First isolated hypoelectronic [In6]6- cluster in insulating Cs22In6(SiO4)4

Article abstract of DOI:10.1002/zaac.201100074

Cs₂₂In₆(SiO₄)₄ was synthesized by the reaction of appropriate starting materials at 673 K, followed by slow cooling to room temperature, in arc-welded tantalum ampoules. According to single-crystal X-ray analysis, the compound crystallizes in a new structure type (P2₁/n(no. 14), a = 14.3533(4), b = 16.1712(4), c = 25.0135(7) A, β = 94.368(1), Z = 4), consisting of [In₆]^6- clusters with the shape of a distorted octahedron or more appropriately described as a condensate of three face sharing tetrahedra. The cluster is the first example of a hypoelectronic isolated [In₆] ^6- indium cluster. The oxosilicate indide can be regarded as a double salt , Cs₆In₆ on one hand and the oxosilicate Cs₄SiO₄ (× 4) on the other, which form the quaternary structure by inhomogeneous intergrowth of partial structures. The electronic structure of Cs₂₂In₆(SiO₄) ₄ was examined by DFT calculations and compared to the one of Rb ₂In₃, which exhibits linked In₆ polyhedra. According to the DOS the title compound is a semiconductor with a band gap of 0.5 eV, which is in agreement with its observed insulating character. [In ₆]^6- is an isolated cluster bearing inert electron pairs at each vertex. In contrast, [In₆]^4- in Rb₂In₃ only exhibits inert pairs at the apical atoms. The four basal atoms are linked to neighboring clusters by covalent bonds forming a 2D network. These bonding scenarios are supported by the analysis of the projected density of states, the electron localization function and the partitioning of the electron density according to Bader. Copyright

Full text of DOI:10.1002/zaac.201100074

ARTICLE
DOI: 10.1002/zaac.201100074
6–
First Isolated “Hypoelectronic” [In ] Cluster in Insulating Cs In (SiO )
6
22
6
4 4
[a]
[a]
[a]
[a]
Vyacheslav Saltykov, Jürgen Nuss, Ulrich Wedig, Dasari L. V. K. Prasad, and
Martin Jansen*[
a]
In Memory of Professor Hans Georg von Schnering
Keywords: Cesium; Indium; Cluster compounds; Density functional calculations; Electron localization function;
Topological analysis of the electron density
Abstract. Cs22In
6 4 4 6 4 4
(SiO ) was synthesized by the reaction of appropri- structure of Cs22In (SiO ) was examined by DFT calculations and
ate starting materials at 673 K, followed by slow cooling to room tem- compared to the one of Rb In , which exhibits linked In polyhedra.
2
3
6
perature, in arc-welded tantalum ampoules. According to single-crystal
X-ray analysis, the compound crystallizes in a new structure type (P2
According to the DOS the title compound is a semiconductor with a
1
/n
band gap of 0.5 eV, which is in agreement with its observed insulating
(
no. 14), a = 14.3533(4), b = 16.1712(4), c = 25.0135(7) Å, β =
6–
character. [In
6
]
is an “isolated” cluster bearing inert electron pairs at
6
–
9
4.368(1), Z = 4), consisting of [In
6
]
clusters with the shape of a
4–
each vertex. In contrast, [In
6
]
2 3
in Rb In only exhibits inert pairs at
distorted octahedron or more appropriately described as a condensate
of three face sharing tetrahedra. The cluster is the first example of a
the apical atoms. The four basal atoms are linked to neighboring clus-
ters by covalent bonds forming a 2D network. These bonding scenarios
are supported by the analysis of the projected density of states, the
6
–
“
hypoelectronic” isolated [In
can be regarded as a “double salt“, Cs
cate Cs SiO
by inhomogeneous intergrowth of partial structures. The electronic sity according to Bader.
6
]
indium cluster. The oxosilicate indide
6
In on one hand and the oxosili-
6
( × 4) on the other, which form the quaternary structure electron localization function and the partitioning of the electron den-
4
4
Introduction
Towards the lighter triels, the energy separation between the
valence s- and p-orbital decreases and the inert pair becomes
available for bonding. Via hybridization, one of the electrons
of an inert pair may participate in an exo- or intercluster bond
whereas the other one additionally contributes to the skeletal
bonding, thus reducing the formal charge of the cluster by one.
