Compounds with the electron-rich [W6Cl18] 2- cluster anion

Article abstract of DOI:10.1021/ic8019843

Cluster compounds of the general formula A₂[W₆Cl ₁₈] containing singly charged A cations (A = K, Rb, Ag, Tl, NH ₄, N(C₂H₅)₄, N(n-C₃N ₇)₄, N(n-C

Full text of DOI:10.1021/ic8019843

Inorg. Chem. 2009, 48, 3825-3831
Compounds with the Electron-Rich [W6Cl18]2- Cluster Anion
Sonja Tragl,† Markus Stro¨bele,† Jochen Glaser,† Cristian Vicent,‡ Rosa Llusar,*,‡
and H.-Ju¨rgen Meyer*,†
Institut fu¨r Anorganische Chemie, UniVersita¨t Tu¨bingen, Ob dem Himmelreich 7,
D-72074 Tu¨bingen, Germany, and Departament de Qu´ımica F´ısica i Anal´ıtica and SerVeis
Centrals d’Instrumentacio´ Cientifica, UniVersitat Jaume I, AV. Sos Baynat s/n,
E-12071 Castello´, Spain
Received October 21, 2008
Cluster compounds of the general formula A2[W6Cl18] containing singly charged A cations (A ) K, Rb, Ag, Tl, NH4,
N(C2H5)4, N(n-C3N7)4, N(n-C4H9)4) and [W6Cl18]2- anions have been synthesized. Compounds were obtained by
W6Cl18 reduction using methanol and the corresponding metal or (alkyl-)ammonium salts. The use of CoCp2 as
reducing agent in inert solvents such as tetrahydrofurane also leads to the ionic compound (Co(C5H5)2)2[W6Cl18].
All compounds described here evidence the existence of octahedral clusters of the M6X12 type with 20 cluster
electrons occupying metal centered states, thus exceeding the conventional number of 16 electrons for this cluster
type. Electrospray ionization (ESI) mass spectra were recorded for the neutral compound W6Cl18 and for the ionic
salts K2[W6Cl18], Cs2[W6Cl18], and (Co(C5H5)2)2[W6Cl18], showing that the cluster W6Cl18 unit in these compounds is
preserved in solution. The base peak in the ESI spectra for all compounds corresponds to the [W6Cl18]2- anion,
so that neutral W6Cl18 is prone to undergo a two electron reduction process. This result is confirmed by cyclic
voltametry experiments, which makes of W6Cl18 a very clean mild oxidizing agent. The preparation of the complete
series of ionic A2[W6Cl18] (A ) K, Cs, CoCp2) clusters allows to systematically investigate their structural trends as
function of the distinctive cations, which is the main focus of the present work.
Introduction
to date in which the “ideal” number of 16 cluster electrons
is exceeded are the tungsten(III) halides W6Cl18 and W6Br18
Octahedral metal clusters (M6) are abundant in metal-rich
halides of the transition metals of group 5 and 6 metals such
as Nb, Ta, Mo, and W. According to the arrangement of the
inner halide atoms X at the M6 cluster, two types of clusters
can be distinguished, the M6X8 type and the M6X12 type.1
Tungsten(III) chloride, W6Cl18, is a representative of the
M6X12 type, that is, 12 chlorine atoms are bridging the M6
octahedron’s edges and 6 terminal chlorine ligands are
located above the corners of the cluster. As summarized by
Hughbanks, the electronic states of M6X12 clusters can be
generally represented by the molecular orbital level diagram
depicted in Figure 1, with the a1g, t1u, t2g, and a2u levels
representing metal-metal bonding molecular orbitals.2 There-
fore, M6X12 clusters should be stable with 14-16 electrons
in M-M bonding states. The only binary compounds known
* To whom correspondence should be addressed. E-mail: rosa.llusar@
qfa.uji.es (R.L.), juergen.meyer@uni-tuebingen.de (H.-J.M.).
†
Universita¨t Tu¨bingen.
Universitat Jaume I.
‡
(1) Scha¨fer, H.; von Schnering, H. G. Angew. Chem. 1964, 76, 833–849.
(2) Hughbanks, T. Prog. Solid State Chem. 1989, 19, 329–372.
Figure 1. Molecular orbital level diagram of a M6X12 type cluster.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3825
10.1021/ic8019843 CCC: $40.75  2009 American Chemical Society
Published on Web 03/18/2009

Tragl et al.
ammonium ions, the salts A2[W6Cl18] precipitated as small black
crystals only a few minutes after mixing the two solutions. All
reactions resulted in yields higher than 80 %.
