The first example of a mixed valence ternary compound of silver with random distribution of Ag(i) and Ag(ii) cations

Article abstract of DOI:10.1039/c5dt00740b

The reaction between colourless AgSbF₆ and sky-blue Ag(SbF₆)₂ (molar ratio 2:1) in gaseous HF at 323 K yields green Ag₃(SbF₆)₄, a new mixed-valence ternary fluoride of silver. Unlike in all other Ag(i)/Ag(ii) systems known to date, the Ag⁺ and Ag²⁺ cations are randomly distributed on a single 12b Wyckoff position at the 4ˉ axis of the I4ˉ3d cell. Each silver forms four short (4 × 2.316(7) ?) and four long (4 × 2.764(6) ?) contacts with the neighbouring fluorine atoms. The valence bond sum analysis suggests that such coordination would correspond to a severely overbonded Ag(i) and strongly underbonded Ag(ii). Thorough inspection of thermal ellipsoids of the fluorine atoms closest to Ag centres reveals their unusual shape, indicating that silver atoms must in fact have different local coordination spheres; this is not immediately apparent from the crystal structure due to static disorder of fluorine atoms. The Ag K-edge XANES analysis confirmed that the average oxidation state of silver is indeed close to +1 1/3. The optical absorption spectra lack features typical of a metal thus pointing out to the semiconducting nature of Ag₃(SbF₆)₄. Ag₃(SbF₆)₄ is magnetically diluted and paramagnetic (μ_eff = 1.9 μ_B) down to 20 K with a very weak temperature independent paramagnetism. Below 20 K weak antiferromagnetism is observed (Θ = -4.1 K). Replacement of Ag(i) with potassium gives K(i)₂Ag(ii)(SbF₆)₄ which is isostructural to Ag(i)₂Ag(ii)(SbF₆)₄. Ag₃(SbF₆)₄ is a genuine mixed-valence Ag(i)/Ag(ii) compound, i.e. Robin and Day Class I system (localized valences), despite Ag(i) and Ag(ii) adopting the same crystallographic position.

Full text of DOI:10.1039/c5dt00740b

Volume 44 Number 24 28 June 2015 Pages 10891–11242
Dalton
Transactions
An international journal of inorganic chemistry
www.rsc.org/dalton
ISSN 1477-9226
PAPER
Zoran Mazej, Wojciech Grochala et al.
The first example of a mixed valence ternary compound of silver with
random distribution of Ag(I) and Ag(II) cations

Dalton
Transactions
PAPER
The first example of a mixed valence ternary
compound of silver with random distribution of
Ag(I) and Ag(II) cations†
Cite this: Dalton Trans., 2015, 44,
10957
a
b,c
a
d
Zoran Mazej,* Tomasz Michałowski, Evgeny A. Goreshnik, Zvonko Jagličić,
e,f
c
b
Iztok Arčon, Jadwiga Szydłowska and Wojciech Grochala*
6 6 2
The reaction between colourless AgSbF and sky-blue Ag(SbF ) (molar ratio 2 : 1) in gaseous HF at 323 K
yields green Ag (SbF , a new mixed-valence ternary fluoride of silver. Unlike in all other Ag(I)/Ag(II)
3
6 4
)
+
2+
systems known to date, the Ag and Ag cations are randomly distributed on a single 12b Wyckoff posi-
tion at the 4¯ axis of the I 4¯ 3d cell. Each silver forms four short (4 × 2.316(7) Å) and four long (4 × 2.764(6)
Å) contacts with the neighbouring fluorine atoms. The valence bond sum analysis suggests that such
coordination would correspond to a severely overbonded Ag(I) and strongly underbonded Ag(II). Thorough
inspection of thermal ellipsoids of the fluorine atoms closest to Ag centres reveals their unusual shape,
indicating that silver atoms must in fact have different local coordination spheres; this is not immediately
apparent from the crystal structure due to static disorder of fluorine atoms. The Ag K-edge XANES analysis
1
3
confirmed that the average oxidation state of silver is indeed close to +1 . The optical absorption spectra
lack features typical of a metal thus pointing out to the semiconducting nature of Ag
magnetically diluted and paramagnetic (μeff = 1.9 μ ) down to 20 K with a very weak temperature indepen-
dent paramagnetism. Below 20 K weak antiferromagnetism is observed (Θ = −4.1 K). Replacement of Ag(I)
with potassium gives K(I) Ag(II)(SbF which is isostructural to Ag(I) Ag(II)(SbF . Ag (SbF is a genuine
3 6 4 3 6 4
(SbF ) . Ag (SbF ) is
B
Received 19th February 2015,
Accepted 18th March 2015
2
6
)
4
2
6
)
4
3
6 4
)
DOI: 10.1039/c5dt00740b
mixed-valence Ag(I)/Ag(II) compound, i.e. Robin and Day Class I system (localized valences), despite Ag(I)
www.rsc.org/dalton
and Ag(II) adopting the same crystallographic position.
Introduction
a
Department of Inorganic Chemistry and Technology, Jožef Stefan Institute, Jamova
3
9, SI–1000 Ljubljana, Slovenia. E-mail: zoran.mazej@ijs.si
Chemical compounds with an element in more than one oxi-
dation state have been labelled as “mixed-valence” systems,
although the expression “mixed oxidation state compounds”
perhaps conveys a clearer idea about what is signified. One
striking feature about many mixed-valence compounds is their
b
CENT, University of Warsaw, Żwirki i Wigury 93, 02–089 Warsaw, Poland.
E-mail: w.grochala@cent.uw.edu.pl
c
Faculty of Chemistry, University of Warsaw, Pasteur 1, 02–093 Warsaw, Poland
1
–3
d
University of Ljubljana, Faculty of Civil and Geodetic Engineering, and Institute of
Mathematics, Physics and Mechanics, Jadranska 19, SI–1000 Ljubljana, Slovenia
e
University of Nova Gorica, Vipavska 13, SI–5000 Nova Gorica, Slovenia
intense colour; some – such as minium (Pb(II)
2 4
Pb(IV)O ), or
f
Department of Low and Medium Energy Physics, Jožef Stefan Institute, Jamova 39,
magnetite (Fe(II)Fe(III) O ) – have been used since antiquity as
2
4
SI–1000 Ljubljana, Slovenia
pigments. Prussian blue, Fe(III)
pared over three centuries ago well before the organic chemists
6 2
)
, photos showing self-supported homo- joined the dye business, was the first mixed-valence inorganic
4 6 3 2
[Fe(II)(CN) ] ·nH O, first pre-
†
Electronic supplementary information (ESI) available: X-ray crystallographic
files in the CIF format, a photo of a Ag(SbF solution in aHF, the Raman spec-
trum of impure blue-green Ag(SbF
geneous pellets used for XANES spectroscopy, XRDP for a powder sample of
Ag (SbF , Raman spectra of Ag (SbF compared to several relevant reference
samples, FIR and MIR absorption spectra for Ag (SbF , Ag(SbF , and AgSbF
and Ag(SbF , M vs. H curves
and NUV absorption spectra for
6 2
)
4
compound synthesized by men for use in paints and inks. In
3
6
)
4
3
6
)
4
5
1
967 Robin and Day (almost simultaneously with Allen and
3
6
)
4
6
)
2
6
,
6
Hush ) published an extensive survey of mixed-valence
systems. Their classification, which remains widely utilized
comparison of magnetic properties of Ag
at various temperatures for Ag(SbF
Ag (SbF Ag(SbF and AgSbF
3
(SbF
)
6 4
6 2
)
6 2
) ,
3
6
)
4
,
)
6 2
6
.