The relativistic stabilization of the 6s orbital in main group
elements of the sixth period leads to the formation of inert
electron pairs (lone pairs), not involved in the chemical bond-
ing. As one of the consequences, thallium exhibits a notable
inclination to form homonuclear clusters with rather high nega-
tive charges. However, after subtracting the inert pairs, the
number of electrons remaining for the skeletal bonding is in
2–
The classical example is B H6 having 6 exo bonds and
6
2
× 6 + 2 (2n + 2) skeletal electrons, forming a Wade closo-
[8]
cluster. Indium is at the borderline between thallium and the
lighter triels and can comply with both situations, even in the
most instances even lower than required for electron deficient
clusters according to Wade’s rules.[
1–6]
Such clusters have been
[9]
[10]
4–
[1]
same compound. In Rb
2
In
3
,
as well as in Cs
2
In
3
,
[In ]
6
denoted as “hypoelectronic” in a merely descriptive sense.
octahedra form a two-dimensional network each displaying
four intercluster bonds. The apical atoms still exhibit an inert
However, closer inspection of the respective electronic struc-
tures (by state of the art quantum mechanical tools) has re-
vealed that, in the cases studied, the thallium cluster anions
may be regarded “electronic precise“ since they have arrived
at closed shell configurations with a significant HOMO–
LUMO gap opening. As the underlying mechanisms, either the
degeneracy of the occupied frontier orbitals is lifted by geo-
metrical distortion (Jahn–Teller effect), or by spin–orbit coup-
ling. In the latter case, the cluster anions can even stay highly
symmetric.[
electron pair. Completely isolated indium clusters are rare.
Nido-[In5]9
– [11]
has quite short intercluster distances, and re-
spective bonding interactions cannot be excluded. Isolated “hy-
7–
[12–14]
poelectronic” [In ] is found in K In and Rb In ,
and
11
8
11
8
11
8
–
[15]
nido-[In4] in Na In.
However, all latter compounds are
2
metallic, and thus it is not possible to unambiguously assign
electrons to the cluster.
4,6,7]
Incorporating different, closed shell complex anions into the
systems under consideration is expected to favor the formation
of isolated hypoelectronic indium or thallium clusters. Such an
approach has recently been proven feasible for the quaternary
*
Prof. Dr. M. Jansen
Fax: +49-711-689-1502
E-Mail: M.Jansen@fkf.mpg.de
[
5]
4–
systems Cs–Tl–Tt–O (Tt = Si, Ge, Sn), where [SiO ] ,
4
[
a] Max-Plank-Institut für Festkörperforschung
4–
4–
[
GeO ] or [SnO3] anions coexist with the hypoelectronic
[Tl6] cluster. The compounds can be regarded as “double
4
Heisenbergstr. 1
6–
70569 Stuttgart, Germany
8
34
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 637, 834–839

6 6 4 4
First Isolated “Hypoelectronic” [In (SiO )
]6– Cluster in Insulating Cs22In
Table 1. The crystallographic and structure refinement data for
Cs22In (SiO
salts” consisting of Cs Tl and respective oxosilicates, -ger-
6
6
6
4 4
) .
manates, or -stannates. Using a similar synthesis approach we
now managed to obtain and characterize Cs In (SiO ) . This Space group (no.); Z
1
P2 /n (14); 4
22
6
4 4
Cell parameters (from
X-ray powder data) /Å
a = 14.3533(4)
b = 16.1712(4)
c = 25.0135(7)
β = 94.368(1)
5789.0(3)
is the first example of an insulating compound, that contains
isolated “hypoelectronic” [In6]6– clusters.