(18 electrons/cluster). Calculations of the electronic structure
of W6Cl18 using the extended Hu¨ckel method were performed
by Na¨gele et al. in 2001 and confirmed the antibonding
character of the electronic states above the a2u level.3 It was
concluded that the low thermal stability and the paramagnetic
behavior of W6Cl18 is the result of the occupation of the
antibonding states by two electrons. In consideration of the
electronic properties, it seems quite likely that W6Cl18 tends
to undergo oxidation processes yielding the hypothetical 16
electron cluster cation [W6Cl18]2+. But instead, W6Cl18 is
reduced in methanolic solution forming anionic species
[W6Cl18]n- (n ) 1 or 2), as previously evidenced by the
crystallographic characterization of (N(n-C4H9)4)[W6Cl18] (19
cluster electrons)4 and A2[W6Cl18] (20 cluster electrons, A
) Cs or N(CH3)4).5
In this paper, the existence of the unexpected anion
[W6Cl18]2- is substantiated by the characterization of further
compounds of the composition A2[W6Cl18] with A ) K, Rb,
Ag, Tl, NH4, N(C2H5)4, N(n-C3N7)4, N(n-C4H9)4, and
Co(C5H5)2. Additional confirmation of the cluster’s readiness
to undergo reduction processes was gathered from electro-
spray ionization mass spectrometry and cyclic voltammetry
measurements.
Larger crystals were grown by the diffusion method described
by Massa.6 A small glass container (V ) 10 mL) was filled
completely with a methanolic solution of W6Cl18 and then sealed
by a lid with a small hole in it. The container was plunged into a
beaker containing a solution of the corresponding alkyl ammonium
halide in a way that the smaller glass container was completely
covered by the liquid. After about 1 week, crystals suitable for single
crystal diffraction measurements were formed.
Preparation of (Co(C5H5)2)2[W6Cl18]. To a tetrahydrofuran
(THF) solution of W6Cl18 was added a two-fold excess of Co(C5H5)2
(CoCp2). The mixture was stirred for 30 minutes and filtrated to
give a brown solution. Slow evaporation of this solution gave prism-
like single crystals of (CoCp2)2[W6Cl18] in quantitative yields.
Physical Measurements. A hybrid QTOF I (quadrupole-hexapole-
TOF) mass spectrometer with an orthogonal Z-spray-electrospray
interface (Waters, Manchester, U.K.) was used. The desolvation gas,
as well as nebulizing gas, was nitrogen at a flow of 800L/h and 20
L/h, respectively. The temperature of the source block was set to 120
°C, and the desolvation temperature to 200°C. A capillary voltage of
3.3 KV was used in the negative scan mode, and the cone voltage
was set to 10 V to control the extent of fragmentation of the identified
ions. Sample solutions (ca. 5 × 10-5 M) were infused via syringe pump
directly connected to the ESI source at a flow rate of 10 µL/min. The
chemical composition of each peak in the scan mode was assigned by
comparison of the isotope experimental pattern with that calculated
using the MassLynx 4.0 program.7 Cyclic voltammetry experiments
were performed with an Echochemie Pgstat 20 electrochemical
analyzer. All measurements were carried out with a conventional three-
electrode configuration consisting of platinum working and auxiliary
electrodes and an Ag/AgCl reference electrode containing aqueous 3
M KCl. The supporting electrolyte was 0.1 M tetrabutylammonium
hexafluorophosphate. E1/2 values were determined as 1/2(Ea + Ec),
where Ea and Ec are the anodic and cathodic peak potentials,
respectively.
Experimental Section
Preparation of W6Cl18. The starting compound W6Cl18 was
synthesized as described previously:3 First, WCl6 (Strem, 99.9 %)
was reduced to WCl4 at 370 °C using aluminum (Strem,
99.999 %) as reducing agent. In a subsequent disproportionation
step, WCl4 was decomposed at 470 °C yielding WCl5 and W6Cl12.
Afterwards, W6Cl12 was oxidized in a chlorine gas flow at 240 °C
for 1.5 h to give W6Cl18.
Preparation of A2[W6Cl18] (A ) K, Rb, Ag, Tl, NH4).
Compounds with the general formula A2[W6Cl18] were obtained
as black crystalline powders or black crystals from formal W6Cl18
reduction using methanol as reducing agent in the presence of the
corresponding A+ salts (A ) K, Rb, Ag, Tl, NH4) salt. In a typical
raction, W6Cl18 was stirred in methanol (Fisher Scientific, Analytical
reagent grade) at RT for approximately 0.5 h giving dark brown
solutions. After filtration, methanolic solutions of KCl (Merck,
99.5 %), RbCl (Fluka, 98 %), AgNO3 (Merck, 99.7 %), Tl2CO3
(Merck, 99 %), or NH4Cl (99.8 %) were added. K2[W6Cl18] was
obtained to form needle-shaped crystals after complete evaporation
of the solvent, whereas microcrystalline Rb2[W6Cl18] precipitated
after heating the reactions mixture, leaving back a clear solution.
