂Cl₇]: triclinic, space group P1 (No. 2); a = 9.649(1), b = 9.624(1), c = 10.133(1) A; α = 77.38(1), β = 76.13(1), γ = 65.61(1)°; V = 824.4(2) A³; Z = 2; T = 143 K; R₁ [I > 2σ(I)] = 0.0358, wR₂ = 0.0792. Structural parameters for the square planar cation [Rh(CO)₄]⁺ are compared to those of isoelectronic [Pd(CO)₄]²⁺ and of [Pt(CO)₄]²⁺. Dissolution of [Rh(CO)₂Cl]₂ in HSO₃F in a CO atmosphere allows formation of [Rh(CO)₄]⁺_(solv). Oxidation of [Rh(CO)₂Cl]₂ by S₂O₆F₂ in HSO₃F results in the formation of ClOSO₂F and two seemingly oligomeric RH(III) carbonyl fluorosulfato intermediates, which are easily reduced by CO addition to [Rh(CO)₄]⁺_(solv). Controlled oxidation of this solution with S₂O₆F₂ produces fac-Rh(CO)₃(SO₃F)₃ in about 95% yield. This RH(III) complex can be reduced by CO at 25 °C in anhydrous HF to give [Rh(CO)₄]⁺_(solv); addition of SbF₅ at -40 °C to the resulting solution allows isolation of [Rh(CO)₄][Sb₂F₁₁]₂, which is found to have a highly symmetrical (D_4h) [Sb₂F₁₁]^- anion. Oxidation of [Rh(CO)₂Cl]₂ in anhydrous HF by F₂, followed in a second step by carbonylation in the presence of SbF₅, is found to be a simple, straightforward route to pure [Rh(CO)₅Cl][Sb₂F₁₁]₂, which has previously been structurally characterized by us. All new complexes are characterized by vibrational and NMR spectroscopy. Assignment of the vibrational spectra and interpretation of the structural data are supported by DFT calculations.

Full text of DOI:10.1021/ic0206903

Inorg. Chem. 2003, 42, 3801−3814
Cationic Carbonyl Complexes of Rhodium(I) and Rhodium(III):
Syntheses, Vibrational Spectra, NMR Studies, and Molecular Structures
of Tetrakis(carbonyl)rhodium(I) Heptachlorodialuminate and -gallate,
[Rh(CO)4][Al2Cl7] and [Rh(CO)4][Ga2Cl7]
,†
Britta von Ahsen,† Christian Bach,† Michael Berkei,† Martin Ko1ckerling,† Helge Willner,*
,§
Gerhard Ha1gele,‡ and Friedhelm Aubke*
Fakulta¨t 4, Anorganische Chemie, Gerhard Mercator UniVersita¨t Duisburg, Lotharstrasse 1,
D-47048 Duisburg, Germany, Anorganische Chemie und Strukturchemie, Heinrich Heine
UniVersita¨t Du¨sseldorf, UniVersita¨tsstrasse 1, D-40225 Du¨sseldorf, Germany, and Department of
Chemistry, The UniVersity of British Columbia, 2036 Main Mall, VancouVer, BC V6T1Z1, Canada
Received November 26, 2002
Dimeric rhodium(I) bis(carbonyl) chloride, [Rh(CO)2(µ-Cl)]2, is found to be a useful and convenient starting material
for the syntheses of new cationic carbonyl complexes of both rhodium(I) and rhodium(III). Its reaction with the
Lewis acids AlCl3 or GaCl3 produces in a CO atmosphere at room temperature the salts [Rh(CO)4][M Cl7] (M) Al,
2
Ga), which are characterized by Raman spectroscopy and single-crystal X-ray diffraction. Crystal data for
[Rh(CO)4][Al2Cl7]: triclinic, space group P1h (No. 2); a ) 9.705(3), b ) 9.800(2), c ) 10.268(2) Å; R ) 76.52(2),
3
â ) 76.05(2), γ ) 66.15(2)°; V ) 856.7(5) Å ; Z ) 2; T ) 293 K; R [I > 2σ(I)] ) 0.0524, wR ) 0.1586. Crystal
1
2
data for [Rh(CO)4][Ga2Cl7]: triclinic, space group P1h (No. 2); a ) 9.649(1), b ) 9.624(1), c ) 10.133(1) Å; R )
3
77.38(1), â ) 76.13(1), γ ) 65.61(1)°; V ) 824.4(2) Å ; Z ) 2; T ) 143 K; R [I > 2σ(I)] ) 0.0358, wR )
1
2
0.0792. Structural parameters for the square planar cation [Rh(CO)4]+ are compared to those of isoelectronic
[Pd(CO)4]2+ and of [Pt(CO)4]2+. Dissolution of [Rh(CO)2Cl]2 in HSO F in a CO atmosphere allows formation of
3
[Rh(CO)4]+(solv). Oxidation of [Rh(CO)2Cl]2 by S O F in HSO F results in the formation of ClOSO F and two seemingly
2
6
2
3
2
oligomeric Rh(III) carbonyl fluorosulfato intermediates, which are easily reduced by CO addition to [Rh(CO)4]+
.
(solv)
Controlled oxidation of this solution with S O F produces fac-Rh(CO)3(SO F)3 in about 95% yield. This Rh(III)
2
6
2
3
complex can be reduced by CO at 25 °C in anhydrous HF to give [Rh(CO)4]+(solv); addition of SbF at −40 °C to
5
the resulting solution allows isolation of [Rh(CO)4][Sb2F ], which is found to have a highly symmetrical (D ) [Sb2F ]-
11
4h
11
anion. Oxidation of [Rh(CO)2Cl]2 in anhydrous HF by F , followed in a second step by carbonylation in the presence
2
of SbF , is found to be a simple, straightforward route to pure [Rh(CO)5Cl][Sb2F ] , which has previously been
5
11 2
structurally characterized by us. All new complexes are characterized by vibrational and NMR spectroscopy.
Assignment of the vibrational spectra and interpretation of the structural data are supported by DFT calculations.
Introduction
diverse metal carbonyl family.4-10 The use of superacids11,12
13,14
Thermally stable σ-bonded metal carbonyl cations1-3 and
their derivatives are a very recent addition to the large and
like HF-SbF5
as reaction media and the development
of useful synthetic routes2,3 have allowed us to generate
σ-carbonyl cations and their derivatives for 16 metals,
ranging from group 12 (Hg) to group 6 (W, Mo).1-3 Salts
are formed with the superacid anions [Sb2F11]- and [SbF6]-.15
* To whom correspondence should be addressed. E-mail: willner@uni-
duisburg.de (H.W.); aubke@chem.ubc.ca (F.A.). Phone: int. 49-203-379-
3309 (H.W.); int. 604-822-3817 (F.A.). Fax: int. 49-203-379-2231 (H.W.);
int. 604-822-2847 (F.A.).
† Gerhard Mercator Universita¨t Duisburg.
(2) Willner, H.; Aubke, F. In Inorganic Chemistry Highlights; Meyer,
G., Naumann, D., Wesemann, L., Eds.; Wiley-VCH: Weinheim,
Germany, 2002; p 195.
‡ Heinrich Heine Universita¨t Du¨sseldorf.
§ The University of British Columbia.
(1) Willner, H.; Aubke, F. Angew. Chem., Int. Ed. Engl. 1997, 36, 2402.
(3) Willner, H.; Aubke, F. Chem.sEur. J. 2003, 9, in press.
10.1021/ic0206903 CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/14/2003
Inorganic Chemistry, Vol. 42, No. 12, 2003 3801