Further details of the crystal structure today, is based upon the local symmetry and strength of the
determination could be found and CIF files may be obtained from Fachinforma-
tionszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (e-mail:
crysdata@fiz-karlsruhe.de; http://www.fiz-karlsruhe.de/request_for_deposited_
ligand fields around metal cations. In Class I systems, cations
are found in sites of different symmetry and/or ligand field
strength (valences are firmly trapped, i.e. localized; such com-
pounds adopt mixed valence). On the other hand, in Class III
the electron density is delocalized over cationic sites and two
3 6 4 2 6 4
data.html). CCDC 429207 (Ag (SbF ) ) and 429208 (K Ag(SbF ) ). For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5dt00740b
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 10957–10968 | 10957

Paper
Dalton Transactions
distinct oxidation states cannot be distinguished (“intermedi-
ate valence”) – such systems often exhibit metallic conduc-
tivity. In some cases it is not possible to make a clear
demarcation between Class I and Class III behaviour. Class II
compounds represent such a rare intermediate case in which
some degree of delocalization is observed although the cat-
ionic sites remain distinguishable. One of the most famous
examples of Class II systems is the so called Creutz–Taube
7
Ru(II)/Ru(III) complex, which reveals Class I or Class III
characteristics depending on the energy/time scale of experi-
ments (“dynamic delocalization”).
Mixed-valence compounds exhibit a broad range of fasci-
nating physicochemical properties which are related to elec-
tron transfer between distinct oxidation states (i.e. inter-
8
valence charge transfer). The most striking of those is high-
Fig. 1 A sample of extremely moisture-sensitive green Ag(I)
enclosed in a prefluorinated 0.3 mm quartz capillary.
2 6 4
Ag(II)(SbF )
temperature superconductivity which results from either hole
9
,10
or electron doping to layered Cu(II) oxides
to an appropriate
1
1,12
degree;
tutions or charge injection
doping may be realized either via chemical substi-
1
3,14
formally leading to introduc-
tion of Cu(III) or Cu(I) to the parent Cu(II) systems. Chemically graphic sites and the local geometries around them differ
doped copper oxides are now the best crystallographically markedly, thus leading in all cases to the Class I behaviour.
researched group of inorganic compounds, with over eight
In the present work, we describe the results of detailed
In examination of green Ag (SbF (Fig. 1) and of a related K Ag-
majority of cases (except for YBCO-like systems) these are (SbF . We show that Ag (SbF – unlike its predecessors – is
Class III systems, where doped charge is equally distributed an example of a mixed valence ternary compound of silver
over all copper sites. (Ag(I) Ag(II)(SbF ) with random distribution of Ag(I) and Ag(II)
1
5
thousand distinct structures available in databases.
3
6
)
4
2
6
)
4
3
6 4
)
2
6 4
)
Silver is a heavier sibling of copper and it is natural to cations on the same crystallographic site in the crystal lattice –
search for superconductivity in doped compounds of Ag(II). which nevertheless preserves a localized-valence character –
Many such attempts were made in the early days of oxocuprate and we discuss possible consequences of this finding.
1
6
superconductivity
(late 1980s–early 1990s) but without
success – in each case only Ag(I) could be introduced into the
oxide matrix, which was detrimental to superconducting pro-
perties. Only later it was realized that the oxide environment is
not appropriate for hosting paramagnetic Ag(II) since the latter
species is an enormously strong oxidizer (thus it cannot be
Experimental
Caution: anhydrous HF (aHF) and some fluorides are highly
toxic and must be handled using appropriate apparatus and
protective gear. Organic matter may ignite upon contact with
obtained by the action of O
2
, and it is readily reduced to
3
1
1
7
compounds of Ag(II).
Ag(I) ) and it also tends to disproportionate to Ag(I) and Ag(III)
1
8,19
in the oxide environment.
Instead, Ag(II) systems with
Materials and methods
3
Reagents. AgNO (Alkaloid Skopje, 99.7%) and BF (Union
fluoride anions were suggested as being analogous to Cu(II)
oxides as evidenced by many similarities between the two
3
families of compounds. Moreover, the quantum mechanical Carbide Austria GmbH, 99.5%) were used as supplied. AgBF4,
calculations have revealed that layered Ag(II) fluorides would AgSbF , AgSb 11 and IF [Ag(SbF ] were prepared by methods
2
have a strikingly similar electronic structure to that of as described in the literature. AgF was obtained by fluori-
6
2
F
6
6 3
)
3
2,33
2
0,21
undoped oxocuprates,
thus supporting the surmise that nation of AgNO in aHF, meanwhile a SbF –HF mixture was
3
5
superconductivity could be generated upon doping to the prepared by fluorination of SbF
3
5
in aHF. Pure SbF was pre-
2
2
former type of material just like it is observed for the latter.
3
pared by fluorination of SbF in a flow reaction, and its purity
Regretfully, mixed-valence compounds of silver are quite was checked by Raman spectroscopy. Ag(SbF ) was prepared
6
2
3
4
rare, with about 20 documented stoichiometries (cf. Table 1 in from AgF
ref. 23). The Ag(II)/Ag(I) system – relevant to electron doping to A freshly prepared Ag(SbF
the parent Ag(II) compound – is limited to two complex com- traces of green particles as observed in some reactions
2
and a SbF
5
–HF mixture as described previous1y.
6 2
)
should be sky-blue without any
2
4–26
pounds with organic ligands,
and two inorganic systems: (cf. ESI†).
2
7,28
29,30
Ag
3
(SO
3
F)
4
and a recently prepared Ag
9
(PO
2
F
2
)
14
.
The
5
Synthetic apparatus. Volatile materials (SbF , aHF) were
first coordination spheres of Ag(I) and Ag(II) in these systems handled in an all-PTFE vacuum line equipped with PTFE
are quite different: in organic systems both types of silver valves. The manipulation of the non-volatile materials sensitive
cations are coordinated by different ligands, while in homo- to moisture was done in a glovebox (M. Braun). The residual
leptic inorganic systems Ag(II) and Ag(I) sit on different crystallo- water in the atmosphere within the dry box never exceeded
10958 | Dalton Trans., 2015, 44, 10957–10968
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
1
ppm. The reactions were carried out in FEP (tetrafluoroethyl- at 77 K. The reaction vessel was brought up to 298 K and a
ene–hexafluoropropylene) reaction vessels (length clear blue solution formed in a few minutes. Hydrogen was
50–300 mm, i.d. 16 mm, o.d. 19 mm) equipped with PTFE added to the final pressure of 4 bar. The blue solution
2
valves and PTFE coated stirring bars. Prior to use, all reaction changed its colour to green immediately after the addition of
vessels were passivated with elemental fluorine. Fluorine was hydrogen. In a few minutes a white solid started to precipitate
used as supplied (Solvay, Germany). Anhydrous HF (Linde, and the solution became colourless. After volatiles were
9
2 6
9.995%) was treated with K NiF (Advance Research Chemi- pumped off at room temperature 0.680 g of white solid was
cals, Inc.) for several hours prior to use.
obtained. The Raman spectrum showed the mixture of AgSbF6
and AgSb 11. In the second experiment 0.855 g (1.41 mmol)
Ag(SbF was dissolved in 6 ml of aHF. The reaction vessel
2
F
Synthetic procedures
6 2
)
Reaction between AgSbF6 and Ag(SbF6)2 in liquid was brought up to 298 K and a clear blue solution formed.
aHF. A mixture of AgSbF
6
(0.62 mmol, 214 mg) and Ag(SbF
6
)
2
After small addition of hydrogen a white solid started to pre-
(0.31 mmol, 180 mg) was loaded in a reaction vessel in a dry cipitate, meanwhile the solution remained blue.
