Synthesis of Au(II) fluoro complexes and their structural and magnetic properties

Article abstract of DOI:10.1021/ja9630654

Gold at ~20°C with F₂ in anhydrous hydrogen fluoride (aHF) acidified with SbF₅ dissolves to a red solution from which orange Au(II)(SbF₆)₂ crystallizes on removal of volatiles. Au(SbF₆)₂ is triclinic with a = 5.300(1) A?, b = 5.438(1) A?, c = 8.768(2) A?, α = 76.872(3)°, β = 88.736(3)°, γ = 68. 109(3)°, V = 227.79(7) A?³, and Z = 1, space group P1. Each Au(II) atom, at 1, is at the center of an elongated octahedron of F ligands; the four F's of the approximately square AuF₄ unit are at 2.09(2) A? x 2 A? and 2.15(2) A? x 2, each F provided by a different SbF₆ species. The two long Au-F interatomic distances are at 2.64(2) A?. The SbF₆ are grossly distorted in their interactions with the Au. A cis pair of F ligands of each SbF₆, make close approach to two different gold atoms, stretching Sb-F to 1.99(2) and 1.94(2) A?. In each case the Sb-F distances trans to these stretched Sb-F bonds are short, being 1.85(2) and 1.84(2) A?, respectively. Magnetic susceptibility measurements show antiferromagnetic coupling with a susceptibility decrease below 13 K. Solvolysis of Au(II)(SbF₆)₂ in aHF is accompanied by disproportionation: 4Au(SbF₆)₂ → Au + Au₃F₈ + 8SbF₅(solv). Fluorination, at ~20°C, of the solution of Au(SbF₆)₂, in SbF₅ acidified aHF, precipitates red crystals of triclinic Au(II){SbF₆{₂Au(II){Au(III)F₄}₂ with a(o) = 5.2345(2) A?, b(o) = 8.1218(1) A?, c(o) = 10.5977(3) A?, α = 100.090(2)°, β = 100.327(2)°, γ = 104.877(2)°, V = 416.63(2) A?³, space group P1, and Z = 1. It is a simple paramagnet. The structure shows two different Au(II) environments, each approximately square-coordinated by F ligands, one being coordinated trans by an F ligand of each of two SbF₆ and similarly by an F ligand from each of two Au(III)F₄ species. The other Au(II) is approximately square-coordinated via bridging F ligands to four different Au(III)F₄ species. Au(II){SbF₆}₂Au(II){Au(III)F₄]₂ with KAuF₄ in aHF yields Au₃F₈ free of metallic gold, the simple paramagnetism of which indicates the formulation Au(II){Au(III)F₄}₂.

Full text of DOI:10.1021/ja9630654

1020
J. Am. Chem. Soc. 1997, 119, 1020-1026
Synthesis of Au(II) Fluoro Complexes and Their Structural and
Magnetic Properties
Scott H. Elder, George M. Lucier, Frederick J. Hollander, and Neil Bartlett*
Contribution from the Chemical Sciences DiVision, Lawrence Berkeley Laboratory, and
Department of Chemistry, The UniVersity of California at Berkeley, Berkeley, California 94720
ReceiVed September 3, 1996X
Abstract: Gold at ∼20 °C with F2 in anhydrous hydrogen fluoride (aHF) acidified with SbF5 dissolves to a red
solution from which orange AuII(SbF6)2 crystallizes on removal of volatiles. Au(SbF6)2 is triclinic with a ) 5.300(1)
Å, b ) 5.438(1) Å, c ) 8.768(2) Å, R ) 76.872(3)°, â ) 88.736(3)°, γ ) 68.109(3)°, V ) 227.79(7) Å3, and Z )
1, space group P1h. Each Au(II) atom, at 1h, is at the center of an elongated octahedron of F ligands; the four F’s of
the approximately square AuF4 unit are at 2.09(2) Å × 2 Å and 2.15(2) Å × 2, each F provided by a different SbF6
species. The two long Au-F interatomic distances are at 2.64(2) Å. The SbF6 are grossly distorted in their interactions
with the Au. A cis pair of F ligands of each SbF6, make close approach to two different gold atoms, stretching
Sb-F to 1.99(2) and 1.94(2) Å. In each case the Sb-F distances trans to these stretched Sb-F bonds are short,
being 1.85(2) and 1.84(2) Å, respectively. Magnetic susceptibility measurements show antiferromagnetic coupling
with a susceptibility decrease below 13 K. Solvolysis of AuII(SbF6)2 in aHF is accompanied by disproportionation:
4Au(SbF6)2 f Au + Au3F8 + 8SbF5(solv). Fluorination, at ∼20 °C, of the solution of Au(SbF6)2, in SbF5 acidified
aHF, precipitates red crystals of triclinic AuII{SbF6}2AuII{AuIIIF4}2 with ao ) 5.2345(2) Å, bo ) 8.1218(1) Å, co )
10.5977(3) Å, R ) 100.090(2)°, â ) 100.327(2)°, γ ) 104.877(2)°, V ) 416.63(2) Å3, space group P1h, and Z ) 1.
It is a simple paramagnet. The structure shows two different Au(II) environments, each approximately square-
coordinated by F ligands, one being coordinated trans by an F ligand of each of two SbF6 and similarly by an F
ligand from each of two AuIIIF4 species. The other Au(II) is approximately square-coordinated via bridging F ligands
to four different AuIIIF4 species. AuII{SbF6}2AuII{AuIIIF4}2 with KAuF4 in aHF yields Au3F8 free of metallic gold,
the simple paramagnetism of which indicates the formulation AuII{AuIIIF4}2.