When a solution of AgNO3 in methanol was added to dissolved
W6Cl18, Ag2[W6Cl18] was formed immediately as a brownish black
precipitate of low crystallinity. The compounds Tl2[W6Cl18] and
(NH4)2[W6Cl18] crystallized as fine powders after evaporation of
methanol from the corresponding solutions.
X-ray Diffraction. The crystal structures of K2[W6Cl18],
(N(C2H5)4)2[W6Cl18], (N(n-C3H7)4)2[W6Cl18], (N(n-C4H9)4)2[W6-
Cl18], and (Co(C5H5)2)2[W6Cl18] were determined from single crystal
diffraction data.
Intensity data of K2[W6Cl18], (N(C2H5)4)2[W6Cl18], (N(n-
C3H7)4)2[W6Cl18], and (N(n-C4H9)4)2[W6Cl18] were recorded on a
Stoe IPDS diffractometer using Mo-KR radiation (λ ) 0.71069 Å,
graphite monochromator). The measurements were performed at
room temperature for K2[W6Cl18] and (N(n-C4H9)4)2[W6Cl18], and
at 210 K for N(C2H5)4)2[W6Cl18] and (N(n-C3H7)4)2[W6Cl18]. The
X-ray intensity data were corrected for absorption, polarization,
and Lorentz effects by the IPDS software X-Red/X-Shape. The data
collection for (CoCp2)2[W6Cl18] was performed at room temperature
on a Bruker Smart CCD diffractometer using graphite-monochro-
mated Mo-KR radiation. The diffraction frames were integrated
using the SAINT package8 and corrected for absorption with
SADABS.9 In all cases, structure solutions using direct methods
and structure refinements were performed using the programs
SHELXS and SHELXL10 of the program package SHELX-97.11
Preparation of A2[W6Cl18] (A ) N(C2H5)4, N(C3H7)4, N(n-
C4H9)4). The alkyl ammonium compounds were also prepared by
adding methanolic solutions of N(C2H5)4Cl (Schuchardt, Mu¨nchen,
99 %), N(n-C3H7)4Br (Merck, 99 %), or N(n-C4H9)4Cl (Merck,
99 %) to W6Cl18 solutions in methanol. With all three alkyl
(6) Massa, W. Kristallstrukturbestimmung; B. G. Teubner: Stuttgart,
Germany, 1994.
(3) Na¨gele, A.; Glaser, J.; Meyer, H.-J. Z. Anorg. Allg. Chem. 2001, 627,
244–249.
(4) Dill, S.; Stro¨bele, M.; Meyer, H.-J. Z. Anorg. Allg. Chem. 2003, 629,
948–850.
(7) Masslynx, 4.0; Waters Ltd., 2000.
(8) SAINT; Bruker Analytical X-Ray Systems: Madison, WI, 2001.
(9) Sheldrick, G. M. SADABS empirical absorption program; Go¨ttingen,
Germany, 2001.
(5) Dill, S.; Glaser, J.; Stro¨bele, M.; Tragl, S.; Meyer, H.-J. Z. Anorg.
Allg. Chem. 2004, 630, 987–992.
(10) Sheldrick, G. M. SHELXTL; Bruker Analytical X-Ray Systems:
Madison, WI, 1997.
3826 Inorganic Chemistry, Vol. 48, No. 8, 2009

Compounds with the Electron-Rich [W6Cl18]2- Cluster Anion
Table 1. Single Crystal Structure Data of K2[W6Cl18] (1), (N(C2H5)4)2[W6Cl18] (2), (N(n-C3H7)4)2[W6Cl18] (3), (N(n-C4H9)4)2[W6Cl18] (4), and
(Co(C5H5)2)2[W6Cl18] (5)
1
2
3
4
5
formula
a, Å
b, Å
K2[W6Cl18]
9.3223(9)
9.3223(9)
8.4528(8)
90
90
120
636.2(1)
1
N2C16H40[W6Cl18]
8.6250(8)
11.653(1)
11.912(1)
61.07(1)
86.53(1)
84.43(1)
1042.7(2)
1
N2C24H56[W6Cl18]
8.7736(9)
12.624(1)
12.755(1)
64.76(1)
79.19(1)
85.27(1)
1255(2)
N2C32H72[W6Cl18]
20.501(2)
28.784(3)
20.369(3)
90
90
90
12020(2)
8
Co2C20H20[W6Cl18]
13.479(1)
13.479(1)
21.874(3)
90
90
90
3974.3(7)
4
c, Å
R, deg
ꢀ, deg
γ, deg
V, Å3
Z
1
fw, g/mol
space group
T, K
1819.40
P31m
2001.70
P1
2113.91
P1
2226.12
Pbca
293(2)
2.460
122.63
0.1464
0.1785
2119.42
I41/amd
293(2)
3.542
193.31
0.0513
0.0776
j
j
j
293(2)
4.749
292.22
0.0288
0.0495
210(2)
3.188
176.51
0.0379
0.0793
210(2)
2.797
146.71
0.0296
0.0696
F
calc, g/cm3
µ, cm-1
a
R1
a
wR2
a all data.