box. Anhydrous HF (4 mL) was condensed onto the reaction
Reaction between Ag(SbF ) and H O in liquid aHF. 0.815 g
6
2
2
6 2
mixture and the reaction vessel was warmed to ambient temp- (1.41 mmol) of Ag(SbF ) was loaded in a glovebox into an FEP
erature. Since some white insoluble solid was still visible, reaction vessel. Then aHF (4 ml) was condensed on the reac-
additional 4 ml of aHF were added. After warming the reaction tion mixture at 77 K and 0.025 g (1.41 mmol) of water was
mixture to ambient temperature, a light blue solution was added. The reaction vessel was warmed to ambient tempera-
obtained. After one day of intense stirring, the volatiles were ture. A clear blue solution formed above the white precipitate.
pumped off at ambient temperature. The final mass of the iso- Volatiles were pumped away at ambient temperature. The
lated green solid was 0.390 mg (calcd for Ag
.394 mg). Raman spectra and X-ray powder diffraction pat- AgSbF
terns of the remaining solid showed not only the presence of Synthesis of high-purity Ag (SbF ) . In
3
(SbF
6
)
4
:
Raman spectrum was in agreement with literature data for
0
6
and Ag(SbF
6 2
) .
a
glovebox
and 1.14 mmol (0.661 g) of Ag-
6 2
) were loaded into a FEP tube (V = 8 ml, 6 mm o.d.). aHF
3
6 4
Ag
3
(SbF
6
)
4
but also the presence of the starting AgSbF
6
and 2.28 mmol (0.784 g) of AgSbF
(SbF
6
Ag(SbF
6
)
2
.
Reaction between AgSbF6 and Ag(SbF6)2 in liquid SbF5. was added at room temperature till the final pressure in the
AgSbF (0.48 mmol, 165 mg) and Ag(SbF (0.24 mmol, tube reached 0.93 bar. Immediately after the first traces of aHF
39 mg) were loaded in a glovebox into the FEP reaction vessel. were added to the bluish AgSbF –Ag(SbF mixture, its colour
Then SbF5 (∼2 ml) was condensed on it at 77 K and the changed from blue to olive green. The bottom of the FEP tube
mixture was warmed up to room temperature. The reaction containing the AgSbF –Ag(SbF mixture was cooled down to
6
6 2
)
1
6
6 2
)
6
6 2
)
was left to proceed for one day. Since there was no visible 77 K. After aHF condensed at the bottom, the upper part of the
change (only bluish insoluble stuff was observed) and the FEP tube was sealed using the small flame of a gas burner
reaction mixture was very viscous, a small amount of using flat-nose pliers. A sealed tube was taken in a dry-box and
gaseous aHF was added to decrease the viscosity of the placed inside a larger glass tube with a ground glass neck. The
liquid phase. After an additional day no change had been glass tube was closed with a glass valve. In that way, the FEP
observed.
6 6 2
tube with the AgSbF –Ag(SbF ) –aHF reaction mixture was sur-
Reaction between AgSb F11 and AgF in gaseous HF. 0.070 g rounded by an inert atmosphere which was preventing the
2 2
(
0.24 mmol) of dark brown AgF and 0.538 mg of colourless diffusion of moisture from the atmosphere to the reaction
2
2
AgSb F11 (0.48 mmol) were loaded in a dry-box into a 35 ml mixture through walls of the FEP tube (an attempt without
FEP reaction vessel. aHF was added at room temperature till using the glass tube filled with argon resulted in complete
the final pressure in the reaction vessel reached 0.93 bar. The reduction of Ag(II) and formation of AgSbF /AgSb F ). An
6
2 11
brown reaction mixture immediately turned to olive green. additional reason for selecting the glass instead of, for
After one day, volatiles were pumped off at room temperature. example, a metal tube was the possibility to monitor the
The Raman spectrum of the isolated green solid showed the colour of the solid inside the FEP tube. A glass tube was con-
3 6 4
presence of starting materials and Ag (SbF ) . Since the inspec- nected to a vacuum system and a small amount of argon was
tion under a microscope showed the presence of dark par- pumped away. Only 0.6 bar were left inside. A closed glass tube
ticles, the treatment with gaseous aHF was repeated. The containing a sealed FEP tube was heated up to 318 K. After two
Raman spectrum of the isolated green solid showed the pres- days the glass tube was cooled down and transferred inside the
ence of AgSbF and Ag (SbF ) , meanwhile inspection under dry-box where it was opened. A small FEP tube was cut and
6 3 6 4
the microscope showed the presence of a predominantly green quickly transferred into a larger FEP reaction vessel. Outside
solid with some blue particles. Attempts to obtain X-ray the glovebox the FEP reaction vessel containing the opened
powder diffraction patterns were unsuccessful.
FEP tube was connected to a vacuum system and Ar and aHF
Reduction of Ag(SbF ) , dissolved in aHF, by elemental were pumped away over the night. After that the sample was
6
2
hydrogen. In the first experiment 0.215 g (1.48 mmol) of AgF
was loaded in a glovebox into an FEP reaction vessel and then placed into a new FEP tube. The whole procedure of
.98 g (4.52 mmol) SbF and aHF (6 ml) were condensed on it adding aHF, sealing, etc. has been repeated. Reaction at 318 K
2
first finely ground in an agate mortar inside the dry-box and
0
5
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 10957–10968 | 10959

Paper
Dalton Transactions
was left to proceed for one day and isolation was done as
3 6 4 2 6 4
Crystal structure determination of Ag (SbF ) and K Ag(SbF ) .
described previously. An olive green solid was isolated. A very Single crystals were immersed in perfluorinated oil (perfluoro-
homogeneous olive-green solid was observed under a micro- decalin, ABCR, 98%) in a dry box, selected under a micro-
scope without any blue particles which could be attributed to scope, and transferred into the cold nitrogen stream of the
Ag(SbF ) . The X-ray powder diffraction pattern of the olive diffractometer. Data for Ag (SbF ) were collected at 150 K on
6
2
3
6 4
green Ag
as the X-ray powder diffraction pattern calculated from the tor, and data for K
crystal structure of Ag (SbF ) . In addition, the Raman spectra AFC7 diffractometer equipped with a Mercury CCD detector
3
(SbF
6
)
4
powder obtained by syntheses was the same an Agilent Gemini A diffractometer with an Atlas CCD detec-
3
(SbF were collected at 200 K on a Rigaku
6 4
)
3
6 4
of the bulk material and of the single crystal of Ag
were virtually identical.
3
(SbF
6
)
4
(both using graphite-monochromated Mo-Kα radiation, λ =
0.71069 Å). A multi-scan absorption correction has been
There are two crucial factors which may impact the fate of applied for both datasets. The structures were solved by direct
3
5
this reaction. The first one is the amount of aHF; if it is too methods using the SIR-92
software (implemented in
3
6
37
large and sufficient that aHF will start to liquefy when the program packages TeXsan and WinGX, respectively) and
reactor is cooled down to ambient temperature, aHF(l) will refined with SHELXL-97 software. The figures were prepared
3
8
3
9
inevitably solvolize part of the Ag
AgSbF and Ag(SbF which is detected as blue staining of the
liquid phase. The second important thing is to avoid the summarized in Table 1.
3
(SbF
6
)
4
product back to using the DIAMOND 3.1 program.