Introduction
in harmony with the low electronegatiVity of a high oxidation
state in an anion, because of its electron richness, and with the
high electronegatiVity of that oxidation state in a cation, as a
consequence of its electron deficit. By analogy with the
palladium system it seemed that a lower oxidation state of gold,
than the Au(III) favored by base,5 might be realizable by
dissolution of the metal in aHF acidified with a strong F-
acceptor. Such has proved to be the case.
In 1992, Herring et al.1 gave clear ESR and magnetic
evidence for Au2+, as a species present in partially reduced
Au(SO3F)3 and as a solvated ion in the strong protonic acid
HSO3F. They reviewed the previous history of Au(II) and
pseudo-Au(II) chemistry and pointed out that genuine Au(II)
compounds are rare, those known previously appearing to
depend2-4 upon extensive delocalization of the unpaired electron
onto the ligands which support that apparent oxidation state. In
the study of Herring et al., the hyperfine splitting due to the
197Au I ) 3/2 nuclear spin confirmed the essentially Au2+ nature
of the ESR active species in the very weak Lewis-base
environment of SO3F-.
Gold dissolves, at ∼20 °C, with F2 in aHF acidified with
SbF5, to give a red solution from which orange crystals of
AuII{SbF6}2 crystallize. Exhaustive fluorination results in total
conversion of the gold to an insoluble crystalline red solid which
is AuII{SbF6}2AuII{AuIII4F}2. The crystal structures of these
materials and their magnetic properties indicate that they are
true Au(II) derivatives. This paper describes these properties
and the attempts to prepare AuF2 by treatment with base in aHF,
or by solvolysis, which have resulted in disproportionation to
gold, and the mixed-valence fluoride AuIIAuIII2F8.
In a recent investigation5 in these laboratories of the room-
temperature dissolution of the noble metals in anhydrous
hydrogen fluoride (aHF) with F2 gas as oxidant, it was observed,
as had been found previously6,7 for the BrF3 solvent system,
that solutions made basic with alkali fluoride generally excited
a high oxidation state of the noble metal, whereas solutions
acidified by strong F- acceptors (such as SbF5 or AsF5) can
stabilize a low oxidation state. Thus palladium metal with alkali
fluoride, in aHF/F2, quickly gave PdIVF62-, whereas in aHF
acidified with SbF5, the final product was PdII{SbF6}2. This is
Experimental Section
Materials. F2 and aHF were used as supplied by Matheson Gas
Products, East Rutherford, NH 07073, and SbF5 as supplied by Ozark
Mahoning Inc., Tulsa, OK 74107, the small HF impurity in the latter
being of no consequence because of its use in aHF solution. To destroy
any water in the aHF it was distilled to any reactor from stock held in
a 100 mL capacity FEP tube containing K2NiF6 (Ozark Mahoning).
Metallic Au, 1.8-2.3 µm powder (99.95%), was used as supplied by
Johnson Matthey, Inc., Seabrook, NH 03874.
X Abstract published in AdVance ACS Abstracts, January 15, 1997.
(1) Herring, F. G.; Hwang, G.; Lee, K. C.; Mistry, F.; Phillips, P. S.;
Willner, H.; Aubke, F. J. Am. Chem. Soc. 1992, 114, 1271.
(2) MacCragh, A.; Koski, W. S. J. Am. Chem. Soc. 1963, 85, 2375; 1965,
87, 2496.
(3) Warren, L. F.; Hawthorne, M. F. J. Am. Chem. Soc. 1968, 90, 4823.
(4) Schlupp, R. L.; Maki, A. H. Inorg. Chem. 1974, 13, 44.
(5) Lucier, G.; Elder, S. H.; Chaco´n, L.; Bartlett, N. Eur. J. Solid State
Inorg. Chem. 1996, 33, 809.
Apparatus and Technique. All reactors were constructed in T
shape from translucent fluorocarbon polymer tubing (FEP) (Chemplast,
Inc., Wayne, NJ 07490) joined with Teflon Swagelok compression
fittings and equipped with Teflon body valves, having Kel-F stems
(6) Sharpe, A. G. J. Chem. Soc. 1953, 197
(7) Bartlett, N.; Rao, P. R. Proc. Chem. Soc. 1964, 393.
S0002-7863(96)03065-X CCC: $14.00 © 1997 American Chemical Society

Synthesis of Au(II) Fluoro Complexes
J. Am. Chem. Soc., Vol. 119, No. 5, 1997 1021
with Teflon tips, as previously described.8 The typical T-reactor had
1/2 in. o.d. FEP tubes, heat pressure sealed at one end and drawn down
3
3
at the open end to /8 in. o.d. to fit a standard /8 in. Swagelok T. A
Teflon valve was joined to one end of the crossing to the T, and the
reactor was linked to the supply and vacuum line via a 1/4 in. o.d. FEP
tube ∼2 ft long. Each reactor was pretreated with F2 (to ∼1400 Torr)
before it was used. The less volatile reagents (e.g., gold powder together
with SbF5) were placed in the tube at the crossing of the T, in the
DRILAB. The aHF was distilled under vacuum to the mixture, at -196
°C, which was then brought to room temperature. As needed, fluorine
was added from the supply to a pressure of ∼1400 Torr total pressure
(of which ∼760 Torr in the T was due to aHF). (Because of the
corrosive effect of acidified aHF on metals, the teflon-valve access to
the metal line was opened briefly and only when a 1400 Torr pressure
had been established in that line.) The reactor was inclined, so that
the crossing arm was nearly horizontal and the other arm nearly vertical.