von Ahsen et al.
A substantial number of these salts have been structurally
characterized.2,3
is isolated,20,21 while, from a dilute solution of SbF5 in HF,
single crystals of the solvate [Ir(CO)6][SbF6]3‚4HF are
obtained.23
The metals in group 9 present a serious challenge to our
synthetic approach.1-3 Of the univalent cations [M(CO)4]+
(M ) Rh, Ir), which are expected to be square planar with
a d8 electron configuration, only for Rh(I) a salt of the type
[Rh(CO)4][1-Et-CB11F11] has recently been obtained and is
structurally characterized.16 For Co(I), salts with a homoleptic
metal carbonyl cation have been unknown until very recently.
To synthesize the corresponding Rh(III) carbonyl com-
pounds, an approach different from the one used for the
syntheses of iridium(III) cationic carbonyl complexes19-23
is needed, because Rh(III) is easily reduced by CO to Rh(I).
Although Rh(SO3F)3 is known, it is exceedingly difficult to
25
synthesize.24 Likewise RhF6 is not as easily obtained as
[Co(CO)4]+
is claimed to exist in strong protonic acids
IrF626,27 and appears to be a far more powerful oxidizer than
IrF6. So far only [Rh(CO)5Cl][Sb2F11]2 is known.21 While
the molecular structure is obtained,21 its synthesis by oxi-
dative carbonylation of [Rh(CO)2Cl]2 with SbF5 as oxi-
(solv)
such as H2SO4, HSO3F, etc.17 The use of a new conjugate
Brønsted-Lewis superacid system formed by the Brønsted
acid11,12 HF and the nonoxidizing krypto Lewis acid (CF3)3-
BCO17a has allowed the isolation of [Co(CO)5][FB(CF3)3],17b
which is structurally characterized. The trigonal bypyramidal
[Co(CO)5]+ is unprecedented among known homoleptic
metal carbonyl cations.1-3 Oxidative interference from SbF5
had previously curtailed all attempts to generate unipositive,
oxidatively sensitive cations of the type [M(CO)n]+ (n ) 4,
5), with M ) Co, Rh, and Ir, in HF-SbF5.
28
dizer produces the adduct 6SbF3‚5SbF5 as an inseparable
byproduct.
The first evidence for [Rh(CO)4]+
is obtained by
(solv)
Raman spectroscopy. In HSO3F solution, [Rh(CO)4]+(solv) is
formed by reduction of small amounts of Rh(SO3F)324 with
CO29 or by addition of CO to [Rh(CO)2Cl]2.30 Since the
solutions are found to be unstable, a solid product is not
isolated.
On the other hand, the generation of cationic, structurally
characterized Ir(III) carbonyl complexes has been rather
The message contained in this brief summary is 2-fold:
(i) Judging by existing precedents16,17 and references cited
there, salts with the [Rh(CO)4]+ cation are obtainable, as long
as nonoxidizing Lewis acids are used in their generation or
by selecting a specific synthetic approach to [Rh(CO)4]-
[Sb2F11], where the oxidizing ability of SbF5 is negated.
(ii) To generate cationic Rh(III) carbonyl complexes, the
oxidation of suitable Rh(I) precursors by strong oxidizing
agents should be more promising than the opposite approach,
which is the reductive carbonylation of higher valent rhodium
species to Rh(III) carbonyls.
18
successful.1-3 The addition of CO to Ir(SO3F)3 in HSO3F
results in the formation of mer-Ir(CO)3(SO3F)3 as the main
product.19 The reaction of this compound with SbF5 in a CO
atmosphere and traces of halocarbon grease produces unex-
pectedly [Ir(CO)5Cl][Sb2F11]2.20 A more straightforward
synthesis of this compound by the oxidative carbonylation
of [Ir(CO)3Cl]x has subsequently been reported.21 Finally,
[Ir(CO)6]3+ is generated by the reductive carbonylation of
IrF6.20,22,23 In SbF5, the polycrystalline salt [Ir(CO)6][Sb2F11]3
(4) Lupinetti, A. J.; Frenking, G.; Strauss, S. H. Prog. Inorg. Chem. 2000,
49, 1.
There is an unique opportunity to synthesize and charac-
terize cationic carbonyl complexes of rhodium, with the metal
in two distinctly different oxidation states and coordination
geometries: square planar Rh(I) (d8); octahedral Rh(III) (d6).
It appears possible to establish a viable redox chemistry in
superacid media,11,12 involving well-defined mononuclear
carbonyl derivatives. This is not possible for other mono-
nuclear metal carbonyl cations and their derivatives, which
are only known with the metal in a single oxidation state.1-3
Even for iridium, where an extensive amount of work is
reported on octahedral Ir(III) carbonyl complexes,19-23 there
is at present only very preliminary Raman spectroscopic
evidence for [Ir(CO)4]+ in molten AlCl3 under a CO
atmosphere.30,31
(5) Ellis, J. E. AdV. Organomet. Chem. 1990, 31, 1.
(6) Cotton, F. A.; Wilkinson, G. In AdVanced Inorganic Chemistry, 5th
ed.; Wiley: New York, 1988; p 58.
(7) Pruchnik, F. P. Organometallic Chemistry of Transition Elements;
Plenum Press: New York, 1990.
(8) Elschenbroich, C.; Salzer, A. Organometallics; VCH: Weinheim,
Germany, 1992.
(9) Crabtree, R. M. In The Organometallic Chemistry of the Transition
Metals, 2nd ed.; Wiley: New York, 1994; p 142.
(10) Werner, H. Angew. Chem., Int. Ed. Engl. 1990, 29, 1077.
(11) Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids; Wiley: New
York, 1985.
(12) O’Donell, T. A. Superacids and Acidic Melts as Inorganic Reaction
Media; VCH: Weinheim, Germany, 1993.
(13) Hyman, H. M.; Quaterman, L.; Kirkpatrick, M.; Katz, J. J. J. Phys.
Chem. 1961, 65, 123.
(14) Gillespie, R. J.; Moss, K. C. J. Chem. Soc. A 1966, 1170.
(15) Zhang, D.; Rettig, S. J.; Trotter, J.; Aubke, F. Inorg. Chem. 1996, 35,
6113.
(16) Lupinetti, A. J.; Havighurst, M. D.; Miller, S. M.; Anderson, O. P.;
Strauss, S. H. J. Am. Chem. Soc. 1999, 121, 11920.
(17) Xu, Q.; Inoue, S.; Souma, Y.; Nakatani, H. J. Organomet. Chem. 2000,
606, 147. (a) Finze, M.; Bernhardt, E.; Terheiden, A.; Berkei, M.;
Willner, H.; Christen, D.; Oberhammer, H.; Aubke, F. J. Am. Chem.
Soc. 2002, 124, 15385. (b) Willner, H.; Bernhardt, E.; Finze, M.;
Lehmann, C. W.; Aubke, F. Angew. Chem., accepted for publication.
(18) Lee, K. C.; Aubke, F. J. Fluorine Chem. 1982, 19, 501.
(19) Wang, C.; Lewis, A. R.; Batchelor, R. J.; Einstein, F. W. B.; Willner,
H.; Aubke, F. Inorg. Chem. 1996, 35, 1279.
(20) Bach, C.; Willner, H.; Wang, C.; Rettig, S. J.; Trotter, J.; Aubke, F.
Angew. Chem., Int. Ed. Engl. 1996, 35, 1974.
(21) Willner, H.; Bach, C.; Wartchow, R.; Wang, C.; Rettig, S. J.; Trotter,
J.; Jonas, V.; Thiel, W.; Aubke, F. Inorg. Chem. 2000, 39, 1933.
(22) v. Ahsen, B.; Bach, C.; Pernice, H.; Willner, H.; Aubke, F. J. Fluorine
Chem. 2000, 102, 243.
(23) v. Ahsen, B.; Berkei, M.; Henkel, G.; Willner, H.; Aubke, F. J. Am.
Chem. Soc. 2002, 124, 8371.
(24) Leung, P. C.; Wong, G.; Aubke, F. J. Fluorine Chem. 1987, 35, 607.
(25) Chernick, C. L.; Classen, H. H.; Weinstock, B. J. Am. Chem. Soc.
1961, 83, 3165.
(26) Ruff, O.; Fischer, J. Z. Anorg. Allg. Chem. 1929, 179, 166.
(27) Hargreaves, G. B.; Peacock, R. D. Proc. Chem. Soc. London 1959,
85.
(28) Nandana, W. A. S.; Passmore, J.; White, P. S. J. Chem. Soc., Dalton
Trans. 1985, 1623.
(29) Wang, C. Ph.D. Thesis, University of British Columbia, 1996.
(30) Bach, C. Ph.D. Thesis, Universita¨t Hannover, 1999.
(31) 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.
3802 Inorganic Chemistry, Vol. 42, No. 12, 2003

Cationic Carbonyl Complexes of Rh(I) and Rh(III)
Table 1. Crystallographic Data for [Rh(CO)4][Al2Cl7] and
[Rh(CO)4][Ga2Cl7]
Experimental Section
(a) Chemicals. [Rh(CO)2Cl]2 (99% purity) and AlCl3 (99.9%
purity) both from Chempur, Karlsruhe, Germany, were used without
further purification. Gallium trichloride was synthesized by the
reaction of gallium metal (99.99% purity; Strem Chemicals Inc.,
Kehl, Germany) with chlorine (technical grade) in a sealed glass
reactor at room temperature, with a Cl2 pressure of about 6 atm.
The product was purified by sublimation in vacuo (10-5 mbar) at
40 °C. F2 (technical grade, Solvay, Hanover, Germany), HSO3F
(technical grade, Bayer AG, Leverkusen, Germany), and SbF5
(Scientific Industrial Association, Moscow, Russia) were used. The
latter two were purified by distillation first at atmospheric pressure
and then in vacuo. Anhydrous HF (99% purity; Allied Signal,
Seelze, Germany) was used without further purification and stored
over a small amount of SbF5 to remove traces of water as [H3O]-
[Sb2F11].15 Bis(fluorosulfuryl) peroxide, S2O6F2,32 was synthesized
by the catalytic (AgF2) fluorination of SO3 (Dupont) with elemental
fluorine, as described.33 Carbon monoxide (99%, Linde Gas,
Hanover, Germany) was purified, by liquefying and evaporation
of CO at -196 °C. 13CO (99% enriched; Deutero GmbH, Kastel-
laun, Germany) was used without further purification.
empirical formula
fw
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C4Al2Cl7O4Rh
517.079
Ph1 (No. 2)
9.705(3)
9.800(2)
10.268(2)
76.52(2)
76.05(2)
66.15(2)
856.7(5)
2
2.005
0.710 73
293
21.88
0.0524
C4Cl7Ga2O4Rh
602.54
Ph1 (No. 2)
9.649(1)
9.624(1)
10.133(1)
77.38(1)
76.13(1)
65.61(1)
824.4(2)
2
2.427
0.710 73
143
53.3
0.0358
Z
D
calc (g/cm3)
λ (Å)
T (K)
linear abs coeff µ (cm-1
R1(Fo)a
)
2
wR2(Fo )b
0.1586
0.0792
2
2
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|. b wR2 ) Σw(F -F 2)2/Σw(F )2.
x
o
c
o
(ii) NMR Spectroscopy. Preparation of [Rh(13CO)x-
+
(CO)4-x
]
(solv), x ≈ 1, Samples for 13C NMR Studies. A 1 mL
volume of a freshly prepared sample of [Rh(CO)4]+ in HO3F was
transferred to a NMR tube equipped with a rotational symmetrical
PTFE valve.34 Subsequently, 13CO was added in vacuo, until a
pressure of 0.5 atm at 25 °C resulted, and then the NMR tube was
sealed. 13C NMR spectra were recorded at variable temperatures
on a Bruker NMR spectrometer MSL 200, operating at 50.3 MHz.
Preparation of Rh(III) Carbonyl Samples for 13C NMR
Studies. 13C NMR spectra were recorded on a Bruker Avance DRX-
300 spectrometer, operating at 75.47 MHz, or on a Bruker Avance
DRX-500 spectrometer, operating at 125.758 MHz. All reactions
were carried out using 13CO during the whole syntheses of the
respective species. Solutions of the rhodium carbonyl compounds
dissolved in HSO3F were transferred directly from the reaction
vessel via 2 mm o.d. FEP tubes into FEP lining, placed in thin
wall NMR glass tubes. The reaction vessel was pressurized by using
dry gaseous argon. The NMR signals were referenced against TMS
using CD2Cl2 films between the FEP lining and the NMR tube as
external standard (δ(13C) ) 54.0 ppm) and lock. All measurements
were carried out at -20 °C.
Warning! Of the chemicals used in this study F2, S2O6F2, SO3,
Cl2, and SbF5 are strongly oxidizing, while HF, HSO3F, and HF-
SbF5 are highly corrosiVe. Most of these and CO are toxic. All
work should be done in well-Ventilated fumehoods, with personal
safety equipment in place.
(b) 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, containing a few mole percent of SbF5, was stored
in a PFA tube (12 mm o.d., 300 mm long), which was heat sealed
at the bottom and connected on top to a stainless steel needle valve
(3762H46Y Hoke, Creskill, NJ). All other volatile reactants 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 two part V-shaped reactor with an approximate
volume of 25 mL, consisting of two PFA tubes (12 mm o.d., 100
mm length) and a PFA needle valve (Fluoroware, MN) was used.
All other reactions were carried out in glass reactors, equipped with
valves with PTFE stems (Young, London). Solid materials were
manipulated inside a drybox (Braun, Munich, Germany), filled with
dry argon, with a residual moisture of less than 0.1 ppm.
(c) Instrumentation. (i) Vibrational Spectroscopy. Infrared
spectra were recorded at room temperature on a IFS-66v FT
spectrometer (Bruker, Karlsruhe, Germany). Two different detectors,
together with a KBr/Ge or a 6 µm Mylar/Ge beam splitter, operating
in the regions 5000-400 or 650-50 cm-1, respectively, were used.
A total of 128 scans were coadded for each spectrum, using an
apodized resolution of 2 or 4 cm-1. The samples were crushed
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
instrument in the region 5000-80 cm-1 with a spectral resolution
of 2 cm-1, using the 1064 nm exciting line (∼500 mW) of a Nd:
YAG laser (Adlas, DPY 301, Lu¨beck, Germany).
(iii) Single-Crystal X-ray Diffraction. Diffraction data of
[Rh(CO)4][Al2Cl7] were collected at 293 K on a Siemens P4 four-
circle diffractometer, whereas those of [Rh(CO)4][Ga2Cl7] were
collected at 143 K on a Nonius Kappa CCD-diffractometer. Both
data collections utilized graphite-monochromated Mo KR radiation
(λ ) 0.71073 Å). The crystallographic data, details on the data
collection, and the refinement procedures are found in Table 1.
During the data collection of [Rh(CO)4][Al2Cl7], the intensity of
two check reflections, which were measured after every 98
reflections, decreased linearly. The data were corrected for this
decay, as well as for absorption effects, with the aid of 2 ψ-scans
and also for Lorentz and polarization effects. The structure was
solved with direct methods (SHELX97)35 in the space group Ph1.
Refinements were done by full-matrix least-squares methods on
F2 with anisotropic thermal displacement parameters for all atoms,
using the SHELX97 program suite.35 For [Rh(CO)4][Ga2Cl7], the
data collection covered almost the whole sphere of reciprocal space
with 3 sets at different κ-angles and 279 frames via ω-rotation
(∆/ω ) 1°) at two times 75 s/frame. The crystal to detector distance
(34) Gombler, W.; Willner, H. Int. Lab. 1984, 14, 84.
(35) Sheldrick, G. M.: SHELXL97-Programs for the Solution and Refine-
ment of Crystal Structures; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(32) Dudley, F. B.; Cady, G. H. J. Am. Chem. Soc. 1959, 79, 513.
(33) Zhang, D.; Wang, C.; Mistry, F.; Powell, B.; Aubke, F. J. Fluorine
Chem. 1996, 76, 83.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3803