6
6 2
)
The corresponding crystal data and refinement results are
diffusion of traces of water and even oxygen through the walls
3 6 4
Powder X-ray diffraction analysis of Ag (SbF ) . Powder X-ray
of the FEP tube which could lead to reduction of Ag(II).
diffraction patterns were obtained using a D8 discover diffracto-
meter from Bruker equipped with a Cu-cathode and parallel
beam setting provided by Göbel mirrors. The sample was
loaded into a thin (0.3 mm diameter, 0.01 mm thick walls)
Crystallography
Growth of single crystals of Ag (SbF6)4 and K Ag(SbF ) . quartz capillary (Hilgenberg) in a dry-box. The obtained
3
2
6 4
Single crystal growth was carried out in double T-shaped powder pattern was analysed using the previously published
apparatus consisting of two FEP tubes (19 mm o.d., and 6 mm crystal structures of AgSbF , Ag(SbF and Ag (SbF (this
o.d.). Around 300 mg Ag(SbF ) was loaded into the wider arm work). The Reflex QPA module of Materials Studio package
6
6
)
2
3
6 4
)
4
0
6
2
of the crystallization vessel in a dry-box. aHF (∼6 mL) was then gave the following results of quantitative phase analysis:
condensed onto the starting material at 77 K. The crystalliza- Ag (SbF 96.6 wt%, AgSbF 3.4 wt%, no Ag(SbF could be
3
6
)
4
6
6 2
)
tion mixtures were brought up to ambient temperature and detected (cf. ESI†).
the clear blue solution, which had developed, was decanted
Instrumental methods
into the narrower arm. The evaporation of the solvent from
this solution was carried out by maintaining a temperature
gradient corresponding to about 10 °C between both tubes for
Raman spectroscopy. Raman spectra were recorded using a
Horiba Jobin Yvon LabRam-HR Raman micro-spectrometer
with a 632.8 nm He–Ne laser exciting beam. The power of the
beam varied from 0.017 mW to 17 mW. Note! The laser power
above 1.7 mW leads to photochemical decomposition of
1
0 days. The effect of this treatment was to enable aHF to be
slowly evaporated from a narrower into a wider tube leaving
the green crystals of Ag (SbF and some colourless powdered
solids. Selected single crystals of Ag (SbF and colourless
3
6 4
)
3
6
)
4
Ag
shown and discussed in the ESI.†
Magnetic measurements. Magnetic measurements for
Ag (SbF and Ag(SbF reference were carried out with a
6 4 6
(SbF ) and formation of AgSbF . All Raman spectra are
3
particles were placed inside 0.3 mm quartz capillaries in a dry-
box and their Raman spectra were recorded. Raman spectra of
−
1
white particles show a narrow very intense band at 1864 cm
,
3
6
)
4
6 2
)
+
characteristic of the presence of the O2 cation (i.e. O SbF ).
2
6
6 6 2
KSbF (0.34 mmol) and Ag(SbF ) (0.17 mmol) were loaded
into the 19 mm o.d. arm of the crystallization vessel inside a
dry-box. The reagent mixture was then transferred to a metal/
Teflon manifold where aHF (5 ml) was condensed onto the
Table 1 Crystal data and refinement results for Ag
SbF
3 6 4 2
(SbF ) and K Ag
(
6 4
)
starting material at −196 °C. The crystallization mixture was Chemical formula
Ag (SbF )
K2Ag(SbF6)4
3
6 4
warmed to ambient temperature and the resulting clear, blue
solution was decanted into the 6 mm o.d. side arm. Evapor-
ation of the solvent from this solution was carried out by main-
taining a temperature gradient of ca. 10 °C between the 6 mm V (Å )
o.d. tube and the 19 mm o.d. tube for one day. Slow distilla-
1
F
w
(g mol−
)
2533.3
2258.1
Space group
I ˉ4 3d
I ˉ4 3d
a (Å)
12.3613(4)
1888.83(18)
4
12.498(1)
1952.3(4)
4
3
Z
D
calc (g cm−3
)
4.454
3.841
tion of aHF from the 6 mm o.d. tube into the 19 mm o.d. tube
resulted in crystal growth inside the 6 mm o.d. tube. Selected
green single crystals were transferred to 0.3 mm quartz capil-
laries inside the dry box. In addition, some colorless crystals
were observed; their Raman spectra correspond to KSbF6.
T (K)
150
0.0297
0.0642
200
0.0836
0.2055
a
R
wR
1
b
2
a
b
R1 is defined as ∑||F | − |F ||∑|F | for I > 2σ(I). wR2 is defined as
o
c
o
2
2 2
2 2 1/2
o c o
[∑[w(F − F ) ]/∑w(F ) ] for I > 2σ(I).
10960 | Dalton Trans., 2015, 44, 10957–10968
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Dalton Transactions
Paper
superconducting quantum interference device magnetometer
The analysis of Ag K-edge XANES spectra was performed
4
1
MPMS-XL-5 from Quantum Design equipped with a 50 kOe with the IFEFFIT program package ATHENA. The relative
magnet, and operating in the temperature range 2–300 K. The Ag K shell contribution in the absorption spectra of the
measurements were performed in 1 kOe magnetic field. Iso- samples was obtained by removing the extrapolated best-fit
thermal magnetization has been measured at several tempera- linear function determined in the pre-edge region (−150 eV to
tures below 20 K between ±50 kOe (ESI†). The data were −50 eV). By conventional normalization, extrapolating the
corrected for sample holder contribution, a temperature inde- post-edge spline background, determined in the range of 50 to
4
2–44
pendent magnetic susceptibility of inner shell electrons 250 eV, the Ag K-edge jump is set to 1.
(
Larmor diamagnetism) as obtained from Pascal’s tables, and
Theoretical methods
a temperature independent paramagnetism.
ESR spectroscopy. The ESR spectra were obtained with an
ESP 300E spectrometer operating at the X-band (frequency
Density functional theory (DFT) calculations. DFT calcu-
lations were carried out for a primitive cell of Ag (SbF ) using
3
6 4
4
5
15
9
.5 GHz) from Bruker for samples sealed under an Ar atmos-
the VASP code as available in the Medea package. To
describe a hypothetical metallic nature of Ag (SbF we have
phere inside a 4 mm thick quartz capillary. Measurements
3
6 4
)
were carried out in the temperature interval 120 K–300 K (at
used the Generalized Gradient Approximation (GGA) and the
1
0–15 K increments); only the spectra recorded at the two
PBEsol functional adopted for solids, with the energy cutoff of
5
00 eV, and k-point mesh of 0.2 Å−
1
.
extreme temperatures are shown. The typical uncertainty of
the determination of the components of the g tensor is ±0.001.