Volume /Å3
–
1
Experimental Section
All experimental operations were performed under dried argon
Molar weight /g·mol
3981.30
4.568
–3
Density (calc.) /g·cm
Crystal form, color
Block, silver grey
0.23 × 0.21 × 0.11
Bruker AXS, APEX SMART CCD
Graphite
(
1
2 2
Schlenk technique, or glovebox with H O, and O < 0.1 ppm; MB Crystal size /mm
50B–G–II, M. Braun GmbH, München, Germany). Cs22In
6
(SiO
4
)
4
Diffractometer
was prepared by reaction of cesium (synthesized from CsCl by reduc- Monochromator
tion with calcium and distilled twice in vacuo[ ) with indium (tear
16]
X-ray radiation, λ /Å
θ range /°
Index range
α
Mo-K , 0.71073
0.82 to 36.45
drops, 99.999 %, ChemPur, Jülich, Germany; each drop was cut into
–22 ≤ h ≤ 23
smaller pieces before being placed into a tantalum ampoule), SiO
Sigma–Aldrich, Riedel-de-Haën, 99 %) and Cs O (synthesized from
2
–
26 ≤ k ≤ 26
(
2
–
41 ≤ l ≤ 41
cesium by oxidation with a slight deficiency of oxygen followed by
[18]
Absorption correction
Total no. reflections, Nall
SADABS
109617
28071
distillation of the cesium excess[ ). The starting materials for the syn-
17]
thesis (total amount of 1–2 g) were weighed out in the molar ratio of Independent reflections, N
2
Cs:Cs O:In:SiO = 22–24:8:6:4, afterwards placed into a tantalum Reflections with I > 2σ(I), N' 20225
–1
2
tube, which was sealed under argon with an arc welder. In order to Absorption coeff., μ /mm
16.106
6752
F(000)
Refinement method
Weighting scheme
prevent oxidation, the tantalum tube was encapsulated in a silica jacket,
under argon. Cs22In (SiO was synthesized by heating the reaction
2
Full-matrix least-squares on F
6
4 4
)
2
2
2
–
1
w = 1/[σ (F
o
) + (0.1024P) +
mixture at a rate of 50 K·h up to 673 K, annealing at this temperature
3
23.8895P],
where P = (F
434
for 2 days, and afterwards cooling to room temperature at a rate of
2
+ 2F 2
c
)/3
–
1
o
2
00 K·h . The unreacted elemental cesium was distilled off at 373 K
Number of free parameters
Goodness-on-fit on F
–
3
in dynamic vacuum of 10 mbar. The atom ratio of heavy elements
2
1.040
in the product was confirmed by using a scanning electron microscope R (N'; N )
0.0841; 0.1072
0.2433; 0.2572
0.988, 0.012
– 7.043 / 10.622
CSD-422639
1
all
(
XL 30 ESEM, Philips, Netherlands, tungsten electrode, 25 kV), wR (N'; N )
2 all
equipped with an integrated EDAX–EDX system (S–UTW–Si(Li)–de- Twin volume fractions
Δρmin; Δρmax /e·Å–3
tector); furthermore, no impurity elements were detected.
Deposition no.
Single crystal X-ray diffraction data were collected with a Smart
APEX–II–diffractometer (Bruker-AXS, Karlsruhe, Germany) at 293 K
α
with graphite-monochromated Mo-K radiation. Semi-empirical ab-
sorption correction (SADABS[ ) was applied. The structure was dependent resistivity was obtained for
a pressed pellet of
18]
[
21,22]
solved by direct methods and refined by full-matrix least-squares fit- Cs22In
6
(SiO
4
)
4
using the van der Pauw method.
The crystalline
ting using SHELXTL program package.[
19]
Experimental details, samples were ground into a powder and subsequently pressed into a
atomic coordinates and equivalent isotropic displacement parameters ~1 mm thick pellet with a diameter of 6 mm. The pellet was afterwards
are presented in Table 1 and Table 2. The sample also was examined connected to four probes of the resistivity measurement setup. A cur-
by X-ray powder diffraction analysis. Powder pattern was collected rent of 10 mA (Keithley 2400 current source) was applied, and the
with a LynxEye – Super Speed Detector with a D8 Advance diffrac- voltage was measured with a Hewlett Packard 34420 nanovoltmeter in
tometer (Bruker AXS GmbH, Karlsruhe, Germany; Mo-Kα1 radiation, a temperature range 5–290 K at 5 K intervals.