von Ahsen et al.
was 3.4 cm. Crystal decay was monitored by repeating the initial
frames at the end of data collection. From the analysis of the
duplicate reflections, there was no indication for any decay. The
structure was also solved by direct methods and successive
difference Fourier syntheses. Refinement applied the full-matrix
least-squares methods in SHELX97.35
(d) Calculations. The calculations were performed with the
GAUSSIAN 98 software package,36 using density functional
theory37 and employing the three-parameter B3LYB hybrid func-
tion,38 which incorporates the Becke exchange function39 and the
Lee, Yang, Parr correlation function,40 in combination with 3-21
g* basis set to optimize the molecular geometries to standard
convergence criteria. The presented vibrational data are unscaled
harmonic wavenumbers and band intensities of the molecules or
ions in the gas phase. No ECP was used for Rh. The calculations
were done as all-electron calculations.
(e) Synthetic Reactions. (i) Improved Synthetic Route to [Rh-
(CO)5Cl][Sb2F11]2. To a 100 mL PFA round-bottom flask, charged
with 50 mg (0.13 mmol) of [Rh(CO)2Cl]2 and fitted with a PFA
adaptor and a PFTE-coated stirring bar, approximately 5 mL of
anhydrous HF was added in vacuo, using a stainless steel vacuum
line. The reactor was kept at liquid N2 temperature. Upon warming
of the reactor to room temperature, [Rh(CO)2Cl]2 remained undis-
solved and no gas evolution (CO) was noted. With the reactor again
cooled to -196 °C, 0.9 mmol of F2 was added, giving rise to a
partial F2 pressure of 0.23 atm at room temperature. After the sample
was stirred for a few minutes, the HF phase turned pale yellow.
The solid material turned brown and began to cling to the reactor
walls. Stirring was continued for a further 2 h, and an additional
amount of F2 (1.3 mmol) was added. After continued stirring for 1
h at room temperature, a brown solid had settled to the bottom,
with the supernatant HF phase colorless. With the reactor at liquid
N2 temperature, the excess F2 was removed in vacuo and about 3
mL of HF and 2 mL of SbF5 were condensed onto the reaction
mixture. Warming of the reaction mixture to room temperature
produced a brown, turbid suspension. Further warming to 70 °C
resulted in a brown solution after 10 min. After the solution was
stirred for a further 30 min, approximately 4.3 mmol of CO was
added to give a total pressure of about 1 atm at 25 °C. After the
sample was stirred for 20 h at -20 °C, a clear, pale yellow solution
resulted, with small amounts of a yellow solid suspended in it.
Removal of all volatiles in vacuo yielded a yellow solid which was
identified as pure [Rh(CO)5Cl][Sb2F11]2 by vibrational spectroscopy
and comparison to the published spectrum.21 The conversion of
[Rh(CO)2Cl]2 into [Rh(CO)5Cl][Sb2F11]2 is quantitative.
1.5 mL of HSO3F and about 0.6 mL of S2O6F2 were condensed
onto the solid, which dissolved on warming to room temperature
to give an orange solution. After the solution was stirred for a further
10 min, the gas phase was removed in vacuo and another 0.5 mL
of S2O6F2 was added. The resulting mixture was stirred at 80 °C
for 24 h, where, according to the vibrational spectra, seemingly
oligomeric Rh(III) carbonyl fluorosulfato species had formed, which
could not be separated cleanly. After removal of all volatiles in
vacuo, about 2 mL of HSO3F was condensed onto the mixture and
1.6 mmol CO was admitted, to give a partial pressure of 2 atm at
25 °C. Stirring the mixture for several minutes at room temperature
resulted in a clear yellow solution of [Rh(CO)4]+(solv). After the
reactor was cooled to liquid-N2 temperature, all excess CO was
removed in vacuo and 0.6 mL of S2O6F2 was added. On warming,
vigorous bubbling was noted. After 1 min, before the reaction
mixture had warmed to room temperature, the excess S2O6F2 and
HSO3F were removed in vacuo and a white, solid material remained,
which was subsequently identified by vibrational and 13C NMR
spectroscopy as fac-Rh(CO)3(SO3F)3, again obtained in about 95%
yield, with a small amount (∼5%) of a brown byproduct dissolved
in HSO3F.
(iii) [Rh(CO)4][Sb2F11]. A 100 mg (0.21 mmol) of fac-Rh(CO)3-
(SO3F)3 was placed in bulb (A) of a two-bulb PFA reactor, fitted
with a stainless steel top. About 1.5 mL of anhydrous HF was added
in vacuo, with the bulb A kept at liquid-N2 temperature. After the
mixture was warmed, the color of the white solid changed to dark-
orange and the supernatant solution turned pale yellow. After the
reaction mixture was stirred for several minutes at room temper-
ature, a red-orange, viscous mass had formed, which was insoluble
in HF. In addition CO gas evolution was observed. After the
addition of 1.2 mmol of CO to give a partial CO pressure of ∼1
atm at 25 °C, the solid mass began to dissolve in HF and the color
changed from orange to yellow. When the process was monitored
by Raman spectroscopy, two bands at 2219 (vs) and 2180 cm-1
(s) were identified as the Raman-active CO stretches of [Rh-
(CO)4]+
.
29,30 After cooling of bulb A to -196 °C and removal
(solv)
of all excess CO in vacuo, 0.5 mL of SbF5 and 1.2 mmol of CO
were added. The mixture was warmed to -40 °C, and HF in the
lower part of the reactor bulb became liquid, while solid SbF5
remained in the upper arm. As soon as the liquid phase came in
contact with solid SbF5, formation of a finely powdered, yellow
solid was noted, which subsequently became suspended in HF. After
repeated shaking of the reactor by hand, all SbF5 had been removed
from the reactor wall. The resulting mixture was then stirred for
12 h, with the reactor at -20 °C. With the temperature maintained
at -20 °C, the supernatant solution containing excess SbF5 was
decanted into bulb B of the double bulb reactor. HF was
subsequently condensed back onto the solid, and the washing and
decanting process was repeated. After all volatiles were removed
in vacuo, the yellow solid residue remaining in the reactor was
identified by vibrational analysis as pure [Rh(CO)4][Sb2F11] again
formed quantitatively.
(ii) fac-Rh(CO)3(SO3F)3. Inside the drybox, 85 mg (0.219 mmol)
of [Rh(CO)2Cl]2 was added to a carefully dried Raman cell of about
20 mL volume, fitted with a magnetic stirring bar. Subsequently
(36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J. A. GAUSSIAN 98 ReV. A.5; Gaussian Inc.:
Pittsburgh, PA, 1998.
(iv) Single-Crystal Growth of [Rh(CO)4][Al2Cl7]. A glass
reactor of about 25 mL volume, fitted with a glass valve with PTFE
stem, was, after careful heating in vacuo, charged inside a drybox
with 38 mg (0.10 mmol) of [Rh(CO)2Cl]2 and 54 mg (0.40 mmol)
of AlCl3. Both reactants had been finely powdered before mixing.
To the reaction mixture, CO was condensed to give at 25 °C a
partial pressure of about 1 atm. After the reactor was warmed to
room temperature, the color of the reaction mixture changed to
bright yellow. The reactor was kept at 25 °C for 3 weeks, and after
this time, red needlelike crystals began to grow from the reactor
wall toward the center. Raman measurements proved clearly that
(37) Kohn, W.; Sham, L. J. Phys. ReV. A 1965, 140, 1133.
(38) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(39) Becke, A. D. Phys. ReV. A 1988, 38, 3098.
(40) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 41, 785.
3804 Inorganic Chemistry, Vol. 42, No. 12, 2003

Cationic Carbonyl Complexes of Rh(I) and Rh(III)
Figure 1. Synthesis of carbonyl compounds of Rh(I) and Rh(III) starting from [Rh(CO)2Cl]2.
this compound differs from the starting material [Rh(CO)2Cl]2, but
unfortunately, there were no suitable crystals for X-ray diffraction
measurements. After a further 1 week, yellow crystallites were noted
in a different location, which grew within 2 weeks to yellow prisms
with edge lengths of about 1 mm. The yellow crystals were identi-
fied by Raman spectroscopy as [Rh(CO)4][Al2Cl7]. Suitable crystals
were filled inside a drybox under a polarizing microscope into
Lindemann glass capillaries and mounted to the goniometer head.
(v) Single-Crystal Growth of [Rh(CO)4][Ga2Cl7]. In a round-
bottom flask of 100 mL volume, 50 mg (0.13 mmol) of finely
ground [Rh(CO)2Cl]2 was combined inside a drybox with 136 mg
(0.77 mmol) of powdered GaCl3. By using a vacuum line, CO was
admitted and the CO pressure inside the reactor was adjusted to 1
atm at 25 °C. After the reactor was warmed to room temperature,
the mixture began to turn yellow within a few minutes. After several
hours, small droplets had formed and melting was noted at the
bottom of the flask. Raman spectroscopy revealed the presence of
[Rh(CO)4][Ga2Cl7] together with some excess Ga2Cl6. After several
days, small crystals had formed in the neck of the reactor. Suitable
crystals were picked under a polarizing microscope in a stream of
dry N2 and loaded into Lindemann glass capillaries, which in turn
were mounted onto the goniometer head.
carbonylations of or halide abstraction from Rh(I) precursors
are employed.
A versatile starting material for many syntheses is found
to be [Rh(CO)2Cl]2. The compound is commercially available
and known since 1943.42 Its molecular structure43 and
vibrational spectra44 are well-known, and [Rh(CO)2Cl]2 has
found in the past extensive use in the syntheses of many
organometallic rhodium(I) compounds.45
A summary of the synthetic reactions discussed here is
shown in Figure 1. With [Rh(CO)2Cl]2 as precursor, four
[Rh(CO)4]+ salts with different anions are obtained, with two
of them, [Rh(CO)4][Al2Cl7] and [Rh(CO)4][Ga2Cl7], structur-
ally characterized. In addition an improved synthetic route
to [Rh(CO)5Cl][Sb2F11]2 is presented, and the previously
unknown compound fac-Rh(CO)3(SO3F)3 is synthesized in
HSO3F. Furthermore, two additional Rh(III) carbonyl fluo-
rosulfate compounds with different compositions and CO
contents are encountered but not isolated. Interconversions
of Rh(I) and Rh(III) species in HSO3F are summarized in
Figure 2.
(vi) [Rh(CO)4][GaCl4]. In a inverted V-shaped glass reactor with
a volume of about 45 mL, 55 mg (0.14 mmol) of [Rh(CO)2Cl]2
was placed in bulb A and 100 mg (0.57 mmol) of GaCl3 in bulb B.
By using a vacuum line, CO was admitted to give inside the reactor
a CO pressure of about 0.9 atm at 25 °C. After the reaction mixture
was kept at room temperature for several days, small yellow
crystallites began to grow at different places inside the reactor,
which were examined by Raman spectroscopy. In the neck of bulb
B, the spectrum of the product was identical to that of [Rh(CO)4]-
[Ga2Cl7], while those recorded of samples near bulb A showed
clearly [GaCl4]- as a counteranion of [Rh(CO)4]+.
(b) Rhodium(III) Carbonyl Derivatives. The previously
reported synthesis of [Rh(CO)5Cl][Sb2F11]2 involves the
21
oxidative carbonylation of [Rh(CO)2Cl]2 in HF-SbF5 with
SbF5 as oxidizer. This results in the formation of the adduct
6SbF3‚5SbF528 as a reduced byproduct. While a clean sepa-
ration of the adduct from the main product is not achieved,
due to similar solubilities of both compounds in HF-SbF5,
crystals of [Rh(CO)5Cl][Sb2F11]2 are picked under a polar-
izing microscope, on account of their different habit and the
structure is subsequently solved.21 The use of F2 as external
oxidizer, employed in the improved synthesis of [Rh(CO)5Cl]-
Results and Discussion
Synthetic Aspects. (a) General Comments. It has become
apparent from the introduction that two important synthetic
routes to superelectrophilic41 metal carbonyl cations and their
derivatives,1-3 reductive and/or solvolytic carbonylation in
superacids, may not be appropriate for the syntheses of either
Rh(I) or Rh(III) carbonyl complexes. Instead oxidative
(41) Olah, G. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 767.
(42) Hieber, W.; Legally, H. Z. Anorg. Allg. Chem. 1943, 251, 96.
(43) Dahl, L. F.; Martell, C.; Wampler, D. L. J. Am. Chem. Soc. 1961, 83,
1761.
(44) Johnson, B. F. G.; Lewis, J.; Robinson, P. W. J. Chem. Soc. 1969,
2693.
(45) Griffith, W. P. In The Chemistry of the Rarer Platinium Metals (Os,
Ru, Ir, Rh); Interscience: New York, 1967; p 384.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3805