UV-vis-NIR spectroscopy. The electronic spectra for the
samples of AgSbF , Ag(SbF ) and Ag (SbF ) were obtained
using a Vertex 80v spectrometer manufactured by Bruker,
using transmission mode. Halogen and deuterium lamps were
6
6 2
3
6 4
Results and discussion
Towards Ag (SbF ) through serendipity and via many failures
3
6 4
used as the radiation sources with a 2–2.5 mm aperture, a Solid Ag(SbF ) and its aHF solution have characteristic sky-
6
2
CaF
2
beamsplitter in the optical path, and a RT-Si diode type blue colour typical of the presence of solvated Ag(II) (Fig. S1,
/aHF
phere onto one CaF2 window and assembled, together with system in 2003, one of us (ZM) has observed that addition of
another clean CaF window and a separator, as a flat-widow hydrogen into a clear blue solution of Ag(SbF dissolved in
air-tight cuvette used for measurements. aHF–SbF , results in the change of colour. The colour of the
X-ray absorption near-edge structure spectroscopy (XANES). blue solution first turned to green and with time to colourless
The Ag K-edge XANES spectra of the Ag(I) Ag(II)(SbF as well with precipitation of AgSbF and AgSb 11. In addition, during
as Ag(I)SbF and IF [Ag(II)(SbF ] reference compounds were routine preparations of Ag(SbF in the last ten years for
obtained in a standard transmission mode at the BM23 beam- different purposes, many times it happened that the iso-
line of European Synchrotron Radiation Facility (ESRF) in Gre- lated dry Ag(SbF was not completely sky-blue but its colour
detector. Powdered samples were loaded under an Ar atmos- ESI†). During the investigations of the Ag(I)/Ag(II)/SbF
5
3
3
2
6 2
)
5
2
6
)
4
6
2
F
6
6
6
)
3
6 2
)
3
2,46,47
6 2
)
noble. A Si(311) double-crystal monochromator was used with varied from greenish-blue to green. It was also observed that,
about 3 eV resolution at 25 514 eV. Harmonics were effectively when dry sky-blue Ag(SbF ) was stored for a few months’ time
6
2
eliminated by using a flat silicon mirror. The three 30 cm long in a glovebox (where the temperature might increase up to
ionisation chamber was filled with Kr, the first at the pressure 303 K) its colour changed to green-blue. The presence of
of 120 mbar, the second and third at the pressure of 420 mbar. Ag(SbF ) as a major phase in these samples could (beside by
6
2
The absorption spectra were obtained within the its typical colour) be unambiguously determined from its
4
8
−
250 eV/+250 eV interval relative to the Ag K-edge. In the characteristic Raman spectrum. However, all samples, where
XANES region equidistant energy steps of 0.5 eV were used a shade of green colour was observed, gave Raman spectra
with an integration time of 1 s per step. In all experiments the with an additional very intense band at 664 cm− (Fig. S2,
exact energy calibration was established with simultaneous ESI†). Analysis of single crystals of this phase solved the
absorption measurements on a 25 μm thick Ag metal foil mystery showing that the green phase corresponds to a
1
6 6 2
placed between the second and the third ionization chamber. new intermediate compound in the AgSbF /Ag(SbF ) system,
The first maximum of the derivative of the Ag foil spectrum i.e. Ag
3
(SbF
6 4
) .
was assigned to 25 514.0 eV.
Moreover, crystallizations of Ag(SbF6)2 from its aHF solu-
The compounds in the form of fine powder were pressed tions resulted sometimes in green crystals of Ag and a
3
(SbF
6
)
4
into self-supported homogeneous pellets (diameter 6 mm, colorless solid (instead of sky-blue crystals of Ag(SbF ) ). The
6 2
thickness less than 1 mm) in a dry-box to prevent hydrolysis Raman spectra of the colorless solid showed a very narrow
−
1
+
and sealed under an inert atmosphere into a thin-walled FEP intense band at 1864 cm which is typical of O
tube (cf. ESI, Fig. S3†). The FEP tube containing the pellet was mode of O SbF .
2 6
2
stretching
Cationic Ag(II) solvated in aHF is known to
mounted on a sample holder in a vacuum chamber of the be one of the strongest oxidizers known. In aHF acidified with
beam-line. The sealed sample was perfectly stable for several AsF , solvated Ag(II) is capable of oxidizing molecular oxygen
hours of the experiment: no sign of hydrolysis was observed at 195 K, but at 213 K the products (O AsF /AgAsF ) re-
4
9
5
2
6
6
5
0
after demounting.
establish the reactants (Ag(II)(sol)/O2(g)/AsF5(sol)). It seems that
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Paper
Dalton Transactions
6 2
in the case of Ag(SbF ) , dissolved in aHF, the silver(II) salt is
only partly reduced, forming Ag(I) Ag(II)(SbF ) and O SbF as
2
6 4
2
6
a by-product. Therefore crystals of Ag
3
(SbF
6
)
4
may also be
formed in situ during the slow reaction of Ag(SbF ) with traces
6 2
of water or oxygen penetrating through the walls of crystalliza-
tion vessels (cf. the Experimental part).
Since the chemical identity of the green phase has been
established, many attempts were made to synthesize larger
quantities of Ag (SbF ) as a high-purity product suitable for
3 6 4
physicochemical characterization. The most obvious path – by
direct reaction between the corresponding amounts of AgSbF6
and Ag(SbF
6
)
2
in liquid aHF (eqn (1)) – has failed
aHFðliqÞ
2
AgSbF6 þ AgðSbF6Þ2 ꢀꢀꢀ! 2xAgSbF6 þ xAgðSbF6Þ2
298 K
ð1Þ
þ ð1 ꢀ xÞAg3ðSbF6Þ4
since instead of the desired pure Ag (SbF ) , the sample con-
3
6 4
taminated with the substrates was always obtained. Several
other approaches were also tested (see the Experimental
3 4
Fig. 2 Top: comparson of the unit cells of the I 4¯ 3d polytype of Si N
51
2 6 4
and that of the Ag & Sb sublattice of Ag(I) Ag(II)(SbF ) . Note the
section) but either no reaction was observed (i), or mixtures of similarity of the N & Sb sublattices. Bottom: the packing diagrams of
several products were obtained (ii–v):
(left) and Ag(SbF (right) (grey circles: silver atoms;
yellow octahedra: SbF units), two projections were shown.
Ag(I)
2
Ag(II)(SbF
6
)
4
6 2
)
6
(i) 2AgSbF
6 6 2 5
+ Ag(SbF ) in liquid SbF → no reaction;
(ii) 2AgBF4 + AgF2 in liquid SbF5 and gaseous aHF →
AgSb2F11 and Ag (SbF ) ;
3
6 4
(iii) 2AgSb
2 2 6 6 2
F11 + AgF in gaseous aHF → AgSbF , Ag(SbF )
and Ag (SbF ) ;
However, despite their A
compounds, none of them is isostructural to Ag
particular, none of the very few quasi-binary A(I)A(II) B
2 4
3 4
B stoichiometry for many of these
3
6 4
5
2
(
iv) Ag(SbF ) + H in liquid aHF → AgSbF and AgSb F ;
3
(SbF
6
)
4
.
In
6
2
2
6
2 11
(v) Ag(SbF
6
)
2
+ H
2
O in liquid aHF → AgSbF
6
and Ag(SbF
6
)
2
.
com-
Finally, it was established that the solid state reaction pounds of elements, which are known to exhibit both the first
between 2AgSbF6 and Ag(SbF6)2 at 318 K in the presence of and second oxidation state (Cu, Ag, Au, Hg), adopt the I4
traces of the aHF mediator in the gas phase provides the polytype; for example, Cu Cl is isomorphic to magnetite
optimum way to prepare high-purity Ag (SbF ) (eqn (2))
(Fe , spinel). Similarly, we could not find good structural
analogies for A(II)A(IV)2B4 compounds where B is a divalent
ð2Þ anion, such as Pt or Pb . However, it turns out that the
crystal structure of Ag (SbF is somewhat similar to that of a
According to quantitative phase analysis based on powder certain metastable polytype of Si N (Fig. 2).
ˉ3d
3
4
3 4
O
3
6 4
aHFðgÞ
2
AgSbF6 þ AgðSbF6Þ2 ꢀꢀꢀ! Ag3ðSbF6Þ4
3
O
4
O
3 4
)
6 4
3
18 K
3
5
1
3
4
X-ray diffraction, the so-obtained product contains 96.6 wt%
The Sb/N atom in these structures occupies the (x, x, x)
1
Ag (SbF ) and 3.4 wt% AgSbF ; Ag(SbF ) could not be position where x = + δ, with δ ≈ 0.03 (N) and δ ≈ 0.06 (Sb); for
3
6 4
6
6 2
2
detected in the powder pattern. This sample has also proven to x = 0 the packing of anions would correspond to the ideal bcc
be suitable for ESR characterization and hence magnetic sublattice. The major difference between both structures is
measurements were performed since Ag(SbF6)2 could not be that the Ag atom is found in the (0, 1/4, 3/8) position while the
detected in the spectra/magnetization curves (recall AgSbF
diamagnetic).