Table 2. Results of the Rietveld Refinement of the Crystal Structure of
Rb2[W6Cl18]
Table 3. Lattice Parameters (in Å), Unit Cell Volume (in Å3), and
Number of Single Indexed Reflections of Compounds
formula
a, Å
Rb2[W6Cl18]
9.2410(2)
8.5197(2)
630.07(2)
1
single indexed
reflections
compound
a
c
V
c, Å
K2[W6Cl18]
Rb2[W6Cl18]
Cs2[W6Cl18]
Ag2[W6Cl18]
Tl2[W6Cl18]
9.228(2)
9.2410(2) 8.5197(2)
9.3210(7) 8.53.02(6) 642.0(1)
9.104(4)
9.140(2)
8.477(2)
625.1(3)
630.07(2)
30
*
*
17
29
33
V, Å3
Z
fw, g/mol
space group
T, K
1912.13
8.414(2)
8.435(1)
8.485(1)
604.0(4)
610.3(3)
621.7(3)
j
P3
293(2)
5-130
5.039
(NH4)2[W6Cl18] 9.198(3)
2θ range, deg
calc, g/cm3
* Lattice parameters were received from Rietveld refinement.
F
RBragg/Rwp
Rexp/Rp
ꢁ2
0.0807/0.1390
0.1187/0.1020
1.36
Results and Discussion
Synthesis and Electrochemistry. The synthesis of the
ionic [W6Cl18]2- cluster is reported by simple reduction of
the corresponding neutral W6Cl18 homologue using methanol
as reducing agent.14 Through this approach it is possible to
prepare a complete series of ionic complexes of general
formula A2[W6Cl18] with various A+ cations. These cations
Anisotropic displacement parameters could be refined for all non-
hydrogen atoms. Crystal structure data of K2[W6Cl18], (N(C2H5)4)2-
[W6Cl18], (N(n-C3H7)4)2[W6Cl18], (N(n-C4H9)4)2[W6Cl18], and
(Co(C5H5)2)2[W6Cl18] are listed in Table 1. Additional tables and
crystallographic data in CIF format are provided in the Supporting
Information.
+
include inorganic K+, Ag+, Tl+, NH4 , Rb+, Cs+, and
The crystal structure of Rb2[W6Cl18] was refined from powder
diffraction data recorded on an Stoe STADI-P powder diffractometer
(transmission geometry, Cu-KR1 radiation (λ ) 1.540598 Å), Ge
monochromator). The structure refinement was performed using the
program package Fullprof/WinPLOTR.12 The atomic coordinates of
Cs2[W6Cl18]5 were chosen as starting positions. Results of the Rietveld
refinement of the structure of Rb2[W6Cl18] are presented in Table 2;
the refined atomic positions and isotropic displacement parameters can
be obtained from the Supporting Information, as well as from the
crystallographic data in CIF format.
Powder XRD patterns of K2[W6Cl18], Ag2[W6Cl18], Tl2[W6Cl18],
and (NH4)2[W6Cl18] (recorded on a Stoe STADI-P powder diffrac-
tometer, Ge-monochromated Cu-KR1 radiation) were indexed
trigonally with the aid of the program system WinXPow13 and the
lattice parameters were refined therefrom. The refined unit cell
parameters are listed in Table 3, alongside with the values for
Rb2[W6Cl18]. The lattice constants of Cs2[W6Cl18]5 are also given
for comparison.
tetraalkylammonium salts (R4)N+ (R ) C2H5, C3H7 and
C4H9). We have also explored the possibility to reduce the
W6Cl18 cluster in “inert” solvents such as THF in the presence
of a mild reducing agent. Hence, the reaction of W6Cl18 with
a two-fold excess of CoCp2 as reducing agent yields the salt-
like (CoCp2)2[W6Cl18] compound. It is remarkably that the
formation of salt-like A2[W6Cl18] complexes proceeds in
almost quantitative yield independently of the reducing agent
used, CH3OH or CoCp2, thus indicating clean W6Cl18
reduction. The structural integrity in solution of the series
of salts A2[W6Cl18] (A ) K, Cs, CoCp2) was proved by
electrospray ionization (ESI) mass spectrometry. ESI mass
spectra of acetonitrile solutions of A2[W6Cl18] (A ) K, Cs,
CoCp2) reveal the presence of a prominent signal centered
at m/z ) 870.6 corresponding to the [W6Cl18]2- anion (see
Figure 2).