von Ahsen et al.
Figure 2. Interconversions of Rh(III) and Rh(I) carbonyl fluorosulfate species in HSO3F and the spectroscopic identification.
[Sb2F11]2 described here, mandates a separation of the
oxidation step from the carbonylation of the oxidized
intermediate. The initial step proceeds according to
vibrational spectrum in the SO3F- region suggests the pres-
ence of bidentate bridging as well as terminal fluorosulfate
groups.18
Heating a solution of compound I in HSO3F to 60-80
°C in the presence of S2O6F2 produces an orange solid,
labeled species II, which is insoluble in HSO3F. This
compound has a single CO stretch at 2198 cm-1. In the
fluorosulfate region, the vibrational spectrum is remarkably
similar to that reported for Rh(SO3F)3,24 which is also orange
in color and almost insoluble in HSO3F. Since the reaction
of S2O6F2 with various metal carbonyls is reported to be a
useful synthetic route to metal fluorosulfates,49 it appears
likely that Rh(SO3F)3 is a component of species II. While
oxidation of [Rh(CO)2Cl]2 by S2O6F2 in HSO3F appears to
be a more facile process than the reported oxidation of
rhodium metal,24 a complete removal of CO from product
II is not achieved here.
HF/F2
n[Rh(CO)2Cl]2 + 2nF2
8 2[Rh(CO)2ClF2]n (1)
25 °C
The dark brown intermediate, tentatively formulated as [Rh-
(CO)2ClF2]n, is found to be sparingly soluble in aHF
(anhydrous HF) but is neither isolated nor characterized. The
limited number of completely characterized metal carbonyl
fluorides known46,47 would justify a complete study of this
compound; however, this is not intended at this time. The
subsequent carbonylation of [Rh(CO)2ClF2]n proceeds cleanly.
The dark brown solid dissolves in HF/SbF5 at 70 °C. The
carbonylation of the Rh(III) compound at -20 °C is
formulated as
HF/SbF2
Hence a clean separation and full characterization of the
new Rh(III) fluorosulfate compounds is not feasible. As can
be seen in Figure 2, species I and II are readily intercon-
verted in HSO3F by reaction with either S2O6F2 or CO. The
carbonylation of species I or II is found to result in a 2e-
[Rh(CO)2ClF2](solv) + 3CO + 4SbF5 -20 °C 8
[Rh(CO)5Cl][Sb2F11]2 (2)
The final product is obtained in pure form as a yellow solid.
The Raman and IR spectrum of [Rh(CO)5Cl][Sb2F11]2 are
presented in the Supporting Information as Figure S1
(Raman) and Figure S2 (IR). The vibrational data have been
included already in our earlier publication, where the
experimental wavenumbers are compared to calculated
values.21
In contrast to the oxidation with F2 in aHF, treatment of
[Rh(CO)2Cl]2 in HSO3F with S2O6F2 takes a different course.
In a first step, chloride substitution takes place and ClOSO2F
is found as a byproduct, identified by Raman and 19F NMR
spectroscopy.48 Depending on the reaction conditions, two
intermediates, labeled I and II in Figure 2, are obtained.
reduction and the formation of [Rh(CO)4]+
.
(solv)
To complete the oxidative conversions in HSO3F, il-
lustrated in Figure 2, the previously unknown Rh(III)
carbonyl fluorosulfate compound fac-Rh(CO)3(SO3F)3 can
be prepared by a controlled oxidation of [Rh(CO)4]+
in
(solv)
HSO3F by S2O6F2. The resulting white solid is isolated and
characterized by vibrational and 13C NMR spectroscopy (vide
infra). The overall formation reaction is formulated as
HSO3F/S2O6F2
[Rh(CO)4]+(solv) + 2S2O6F2 + SO3F-
8
-10 °C
fac-Rh(CO)3(SO3F)3 + CO2 + S2O5F2 (3)
Species I, tentatively formulated as [Rh(CO)2(µ-SO3F)3-x
-
(SO3F)x]n, is obtained at 25 °C as a light yellow solid. There
are three CO stretches observed, which are IR and Raman
active. Their range between 2204 and 2230 cm-1 is consistent
with expectations21 for a Rh(III) carbonyl complex. The
The exclusive formation of fac-Rh(CO)3(SO3F)3 from square
planar [Rh(CO)4]+
appears to reflect the trans-directing
(solv)
ability of CO.50 According to the overall reaction (eq 3),
both a 2e- oxidation (Rh(I) to Rh(III)) by S2O6F2 or by two
(46) Doherty, N. N.; Hoffman, N. W. Chem. ReV. 1991, 91, 553.
(49) DeMarco, R. A.; Shreeve, J. M. AdV. Inorg. Chem. Radiochem. 1974,
16, 115 and references herein.
(50) Wilkins, R. G. In Kinetics and Mechanism of Reactions of Transition
Metal Complexes, 2nd ed.; VCH: Weinheim, Germany, 1991; and
references therein.
(47) Murphy, E. F.; Murugavel, R.; Roesky, H. W. Chem. ReV. 1997, 97,
3425.
(48) Qureshi, A. M.; Levchuk, L. E.; Aubke, F. Can. J. Chem. 1971, 49,
35.
3806 Inorganic Chemistry, Vol. 42, No. 12, 2003

Cationic Carbonyl Complexes of Rh(I) and Rh(III)
•SO3F radicals33,49 and a nucleophilic substitution of CO by
SO3F- must occur. In the absence of any information on
intermediates, nothing meaningful regarding the sequence
of both events, the mechanism of oxidation (molecular or
radical), or any rearrangement reactions can be said at this
time.
A precedent is reported in the fluorination of Ir4(CO)12
by XeF2 in HF, which results in the formation of fac-
Ir(CO)3F3 as a main product, which is structurally character-
ized by EXAFS.51 On the other hand, the addition of CO to
Ir(SO3F)318 in HSO3F produces mer-Ir(CO)3(SO3F)3 as main
product, which is also structurally characterized.19 All these
reactions, which illustrate the trans-directing ability of CO50
as a common feature,40,51take place in the Brønsted super-
acids11,12 HSO3F52 and HF.
[M2Cl7]- (M ) Al, Ga) is found as counteranion. If the
carbonylation reaction is carried out in a V-shaped reactor,
using Ga2Cl6 as Lewis acid, the [Rh(CO)4]+ cation crystal-
lizes with two different counteranions, [Ga2Cl7]- or [GaCl4]-,
depending on the concentration of gallium trichloride (see
Experimental Section). The two anions can be clearly
identified by Raman spectroscopy due to differences in band
positions and intensities.
The use of AlCl3 as Lewis acid has a precedent in the
early pioneering studies by Fischer54,55 and Hieber,56-58 which
have resulted in the discovery of the first homoleptic metal
carbonyl cations. They are formed by group 7 metals Mn,
Tc, and Re, according to the general equation
85-100 °C
M(CO)5X + M′X3 + CO ∼300 atm of CO8 [M(CO)6][M′X4]
Finally, as seen in Figure 2, the direct solvolytic carbo-
nylation of the precursor [Rh(CO)2Cl]2 in HSO3F proceeds
according to
(6)
M ) Mn,54,55 Tc,58 Re;56,57 M′ ) Al, Fe; X ) Cl, Br
[Rh(CO)2Cl]2 + 2HSO3F + 4CO 1 atm of CO, 25 °C8
As can be seen, the reaction conditions in these early
studies54-58 are much more severe than the growth of single
crystals of [Rh(CO)4][M2Cl7] (M ) Al, Ga) from the gas
phase at 1 atm of CO and 25 °C described here. It is
interesting to note that early attempts to extend this approach
to the syntheses of salts with [M(CO)6]2+ (M ) Fe, Os)
cations had failed.59 It took almost 30 years until the group
8 cations could be obtained with SbF5 as Lewis acid.2,3 The
resulting cations are structurally characterized as [Sb2F11]-
or [SbF6]- salts.23,60
HSO3F
2[Rh(CO)4]+(solv) + 2HCl + 2SO3F-
(4)
(solv)
and produces the solvated cation [Rh(CO)4]+(solv), which is
then oxidized by S2O6F2 to fac-Rh(CO)3(SO3F)3. This
shortcut would allow one to bypass formation of the
intermediates I and II in a much simpler synthetic approach.
The generation of an isolable Rh(I) carbonyl fluorosulfate
of the possible composition Rh(CO)3(SO3F) is not achieved
by solvent removal from a solution of [Rh(CO)4]+
in
(solv)
A plausible pathway for the chloride abstraction from
[Rh(CO)2Cl]2 is suggested by a previous study61 of the
equilibrium
HSO3F. Removing the CO atmosphere from the reaction
vessel results immediately in the formation of a dark brown,
oily solution. A specific, well-defined product is not isolated.
(c) Tetrakis(carbonyl)rhodium(I) Cation, [Rh(CO)4]+,
in Solid-State Compounds. The existence of square planar
[Rh(CO)4]+ in solid-state compounds is demonstrated by the
recent synthesis of [Rh(CO)4][1-EtCB11F11],16 via carbony-
lation of the arene complex [(η6-C6H6)Rh(CO)2][1-Et-
CB11F11],53 and the subsequent structural characterization.16
In this study a simpler, more direct route of the chloride
abstraction from [Rh(CO)2Cl]2 by the Lewis acids M2Cl6 (M
) Al, Ga) followed by CO addition is chosen and found to
proceed according to
[Rh(CO)2Cl]2 + 2CO h 2Rh(CO)3Cl
(7)
Subsequent halide abstraction from monomeric Rh(CO)3Cl
and CO addition are viewed as steps in a facile route to
[Rh(CO)4][M2Cl7] (M ) Al, Ga).
While the synthesis of [Rh(CO)4]+ salts by an estab-
lished organometallic approach62,63 is accomplished in a
straightforward manner, the synthesis of [Rh(CO)4][Sb2F11]
is a far more challenging task. The halide abstraction from
[Rh(CO)2Cl]2 by SbF5 is not feasible, on account of the
oxidizing prowess of SbF5. Hence a more circuitous, two
step procedure is necessary. The starting material chosen is
fac-Rh(CO)3(SO3F)3, whose synthesis is reported in the
preceding part of this section (see also Figure 2).
25 °C
[Rh(CO)2Cl]2 + 2M2Cl6 + 4CO 1 atm of CO8
2[Rh(CO)4][M2Cl7] M ) Al, Ga (5)
In a reductive carbonylation, fac-Rh(CO)3(SO3F)3 is
As discussed in the Experimental Section, the reaction is
performed in a glass reactor without a solvent. The reactants
are found to give crystalline products, suitable for single-
crystal X-ray diffraction. Since molecular M2Cl6 (M ) Al,
Ga) is involved in the reaction and is used in a modest excess,
converted in anhydrous HF to [Rh(CO)4]+ according to
(54) Fischer, E. O.; O¨ fele, K. Angew. Chem. 1961, 73, 581.
(55) Fischer, E. O.; Fichtel, K.; O¨ fele, K. Chem. Ber. 1962, 95, 249.
(56) Hieber, W.; Kruck, T. Angew. Chem. 1961, 73, 580.
(57) Hieber, W.; Kruck, T. Z. Naturforsch., B 1961, 16, 709.
(58) Hieber, W.; Lux, F.; Herget, C. Z. Naturforsch., B 1965, 20, 1159.
(59) Hieber, W.; Frey, V.; John, P. Chem. Ber. 1967, 100, 1961.
(60) 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.
(51) Brewer, S. A.; Brisdon, A. K.; Holloway, J. H.; Hope, E. G.; Peck, L.
A.; Watson, P. G. J. Chem. Soc., Dalton Trans. 1995, 18, 2945.
(52) Thompson, R. C. In Inorganic Sulphur Chemistry; Nickless, G., Ed.;
Elsevier: Amsterdam, 1968; p 587.
(53) Ivanov, S. V.; Rockwell, J. J.; Polyakov, O. G.; Gaudinski, C. N.;
Anderson, O. P.; Solntsev, K. A.; Strauss, S. H. J. Am. Chem. Soc.
1998, 120, 4224.
(61) Morris, D. E.; Tinker, H. B. J. Organomet. Chem. 1973, 49, C 53.
(62) Abel, E. W.; Tyfield, S. P. AdV. Organomet. Chem. 1970, 8, 117.
(63) Beck, W.; Su¨nkel, K. Chem. ReV. 1988, 88, 1405.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3807