This maximized the F2-liquid aHF interface and permitted the
spreading of the metal along the bottom side of the tube. The mixture,
at ∼20 °C, was vigorously agitated by a sideways flicking of the tube
by a properly placed rotating arm. As fluorine was consumed
(measured intermittently against the 1400 Torr of the supply line) it
was replenished periodically. Solutions [of, for example, Au(SbF6)2]
in the aHF were effectively separated from insolubles (e.g., Au) by
decantation of the solution to the other arm. In some cases where the
insoluble residue was not sensitive to solvolysis by aHF (e.g., Au3F8),
the aHF was back-distilled from the decanted solution to the reactor
limb and cooled to -196 °C, and the thawed aHF was then used to
wash the insoluble solid free of aHF-soluble contaminents. This could
be repeated as often as necessary. Manipulation of all solids and SbF5
was carried out in a Vacuum Atmospheres Corp. DRILAB with a dry
argon gas atmosphere.
Figure 1. Reciprocal of molar susceptibility (at 5 kG) versus
temperature for Au(SbF6)2; µeff (for 50-280 K) ) 2.37µB.
The X-ray Single-Crystal Structure of Au(SbF6)2. The crystal
used in the data collection is described in Table SI (Supporting
Information), where other pertinent data are also given.
Structural Solution and Refinement. The structure was solved
by analysis of similar structures11,12 and expanded using Fourier maps.
All atoms were refined anisotropically. The final cycle of full-matrix
least-squares refinement13 was based on 555 observed reflections (I >
3.00σ(I)) and 70 variable parameters and converged (largest parameter
shift was 0.00 times its esd) with unweighted and weighted agreement
factors of
R )
||Fo| - |Fc||/Σ|Fo| ) 0.057
Single Crystal and Powder Containment for X-ray Diffraction.
Because of the easy hydrolysis of the gold fluoro complexes, single
crystals and powders (packed by quartz ram-rods) were loaded into
thin-walled quartz capillaries (Charles Supper Co., 15 Tech Circle,
Natick, MA 01760) which had been vacuum dried at 450 °C. Loading
techniques were as described9 for AgF3. Single crystals were selected
and manipulated in the DRILAB, with the aid of a microscope, and to
facilitate the secure holding of a crystal, the commercial capillaries
were further drawn down and tapered. The capillaries (for both single-
crystal and powder) were plugged with KelF grease, removed from
the DRILAB, and sealed by drawing down in a small flame.
∑
2
Rw ) {( w(|Fo| - |Fc|)2/ΣwFo )}1/2 ) 0.065
∑
The goodness of fit indicator14 was 2.70. The weighting scheme
was based on counting statistics and included a factor (p ) 0.032) to
downweight the intense reflections. Plots of ∑w(|Fo| - |Fc|)2 versus
|Fo|, reflection order in data collection, sin θ/λ, and various classes of
indices, showed no unusual trends. The maximum and minimum peaks
on the final difference Fourier map corresponded to 1.53 and -2.87
e-/Å3, respectively.
Neutral atom scattering factors were taken from Cromer and Waber.15
Anomalous dispersion effects16 were included in Fc, and the values for
∆f ′ and ∆f ′′ were those of Creagh and McAuley.17 The values for
the mass attenuation coefficient are those of Creagh and Hubbel.18 All
calculations were performed using the teXsan19 crystallographic
software package of Molecular Structure Corp. Final unit cell
parameters are in Table 1, atomic coordinates in Table SII, and
anisotropic displacement parameters in Table SIII. Interatomic dis-
tances and angles are in Table SIV.
Solvolysis of Au(SbF6)2 in aHF. Addition of aHF to solid
Au(SbF6)2 at ∼20 °C rapidly produced a dark brown solid and a pale
(11) Gantar, D.; Leban, I.; Frlec, B.; Holloway, J. H. J. Chem. Soc.,
Dalton Trans. 1987, 2379.
X-ray Powder Diffraction Photographs (XRDP) were obtained
using Ni-filtered Cu KR radiation using General Electric Co. Precision
Cameras (circumference 45 cm, Straumanis loading).
Magnetic Measurements were made using a Superconducting
Quantum Interference Device (SQUID) magnetometer as previously
described.10
Synthesis of Au(SbF6)2. One arm of a FEP T-reactor was charged
with Au (6.8 mmol) and SbF5 (∼11 mmol) in the DRILAB. With the
reactor attached to the vacuum line, aHF (∼5 g) was added to the
charge. Fluorine was added to 800 Torr partial pressure in two aliquots,
amounting to ∼3.5 mmol, over a 1.5 h period, with vigorous agitation
of the tube contents, at ∼20 °C. An intense raspberry-red solution
was produced, and fluorination was halted at the first sign of red
Au(SbF6)2Au(AuF4)2 crystals, the clear red solution then being decanted
into the other leg of the reactor and the volatiles removed under vacuum
to give golden-yellow, crystalline Au(SbF6)2 (1.5 mmol). The remain-
ing aHF-insoluble residue was mainly metallic Au, with some
Au(SbF6)2Au(AuF4)2.
(12) Lucier, G.; Mu¨nzenberg, J.; Casteel, W. J., Jr.; Bartlett, N. Inorg.
Chem. 1995, 34, 2692.
(13) Least-squares function minimized: ∑w(|Fo| - |Fc|)2, where w )
1/σ2(Fo) ) 4Fo /σ2(Fo ).
2
2
(14) Standard deviation of an observation of unit weight: [∑w(|Fo| -
|Fc|)2/(No - Nv)}1/2, where No ) number of observations and Nv ) number
of variables.
Magnetic Susceptibility for Au(SbF6)2. The magnetic susceptibility
of Au(SbF6)2 exhibited an unexpected antiferromagnetic departure from
Curie law behavior, with a Nee´l temperature of ∼13 K, as indicated in
Figure 1.
(15) Cromer, D. T.; Waber, J. T. International Tables for X-ray
Crystallography; The Kynoch Press: Birmingham, England, 1974; Vol.
IV, Table 2.2 A.
(16) Ibers, J. A.; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781.