λ = 0.7093 Å, 4 < 2Θ < 40 degrees, 20 h of measure time). The sample
was sealed in a glass capillary of 0.2 mm diameter to prevent oxida-
tion. Lattice parameters were calculated and refined using TOPAS-Aca- Theoretical Methods
demic software.[ Le Bail method was used for the refinement proce-
20]
First principles density functional theory (DFT) calculations
dure. For the refined X-ray pattern the GOF is 1.53, Rexp = 2.69 %,
were performed on Cs In (SiO ) and Rb In to investigate
22
6
4 4
2
3
R
p
wp = 4.13 %, R = 2.78 %. The calculated lattice parameters were
the electronic structure and bonding, paying particular atten-
used for calculation of interatomic distances. Further details of the
crystal structure investigations may be obtained from Fachinforma- tion to the electronic requirements of isolated and linked In
tionszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany clusters. The calculations were done with the Vienna ab initio
6
[23]
(
Fax: +49-7247-808-666; E-Mail: crysdata(at)fiz-karlsruhe.de, http:// simulation package (VASP), employing the Perdew–Burke–
www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting the
appropriate CSD number given in Table 1.
Ernzerhof (PBE) functional of generalized gradient approxima-
[24]
tion (GGA).
The projector-augmented wave (PAW)
[
25,26]
method
was used to treat the core-valence electron inter-
actions where cesium (5s ,5p ,6s ), rubidium (4s ,4p ,5s ), in-
dium (5s 5p ), silicon (3s ,3p ), and oxygen (2s 2p ) are va-
lence states. The plane-wave basis was chosen at the accurate
The thermal stability was studied using Differential Scanning Calorim-
etry (DSC 404 C Pegasus, Netzsch GmbH, Selb, Germany). The speci-
men (~0.015 g) was placed in an aluminum crucible with a lid, cold-
sealed under argon to prevent oxidation and heated to 723 K at a rate
2
6
1
2
6
1
2
1
2
2
2
4
–
1
of 10 K·min , and afterwards cooled down to room temperature with precision level of VASP-PAW atomic potentials concerning the
the same rate. The whole process was run under argon. Temperature- cut-offs, albeit we had 192 atoms in the unit cell of
Z. Anorg. Allg. Chem. 2011, 834–839
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
835

V. Saltykov, J. Nuss, U. Wedig, D. L. V. K. Prasad, M. Jansen
ARTICLE
Table 2. Atomic coordinates and equivalent isotropic displacement pa-
were calculated using a quick projection Scheme implemented
in VASP. The ELF is computed from the excess kinetic-energy
density due to the Pauli principle, taking values between 0 and
2
rameters Ueq /Å of Cs22In
6
(SiO
4
)
4
.
Atom x
y
z
U
eq
1. Regions with high ELF values are related to covalent bonds,
Cs1 0.37405(6)
Cs2 0.13298(6)
Cs3 0.72691(6)
Cs4 0.17418(7)
Cs5 0.52882(6)
Cs6 –0.12467(7)
Cs7 0.85224(7)
Cs8 –0.03704(7)
Cs9 0.03429(7)
Cs10 0.78462(8)
Cs11 0.27552(8)
Cs12 0.09552(7)
Cs13 0.32164(7)
Cs14 0.46600(7)
Cs15 0.21356(7)
Cs16 –0.23657(9)
Cs17 0.0475(1)
Cs18 0.8697(1)
Cs19 0.03322(8)
Cs20 0.4767(1)
Cs21 0.6640(2)
Cs22 –0.29445(7)
In1 0.5287(1)
In2 0.58373(9)
In3 0.39333(9)
In4 –0.0501(1)
In5 0.59424(9)
In6 0.3973(1)
Si1 0.9533(2)
O11 0.9256(9)
O12 0.9862(9)
O13 0.0416(9)
O14 0.8604(9)
Si2 0.2994(2)
O21 0.3806(7)
O22 0.2992(7)
O23 0.1980(7)
O24 0.3175(7)
Si3 0.2029(2)
O31 0.1215(8)
O32 0.1848(7)
O33 0.1986(8)
O34 0.3043(8)
Si4 0.4962(2)
O41 0.5510(8)
O42 0.4982(8)
O43 0.3884(8)
O44 0.547(1)
0.79928(5)
0.69367(5)
0.61985(6)
0.83119(6)
0.63658(5)
0.62962(6)
0.75246(7)
0.86325(6)
0.50620(6)
0.87738(6)
0.87592(7)
0.66429(8)
0.63481(7)
0.54771(8)
0.58567(7)
0.93279(7)
0.91028(7)
0.52675(7)
0.9550(1)
0.00164(8)
0.8544(1)
0.66533(6)
0.77940(9)
0.62518(8)
0.69078(9)
0.74516(9)
0.80058(9)
0.87593(9)
0.7175(2)
0.6423(8)
0.8010(9)
0.6829(7)
0.7367(8)
0.5241(2)
0.5000(6)
0.4581(6)
0.5197(6)
0.6189(6)
0.9636(2)
0.9932(6)
0.8649(5)
0.0207(6)
0.9744(7)
0.2108(2)
0.1432(7)
0.3013(7)
0.1790(8)
0.2101(8)
0.02913(3)
0.0279(2)
–0.01768(4) 0.0311(2)
inert electron pairs and anionic shells. The Bader charge por-
tioning was done by integrating the charge density within the
atomic basins that are regions separated by the zero-flux surfa-
0.11853(4)
0.11924(4)
0.0341(2)
0.0335(2)
–0.01986(4) 0.0321(2)
–0.00525(4) 0.0356(2)
[29]
ces of the electron density.