Remarkably, when acetonitrile solutions of the neutral
W6Cl18 cluster were subjected to ESI, the [W6Cl18]2- anion
was also observed as the base peak. For the neutral W6Cl18
cluster, this ionization mechanism based on the formal two
(11) Sheldrick, G. M. SHELX-97: program package for the solution and
refinement of crystal structures; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(12) Rodr´ıguez-Carvajal, J.; Roisnel, T. FullProf and WinPLOTR - Windows
Applications for Powder Diffraction Patterns Analysis; France, 2006.
(13) WinXPow Diffractometer Software, version 1.10; Stoe&Cie GmbH:
Darmstadt, Germany, 2001.
(14) Ornelas, C.; Aranzaes, J. R.; Salmon, L.; Astruc, D. Chem.sEur. J.
2008, 14, 50–64.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3827

Tragl et al.
Figure 2. ESI mass spectrum of acetontrile solutions of W6Cl18 at cone
voltages Uc ) 10 V. The inset shows the experimental and simulated isotopic
pattern for the [W6Cl18]2- anion.
Figure 4. [W6Cl12i]Cl6a cluster unit in K2[W6Cl18]; black: tungsten, white:
chlorine. The probability factor of the thermal ellipsoids is 50%.
Figure 3. Cyclic voltammogram for W6Cl18 in CH3CN at a scan rate of
500 mV/s.
analogous to that registered for the series of ionic A2[W6Cl18]
(A ) K, Cs, CoCp2) complexes. A comparison between the
integrated intensity for the reduction and oxidation wave
represented in Figure 3 indicated that the number electrons
involved in the reduction process at -0.15 V is ap-
proximately twice the number transferred in the oxidation
of W6Cl18 at 0.71 V. This can be schematically represented
as follows:
electron reduction is rare, as compared with other metal
halide complexes. Typically metal halide cluster complexes
tend to ionize through halide abstraction (formally a M-Cl
cleavage) or adduct formation.15 For example, the ESI mass
spectrum of the cluster Ta4S9Br8 reveals the presence of
adducts of general formula [Ta4S9Br8 · Br]-.16 In the present
case, it is clear that neutral W6Cl18 is very prone to undergo
two electron reduction processes upon ESI conditions in
excellent agreement with the cyclic voltammetry results (see
below). This remarkable stability contrast with that observed
for the homologous carbon centered cluster, namely
[W6CCl18]2- which readily degrades to afford the lower
nuclearity cluster species as judged by ESI-MS.17 It is also
noticeable in the present work, the soft ionization provided
by ESI-MS which allows the observation of intact hexa-
nuclear dianions upon ESI-MS of W6Cl18. Electron impact
(EI) mass spectrometric investigations of W6Cl18 were first
reported four decades ago revealing an assortment of cations
of general formula [WClx]+ (x ) 1-3), [W2Cly]+ (y ) 5, 6)
and [W3Clz]+ (z ) 8, 9).18
-
-
[W6Cl18]+ y+e z W6Cl18 y+2e z [W6Cl18]2-
(1)
-e-
-2e-
An extension of the investigated potential range shows
two extra redox waves, at -0.95 V and +1.35 V, corre-
sponding to one reduction and one oxidation process,
respectively. Both waves are fully irreversible suggesting
significant structural or chemical changes upon addition or
removal of electrons at these potentials. The preparation of
the complete series of ionic A2[W6Cl18] (A ) K, Cs, CoCp2)
allows to systematically investigate their structural trends as
function of the distinctive cations which is the main focus
of the present work.
Cyclic voltammetry experiments of W6Cl18 were per-
formed in acetonitrile. The cyclic voltammogram shows two
quasi reversible waves between 1.0 V and -0.6 V versus
Ag/AgCl, as represented in Figure 3, which correspond to
one reduction process at E1/2 ) -0.15 V (∆E ) 85 mV)
and one oxidation process at 0.71 V (∆E ) 67 mV).
The first process can be tentatively assigned to a two-
electron reduction based on the ESI-MS results and the
isolation of [W6Cl18]2- species upon reduction with CoCp2.
As a matter of fact, the ESI mass spectrum for W6Cl18 is
Crystal Structures of the Ternary Compounds
A2[W6Cl18] with A ) K, Rb, Cs, Ag, Tl and of
(NH4)2[W6Cl18]. All crystal structures of the A2[W6Cl18] type
are built up from monovalent A+ cations and [W6Cl18]2-
anions. As an example, the [W6Cl18]2- ion in K2[W6Cl18] is
displayed in Figure 4.
j
The structure of K2[W6Cl18] (space group P31m) can be
described as a stuffed tungsten carbide structure. The
[W6Cl18] cluster units form a primitive hexagonal lattice and
the potassium atoms are positioned in the centers of all
trigonal prismatic voids of this lattice. A similar arrangement
(15) (a) Henderson, W.; Evans, C. Inorg. Chim. Acta 1999, 294, 183–192.