von Ahsen et al.
HF, 25 °C
fac-Rh(CO)3(SO3F)3 + 2CO
8
1 atm of CO
[Rh(CO)4]+(solv) + SO3F- + CO2 + S2O5F2 (8)
with the progress of the reaction monitored by Raman
spectroscopy. After cooling of the reaction mixture with
liquid nitrogen, excess CO is removed in vacuo and SbF5 is
added to the mixture, together with additional CO. The
mixture is warmed first to -40 °C and then to -20 °C
according to
[Rh(CO)4]+(solv) + 2HF + 2SbF5 -40 to -20 °C, 1 atm of CO8
HF/SbF5, 12 h
[Rh(CO)4][Sb2F11] + H2F+ (9)
Figure 3. Molecular structure of [Rh(CO)4][Ga2Cl7] with 50% probability
thermal ellipsoids shown (ORTEP plot).
There are a number of measures taken to ensure formation
of pure [Rh(CO)4][Sb2F11]: (i) The CO atmosphere is seen
to stabilize the [Rh(CO)4]+ cation in solution, prevents CO
dissociation, and provides a reducing atmosphere. (ii) An
acidic medium is maintained to prevent nucleophilic attack
on [Rh(CO)4]+ by either F- or SO3F-. (iii) The reaction
temperature is kept as low as possible to avoid oxidation of
[Rh(CO)4]+(solv) by SbF5 and to promote salt formation, which
does not seem to require elevated temperatures. (iv) The use
of an inverted V-shaped reactor allows removal of excess
SbF5 from the reaction mixture, so that subsequent oxidation
of the rhodium(I) carbonyl compound is avoided.
The synthesis of [Rh(CO)4][Sb2F11] is important because
in most salts of metal carbonyl cations [Sb2F11]- is usually
the counteranion1-3 as e.g. in structurally characterized
[Pd(CO)4][Sb2F11]2.31 With [Pd(CO)4]2+ and [Rh(CO)4]+
isoelectronic and isosteric, the effect of a lower complex
charge on the conformation of the [Sb2F11]- anion can be
studied. Unfortunately, the sensitivity of [Rh(CO)4]+ to
oxidation by SbF5 does not allow much flexibility in
temperature, which would be necessary in recrystallization
attempts from HF-SbF5.2,3 Hence we have to rely on
vibrational data for the characterization of [Rh(CO)4][Sb2F11].
Figure 4. Packing diagram of [Rh(CO)4][Al2Cl7].
[(Me6C6)3Zr3Cl6][Al2Cl7]2, a related precedent, both a stag-
gered and an eclipsed form of the two [Al2Cl7]- anions are
reported.64
Of the two isostructural [Rh(CO)4][M2Cl7] salts (M ) Al,
Ga), the [Ga2Cl7]- salt has surprisingly a slightly smaller
unit cell volume V by about 3.8% although the square planar
[Rh(CO)4]+ cations have identical bond parameters (see
Tables 2 and 3), while for the anions, [Ga2Cl7]-, the M-Cl
internuclear distances are slightly longer than those in
[Al2Cl7]-. There are only very few, weak interionic C--Cl
contacts to [Rh(CO)4]+ in both [M2Cl7]- (M ) Al, Ga) salts,
which are insignificantly shorter than the sum of the van
der Waals radii of 3.45 Å.65
In summary, there is a viable redox chemistry of Rh(III)
and Rh(I) carbonyl complexes, which can be exploited in
synthetic applications, once the synthetic potential of [Rh-
(CO)2Cl]2 is fully recognized.
Structural Aspects. Crystallographic details for [Rh(CO)4]-
[Al2Cl7] and [Rh(CO)4][Ga2Cl7] are found in Table 1. The
molecular structure of [Rh(CO)4][Ga2Cl7] is shown in Figure
3, and the packing of [Rh(CO)4][Al2Cl7] is depicted in Figure
4. Selected bond lengths and angles for both salts are listed
in Table 2, while in Table 3 structural and spectroscopic
properties of the cations in [Rh(CO)4][M2Cl7] (M ) Al, Ga),
[Rh(CO)4][1-EtCB11F11],16 and [M(CO)4][Sb2F11]2 (M ) Pd,
Pt)31 are collected and compared.
It hence appears that shrinkage at lower temperatures and
more efficient packing of the constituent ions are contributing
factors to the observed slightly smaller unit cell volume of
[Rh(CO)4][Ga2Cl7] compared to that of [Rh(CO)4][Al2Cl7].
Consistent with the assumption of greater packing efficiency,
the Ga-Cl-Ga bridge angle in the dinuclear anion is with
112.9(1)° more acute than the corresponding Al-Cl-Al
As can be seen from the crystallographic data in Table 1,
both salts [Rh(CO)4][Al2Cl7] and [Rh(CO)4][Ga2Cl7] are
isostructural. They crystallize in the triclinic space group P1h
(No.2), and the dinuclear anions [M2Cl7]- have staggered
conformations (see Figure 3). In the molecular structure of
(64) Stollmaier, F.; Thewalt, U. J. Organomet. Chem. 1981, 208, 327.
(65) Bondi, A. J. Phys. Chem. 1964, 68, 441.
3808 Inorganic Chemistry, Vol. 42, No. 12, 2003

Cationic Carbonyl Complexes of Rh(I) and Rh(III)
There is on account of relativistic effects67 remarkably little
difference in structural data and spectroscopic properties for
the isostructural pair [Pd(CO)4][Sb2F11]2 and [Pt(CO)4]-
[Sb2F11]2, with the unit cell volume for the latter slightly
smaller by about 0.4%.31
Table 2. Selected Internal Bond Parameters for [Rh(CO)4][Al2Cl7] and
[Rh(CO)4][Ga2Cl7]
[Rh(CO)4][Al2Cl7]
[Rh(CO)4][Ga2Cl7]
Bond Distances (Å)
1.97(1)
Rh(1)-C(1)
Rh(1)-C(2)
C(1)-O(1)
C(2)-O(2)
Rh(2)-C(3)
Rh(2)-C(4)
C(3)-O(3)
C(4)-O(4)
M(1)-Cl(2)a
M(1)-Cl(3)
M(1)-Cl(4)
M(1)-Cl(1)
M(2)-Cl(1)
M(2)-Cl(5)
M(2)-Cl(6)
M(2)-Cl(7)
1.968(6)
1.975(7)
1.117(5)
1.112(6)
1.954(6)
1.957(8)
1.134(5)
1.129(7)
2.145(2)
2.150(1)
2.162(1)
2.283(1)
2.341(2)
2.134(2)
2.152(2)
2.147(1)
1.93(1)
1.11(1)
1.13(1)
1.97(1)
1.93(1)
1.10(1)
1.13(1)
2.084(4)
2.097(3)
2.104(3)
2.235(3)
2.278(3)
2.090(3)
2.091(4)
2.092(3)
There is another important difference between the [Rh-
(CO)4]+ salts and homologous [M(CO)4]2+ (M ) Pd, Pt)
salts. In [M(CO)4][Sb2F11]2 (M ) Pd, Pt)31 as well as in other
structurally characterized fluoroantimonate salts of super-
electrophilic41 σ-metal carbonyl cations,1-3,21,23,60 the distorted
[Sb2F11]- anions are involved in the formation of extended
structures by significant interionic C--F interactions.1-3 For
[Rh(CO)4][M2Cl7] (M ) Al, Ga) the dinuclear anions are
formed on account of the experimental conditions (see
synthesis section) and are only marginally involved in very
weak interionic contacts; the [Rh(CO)4]+ cation can also have
the mononuclear [GaCl4]- as counteranion. Further evidence
for our view comes from a vibrational analysis of [Rh(CO)4]-
[Sb2F11] discussed in the next section, where the [Sb2F11]-
anion has D4h symmetry and is not distorted by interionic
contacts to [Rh(CO)4]+.
For the recently reported [Rh(CO)4][1-EtCB11F11], five
interionic Rh--F contacts in the range of 3.220(9)-3.588(9)
Å are claimed as evidence for “nonclassical” behavior.16
These contacts, together with an equally weak Rh-H
interaction, are claimed to be responsible for the relatively
low νCO value of 2138 cm-1 compared to 2162 cm-1 reported
for the same IR-active vibration in matrix isolated [Rh-
(CO)4]+.68
We have recalculated the interionic contacts for [Rh(CO)4]-
[1-EtCB11F11] from the deposited data for the salt and come
to a very different conclusion on two accounts: (i) The
reported16 Rh--F contacts are far too long to be of any
significance and are comparable or longer than the estimated
sum of the van der Waals radii.65 On the basis of their length
and the atomic charge distribution in [Rh(CO)4]+ (vide
supra), they are judged to be repulsive interactions. (ii) There
are however 14 C--F contacts in the range of 2.926-3.155
Å, comparable or slightly shorter than the sum of the van
der Waals radii65 which are not mentioned in ref 16. Since
C--F contacts imply a slight electron transfer from the anion
to the π* CO MO’s,2,3 they are most likely responsible for
the observed shift of νCO (Eu) in the solid state compared
with matrix data.61
In summary, significant interionic or inter- as well as
intramolecular contacts to the metal, claimed to be an
essential feature of “nonclassical” M-CO bonding,4 are not
observed in σ-carbonyl cations1-3 and the [Rh(CO)4]+16 salts
discussed here are no exception regardless of anion. The
claim of “nonclassical” behavior for [Rh(CO)4][1-EtCB11F11]
is based on an erroneous interpretation of the structural data.16
Vibrational Spectroscopy. The use of vibrational spec-
troscopy in this study is limited for a number of reasons:
(i) Our main emphasis is on synthetic aspects, in particular
on the development of a viable and synthetically useful redox
Bond Angles (deg)
179.1(9)
Rh(1)-C(1)-O(1)
Rh(1)-C(2)-O(2)
Rh(2)-C(3)-O(3)
Rh(2)-C(4)-O(4)
M(1)-Cl(1)-M(2)
178.5(5)
177.4(5)
176.7(5)
177.8(5)
112.9(1)
179.5(9)
178.2(9)
178.9(9)
115.5(1)
a M ) Al and Ga.
angle of 115.5(1)°, with both anions in an eclipsed confor-
mation. A similarly wide bridge angle of 116.1(4)° is found
for the staggered conformation of [Al2Cl7]- in [(Me6C6)3-
Zr3Cl6][Al2Cl7]2.64
Selected structural (d(M-C), d(C-O)) and vibrational
(νCO, fCO) data for the square planar cations [Rh(CO)4]+ and
[M(CO)4]2+ (M ) Pd, Pt)31 are listed in Table 3 and are
compared to calculated data.31 Also listed are the partial
atomic charges for the M-C-O moieties, obtained by DFT
methods.31
As can be seen, the structural data for the cations in all
three [Rh(CO)4]+ salts are identical within estimated standard
deviations and agree well with calculated parameters for [Rh-
(CO)4]+(g).31 The Rh-C distances are all longer than the
mean distance dm (1.846 Å) or the upper quartile qu value
(1.869 Å) from the Cambridge index,66 based on 238
samples. This reflects the diminished M f CO π-back-
bonding, apparent also from the short C-O bond lengths in
all three salts. Likewise, the vibrational properties (νCO, fCO
)
of [Rh(CO)4]+ show only very slight anion dependency.
Agreement of experimental with calculated data is reason-
able, and a complete vibrational analysis of [Rh(CO)4]+ will
be presented in the next section.
In summary all experimental evidence, supported by DFT
calculations, points in the case of [Rh(CO)4]+ to a unipositive
homoleptic σ-carbonyl cation with electrophilic carbonyl
carbons.1-3 This conclusion is in harmony with the calculated
atomic charges from a natural population analysis31 with qRh
) -0.15, qC ) 0.56, and qO ) -0.28.31
As expected, the increased oxidation state of M in the
group 10 salts [M(CO)4][Sb2F11]2 (M ) Pd, Pt)31 results in
even longer M-C and shorter C-O bond lengths, while
ν
CO(avg) is found about 90 cm-1 higher than for [Rh(CO)4]+.
(66) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, P.
(67) Pyykko¨, P. Chem. ReV. 1988, 88, 563.
(68) Zhou, M.; Andrews, L. J. Phys. Chem. A 1999, 103, 7773.
G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3809