(17) Creagh, D. C.; McAuley, W. J. International Tables for Crystal-
lography; Wilson, A. J. C., Ed.; Kluwer Academic Publishers: Boston,
MA, 1992; Vol. C, Table 4.2.6.8, pp 219-222.
(18) Creagh, D. C.; Hubbell, J. H. International Tables for Crystal-
lography; Wilson, A. J. C., Ed.; Kluwer Academic Publishers: Boston,
MA, 1992; Vol. C, Table 4.2.4.3, pp 200-206.
(19) teXsan: Crystal Structure Analysis Package, Molecular Structure
Corp. (1985 and 1992).
(8) Zˇemva, B.; Hagiwara, R.; Casteel, W. J., Jr.; Lutar, K.; Jesih, A.;
Bartlett, N. J. Am. Chem. Soc. 1990, 112, 4846.
(9) Zˇemva, B.; Lutar, K.; Jesih, A.; Casteel, W. J., Jr.; Wilkinson, A. P.;
Cox, D. E.; Von Dreele, R. B.; Borrmann, H.; Bartlett, N. J. Am. Chem.
Soc. 1991, 113, 4192.
(10) Casteel, W. J., Jr.; Lucier, G.; Hagiwara, R.; Borrmann, H.; Bartlett,
N. J. Solid State Chem. 1992, 96, 84.

1022 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
Elder et al.
Table 1. Crystallographic Data for Au(SbF6)2 and
Au(SbF6)2Au(AuF4)2
) 2.8(3) × 10-6) in the final cycles of least-squares. The final cycle
of full-matrix least-squares refinement13 was based on 1306 observed
reflections (I > 3.00σ(I)) and 122 variable parameters. Agreement
factors are given in Table 1. The weighting scheme was based on
counting statistics and included a factor (p ) 0.031) to downweight
the intense reflections. Plots of ∑w(|Fo| - |Fc|)2 versus |Fo|, reflection
order in data collection, sin θ/λ, and various classes of indices showed
no unusual trends. The maximum and minimum peaks on the final
difference Fourier map corresponded to 2.86 and -2.64 e-/Å3,
respectively. Neutral atom scattering factors, anomalous dispersion
effects, values for ∆f ′ and ∆f ′′, and mass attenuation coefficients were
obtained as for the Au(SbF6)2 structure. All calculations used the same
software package. Final unit cell parameters are in Table 1, atomic
coordinates in Table SVI, and anisotropic displacement parameters in
Table SVII. Interatomic distances and angles are given in Table SVIII.
empirical formula
formula weight
AuSb2F12
668.45
Au4Sb2F20
1411.34
1780 (3.0-45.0°)
no. of reflns used for unit 686 (3.0-45.0°)
cell determn (2θ range)
lattice params
a (Å)
b (Å)
c (Å)
5.300(1)
5.438(1)
8.768(2)
76.872(3)
88.736(3)
68.109(3)
227.79(7)
5.2345(2)
8.1218(1)
10.5977(3)
100.090(2)
100.327(2)
104.877(2)
416.63(2)
R (deg)
â (deg)
γ (deg)
V (Å3)
space group
Z
P1h (no. 2)
1
D
calcd (g/cm3)
4.872
5.625
Au(SbF6)2Au(AuF4)2 with LiF in aHF. LiF (0.58 mmol) in aHF
(∼3 g) was added to Au(SbF6)2Au(AuF4)2 (0.29 mmol) at ∼20 °C and
the mixture agitated for ∼20 h to ensure complete interaction of the
large-particle gold compound with the solution. An insoluble brown
sediment was produced beneath a colorless solution. The latter was
decanted, the insoluble solid was washed four times with back-distilled
aHF, and all volatiles were removed. XRDP showed the soluble
product to be LiSbF6 with LiAuF4 and the brown solid to be like10
Ag(AuF4)2. Similar treatment of Au(SbF6)2Au(AuF4)2 with a 10-fold
molar excess of LiF in aHF produced an insoluble residue of gold and
a mixture of LiAuF4 and LiSbF6 from the aHF solution and washings.
F(000)
289.00
221.55
1196.00
385.87
Mo KR (λ ) 0.710 69 Å)
graphite monochromated
296
µ(Mo KR) (cm-1
radiation
)
temp (K)
residuals: R, Rw, Rall
goodness of fit indicator
0.057, 0.065, 0.075 0.049, 0.069, 0.050
2.70
3.11
Conversion of Au(SbF6)2Au(AuF4)2 to Au3F8. One arm of a FEP
T-reactor was charged with Au(SbF6)2Au(AuF4)2 and KAuF4 with the
latter in greater than 3-fold molar excess. Addition of aHF dissolved
the KAuF4 but not the Au(SbF6)2Au(AuF4)2. Prolonged agitation of
the mixture converted the red macrocrystalline solid to a golden yellow
solid. The solution containing excess KAuF4 was decanted from the
solid which was washed once with aHF. XRDP of the yellow solid
indicated a close structural relationship10 with Ag(AuF4)2 and9 Ag(AgF4)2.
The diffraction data for Ag3F8, AgAu2F8, and Au3F8 are given in Tables
SIX, X, and XI. The XRDP of all Au3F8 preparations, whether
containing metallic gold or not, had the dark background typical of
XRDP of poorly crystalline material, and only the stronger lines of the
diffraction pattern were observed. This pattern, however, roughly
matched the stronger line pattern of AgAu2F8 in relative line intensities
(see Tables SX and SXI). The most complete pattern of the set, was
that of Ag3F8 (see Table SIX). Each of these patterns has been indexed
on the basis of a hexagonal unit cell containing nine formula units.