(b) Henderson, W.; Nicholson, B. K.; McCaffrey, L. J. Polyhedron
1998, 17, 4291–4313.
(16) Sokolov, M. N.; Gushchin, A. L.; Abramov, P. A.; Virovets, A. V.;
Peresypkina, E. V.; Kozlova, S. G.; Kolesov, B. A.; Vicent, C.; Fedin,
V. P. Inorg. Chem. 2005, 44, 8756–8761.
(17) Burgert, R.; Koch, K.; Schno¨ckel, H.; Weisser, M.; Meyer, H.-J.; von
Schnering, H. G. Angew. Chem., Int. Ed. 2005, 44, 265–269.
(18) Rinke, V. K.; Scha¨fer, H. Angew. Chem. 1967, 79, 650–651.
j
is found for Rb2[W6Cl18] (space group P3), but because of
their larger radii, the Rb+ ions are shifted out of the centers
of the prismatic voids, thereby minimizing repulsive interac-
tions between adjacent cations. Additionally, the cluster units
are rotated around the threefold axis. As expected, this
distortion of the K2[W6Cl18] structure is observed more
3828 Inorganic Chemistry, Vol. 48, No. 8, 2009

Compounds with the Electron-Rich [W6Cl18]2- Cluster Anion
Figure 5. Comparison of the crystal structures of K2[W6Cl18] (top),
Rb2[W6Cl18] (middle), and Cs2[W6Cl18] (bottom). Left column: viewing
direction along [110], right column: viewing direction along [001]. For
the purpose of clarity, the inner chlorine atoms are not shown.
Figure 7. Comparison of the crystal structures of (N(CH3)4)2[W6Cl18] (top),
(N(C2H5)4)2[W6Cl18] (middle), (N(n-C3H7)4)2[W6Cl18] (bottom). For the purpose
of clarity, inner chlorine atoms and hydrogen atoms are not shown.
+
radius for this C.N. was used instead. For NH4 , the ionic
radius determined by Khan and Baur for the C.N. 8 was
applied.20 As can be seen in Figure 6, an almost linear
correlation is found between the cubes of the ionic radii (r3)
of A+ and the unit cell volumes for K2[W6Cl18], Rb2[W6Cl18],
Cs2[W6Cl18], Ag2[W6Cl18], and (NH4)2[W6Cl18]. However,
for Tl2[W6Cl18] a significant deviation is observed. It could
be suspected that Tl+ allows for covalent interactions with
the surrounding chlorine atoms, causing shorter Tl-Cl
distances and a decrease of the unit cell volume. This
observation is consistent with the behavior in the homologous
series of binary (Li, Na, K, Rb, Cs, Ag, Tl) halides where
the Shannon radius of Tl+ leads to similar deviations for
Tl+ halides with higher polarizable anions.
Figure 6. Plot of the unit cell volumes V(EZ) of Ag2[W6Cl18], Tl2[W6Cl18],
(NH4)2[W6Cl18], K2[W6Cl18], Rb2[W6Cl18], and Cs2[W6Cl18] versus the cubes
of the ionic radii (r3) of the cations for the C.N. 8 (Ag+, NH4+) or C.N. 12
(Tl+, K+, Rb+, Cs+). The values for Tl+/Tl2[W6Cl18] were not taken into
account for the linear fit.
distinctly for Cs2[W6Cl18]. The structure of Cs2[W6Cl18] can
be related to the anti-(1T-)CdI2 type.5 A comparison of the
crystal structures of K2[W6Cl18], Rb2[W6Cl18] and
Cs2[W6Cl18] is displayed in Figure 5.
As powder patterns of Ag2[W6Cl18], Tl2[W6Cl18], and
(NH4)2[W6Cl18] were indexed trigonally with lattice parameters
similar to those found for K2[W6Cl18], Rb2[W6Cl18], and
Cs2[W6Cl18] (see Table 3), they can be expected to exhibit similar
crystal structures. In Figure 6, the unit cell volumes of K2[W6Cl18],
Rb2[W6Cl18], Cs2[W6Cl18], Ag2[W6Cl18], Tl2[W6Cl18], and
(NH4)2[W6Cl18] are plotted against the cubes of the ionic radii (r3)
of K+, Rb+, Cs+, Ag+, Tl+, and NH4+.
Crystal Structures of the Alkyl Ammonium Com-
pounds A2[W6Cl18] with A ) N(CH3)4, N(C2H5)4, N(C3-
H7)4, N(n-C4H9)4. As described by Dill et al. in 2004,5
(N(CH3)4)2[W6Cl18] crystallizes in the trigonal space group
+
j
P3m1. The arrangement of the N(CH3)4 cations and
[W6Cl18]2- anions corresponds to the positions of iodine and
cadmium, respectively in anti-(1T-)CdI2. As shown in Figure
7, the atomic arrangements found for the triclinic ammonium
compounds A2[W6Cl18] (A ) N(C2H5)4 and N(C3H7)4) can
be regarded as distortion variants of this structure.