von Ahsen et al.
Table 3. Comparison of Structural and Spectroscopic Parameters for Homoleptic D4h Cations [M(CO)4]n+ (n ) 1, 2)
bond lengths
CO stretching vibratns
13C NMR data
atomic chargesb
qM qC qO
d(M-C)avg d(C-O)avg ν1, ν6 (Ra)
ν
13 (IR) ν˜(CO)avg
fCO
δ(13C) 1JMC
compd
(Å)
(Å)
(cm-1
)
(cm-1
2138
2210, 2167e 2137d
1.123(6) 2209, 2166e 2137d
)
(cm-1
)
(102 N m-1
)
(ppm)
(Hz)
ref
[Rh(CO)4][1-Et-CB11F11] 1.951(6)
1.118(7) 2215, 2176
1.12(1)
2167
2163
16
[Rh(CO)4][Al2Cl7]
[Rh(CO)4][Ga2Cl7]
[Rh(CO)4]+(calc)c
[Pd(CO)4][Sb2F11]2
[Pd(CO)4]2+(calc)c
[Pt(CO)4][Sb2F11]2
[Pt(CO)4]2+(calc)c
1.95(1)
1.964(7)
1.967
1.992(6)
2.016
19.00a
173.5f
61.1f
this work
this work
}
2162
2147
2259
2234
2261
2234
1.135
1.106(6) 2278, 2263
1.125 2258, 2236
1.110(9) 2289, 2267
2198, 2150
2119
2249
2221
2244
2216
18.50
20.63a
19.86
20.64a
19.79
-0.15 0.56 -0.28 31
144.0g
31
0.43 0.56 -0.17 31
31
1.982(9)
2.009
137.0g 1550
1.125
2265, 2237
0.42 0.55 -0.15 31
b
d
a Approximation according to Cotton and Kraihanzel. Values in units of e. c Calculated data at the BP86/ECP2 level. [Sb2F11]- salt. e The B1g mode
(ν6) shows clearly a shoulder at higher frequency caused by crystal packing effects. f In HSO3F at -83 °C. MAS at room temperature.
g
chemistry of Rh(I) and Rh(III) carbonyl compounds in the
Brønsted superacids11,12 anhydrous HF and HSO3F,52 with
as counteranions as discussed above, with significant inte-
rionic contacts absent.
the help of powerful oxidizers such as elemental fluorine6
As seen from the vibrational data for [Rh(CO)4][Sb2F11]
and [Rh(CO)4][M2Cl7], (M ) Al, Ga) listed in Table 4, the
observable fundamentals show little anion dependency and
agreement with calculated data is good. As previously ob-
served1-3,21,23,31,60 and discussed, νCO is in calculations slightly
underestimated, usually by about 15-20 cm-1. The Raman
spectrum of [Rh(CO)4][Al2Cl7] is shown in Figure 5.
32,33,50
and bis(fluorosulfuryl) peroxide, S2O6F2.
In this chal-
lenging undertaking, vibrational spectroscopy has been very
useful in (a) monitoring reactions in situ, primarily by Raman
spectroscopy, and (b) identifying intermediates such as e.g.
species I and II shown in Figure 2, which are neither isolated
nor more fully characterized.
Only a limited number of fundamentals of [Rh(CO)4]+ are
observed for three reasons: (i) For [Rh(CO)4][M2Cl7] (M )
Al, Ga), only Raman data are available. (ii) As the bands
for the [M2Cl7]- anions (M ) Al, Ga) listed in Table 5 and
for [Sb2F11]- (D4h) in Table 6 show, extensive overlap of
anion and cation bands does, in some instances, not permit
an unambiguous assignment. (iii) As the calculated intensities
for [Rh(CO)4]+ vibrations show (see Table 4), a substantial
number of fundamentals will be unobservable. The calculated
wavenumbers also reveal that some fundamentals will be
expected to be below 100 cm-1.
(ii) One of the isolated and previously structurally
characterized rhodium(III) compounds, [Rh(CO)5Cl][Sb2F11]2,21
has been the subject of a complete vibrational analysis of
the cation,21 which includes experimental vibrational data,
obtained on the much purer compound, whose synthesis is
reported here. The vibrational spectra (Raman, IR) of [Rh-
(CO)5Cl][Sb2F11]2 are presented in the Supporting Informa-
tion (Figure S1 and S2).
(iii) In a recent, independent study,69 which deals primarily
with the structural characterization of the highly symmetrical
(D2h) adduct pyrazine‚2SbF5, a comparison of structural and
spectroscopic properties of the related high-symmetry (D4h)
conformer of the anion [Sb2F11]-, found in [Au(CO)2]-
[Sb2F11]70 and [H3F2][Sb2F11],71 is reported. The vibrational
comparison of [Sb2F11]- (D4h) found in [Au(CO)2][Sb2F11]70
and in [Rh(CO)4][Sb2F11], whose synthesis is described here,
is included, together with the results of DFT calculations,69
as a part of the vibrational characterization of [Rh(CO)4]-
[Sb2F11]. The vibrational spectra of [Rh(CO)4][Sb2F11] are
shown as Figure S3 in the Supporting Information.
With the help of calculated wavenumbers, Raman and IR
intensities listed in Table 4, and close analogy to the
assignments reported for isoelectronic [Pd(CO)4]2+ and the
related cation [Pt(CO)4]2+,31 a satisfactory vibrational as-
signment for square planar [Rh(CO)4]+ is achieved. This
assignment is complemented by previously reported force
constants,31 quoted in 102 N m-1: fCO(exp), 19.00; fCO(calcd)
18.50; fM-C(calcd), 2.18.
,
Differences in band positions and force constants between
[Rh(CO)4]+ and [M(CO)4]2+ (M ) Pd, Pt)31 are simply due
to the differences in oxidation states of the central metal ions
or the complex charges, which cause a shift of all funda-
mentals for [M(CO)4]2+ (M ) Pd, Pt) to higher wavenum-
bers.31 Strong interionic F--C contacts for [M(CO)4][Sb2F11]2
(M ) Pd, Pt)31 and their absence in [Rh(CO)4]+ salts have
very little effect on the vibrational spectra of the cations.31
Finally, an IR band at 2162 cm-1, reported in the matrix60
and attributed to [Rh(CO)4]+, is considerably higher than the
corresponding bands reported here or elsewhere16,31 for [Rh-
(CO)4]+ and should be viewed with some skepticism.
(iv) A preliminary vibrational analysis of [Rh(CO)4]+,
supported by DFT calculations, has been published recently31
in the context of the complete vibrational and structural
characterization of the related salts [M(CO)4][Sb2F11]2, (M
) Pd, Pt).31 The comparison is motivated by the fact that
[Rh(CO)4]+ and [Pd(CO)4]2+ 31 are isoelectronic as well as
isostructural. However, in contrast to superelectrophilic41
square planar [Pd(CO)4]2+ which is only known as the
[Sb2F11]- salt31 and forms an extended structure with
significant C--F contacts, [Rh(CO)4]+ is also known with
[1-Et-CB11F11]-,16 [M2Cl7]- (M ) Al, Ga), and [Sb2F11]-
It is interesting to mention that in the Raman spectra of
solid [Rh(CO)4][M2Cl7] (M ) Al, Ga) there is a shoulder at
the B1g CO stretching vibration (ν6) to higher wavenumbers
(see Table 4). This unresolved splitting is thought to be
(69) Sham, I. H. T.; Patrick, B.; v. Ahsen, B.; v. Ahsen, S.; Willner, H.;
Thompson, R. C.; Aubke, F. Solid State Sci. 2002, 4, 1457.
(70) Willner, H.; Schaebs, J.; Hwang, G.; Mistry, F.; Jones, R.; Trotter, J.;
Aubke, F. J. Am. Chem. Soc. 1992, 114, 8972.
(71) Mootz, D.; Bartmann, K. Angew. Chem., Int. Ed. Engl. 1988, 27, 391.
3810 Inorganic Chemistry, Vol. 42, No. 12, 2003