Although this indexing should be regarded as tentative the formula
unit volume for each of Ag3F8 and AgAu2F8 is within 2 Å3 of the
formula unit volume (FUV) obtained by the sum FUV(AgF2) +
2[FUV(AF3)]. The hexagonal unit cells are Ag3F8, ao ) 12.79(1), co
) 9.95(1) Å; AgAu2F8, ao )12.92(1), co ) 10.43(1) Å; Au3F8, ao )
12.90(2), co ) 10.81(2) Å. The weight balances for two independent
preparations are in accord with the composition Au3F8 for the golden-
yellow solid and XRDP established the presence of both KSbF6 and
KAuF4 in the aHF-soluble products, consistent with the overall reaction.
Figure 2. Reciprocal of molar susceptibility ([, 5 kG; 0, 40 kG)
versus temperature for Au(SbF6)2Au(AuF4)2; µeff ) 2.24 µB.
pink solution. Decantation of the pale pink solution [of Au(SbF6)2 in
SbF5-rich aHF] left a brown solid insoluble in aHF.
Au(SbF6)2 in aHF with 2KF. KF (0.2 mmol) in aHF (∼5 g) was
added to Au(SbF6)2 (0.1 mmol) to produce a dark brown solid from
which a colorless solution containing KSbF6 was decanted. The
insoluble brown solid was washed five times with back-distilled aHF.
XRDP indicated that the insoluble solid was structurally related10 to
Ag(AuF4)2. The XRDP also contained the pattern of elemental gold,
the lines being faint and broad. The soluble product was KSbF6.
Synthesis of Au(SbF6)2Au(AuF4)2. The experimental arrangement
for the synthesis of this compound was the same as in the synthesis of
Au(SbF6)2, with the difference that more fluorine was added and the
fluorination continued until the supernatant solution was almost
colorless. With Au (5.7 mmol) and SbF5 (10 mmol) in aHF (∼5 g),
F2 was added to the reactor in 8 aliquots over a 5 h period, amounting
to 8 mmol, which was agitated for a total of 21 h at ∼20 °C. The red,
highly crystalline Au(SbF6)2Au(AuF4)2 (2.7 mmol, 95% yield) was freed
of any soluble products by decantation of the supernatant solution and
by one wash with aHF (∼5 g).
Au(SbF6)2Au(AuF4)2 + 2KAuF4 f 2Au3F8 + 2KSbF6 (1)
In reaction a, 61.8 mg of Au(SbF6)2Au(AuF4)2 (0.0438 mmol) treated
with 54.5 mg of KAuF4 (0.175 mmol) in aHF (∼3 g) gave 65.3 mg of
Au3F8 and 53.3 mg of KAuF4 with KSbF6. Equation l requires 65.1
mg of Au3F8 (0.0876 mmol) and 27.3 mg of KAuF4 (0.0874 mmol)
with 24.1 mg of KSbF5 (0.0876 mmol), total 51.4 mg. In b, 52.2 mg
of Au(SbF6)2Au(AuF4)2 (0.037 mmol) with 38.1 mg of KAuF4 (0.122
mmol) in aHF (∼10 g) gave 59.5 mg of Au3F8 and 39.8 mg of KAuF4
with KSbF6. Equation 1 requires 55.0 mg of Au3F8 (0.074 mmol)
and 15.0 mg of KAuF4 (0.048 mmol) with 20.3 mg of KSbF6 (0.074
mmol), total 35.3 mg. The magnetic susceptibility of the sample of
Au3F8 prepared in a obeyed the Curie law as shown in Figure 3.
Magnetic Susceptibility for Au(SbF6)2Au(AuF4)2. The magnetic
susceptibility of this solid obeyed the Curie law as shown in Figure 2.
The X-ray Crystal Structure of Au(SbF6)2Au(AuF4)2. The crystal
used in the data collection is described in Table SIV, where other
pertinent data are also given.
Structure Solution and Refinement. The structure was solved by
direct methods.20 Information on the collection of data and the
refinement are given in Table SV. All atoms were refined anisotro-
pically. A correction for secondary extinction was applied (coefficient
(20) SIR92: Altamare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr., manuscript
in preparation.

Synthesis of Au(II) Fluoro Complexes
J. Am. Chem. Soc., Vol. 119, No. 5, 1997 1023
Ag(MF6)2. The evident, powerful F--attracting ability of the
A(II) cations must derive from high effective nuclear charge,
in spite of these d9 species having only one electron hole in the
valence d shell. The F-ligand arrangements about the two A(II)
species, illustrated in Figure 4, show close similarity in the xy
plane, but differ appreciably in the local z axis direction.
Since the Au-F and Ag-F distances of the xy plane differ
by no more than one ESD, the effective size of the A(II) must
be essentially the same in this plane. This must be an accident
of cancelling opposing effects. In the metals, the effective size
of the gold atom, measured by the formula unit volume of the
cubic close packed structure,24 is slightly less than that of the
metallic silver atom (16.97 versus 17.05 Å3). This derives from
a combination of the lanthanide contraction and the impact of
the relativistic effect.25,26 As a consequence of the latter, the
atomization enthalpy27 of gold is greater than that of silver
[∆Hf°(A(g)): A ) Au, 87.5; A ) Ag, 68 kcal mol-1]. This is
because, in the higher nuclear-charge gold atom, the binding
of the s orbital electrons is enhanced, and the valence electron
for the metallic bonding has this character. The slightly smaller
metal-atom size of gold relative to silver must, in some measure,
represent this greater bonding energy for gold. The separated
gaseous atoms should not have the same relative size as in the
metal. In the simple A(III) systems the relative sizes are
reversed. Thus, in the {AIIIF4}- ions, the Au-F interatomic
distance is slightly larger28 than the Ag-F one29 [Au-F,
1.915(3); Ag-F, 1.889(3) Å], and the same holds for the close
set of four F ligands (approximately square planar) in the
trifluorides,9 although the difference there is not significant. This
switch in relative size from A(0) to A(III) is in harmony with
the involvement of valence d electrons of A in the A(III)
fluorospecies bonding. The enhancement of the binding of gold
s electrons, destabilizes the d electrons of gold relative to their
silver counterparts. Thus, although the valence s electron of
the gold [via the a1g orbital of the D4h symmetry (AuF4)- ion]
provides enhanced binding relative to the situation in the silver
analogue, the d orbitals of the Au atoms provide less binding
energy benefit than in the case of silver. The adverse effects
on the binding in AuF4- appear to outweigh the benefit derived
from the tighter binding of the a1g bonding electron pair. But
A(II) in the A(SbF6)2 compounds has an antibonding electron
to be associated with the plane (xy) containing the four close
ligands. This should weaken the Au(II)-F bonding more than
that of Ag(II)-F and enhance the difference already noted for
the A(III)-F interatomic distances. Because of the lower
precision realized in the Au(SbF6)2 structure, the xy plane Au-F
and Ag-F distances are not significantly distinguished from
one another. It is possible however that AuIIF could be as much
as 0.07 Å longer than AgIIF, but not more. The single σ*
Figure 3. Reciprocal of molar susceptibility (at 5 kG) versus
temperature for Au(AuF4)2; µeff ) 2.05 µB.