As the alkali metal ions in K2[W6Cl18] and Cs2[W6Cl18]
are coordinated by 12 chlorine atoms, the ionic radii of K+,
Rb+, Cs+, and Tl+ for the coordination number (C.N.) 12
were used as they were published by Shannon.19 In the case
of the silver ions, the highest given C.N. is 8, so the ionic
For (N(n-C4H9)4)2[W6Cl18], crystallizing in the space group
Pbca, a different structure was determined: If only their
centers of gravity are taken into account, the clusters form
a packing arrangement of a rhombic dodecahedra, that is,
each cluster is surrounded by 14 others. The (N(n-C4H9)4)+
(19) Shannon, R. D. Acta Cryst. 1976, A32, 751–767.
(20) Khan, A. A.; Baur, W. H. Acta Cryst. 1972, B28, 683–693.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3829

Tragl et al.
Figure 8. Arrangement of the W6Cl18 cluster units and the N(n-C4H9)4+ ions in the crystal structure of (N(n-C4H9)4)2[W6Cl18] (left hand side, chlorine and
hydrogen atoms are omitted for clarity). On the right hand side, only the clusters’ centers of gravity (black) and the nitrogen positions (light gray) are shown.
To elucidate the structural properties, connecting lines between the clusters’ centers (black) and the nitrogen atoms (white) are drawn.
ions are surrounded tetrahedrally by the cluster anions,
whereas the clusters are coordinated by 8 n-butyl ammonium
ions forming a trigonal dodecahedral arrangement (Figure
8).
Crystal Structure of (Co(C5H5)2)2[W6Cl18]. If only the
positions of Co and of the centers of gravity of the clusters
are considered, the crystal structure of (CoCp2)2[W6Cl18] can
be described as a distorted variant of the MgCu2 struc-
ture:21 The clusters are arranged in a diamond lattice like
the Mg atoms in MgCu2, and the Co atoms of the CoCp2
units are arranged to form tetrahedra linked through all
corners similar to the Cu atoms in the MgCu2 structure. A
comparison of the crystal structures of MgCu2 and
(CoCp2)2[W6Cl18] is shown in Figure 9, where the clusters
are shown by their W6 octahedra and the cations by their
Co atoms only.
Dimensions of W6 clusters in W6Cl18 and [W6Cl18]n-.
The crystal structure of W6Cl18 has been determined from
powder and from single crystal diffraction data.3,5,22 The
structure of the W6Cl18 unit was previously elucidated from
single crystals of W6Cl18(DMSO)4.23 Attention is paid to
Figure 9. Comparison of the crystal structures of MgCu2 (top) and
(CoCp2)2[W6Cl18] (bottom). The smaller unit cell edges of (CoCp2)2[W6Cl18]
dimensional changes of the W6 octahedra in [W6Cl18]n- (n
in the bottom drawing are drawn in gray, with W atoms shown as black
and Co atoms as gray spheres. For reasons of clarity, chlorine atoms and
atoms of the Cp ligands are omitted.
) 0-2) structures, as the occupation of M-M antibonding
orbitals should weaken the W-W bonds. A distortion of the
tungsten cluster could be regarded if the antibonding eu level
In contrast to the feasibility outlined above, no significant
is partly occupied. However, we may note, that because of
increase of the W-W bond lengths is observed when the
the small energy separation between eu and t2g levels (in the
number of electrons in antibonding states rises from 2 in
order of 0.1 eV in [W6Cl18]n-) the sequence of these states
W6Cl18 and W6Cl18(DMSO)4 to 4 in compounds with the
(Fig. 1) may change. In Table 4, a comparison of selected
formula A2[W6Cl18]: for neutral W6Cl18, the W-W bond
structural data of W6Cl18 and its derivatives is given along
lengths vary from 2.866(1) to 2.9021(5) Å, for anions
with the number of cluster electrons and basic structure
[W6Cl18]2-, W-W distances between 2.832(3) Å (powder,
determination characteristics.
293 K) and 2.9185(4) Å (single crystal, 210 K) are found.
There is no pronounced elongation of the W-W bonds found
for 20 electron clusters when compared with an 18 electron
(21) Friauf, J. B. J. Am. Chem. Soc. 1927, 49, 3107–3114.
(22) Siepmann, R.; von Schnering, H. G.; Scha¨fer, H. Angew. Chem., Int.
Ed. Engl. 1967, 6, 637.