0.25130(4)
0.01724(4)
0.0360(2)
0.0339(2)
–0.11306(4) 0.0353(2)
Results and Discussion
0.13266(4)
–0.10828(4) 0.0425(2)
0.24757(4) 0.0380(2)
0.0381(2)
Cs In (SiO ) was synthesized by the reaction of the appro-
22
6
4 4
priate starting materials at 673 K, followed by slow cooling to
room temperature, in arc-welded tantalum ampoules. The
product was obtained as silver gray brittle crystals. According
–0.11692(5) 0.0414(2)
0.10751(4)
0.11660(4)
0.0416(2)
0.0392(2)
to powder diffraction X-ray analysis, Cs In (SiO ) was ob-
–0.14100(4) 0.0431(2)
22
6
4 4
0.24770(5)
0.25424(4)
0.0477(3)
0.0463(3)
tained as a single phase. The synthesized compound is ex-
tremely sensitive to moisture and air, and is stable at least up
to 723 K in an argon atmosphere. The specific electrical resist-
–0.10456(5) 0.0517(3)
0.10476(8)
0.0672(5)
ivity is temperature independent and is in the range of 5–8 ×
–0.00828(6) 0.0715(5)
–0.13420(4) 0.0367(2)
7
1
0 Ω·cm, which is typical for insulating materials.
0.16664(5)
0.25335(5)
0.23686(5)
0.0422(3)
0.0388(2)
0.0429(3)
According to the results of an X-ray single-crystal structure
analysis (Table 1 and Table 2), Cs In (SiO ) crystallizes in a
22
6
4 4
new structure type (Figure 1), consisting of octahedral [In6]
clusters, [SiO ] oxoanions and isolated cesium cations. Similar
–0.15291(5) 0.0427(3)
0.28078(6)
0.24565(6)
0.1251(1)
0.1689(5)
0.1605(5)
0.0913(5)
0.0847(5)
–0.0046(1)
–0.0437(4)
0.0470(4)
–0.0394(4)
0.0194(4)
0.0117(2)
0.0509(4)
–0.0053(4)
–0.0428(4)
0.0446(5)
0.1306(2)
0.1724(4)
0.1601(6)
0.1219(5)
0.0755(5)
0.0450(3)
0.0452(3)
0.0140(4)
0.037(2)
0.042(3)
0.038(2)
0.038(2)
0.0166(5)
0.025(2)
0.026(2)
0.026(2)
0.028(2)
0.0178(5)
0.031(2)
0.028(2)
0.029(2)
0.036(2)
0.0201(5)
0.031(2)
0.038(3)
0.040(3)
0.047(4)
4
[
5]
to oxotetralide thallides, Cs22In
(SiO ) can be regarded as a
6
4 4
“
double salt”. On the basis of an ionic description, Cs In on
6
6
one hand and the oxosilicates Cs SiO ( × 4) on the other,
4
4
constitute binary and ternary, respective, partial structures,
which form the quaternary structure by inhomogeneous
[
30]
intergrowth
(Figure 1a). The shape of the orthosilicate(IV)
anion deviates from the ideal tetrahedral arrangement
105.9(7)° ≤ ∠O–Si–O ≤ 113.5(8)°), similarly to other known
(
orthosilicates. The Si–O distances range from 1.61 to 1.67 Å
and are also similar to the distances in other known orthosili-
[
31–35]
cates containing tetrahedral SiO units
recently synthesized Cs Tl SiO .