Figure 4. Comparison of bonded-atom interatomic distances (Å) for
Au(SbF6)2 and Ag(SbF6)2, values for the latter in italics.
Results and Discussion
Au(SbF6)2 is isostructural with Ag(SbF6)2, which was first
prepared and described by Gantar et al.11,21 Figure 4 compares
the interatomic distances for the two compounds. In both
compounds, the noble-metal atom is at the center of an elongated
octahedron of F ligands, each of which is provided by a different
SbF6 species. The AF6 distortion (first-order Jahn-Teller) is
attributable to the greater antibonding effect of a pair of electrons
2
located in the sigma antibonding orbital (σ*) designated dz
(elongation axis z) compared with the single antibonding
2
2
electron in the σ* orbital dx -y . Indeed, it is evident from the
2
2
electron (identified loosely with the dx -y orbital) therefore does
not appear to have a large impact on the ligand geometry. This
-
gross distortion of the SbF6 in each of these structures that
the Au(II) or Ag(II) atom [A(II)] strongly attracts the F ligands
of the four SbF6 species associated with the xy plane of the
A(II). It can be seen that the A(II) centers are withdrawing F-
from the SbF6-, which is one of the poorest F- donors
known.22,23 The weaker F- acceptor AsF5, in aHF, is not able
to bring about the oxidation of Au to Au(II). Nor is AsF5 able
to stabilize Ag(AsF6)2, since systems of that composition in aHF
lose AsF5 on crystallization to yield11,21 the F- bridged chain-
cation salt (AgF)n+n(AsF6-)n, and only the poorest F- donor
anions (MF6)- have proved12 to be capable of stabilizing
2
is not so for the dz electron pair.
The difference of ∼0.21 Å in the Au-F and Ag-F distances
along the z axis expresses the highly antibonding influence of
3
2
the dz electron pair in the case of Au(II). This and the ∼8 Å
greater formula unit volume of Au(SbF6)2 (227.8 Å3) versus
(24) Wyckoff, R. W. G. Crystal Structures; Interscience Publishers:
London and Sydney, 1963; Vol. 1.
(25) Pitzer, K. Acc. Chem. Res. 1979, 12, 271.
(26) Pyykko, P.; Desclaux, J. P. Acc. Chem. Res. 1979, 12, 276.
(27) NBS Technical Note 270-4, U.S. Department of Commerce, National
Bureau of Standards, 1969.
(28) Engelmann, U.; Mu¨ller, B. G. Z. Anorg. Allg. Chem. 1991, 598/9,
103.
(21) Gantar, D.; Frlec, B.; Russell, D. R.; Holloway, J. H. Acta
Crystallogr. 1987, 99, 685.
(22) Mallouk, T. E.; Rosenthal, G. L.; Mu¨ller, G.; Brusasco, R.; Bartlett,
N. Inorg. Chem. 1984, 23, 3167.
(29) Lutar, K.; Milic´ev, S.; Zˇemva, B.; Mu¨ller, B. G. Bachmann, B.;
Hoppe, R. Eur. J. Solid State Inorg. Chem. 1991, 28, 1335.
(23) Christe, K. O.; Dixon, D. A. J. Am. Chem. Soc. 1992, 114, 2978.

1024 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
Elder et al.
Figure 5. Structural features of AuII(SbF6)2AuII{AuIIIF4}2: (a) sheets of composition AuF2, with sandwiched SbF6; (b) interatomic distances (Å),
σ ≈ 0.01 Å, for the AuF2 composition sheet; Roman numerals indicate oxidation states; (c) labeling for the structural unit.
3
2
Ag(SbF6)2 (219.7 Å ) shows that the Au(II) dz electron pair
has greater effective size than its Ag(II) counterpart. This is
similar to Au(III) versus Ag(III) in the trifluorides.9 It is also
in harmony with the ease of addition30,31 of F2 to the low-spin
d8 configuration of Au(III) in AuF4- in its oxidation to AuVF6-
and the failure to similarly oxidize Ag(III).
latter pendant above and below the AuF2 composition sheet.
The SbF6 groups are sandwiched between the AuF2 composition
sheets, filling the available space between them, the entire
arrangement being a highly close-packed one, the sheets
puckering slightly to help make it so.