6
is Si atom occupies the (0, 1/4, 7/8) one, which leads to a
different bonding pattern. Each compound, however, may
be viewed as a defective and severely distorted AB polytype,
Crystal structures of Ag (SbF ) and related K Ag(SbF )
3
6 4
2
6 4
□
[
A A ]B .
3
4
The title compound has been found to crystallize in the body-
centered cubic cell (I ˉ4 3d), with a = 12.3613(4) Å (Table 1). The Ag
related potassium compound, green K(I) Ag(II)(SbF
, is iso- all silver atoms are found in a single crystallographic site
The most interesting feature of the crystal structure of
3
(SbF of relevance to its mixed-valence character is that
6 4
)
2
6 4
)
morphic, albeit with a larger unit cell vector of a = 12.498(1) Å, (occupying a 12b Wyckoff position at the 4
ˉ axis) thus reflecting
consistent with the larger ionic radius of K(I) than that of Ag(I). a random distribution of the Ag(I) and Ag(II) cations (in the
There are four formula units inside the unit cell. There is only 2 : 1 ratio). Thus, irrespective of a noticeable difference in ionic
one Ag atom, one Sb atom and two crystallographically inde- radii of Ag(I) and Ag(II), all silver cations are found in an identi-
pendent F atoms inside the irreducible cell (Fig. 2).
cal environment of eight fluorine atoms (Fig. 3). Such a feature
1
5
Inspection of the Medea database returns over 500 crystal would imply that Ag
(SbF
)
is in fact an intermediate valence
3
6
4
1
structures of inorganic compounds containing at least one (Class III) system, with an average oxidation state of Ag of +1 ,
3
metal atom, which adopt the I ˉ4 3d space group (no. 220). while rendering Ag
3 6 4
(SbF ) the first Ag(II)/Ag(I) system of this
10962 | Dalton Trans., 2015, 44, 10957–10968
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
5
9
the valence bond sum analysis suggests that such coordi-
nation corresponds to a severely overbonded Ag(I) (VBS =
1.288) and strongly underbonded Ag(II) (VBS = 1.252). This
alone is a peculiar but not disqualifying factor, since a silver
site should indeed have bonding characteristics typical of
1
3
+
1
60
Ag
.
However, closer inspection of thermal ellipsoids of fluo-
rine atoms next to silver cations reveals an unusual flattened
shape, indicating a possibility that F atoms might exhibit
static disorder, and thus there might be at least two types of
silver cations in the structure, each with a different coordi-
nation sphere. One site would have a markedly covalent Ag(II)–
Fig. 3 The 4 + 4 coordination sphere of silver and the closest contacts
−
4
F bonding with a nearly square-planar AgF unit, while the
to the [SbF
6
]
anion in the crystal structure of Ag
3
(SbF
6
)
4
(dashed lines:
longer Ag1⋯F2 contacts; outlined bonds: shorter Sb1–F2 distances; other would resemble a typical Ag(I)–F site with eight ligands
6
1
thermal ellipsoids are drawn at the 40% probability level).
in a more ionic environment (Fig. 1). This surmise is further
supported by the fact that the latter site – corresponding to
Ag(I) – may be substituted with K(I) in a new distinct com-
6
2
pound, K Ag(SbF ) (Table 1).
2
6 4
Based on what was discussed above, one may suspect that –
despite a single crystallographic site for Ag in Ag (SbF – this
compound is not a genuine Class III system. However, it is still
unclear at this stage whether Ag (SbF is a well-defined
3
6 4
)
3
6 4
)
frozen-valence Class I or rather a Class II system. Since
vibrational spectra – albeit different for Ag (SbF ) , Ag(SbF )
6 2
3
6 4
and AgSbF
dation states, we have further studied Ag
6
(cf. ESI†) – are not unequivocally indicative of oxi-
(SbF with a range
3
6 4
)
of physicochemical methods more sensitive to the nature of
silver cations present in this compound.
X-ray absorption near-edge structure spectroscopy (XANES) of
Ag (SbF ) and Ag(I)SbF , and IF [Ag(II)(SbF ) ] references
Fig. 4 The electronic density of states of Ag
3
(SbF
6
)
4
calculated while
3
6 4
6
6
6 3
assuming its intermediate-valence (Class III) metallic character (GGA/
PBEsol calculation). Partial DOS (contribution from Ag4d – blue and F2p
states – mauve) is also shown.
3 6 4
The average value of the Ag oxidation state in the Ag (SbF )
sample was obtained by Ag K-edge XANES analysis. The
binding energies of the valence orbitals and therefore the
energy position of the edge and the pre-edge features in the
XANES spectrum are known to be correlated with the valence
state of the absorbing atom in the sample. Namely, each
type. What is more, Ag
3 6 4
(SbF ) would then be analogous to a absorption feature in the XANES spectrum is shifted to higher
family of the long known silver clathrate salts Ag O X (X = energies with an increasing oxidation state of a given element;
7
8
NO
3 2 3
, HF , HCO , etc.) which also crystallize in cubic lattices, this feature is now routinely used to deduce the valence
4
2,43,63,64
and they are comproportionated, metallic and superconduct- state.
For example, examining the XANES spectra
5
3
65
32
ing, with an average oxidation state of silver of +2.43. Indeed, of two reference compounds AgSbF6
in the case of comproportionation Ag (SbF would be met- (Fig. 5) with a known crystal structure and a Ag valence state,
allic, with a substantial density of states at the Fermi level we found out that the Ag K-edge shifts by about 5.4 eV per
Fig. 4) available for Bose–Einstein condensation, and thus valence from Ag(I) to Ag(II).
and IF [Ag(SbF ) ]
6 6 3
3
6 4
)
(
likely even superconducting below a certain temperature.
However, it turns out that the situation for Ag (SbF
If the sample contains the same element in two or more
is sites with a different local structure and/or valence state, the
3
6
)
4
more complex than that. The first coordination sphere of silver measured XANES spectrum is a linear combination of individ-
consists of four short bonds to fluorine (2.316(7) Å) which are ual XANES spectra of individual sites. In such a case the rela-
arranged in a close-to square planar geometry, and four tive amounts of cations at each site can be determined by the
additional long contacts at 2.764(6) Å (Fig. 1). The short con- linear combination fit using XANES spectra of appropriate
tacts are too long for typical Ag(II)–F bonds (in Ag(SbF
6
)
2
:
reference compounds, i.e. compounds that contain the
in element with the same valence state and the same or very
AgFAsF : 1.995(5)–2.004(5) Å; AgF : 2.086 Å ) and simul- similar local structure to the element on each site in the
3
4
32
2
.095(5)–2.132(4) Å;
6 6 3
in [IF ][Ag(SbF ) ]: 2.11(1) Å;
5
4
55
6
2
4
3,63,64
taneously too short for Ag(I)–F bonds (the shortest Ag–F bond sample.
We applied this procedure to the Ag XANES
4 6
are 2.561(4) Å, in AgSbF 2.505(2) Å, in spectrum measured for the Ag (SbF ) sample. The spectrum
5
6
57
lengths in AgBF
AgSb2F11 2.472(8) Å and in AgF 2.55 Å ). Not surprisingly, may indeed be described as a linear combination of the
3
6 4
3
3
58
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Paper
Dalton Transactions
3 6 4 6
Fig. 6 The ESR spectra (X-band) of Ag (SbF ) and Ag(II)SbF .
6
6
AgZrF (g = 2.58, g = 2.11) and AgSnF (g = 2.65, g = 2.12)
6
k
⊥
6
k
⊥
6
6,69
in agreement with the weaker ligand field
exerted by
−
weakly coordinating SbF anions than those of the more elec-
6
tron-rich MF6− ones (M = Zr, Sn).