Cationic Carbonyl Complexes of Rh(I) and Rh(III)
Table 4. Experimental and Calculated Wavenumbers (cm-1) and Band Intensities for the Cation [Rh(CO)4]+ (D4h) with Three Different Anions
BP86/ECP2a
[Rh(CO)4][Sb2F11]
[Rh(CO)4][Al2Cl7]
[Rh(CO)4][Ga2Cl7]
vibrational modes
calc
intb
expt
int
exptc
int
exptc
int
ν1
ν6
A1g
B1g
ν(CO)
ν(CO)
2198
2150
201
287
2214
2174
s
vs
2167
s
2166
s
ν13
ν14
ν8
Eu
Eu
B2g
A2u
B2u
A1g
ν(CO)
2119
545
481
437
430
423
1677
130
0.1
3.4
i
2137
543
vs
s
δ(MCO)
δ(MCO)
δ(MCO)
δ(MCO)
ν(MC)
n.o.
i
n.o.
n.o.
ν4
465
m
ν10
ν2
18.7
406d
331
306
vw
s
405d
w
408d
308
w
ν7
ν15
ν3
ν12
ν9
ν16
ν5
ν11
B1g
Eu
A2g
Eg
B2g
Eu
A2u
B2u
ν(MC)
ν(MC)
404
353
320
316
91
86
54
30
1.3
82
i
0.3
8.1
0.5
0.5
i
δ(MCO)
δ(MCO)
δ(CMC)
δ(CMC)
δ(CMC)
δ(CMC)
i
i
i
vw
n.o.e
n.o.
vvw
n.o.
n.o.
n.o.
i
n.o.
b
a Reference 31. Abbreviations: s ) strong; m ) medium; w ) weak; sh ) shoulder; v ) very; n.o. ) not observed; i ) inactive mode. Intensities in
d
km mol-1 (IR; bold type) or Å4 amu-1 (Raman). c Only Raman data are available. ν2 and ν7 may be accidentally degenerate. e Overlap with ν4 vibration
from [Al2Cl7]- (see Table 5).
Ga) anions in Table 5. For the staggered conformers of C2
symmetry, found in the molecular structures reported here,
21 nondegenerate (11 A; 10 B) fundamentals are expected
in the Raman or the IR spectra. As seen in Table 5, for
[Al2Cl7]- 15 bands are detected compared to 12 bands for
[Ga2Cl7]-, but this number is reduced by two instances of
accidental degeneracy and one coincidence with a cation
band. For both anions, four fundamentals are expected to
fall below 90 cm-1 with low intensities. Hence, a satisfactory
agreement between results of DFT calculations and experi-
mental observations is achieved for both [Al2Cl7]- and
[Ga2Cl7]-.
The calculated and experimental wavenumbers for the D4h
conformer of [Sb2F11]- in [Rh(CO)4][Sb2F11] and [Au(CO)2]-
[Sb2F11],70 listed in Table 6, reflect a much simpler situation.
While a detailed structural and vibrational analysis of
[Sb2F11]- (D4h) is reported elsewhere,69 three salient points
are mentioned here: (i) The D4h conformer of [Sb2F11]- is
Figure 5. Raman spectrum of [Rh(CO)4][Al2Cl7] (single crystal).
caused by different orientations of the two [Rh(CO)4]+
cations, in the unit cell, relative to the [M2Cl7]- anions (M
) Al, Ga) in the unit cell of the crystal. On the other side,
if gallium trichloride is used as Lewis acid in a great excess,
the resulting mixture of [Rh(CO)4][Ga2Cl7] in Ga2Cl6 be-
comes liquid at room temperature, due to its low melting
point, and the shoulder disappears.
For the related square planar cation [Ir(CO)4]+, vibrational
spectra detected in molten AlCl3 are limited to the CO
stretching range. The reported bands at 2216 (A1g), 2170
(B1g), and 2125 (Eu) cm-1 compare well to the corre-
sponding bands for [Rh(CO)4][Sb2F11] at 2214, 2174, and
2137 cm-1. Similar close correspondence is found for the
pairs [M(CO)5Cl]2+ (M ) Rh, Ir),21 [M(CO)4]2+ (M ) Pd,
Pt)31 (see Table 3), and [M(CO)6]2+ (M ) Ru, Os),1 all
stabilized by [Sb2F11]- in isostructural salts.1-3,21,31
easily recognized, because there are in the Sb-F stretching
region (800-500 cm-1) 3 IR- and 3 Raman-active bands,
which are mutually exclusive. (ii) On increase of the oxida-
tion state of M as e.g. in [Pd(CO)4][Sb2F11]2,31 the electro-
philicity of the carbonyl carbon increases, significant C--F
contacts are found, the [Sb2F11]- distorts to C2V and C1, and
the vibrational spectrum in the Sb-F region becomes more
complicated.31 (iii) The ability of the dioctahedral [Sb2F11]-
anion to distort by bending of and rotation about the C4 axis,
to facilitate the formation of the C--F interionic contacts, is
unique and is documented in more than 10 molecular struc-
tures of salts with σ-metal carbonyl cations.2,3 At the core
of this flexibility are weak, linear Sb-Fb-Sb 3c-4e- bonds
which allow rotational as well as bending deformation. The
ditetrahedral anion [M2Cl7]- (M ) Al, Ga) does not have
the same potential as counteranion,64 and as seen in Table
5, a low-symmetry conformation gives rise to extensive band
proliferation even in the absence of significant interionic
contacts, as is the case for [Rh(CO)4][M2Cl7] (M ) Al, Ga).
The vibrational analyses of [Rh(CO)4][Al2Cl7] and
[Rh(CO)4][Ga2Cl7] are completed by the listing of calculated
and experimental wavenumbers for the [M2Cl7]- (M ) Al,
Inorganic Chemistry, Vol. 42, No. 12, 2003 3811

von Ahsen et al.
Table 5. Experimental and Calculated Wavenumbers (cm-1) and Band Intensities for the Anions [Al2Cl7]- and [Ga2Cl7]-
[Al2Cl7]- [Ga2Cl7]-
intb
vibrational modesa
B3LYP
exptc
int
B3LYP
intb
exptc
425
int
ν12
ν1
B
A
A
B
A
B
B
A
A
B
B
A
B
A
B
A
A
B
A
A
B
νas(MCl3)
567
567
551
546
438
392
335
308
197
186
173
160
154
146
120
94
0.8
0.3
1.5
1.0
6.2
0.2
0.1
10.4
0.4
1.4
1.5
3.0
3.2
1.6
0.4
1.7
2.1
0.9
0.2
0.0
0.0
559
559
537
519
438
388
336
311
195
182
173
164
152
vw
vw
w
413
411
401
398
352
339
291
263
157
143
137
131
126
125
107
82
2.5
0.8
4.5
2.7
23.6
0.9
0.2
7.0
0.3
2.1
1.2
3.0
2.9
2.3
0.7
2.7
2.7
1.0
0.3
0.0
0.1
vw
νas(MCl3)
νas(MCl3)
νas(MCl3)
νs(MCl3)
νs(MCl3)
νas(MClM)
νs(MClM)
δ(MCl3)
n.o.d
ν2
ν13
ν3
397e
w
w
m
371
332
289
261
162
149
143
139
126f
vs
vw
vw
w
ν14
ν15
ν4
vw
vw
s
vvw
w
sh
m
w
ν5
w
ν16
ν17
ν6
ν18
ν7
ν19
ν8
ν9
ν20
ν10
ν11
ν21
δ(MCl3)
sh
vs
vs
m
δ(MCl3)
δ(MCl3)
δ(MCl3)
δ(MCl3)
n.o.
n.o.
δ(MCl3)
101
sh
δ(MCl3)
106
98
w
w
n.o.
n.o.
n.o.
n.o.
n.o.
n.o.
δ(MCl3)
88
86
40
21
75
73
37
18
δ(MCl3)
n.o.
n.o.
n.o.
n.o.
δ(MClM)
τ(MClMCl)
τ(MClMCl)
17
18
a Assuming C2 symmetry. Abbreviations: see Table 4. b Raman intensities in Å4 amu-1
.
c Raman emissions only. d ν1 overlap with ν2/ν7 vibration from
f
[Rh(CO)4]+ cation. e ν2 and ν13 may be accidentally degenerated. ν7 and ν18 may be accidentally degenerated.
Table 6. Experimental and Calculated Wavenumbers (cm-1) and Band Intensities for the [Sb2F11]- Anion Assuming D4h Symmetry
B3LYP
[Rh(CO)4][Sb2F11]
expt int
[Au(CO)2][Sb2F11]
expt int
vibrational modes
calc
inta
ν20
ν16
ν1
Eu
Eg
νas(SbFeq)ip
779
775
742
739
702
697
679
678
627
319
318
301
299
294
283
271
287
0.0
5.4
188
8.3
28.8
4.1
i
144
157
0.3
3.6
i
691
s
689
vs
νas(SbFeq)op
ν(SbFax)ip
ν(SbFax)op
νs(SbFeq)op
νs(SbFeq)ip
νa(SbFeq)ip
νa(SbFeq)op
νa(SbFSb)
δ(SbFSb)
δ(SbFeq)
n.o.
n.o.
i
n.o.
n.o.
i
A1g
A2u
A2u
A1g
B1g
B2u
A2u
Eu
A1g
B2g
B1u
Eg
692
664
m
m
697
662
s
sh
ν6
ν7
ν2
ν10
ν14
ν8
ν21
ν3
ν13
ν12
ν17
ν22
ν9
651
598
s
w
653
598
vs
m
489
303
312
297
m
m
vvw
w
503
307
312
293
m
s
sh
m
δ(SbFeq)
δ(SbFeq)
δ(FSbFax)
δ(FSbFax)
δ(SbFeq)
i
i
1.7
46.6
206
273
274
242
229
273
274
242
229
vw
m
br
w
vw
m
br
w
279
267
230
229
279
267
230
229
w
s
sh
m
w
s
Eu
A2u
ν17
ν22
ν9
ν18
ν23
ν11
ν15
ν4
Eg
Eu
A2u
Eg
Eu
B1g
B2u
A1g
Eg
A1u
Eu
δ(FSbFax)
294
283
271
237
200
158
154
143
119
51
1.7
δ(FSbFax)
46.6
206
3.0
δ(SbFeq)
sh
m
δ(SbFeq)
δ(FaxSb‚‚‚SbFax)
δ(SbFeq)
n.o.
n.o.
i
n.o.
n.o.
i
n.o.
n.o.
i
δ(SbFeq)
νs(SbFSb)
δ(FaxSb‚‚‚SbFax)
τ(FeqSb‚‚‚SbFeq)
δ(SbFSb)
0.9
0.2
i
131
w
ν19
ν5
ν24
n.o.
i
n.o.
32
0.1
n.o
a Raman activities in Å4 amu-1; IR activities in km mol-1 (bold type). Abbreviations: see Table 4; ip ) in phase, op ) off phase.
The final example in this section is fac-Rh(CO)3(SO3F)3,
whose synthesis is described here. In the absence of a
structural characterization, a vibrational analysis of this
compound is essential. Therefore, experimental (Raman, IR,
and estimated intensities) and calculated (wavenumbers and
Raman and IR band intensities) vibrational data to about 300
cm-1 are listed in Table 7. An assignment included here
assumes C3V symmetry. The vibrational spectra of fac-Rh-
(CO)3(SO3F)3 are shown in Figure 6.
Identification of the compound as fac-Rh(CO)3(SO3F)3
rests in part on the observation of a single resonance in the
13C NMR spectrum (see Figure 2) (δ(13C), 149.4 ppm; 1JRh-C
,
57.0 Hz) found for solutions in HSO3F and the observation
of two bands in the CO stretching region, ν1(A) at 2243 (IR
and Raman) and ν21(E) at 2223 (IR) or 2222 (Raman) cm-1,
with ν(CO)avg of ∼2230 cm-1 for the isolated product. These
values seem to be at variance with reports for fac-Ir(CO)3-
19
(SO3F)3 with 2233 (A), 2156 (E), and ν(CO)avg. at 2181
3812 Inorganic Chemistry, Vol. 42, No. 12, 2003