Justification for these Au oxidation-state assignments is as
follows: Each Au atom is approximately square-coordinated
by four F ligands, but the three AuF4 species are not chemically
equivalent. The longer Au-F interatomic distances of ∼2.1
Å, associated with Au1 and Au3, point to these being Au(II)
species. The interatomic distances for the bridging F ligands
close to Au2, with Au-F ranging from 1.91(1) to 1.96(1) Å
are slightly shorter than comparable bonds in AuF3, where9 two
F ligands (of the approximately square array) bridge equally
two gold atoms, and have Au-F ) 1.998(2) Å. The one
nonbridging F ligand associated with Au2 at Au-F ) 1.890(9)
Å is not significantly different from the “nonbridging” distances
of the square ligand arrangement of AuF3, where Au-F )
1.876(3) Å. It is therefore appropriate to assign Au2 as Au(III).
The Curie law paramagnetism of the Au(SbF6)2Au(AuF4)2
displayed in Figure 2 establishes that the Au(II) centers cannot
share a common bridging F ligand since that32 would be
associated with strong antiferromagnetic coupling of the Au(II)
Even though extensive oxidation of gold by F2, in strongly
acidified aHF, to Au(SbF6)2 occurs easily, prolonged exposure
of the solution of this highly soluble salt to F2 does produce an
insoluble further oxidation product. Because of its slow
formation and its very low solubility in aHF, the red product is
highly crystalline and the X-ray single-crystal structure, repre-
sented in Figure 5, shows it to be AuII{SbF6}2AuII{AuIIIF4}2.
The low solubility of the material can be attributed to the
polymeric sheet component of the structure, which has the
overall composition AuF2. As Figure 5 shows, this sheet
involves two different Au(II) species, each at a center of
symmetry, 1h. One is linked in a square array of four F ligands
shared with four separate square AuIIIF4 units, the other joined
to two different AuIIIF4 groups and to two SbF6 groups, the
(30) Leary, K.; Bartlett, N. J. Chem. Soc., Chem. Commun. 1972, 903.
Leary, K.; Zalkin, A.; Bartlett, N. Inorg. Chem. 1974, 13, 775. Bartlett, N.;
Leary, K. ReV. Chim. Miner. 1976, 13, 82.
(31) Lutar, K.; Jesih, A.; Leban, I.; Zˇemva, B.; Bartlett, N. Inorg. Chem.
1989, 28, 3467.
(32) Goodenough, J. B. Magnetism and the Chemical Bond;
Wiley-Interscience: New York, 1963; p 170.

Synthesis of Au(II) Fluoro Complexes
J. Am. Chem. Soc., Vol. 119, No. 5, 1997 1025
-
3AuII{AuIIIF4}2 + 8F- f Au + 8AuIIIF4
(3)
unpaired electrons. The oxidation-state designation indicated
by the crystal structure is therefore in harmony with the magnetic
properties, for which the formulation AuII{SbF6}2AuII{AuIIIF4}2
is apt.
This instability with respect to the metal and the A(III) oxidation
state, is not observed in either Ag3F8 or AgF2, the latter
9
withstanding attack34 by a saturated solution of KF in aHF over
many days at 20 °C. These and other differences are attributable
to the relativistic effect. Because of the greater binding energy
of the valence s electron of gold, the atomization and first
ionization enthalpies of gold are higher than for silver.
However, from the same cause, the binding energies of the gold-
atom d electrons are lowered. The relative ease of formation5,31
If, by subtle reduction, the AuF2 composition sheets of the
Au(SbF6)2Au(AuF4)2 structure were freed of their pendant SbF6
groups, the nonbridging F ligands (F3) of the Au2 AuF4 groups
could then make closer approach to Au3 (making two Au3-
F3 connections, indicated by the broken lines in Figure 5b) to
complete its four coordination, intra sheet. The result would
be a puckered sheet of the kind found in AgF2, as recently
described by Hoppe and his co-workers.33
-
-
of AuF4 and AuF6 are testimony to this, as is the high
thermodynamic stability of AuF3 (∆Hf°298 ) -83 kcal mol-1 35
)
The attempted preparation of AuF2, by addition, at ∼20 °C,
of an alkali fluoride solution in aHF to Au(SbF6)2, in 2:1 molar
ratio, rapidly produced a light brown solid insoluble in aHF,
from which KSbF6 was removed by its dissolution in the aHF.
XRDP showed that the brown solid contained some metallic
gold, but the dominant pattern resembled that9 of Ag(AgF4)2
and was particularly close to the pattern10 of Ag(AuF4)2. [Both
Ag(AgF4)2 and Ag(AuF4)2 had been previously prepared9,10 in
these laboratories by precipitation from aHF by mixing a soluble
Ag2+ salt with twice the molar quantity of a soluble (AF4)-
salt.] This indicated that the mixed oxidation state material
AuII{AuIIIF4}2 had probably been produced, the overall reaction
being
which stands in contrast to the thermodynamic instability of
AgF3, which loses fluorine in aHF at room temperature.9
AgF2 does not behave chemically like AuF2. It does not
disproportionate to Ag and AgII{AgIIIF4}2 and, in highly basic
aHF, does not give Ag and AgF4-. These differences between
AgF2 and AuF2 must be associated with the higher excitation
energy involved in making Ag(III), compared with Au(III), as
well as the energy of forming metallic A from A(II). The latter
is more favorable for Au than for Ag. A rough measure of this
energy is given by the sum of the atomization enthalpy and the
first and second ionization potentials for the two elements. The
sums are as follows: Ag, 738.3; Au, 773.6 kcal mol-1. These
sums can also give us a rough assessment of the enthalpy of
formation of AuF2, since ∆Hf°298 of AgF2 is known (-87.3 kcal
mol-1).27 Making the approximation that AgF2 and AuF2 are
ionic and structurally similar, the differences in their enthalpies
of formation equate with differences in the sum quoted above,