2
3 6 4
Fig. 5 The Ag K-edge XANES spectrum of a Ag (SbF ) (black dots) and
3 6 4
On the other hand, the ESR spectrum of Ag (SbF )
best-fit linear combination (magenta line) of XANES profiles of Ag(I)SbF
blue line) as a reference for Ag(I) and IF [Ag(II)(SbF ] (red line) as a
6
recorded at room temperature (Fig. 6) shows the presence of a
broad signal at g = 2.333, with some weak additional features.
When the temperature is decreased to 120 K, the signal shifts
slightly to g = 2.324 but without splitting. The measured g
(
6
6 3
)
reference for Ag(II), with the relative weight of 68% and 32%,
respectively.
value is similar to the arithmetic average of g , g⊥1, and g⊥2
k
6 2
obtained for Ag(SbF ) , i.e. 2.318 (at 120 K). This result clearly
suggests that (i) distinct paramagnetic Ag(II) cations are
present in the crystal structure of Ag (SbF ) while (ii) the local
environment of Ag(II) possibly takes the form of a square plane
albeit either substantial local disorder or/and fast relaxation,
which broadens the signal and prevents the separation of
g components.
XANES profiles of AgSbF
6
(containing Ag(I) in the 8-co-
] (containing Ag(II) in a close-
3
6 4
ordinated site) and IF [Ag(SbF )
6
6 3
to square-planar site) (Fig. 5). The fit yields the relative
amounts of Ag(I) and Ag(II) of, respectively, 68% and 32%, with
an uncertainty of about 2%; obtained values are in excellent
agreement with the expected ones of 66.6% and 33.3% for the
proposed structure of the sample.
Thus, XANES results have nicely confirmed not only the
average oxidation state of silver (+1 ) in Ag (SbF ) , but also
that the Ag cations in the structure are present in two valence
states Ag(I) and Ag(II) in a relative ratio of 2 to 1.
The magnetic susceptibility of Ag (SbF ) has been investi-
3
6 4
gated between 2 K and 300 K in a constant magnetic field H =
000 Oe. The molar magnetic susceptibility free from diamag-
1
1
3
6 4
70
3
netic core contribution as a function of temperature of
Ag (SbF is plotted in Fig. 7. The susceptibility monotonically
3
6 4
)
increases with decreasing temperature and obeys a Curie–
ESR spectra and magnetochemistry of Ag (SbF ) and
3
6 4
6 2
Ag(SbF )
One of the key features which distinguish Ag(II) from Ag(I) –
apart from voracious oxidizing properties of Ag(II) – is the pres-
9
ence of one unpaired electron in the former species (4d ),
1
0
while the latter is diamagnetic (4d ). This allows easy detec-
tion of the presence of paramagnetic Ag(II) in the samples by
applying electron spin spectroscopy and magnetometry.
6 2
The ESR spectrum of the Ag(II)(SbF ) reference sample
taken at room temperature (Fig. 6) shows the presence of a
familiar two-constituent signal with the following components
of the g tensor: g
ture is decreased to 120 K, the higher-field signal is split into
two, with g = 2.702, g = 2.149, and g⊥2 = 2.103. Such com-
k ⊥
= 2.699 and g = 2.161. When the tempera-
k
⊥1
ponents of the rhombic g tensor with g
k
≫ g⊥1 ≈ g⊥2 suggest
that the local environment of Ag(II) takes the slightly distorted
6
6–68
square-planar form (i.e. a strongly elongated octahedron)
consistent with the previously determined crystal structure of
this compound (Fig. 2). The absolute values of the g tensor
Fig. 7 Temperature-dependent susceptibility χ(T) and inverse suscepti-
bility χ (T) (inset) of Ag (SbF ) as measured in a magnetic field H =
3 6 4
3
4
−1
components are larger than the corresponding values for 1000 Oe. The full line in the inset is a fit with the Curie–Weiss law.
10964 | Dalton Trans., 2015, 44, 10957–10968
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
Weiss law as can be observed from linear χ− vs. T dependence quasi-tetragonal, the assignment is based on the literature
1
(
inset in Fig. 7). In the 50–300 K temperature range the data data for related compounds of Ag(II) exhibiting D
local
h
4
6
6–69
−1
were fitted to the Curie–Weiss law with the Curie constant C = symmetry.
0
of the Curie constant is consistent with the spin-only value for to the 2b →3b , while the one at 17.83k cm (III) to the
The broad band centered at 9.95k cm (I) is
.46 emu K mol− (μeff = 1.9 B.M.) and θ
1
= −4.0 K. The value assigned to the 5a1g→3b1g transition, that at 13.64k cm (II)
−1
p
−
1
2
g
1g
one Ag(II) per formula unit. This result further confirms that 3e
one out of three silver cations corresponds to paramagnetic immediately derive the values δ
g
→3b1g. From these energies of d–d transitions one may
6
6
1
and δ
2
governing the split-
2
2
Ag(II) while the remaining two cations are diamagnetic Ag(I). ting of the, respectively, E and T terms, as follows: δ =
g
2g
1
is characteristic of weak antiferro- 2.49k cm− and δ
1
= 1.40k cm , as well as the value of the
−1
The negative value of θ
magnetic interactions. Further comparison of magnetic pro- ligand-field splitting energy, Δ = 11.45k cm . The value of
p
2
−
1
−
1
perties of Ag (SbF ) with those of parent Ag(SbF ) is given in Δ falls in the 9.2–12.0k cm range typical of compounds of
3
6 4
6 2
6
6
ESI.†
Concluding this section we note that ESR spectroscopy as
well as magnetometry have helped to unequivocally establish not been assigned in the past; judging from its energy and
Ag(II).
The broad absorption seen at ca. 20–21.5k cm− (IV) has
1
6
7,69
Ag
3
(SbF
6
)
4
as a mixed-valence (Class I) compound with loca- using the previously computed orbital scheme
and selec-
lized oxidation states, i.e. Ag(I)
2
Ag(II)(SbF , and despite the tion rules, we tentatively assign it to the charge-transfer (CT)
6 4
)
same crystallographic site occupied by both types of cations.
transition 2e →3b involving axial F atoms, F →Ag(II). Using
g 1g ax
the same scheme, the absorption seen at ca. 25.9–27.6k cm
−
1
(
(
V–VI) may be assigned to another Fax→Ag(II) CT transition
4a →3b ) which is split since local symmetry around Ag(II) is
Electronic spectra of Ag (SbF ) , Ag(SbF ) , and AgSbF in the
UV-vis-NIR range
3
6 4
6 2
6
1g
1g
in fact slightly departing from ideal D4h (equatorial Ag–F bond
lengths are 2.09 Å and 2.13 Å). The CT transitions involving
Electronic spectra – in conjecture with the ESR spectra and
quantum mechanical calculations – have proven in the past to
be very useful for analysis of chemical bonding in the Ag(II)
67,69
equatorial F atoms, Feq.→Ag(II), have been predicted
to fall
in the region 35–41k cm− and, indeed, some absorption is
1
6
6–69
systems.
the samples of Ag
Ag(SbF ) and AgSbF ) in the NUV-vis-NIR range (5–50k cm
Here we have studied the electronic spectra for
seen in this part of the NUV region (ESI†).
3
(SbF and two reference samples
6 4
) ,
The electronic spectrum of Ag (SbF ) is rather poor in
−
1
3
6 4
(
6
2
6
6 2
bands as compared to that of Ag(SbF ) . The bands I–III are
or ca. 0.6–6.2 eV).
hard to detect, which likely originates from substantial static
disorder of F atoms around Ag(II) in this compound, and thus
to excessively broad absorption bands. An unstructured
shoulder may be seen at ca. 21.3k cm− possibly corresponding
to the CT transition analogous to IV seen for Ag(SbF ) . The
The reflectance spectra in the vis-NIR region are presented
in Fig. 8, while the less informative NUV region is shown in
ESI.†
1
The spectrum of Ag(SbF
6 2
) is rich in bands (I–VI in Fig. 8).