Cationic Carbonyl Complexes of Rh(I) and Rh(III)
(i) As discussed for [M(CO)4]+ (M ) Rh, Ir), vibrational
data for isostructural Rh and Ir carbonyl derivatives differ
very little.2,3 (ii) Replacement of one or more CO ligands in
Table 7. Experimental and Calculated Vibrational Data for
fac-Rh(CO)3(SO3F)3 to 280 cm-1
exptl data
calcd data
int IR
homoleptic metal carbonyl cations by anionic ligands (e.g.
IR
rel
int
Ra
(cm-1
rel
int
B3LYP
(cm-1
int Ra
(cm-1
)
)
)
(km mol-1
)
(Å4 amu-1
)
2,3
Cl-, SO3F-, etc.) results in a gradual decrease of ν(CO)avg
.
2243
2223 vs
1392 vs
s
2243 vs
2222
1391 sh
2179.3
2161.6
1398.7
1394.3
1161.1
1155.5
957.3
935.5
819.0
812.7
656.2
633.1
550.8
550.1
506.4
501.2
487.8
453.6
433.7
431.6
421.0
398.7
376.4
355.8
339.0
317.9
289.1
279.8
124.1
180.4
376.2
278.7
88.9
459.3
15.7
1155.6
244.8
0.3
37.5
10.2
0.2
249.2
40.5
1.6
51.5
60.2
6.0
1.9
4.3
0.0
3.8
0.3
1.0
168.5
59.2
116.5
109.1
66.9
20.9
25.2
6.6
9.9
2.2
49.9
15.3
1.5
9.0
0.7
3.9
0.2
1.6
3.2
6.2
2.4
Hence the observed band positions for fac-Rh(CO)3(SO3F)3
m
agree with expectations, while those reported for fac-Ir(CO)3-
19
(SO3F)3 are doubtful, because they are almost 50 cm-1
1375
1219
1203
1042
1001
806
834
652
637
599
590
549
555
533
503
456
445
430
n.o.
398
n.o.
374
n.o.
298
281
m
m
m
w
w
w, br
w
vs
sh
w
w
sh
w
vw
w
sh
m
1202 vs, br
below those of fac-Rh(CO)3(SO3F)3.
The vibrational spectra in the SO3F-stretching region
(1400-800 cm-1) are consistent with the expectations for a
monodentate -OSO2F group bound to a trivalent metal
center in both fac-Rh(CO)3(SO3F)3 and mer-Ir(CO)3(SO3F)3.19
The band distribution with νas(SO2) at 1380 cm-1, ν(SO2)sym
at 1210 cm-1, ν(SO--M) at 1040-990 cm-1, and ν(SF) at
∼830 cm-1 is similar to findings for other structurally
characterized metal(III) fluorosulfato complexes such as
[Au(SO3F)4]- 15 and [Au(SO3F)3]2.72
1044
1009 vs
s
809
n.o.
653
636
n.o.
590
548
556
529
500
453
vs
m
m
sa
s
sh
s
s
There is more extensive band proliferation in the
m, br
19
ν(SO--M) and ν(SF) regions for mer-Ir(CO)3(SO3F)3 be-
431
n.o
409
n.o
372
311
n.o
b
vw
sh
cause there are two geometrically different -OSO2F groups
in this compound. Finally, for fac-Rh(CO)3(SO3F)3 there is
reasonable agreement between experimental and calculated
wavenumbers, which leaves little doubt regarding the pro-
posed structure, even without a conclusive structure deter-
mination.
0.8
3.8
0.2
10.2
0.8
w, br)
s
s
vw, br
m, br
54.1
0.5
16.4
s
s
11.3
12.0
sh, br
In summary, as demonstrated by us here and elsewhere,21,31
vibrational spectroscopy in conjunction with DFT calcula-
tions is a very powerful structural tool, which is used here
in three different ways: (i) For structurally characterized
compounds ([Rh(CO)4][M2Cl7] (M ) Al, Ga), [M(CO)5Cl]-
[Sb2F11]2 (M ) Rh, Ir),21 and [M(CO)4][Sb2F11]2 (M ) Pd,
Pt)31) it confirms that the single crystals used in structural
studies21,31 and the bulk compositions are identical. (ii) For
these compounds it permits an unambiguous confirmation
of the square planar [M(CO)4]- cations31 (M ) Rh, Pd, Pt)
and the observed anions [Sb2F11]- and [M2Cl7]- (M ) Al,
Ga). (iii) For compounds which are so far not structurally
characterized ([Rh(CO)4][Sb2F11], fac-Rh(CO)3(SO3F)3) it
confirms both the postulated structures and the composition.
σ-bonded metal carbonyl cations and their deriva-
tives1-3,19-21,23,31,66,70 are a new class of metal carbonyls. A
complete characterization of their salts is essential in our
view.
a Band is split into a doublet (593/588 cm-1) due to removal of
degeneracy; a very broad and unresolved shoulder to lower frequencies
appear at the band at 311 cm-1
.
13C NMR Spectroscopy. The use of 13C NMR spectros-
copy in the structural analysis of fac-Rh(CO)3(SO3F)3 has
already been discussed. Complementing the structural char-
acterization of [Rh(CO)4][M2Cl7] (M ) Al, Ga), the solvated
[Rh(CO)4]+, cation generated from [Rh(CO)2Cl]2 in a 13C
atmosphere is characterized by temperature-dependent 13C
NMR in HSO3F. At ambient temperature (23 °C) a sharp
singlet is observed, located at δC ) 172.5 ppm. Upon cooling,
a characteristic line broadening takes place, indicating a
dynamic behavior of the [Rh(CO)4]+ system. After coales-
cence is passed at -68 °C, a doublet structure is resolved.
Figure 6. Vibrational spectra of fac-Rh(CO)3(SO3F)3 (IR, top; Raman,
bottom).
cm-1. However in that study, the structurally characterized
mer-isomer is the main component, with ν(CO)avg at 2218
cm-1, while the fac-Ir(CO)3(SO3F)3 is always present in a
mixture and the vibrational assignment is tentative and
uncertain.19
The reported data for [M(CO)5Cl]2+ (M ) Rh, Ir)21 have
ν(CO)avg at 2249 cm-1 for the rhodium cation and at 2246
cm-1 for its iridium(III) counterpart and at 2268 cm-1 for
homoleptic [Ir(CO)6]3+.23 This reflects two general trends:
(72) Willner, H.; Rettig, S. J.; Trotter, J.; Aubke, F. Can. J. Chem. 1991,
69, 391.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3813

von Ahsen et al.
At the lowest possible temperature (just above the freezing
point), a splitting of 61.1 Hz is identified, corresponding to
the absolute value of the characteristic rhodium carbon
of [Rh(CO)2Cl]2 by S2O6F232,49 in HSO3F11,12,52 results in the
formation of two, seemingly oligomeric rhodium(III) car-
bonyl fluorosulfates, which are identified and characterized
by vibrational and 13C NMR spectroscopy but not cleanly
separated and isolated. Reductive carbonylation of these
intermediates produces [Rh(CO)4]+(solv) which is subsequently
oxidized by S2O6F232,49 to previously unknown fac-Rh(CO)3-
(SO3F)3, characterized by vibrational spectroscopy, DFT
calculations, and 13C NMR.
Finally, reductive carbonylation of fac-Rh(CO)3(SO3F)3
in HF at 25 °C followed by reaction with SbF5 at -20 °C
provides a clean synthetic route to [Rh(CO)4][Sb2F11], which
is analyzed by vibrational spectroscopy, supported by DFT
calculations.
1
coupling constant JRhC in [Rh(CO)4]+. (It is interesting to
note that for fac-Rh(CO)3(SO3F)3 a value of 57.0 Hz for 1JRhC
is found.) A detailed analysis of the NMR data and an
explanation for the dynamic nature of the 13C NMR spectrum
of [Rh(CO)4]+
in HSO3F will be presented in a further
(solv)
study,73 which will include additional examples of dynamic
13C NMR in metal carbonyl species in HSO3F as solvent.
13C NMR data for [Rh(CO)2(µ-SO3F)3-x(SO3F)x]n (I) and
fac-Rh(CO)3(SO3F)3 are listed in Figure 2 and will not be
discussed here.
Summary and Conclusions
One of the most surprising aspects of the oxidation
reactions described here is the resilience of the Rh(III)-CO
moieties toward the strongest Brønsted superacids HF11-14
and HSO3F11,12,52 in combination with powerful molecular
oxidizing agents such as F2 and S2O6F2.49
Rhodium is found to be the first and so far only metal to
form thermally stable, fully characterized metal carbonyl
cations1-3 and their derivatives, with rhodium in two
distinctly different oxidation states. Starting from dimeric
[Rh(CO)2Cl]242-45 halide abstraction by the Lewis acids11,12
AlCl3 and GaCl3 in the presence of CO allows under mild
conditions formation of [Rh(CO)4][M2Cl7] (M ) Al, Ga).
Both salts are characterized by single crystal X-ray diffraction
and a complete vibrational analysis, supported by DFT
calculations. Structural and vibrational properties in the CO
stretching region of the square planar [Rh(CO)4]+ (d8) cation
are essentially identical to those reported earlier for [Rh-
Acknowledgment. This paper is dedicated to Professor
Jean´ne M. Shreeve on the occasion of her upcoming birth-
day. Financial support by the Deutsche Forschungsgesell-
schaft, the Fonds der Chemischen Industrie, the Natural Sci-
ences and Engineering Research Council of Canada, the
Alexander von Humboldt Foundation, and the Heinrich Hertz
Stiftung is gratefully acknowledged. Dr. G. Balzer (Univer-
sity Hannover) is thanked for his help in recording 13C NMR
spectroscopy. Dr. Stefan von Ahsen is thanked for doing the
DFT calculations.
(CO)4][1-Et-CB11F11].16 For [Rh(CO)4]+
in HSO3F, the
(solv)
13C NMR spectrum is temperature dependent and a dissocia-
tive exchange pathway is suggested.73
Supporting Information Available: Raman (Figure S1) and
IR spectra (Figure S2) of [Rh(CO)5Cl][Sb2F11]2 and vibrational
spectra of [Rh(CO)4][Sb2F11] (Raman, IR; Figure S3). This material
is available free of charge via the Internet at http://pubs.acs.org. In
addition, complete structure reports are available from Fachin-
formationszentrum Karlsruhe D-76344 Eggenstein-Leopoldshafen
Germany, Hermann von Helmholtz Platz 1. E-mail: crystdata@
Fiz.Karlsruhe.de; Fax: int 49-7247-806-666. Deposit numbers:
CSD 412960 ([Rh(CO)4][Al2Cl7]) and CSD 412961 ([Rh(CO)4]-
[Al2Cl7]).
Oxidation of [Rh(CO)2Cl]2 to Rh(III) carbonyl complexes
is achieved by the strongly ionizing and oxidizing solutions
of F2 in the Brønsted superacid HF11,12 or those of bis-
(fluorosulfuryl) peroxide, S2O6F2,32,49 in the superacid
HSO3F.11,12,52 Oxidation of [Rh(CO)2Cl]2 by F2 in HF,
followed by carbonylation in HF-SbF51-3 in a second step,
leads in an improved method to the previously known and
structurally characterized [Rh(CO)5Cl][Sb2F11]2.21 Oxidation
(73) Bach, C.; Bodenbinder, M.; Bley, B.; Willner, H.; Aubke, F.; Ha¨gele,
G. Inorg. Chem., manuscript in preparation.
IC0206903
3814 Inorganic Chemistry, Vol. 42, No. 12, 2003