combined with lattice energy differences. Because of the
similarity of the Au(II) and Ag(II) radii in the xy plane, it can
be expected that the binding together of the A2+ and F- in the
puckered sheets of the AgF2 structure33 could provide the
greatest part of the lattice energy, and this contribution would
be the same for both AgF2 and AuF2. As has been noted earlier,
4Au(SbF6)2 + 8KF f Au + Au(AuF4)2 + 8KSbF6 (2)
Even the solvolysis of Au(SbF6)2 by aHF gave the same
insoluble brown solid mixture of gold and the conjectured
Au(AuF4)2. Firmer support for that formulation came, however,
with the preparation of the Au(AuF4)2, by metathetical interac-
tion of Au(SbF6)2Au(AuF4)2 with a nearly 4-fold molar excess
of KAuF4 dissolved in aHF. Since the former compound is of
very low solubility in aHF and slow to solvolyze, the complete
replacement of SbF6 by AuF4 appeared to be the predominant
interaction (see eq 1). The magnetic behavior of the product
(see Figure 3) is almost the same as that of Au(SbF6)2Au(AuF4)2
(see Figure 2), and as in that compound, it can be safely
concluded that Au(II) centers cannot be joined by a bridging F
ligand in common. It is also highly probable that each gold
atom [whether Au(II) or Au(III)] will be square-coordinated by
F ligands. These considerations, and stoichiometic require-
ments, lead to the expectation that each AuIIF4 shares each of
its F ligands with four AuIIIF4, and each of the latter (on average)
shares with two AuIIF4. Indeed, if the hexagonal indexing of
the Ag(AgF4)2, Ag(AuF4)2, and Au(AuF4)2 diffraction data is
correct (Tables SIX, SX, and SXI), the A(II) atoms will probably
be located in 2-fold axial-symmetry sites of the hexagonal unit
cell (1/2, 0, z, etc). This would allow a square AIIF4 group to
share each of its four F ligands with each of four A(III) atoms
(in positions x, y, and z). The latter, also in a roughly square
F-ligand environment {AIIIF4} should be linked by two F bridges
(probably in cis arrangement, as in the trifluorides9) to two A(II)
atoms. The ao dimension (∼12.9 Å) is in harmony with either
12-membered rings of alternating A(II) and A(III), bridged by
F [i.e., {-F-AuII-F-AuIII-}3] or by such a 12-atom sequence
forming one turn in a helical structure (not unlike segments of
the AgF3 and AuF3 structures9).
3
2
the dz electron pair of Au(II) is ∼8 Å larger in effective volume
than that of Ag(II). Separation of the puckered sheets of AuF2
should therefore be greater than in the AgF2 case. That part of
the lattice energy, which derives from sheet-to-sheet attraction,
will consequently be less for AuF2 than for AgF2. But the sheet-
to-sheet distances, even in AgF2, are large33 and indicative of
weak binding. Even if the lattice energies were the same, the
sum of the atomization and first two ionization enthalpies favors
AgF2 over AuF2 by 35.3 kcal mol-1. We can expect the impact
of lattice energy36 to increase this preference for AgF2, but
probably by not more than another 40 kcal mol-1. Therefore
the estimated ∆H°298 of disproportionation (3AuF2 f Au +
2AuF3) ranges from -10 kcal mol-1 [with no difference in the
AgF2 and AuF2 lattice energies (U)] to -130 kcal mol-1 [with
U(AgF2) - U(AuF2) ) -40 kcal mol-1]. The change in
entropy37 for the disproportionation should be close to 0.
Thermodynamic instability of AuF2 with respect to metallic gold
(34) AgF2 showed no sign of interaction with a KF/HF mixture of ∼20:1
molar ratio over 5 days at ∼20 °C. Dr. M. Whalen, U.C. Berkeley,
unpublished observation.
(35) Woolf, A. A. J. Chem. Soc. 1954, 4694.
(36) The Madelung part of the lattice energy is inversely proportional
to the sum of the ionic radii. The latter can be roughly approximated37 by
the cube root of the effective formula unit volume (FUV). FUV(AgF2) (see
ref 33) is 42 Å3. Assuming FUV(AuF2) to be 8 Å3 greater, the lattice energy
of AuF2 would be 0.944 times the lattice energy (U) of AgF2. From the
Born-Haber cycle, the latter is -701 kcal mol-1; therefore U(AuF2) would
Treatment of Au(AuF4)2 with excess alkali fluoride in aHF
destroys all Au(II) according to the equation
be ∼-661 kcal mol-1
.
(37) Thrasher, J. S., Strauss, S. H., Eds. Inorganic Fluorine Chemistry:
Toward the 21st Century; ACS Symposium Series, 555; American Chemical
Society: Washington, DC, 1994; Chapter 2.
(33) Jesih, A.; Zˇemva, B.; Bachmann, B.; Becker, St.; Mu¨ller, B. G.;
Hoppe, R. Z. Anorg. Allg. Chem. 1990, 588, 77.

1026 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
Elder et al.
and AuF3 is therefore indicated. But Au(III) must be energeti-
cally more favorably placed when in an anion (AuF4)-, and
this may be why the solvolysis of Au(SbF6)2 by aHF produces
gold with AuII{AuIIIF4}2 and not with AuF3.
Science Foundation under Grant CHE-9302414 awarded to
S.H.E. The authors also thank Professor Kenneth Pitzer for a
careful reading of the manuscript and helpful comments and
Dr. Horst Borrmann for his critical evaluation of the crystal-
lographic work and for his assistance in compiling the final
document.
Acknowledgment. This paper is dedicated to Professor Hans
Georg von Schnering on the occasion of his 65th birthday. The
authors gratefully acknowledge the support of this work by the
Director, Office of Energy Research, Office of Basic Energy
Sciences, Chemical Sciences Division of the U.S. Department
of Energy under Contract Number DE-AC-03-76SF00098. This
material is also based upon work supported by the National
Supporting Information Available: Listings of crystal data,
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