6
2
Since local geometry at the Ag(II) site in this compound is
strong band (#) peaking at 24.67k cm− (3.06 eV, 405 nm) is
the only preeminent feature of the absorption spectrum. Such
a strong feature is absent in the spectrum of Ag(SbF6)2 and
1
6
also the AgSbF reference. Since mixed-valence compounds
often exhibit a new strong electronic absorption band due to
the inter-valence charge transfer (IVCT) transition, it is tempt-
ing to assign the 405 nm absorption band to the vertical
Ag(I)→Ag(II) CT process. The energy of this transition is quite
large (3 times larger than that of the d–d transitions) and thus
the valences are firmly localized in Ag (SbF ) , while the elec-
3 6 4
tronic conduction due to electron hopping between Ag(I)
and Ag(II) sites must be difficult. Indeed, single crystals of
Ag (SbF ) are quite transparent to light and they do not
3 6 4
exhibit metallic luster, typical of metals and narrow-band gap
semiconductors.
Conclusions and prospect
3 6 4
We have prepared and studied Ag (SbF ) , which is one of five
currently known mixed-valence compounds of Ag(II)/Ag(I). This
compound crystallizes in the cubic system (I ˉ4 3d cell) with just
one crystallographic position for silver cations, which finds no
precedence in the chemistry of Ag(II)/Ag(I) compounds of
Fig. 8 Electronic absorption spectra of Ag
3
(SbF
6
)
4
,
Ag(SbF
6 2
) , and
−1
AgSbF
6
in the vis-NIR region. The 5–8.5k cm region is predominated
by instrumental artefacts and it has not been shown. Starlet marks the
artefact, while Roman numbers and # indicate bands which are dis-
cussed in the text.
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 10957–10968 | 10965

Paper
Dalton Transactions
silver. A number of physicochemical methods applied have
confirmed the mixed- (Class I) and not an intermediate-
valence (Class III) character of this compound. The localized-
2 J. P. Fackler Jr., Mixed Valence Compounds, in Encyclopedia
of Inorganic Chemistry, ed. R. B. King, John Wiley & Sons,
Chichester, 2005, vol. 5.
oxidation state character, i.e. Ag(I)
2
Ag(II)(SbF
6
)
4
, cannot be
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deduced from X-ray data due to substantial static disorder of
−
fluorine atoms of the SbF
6
anions, which leads to a mislead-
ing averaged bonding picture. The strong absorption band
seen at 3.05 eV in the electronic spectrum of Ag (SbF ) has
been tentatively assigned to the IVCT transition, Ag(I)→Ag(II).
3
6 4
If the energy of this transition were substantially decreased
6 G. C. Allen and N. S. Hush, in Progress in Inorganic
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7
1
due to application of a large external pressure, a fraction of
Ag(4d) electrons should become itinerant thus resulting in
metallic conductivity, like it has been observed for other proto-
typical mixed-valence systems.
Since Ag
3
(SbF
6
)
4
may be obtained from 2AgSbF
6
+ Ag(SbF
6
)
2
8 R. A. Marcus, J. Chem. Phys., 1956, 24, 966–978.
9 J. G. Bednorz and K. A. Müller, Z. Phys. B: Condens. Matter,
1986, 64, 189–193.
precursors, it seems the enthalpy factor is slightly in favour of
the formation reaction; this process may be viewed as a Lewis
acid/Lewis base reaction, with Ag(SbF
acid, similar to the analogous case of Ag
Moreover, the volume of Ag (SbF ) per formula unit 11 E. Dagotto, Rev. Mod. Phys., 1994, 66, 763–840.
6
)
2
playing the role of an 10 Y. Tokura, H. Takagi and S. Uchida, Nature, 1989, 337, 345–
3
(SO F) formation. 347.
3
4
3
6 4
3
(
(
∼472.21 Å ) is slightly larger than that of the reactant mixture 12 K. J. Fijałkowski and W. Grochala, Dalton Trans., 2008,
∼454.93 Å ) which points to an additional small entropic
3
5447–5453.
−1
stabilization of Ag (SbF ) (of about 9.2 kJ mol in ST factor) 13 C. H. Ahn, et al., Rev. Mod. Phys., 2006, 78, 1185–1212.
3
6 4
7
2
with respect to the substrates. The quest for new mixed- 14 A. T. Bollinger, G. Dubuis, J. Yoon, D. Pavuna, J. Misewich
valence compounds of Ag(II)/Ag(I) – and in particular the Class
III systems – is on.
and I. Božović, Nature, 2011, 472, 458–460.
15 The Infomatica 3.1 database of Medea package lists over
4
.1 thousand structures of quaternary (4-element) copper
oxides A Cu (excluding systems such as complex
sulphates, nitrates etc.), over 3.0 thousand systems built of
elements, over 1.3 thousand combinations of 6 and
more-elements. © Materials Design (2014).
t
B
x
y z
O
5
Acknowledgements
Z.M., E.G., Z.J. and I.A. gratefully acknowledge the Slovenian 16 These attempts left rather little trace in the literature, since
Research Agency (ARRS) for the financial support of the
present study within the research program: P1-0045 Inorganic
Chemistry and Technology, P1-0112 Studies of atoms, mole-
cules and structures with photons and particles, and P2-0348
New imaging and analytic methods. The project ‘AgCENT:
novel unique magnetic and electronic materials based on Ag(II)
failed experiments are less frequently reported than the
successful ones. J. Wang and M. Sayer, Cryogenics, 1993,
33, 1164–1169; A. K. Tyagi, S. Tyagi and T. P. Sharma,
Mater. Sci. Eng., B, 1997, 45, 88–97; J. E. Rodriguez,
A. Marino and J. Giraldo, Physica C, 1997, 282, 1253–
1254.
compounds’ is financed by the Polish National Science Centre 17 W. Grochala and R. Hoffmann, Angew. Chem., Int. Ed.,
NCN, project UMO-2011/01/B/ST5/06673). The selected 2001, 40, 2742–2781.
measurements were carried out by using the CePT infrastruc- 18 K. Yvon, A. Bezinge, P. Tissot, et al., J. Solid State Chem.,
ture financed by the EU European Regional Development Fund 1986, 65, 225–230.
within the Operational Programme “Innovative economy” 19 M. Jansen and P. Fischer, J. Less-Common Met., 1988, 137,
for 2007–2013 (POIG.02.02.00-14-024/08-00). DFT calculations 123–131.
(
were carried out using ICM supercomputers (grant G29-3). 20 W. Grochala, Scr. Mater., 2006, 55, 811–814.
Access to beamline BM23 of European Synchrotron Radiation 21 X. Yang and H. Su, Sci. Rep., 2014, 4, 5420.
Facility within the project LS 2209 is acknowledged. We would 22 W. Grochala, J. Mater. Chem., 2009, 19, 6949–6968.
like to thank Gleb Parakhonskiy and Olivier Mathon of ESRF 23 W. Grochala, J. Mol. Model, 2011, 17, 2237–2248.
for expert advice on beamline operation.
24 Q.-M. Wang, H. K. Lee and T. C. W. Mak, New J. Chem.,
002, 26, 513–515.
5 Q.-M. Wang and T. C. W. Mak, Chem. Commun., 2001, 807–
08.
2
2
2
8
6 A system erroneously claimed to contain Ag(II) and Ag(I)
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4
4
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 10957–10968 | 10967

Paper
Dalton Transactions
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