(23) Zheng, Y.-Q.; Jonas, E.; Nuss, J.; von Schnering, H. G. Z. Anorg.
cluster. The small deviations observed in the W-W bond
Allg. Chem. 1998, 624, 1400–1404.
lengths of [W6Cl18]n- compounds can be explained by
3830 Inorganic Chemistry, Vol. 48, No. 8, 2009

Compounds with the Electron-Rich [W6Cl18]2- Cluster Anion
Table 4. Interatomic Distances, Space Group, Structure Determination Method, Temperature, and Number of Cluster Electrons of Compounds
W-W distances
compound
min
max
space group
methodb, T in K
electrons per cluster
3
j
R3
W6Cl18
2.866(1) Å
2.8915(5) Å
2.879(1) Å
2.8568(6) Å
2.832(3) Å
2.842(3) Å
2.8703(6) Å
2.8732(4) Å
2.8889(5) Å
2.8827(5) Å
2.856(1) Å
2.8687(5) Å
2.877(2) Å
2.9021(5) Å
2.882(1) Å
2.8934(6) Å
2.873(2) Å
2.862(2) Å
2.9054(6) Å
2.8955(5) Å
2.9185(4) Å
2.9076(5) Å
2.888(1) Å
2.8704(5) Å
R3
p, 293
sc, 293
sc, 293
sc, 210
p, 293
p, 293
sc, 293
sc, 293
sc, 210
sc, 210
sc, 293
sc, 293
18
18
18
19
20
20
20
20
20
20
20
20
5
j
W6Cl18
23
W6Cl18(DMSO)4
P21/n
C2/c
(N(n-C4H9)4)[W6Cl18]4
Cs2[W6Cl18]5
j
P3
Rb2[W6Cl18]a
P3
j
K2[W6Cl18]a
P31m
j
(N(CH3)4)2[W6Cl18]5
(N(C2H5)4)2[W6Cl18]a
(N(n-C3H7)4)2[W6Cl18]a
(N(n-C4H9)4)2[W6Cl18]a
(Co(C5H5)2)2[W6Cl18]a
P3m1
j
j
P1
j
P1
Pbca
I41/amd
a This work. b sc ) single crystal diffraction, p ) powder diffraction.
packing effects of clusters rather than by electronic desta-
bilization effects due to the occupation of antibonding states.
This assumption is supported by investigations concerning
the 14 electron cluster anions [W6O6Cl12]2- and [W6O7Cl11]3-
published by Crawford and Long.24 These compounds also
exhibit the structural characteristics of M6X12 type clusters,
but some of the inner, edge-bridging chlorine ligands are
replaced by oxygen atoms. The W-W distances of chloro-
bridged cluster edges vary from 2.869(1) to 2.914(5) Å and
are in good agreement with the W-W bond lengths in
W6Cl18 units. In contrast, the W-W bond lengths of oxo-
bridged edges are significantly shorter and amount to
approximately 2.7 Å.
The mild synthetic conditions might be the reason for the
stabilization of the possibly metastable [W6Cl18]n- ions.
Although theoretical calculations on various levels of
complexity always revealed antibonding characteristics of
the energy levels above the a2u level, experimental evidence
is found for the observation of energetically stable [W6Cl18]n-
species: cluster anions [W6Cl18]n- seem to be formed
spontaneously when the neutral cluster is dissolved in
methanol or THF, and the results of the ESI-MS and cyclic
voltammetry examinations also confirmed that W6Cl18 is
prone to undergo reduction processes. No significant dimen-
sional changes or distortions are obtained for the different
[W6Cl18]n- species.
Conclusion
Acknowledgment. This work was partially supported by
the Spanish Ministerio de Educacio´n y Ciencia (MEC,
Project CTQ2005-09270-C02-1), Generalitat Valenciana
(GV/2007/106) and Fundacio´ Caixa Castello´-Bancaixa (Grant
P1.1B2007-12). The authors thank the Servei Central
d’Instrumentacio´ Cient´ıfica (SCIC) of the Universitat Jaume
I for X-ray measurements on (CoCp2)2[W6Cl18].
In addition to the already known compounds containing
anionic [W6Cl18]n- species,4,5 it could be shown in the present
work, that the 20 cluster electron anion [W6Cl18]2- can be
crystallized with a variety of monovalent cations yielding
salt-like products of the composition A2[W6Cl18]. The
formation of these compounds is not limited to the use of
methanol as reducing agent, but other chemical reducing such
as CoCp2 in THF solutions cleanly afford (CoCp2)2[W6Cl18]
in quantitative yield.
Supporting Information Available: Additional tables and
crystallographic data in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
(24) Crawford, N. R. M.; Long, J. R. Inorg. Chem. 2001, 40, 3456–3462.
IC8019843
Inorganic Chemistry, Vol. 48, No. 8, 2009 3831