The new cesium indide silicate contains [In6] clusters,
which can be characterized as strongly distorted octahedra
(Figure 1a), with eleven shorter In–In distances ranging from
as well as in the
4
[5]
10
6
4
6–
2
.85 to 3.36 Å and one very long In–In distance of 3.78 Å
(
Figure 2a). These distances are similar to those in known In -
6
[9]
units: d(In–In) = 2.88–3.09 Å in Rb In , d(In–In) = 2.89–
2
3
[10]
[36]
3
.15 Å in Cs In , d(In–In) = 2.91–3.12 Å in K Na In ,
2 3 3 26 48
[37]
and d(In–In) = 2.897–3.15 in KNa In . In contrast to Rb In3
3
9
2
Cs In (SiO ) . Single-point calculations were done with the and Cs In , where the “intercluster” In–In distances (2.808 and
2
2
6
4 4
2
3
experimental crystal structure parameters with a 4 × 4 × 4 2.833 Å, respectively) are even shorter than the intracluster
Monkhorst–Pack k-point mesh and a Gaussian smearing of separations, the intercluster distances in Cs In (SiO ) are
22
6
4 4
0
1
.05 eV until the self-consistence convergence of 0.1 × substantially larger ( ≥ 6.6 Å).
–6
0
eV was achieved.
A detailed analysis of the bond lengths (Figure 2) shows that
To extract meaningful chemical information, the converged the In cluster is much more distorted than the corresponding
6
[2,38]
6–
Kohn–Sham eigenstates were analyzed in three ways by look- Tl polyanions.
. Thus, the [In6] cluster can be more ap-
6
ing at the density of states (DOS), electron localization func- propriately described as a condensation of three face sharing
[27]
tion (ELF) and partitioning of the electron density according tetrahedra (Figure 2a), and if the In polyhedron would be de-
6
to Bader.[ The generated DOS relies on linear combination scribed as a distorted closo-cluster according to Wade’s
28]
[8]
of basis functions used to construct the electron density. The notation, it would require 6 × 4 + 2 = 26e (inert-electron
site and orbital angular momentum (l) projected DOS (PDOS) pairs included), which is in conflict with the 24e available. In
8
36
www.zaac.wiley-vch.de
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 834–839

6 6 4 4
First Isolated “Hypoelectronic” [In (SiO )
]6– Cluster in Insulating Cs22In
6
–
4–
Figure 2. The [In
6
]
(a) and [In
6
]
6 4 4
(b) clusters in Cs22In (SiO ) and
Rb In , respectively. Dotted line emphasizes the In–In distance, which
2
3
is much too long as to be assigned to a homoatomic bond. Bader char-
ges of indium atoms are under the labels of atoms (bold-italic).
Figure 1. Perspective representations of the crystal structures of mental observations on electrical resistivity measurements of
(SiO (a) and Rb In
violet spheres – indium atoms, green spheres – cesium or rubidium
atoms, SiO tetrahedra in gray, red spheres – oxygen atoms); horizontal
black lines at the right hand side emphasize the intergrowth type of
structure, consisting of Cs In and 4 × Cs SiO slabs.
Cs22In
6
4
)
4
2
3
(b) (green lines mark the unit cell edges, the title compound. The closed shell electronic structure and
thus the band gap originate from a Jahn–Teller type distortion
4
6–
as observed in a similarly distorted [Tl ]
cluster in
6
[2,6]
Rb Tl O .
The inspection of the PDOS clearly shows the
10
6
2
6
6
4
4
predominant In(5p) states around the Fermi level (E ). The
F
separated part of the band structure below E (E = 0 eV to
F
F
–
1.3 eV) represents 24 bands, i.e. 6 bands per indium cluster,
this regard, the present [In ]6– cluster is hypoelectronic and corresponding to its 2n (2 × 6 = 12) skeletal electrons. In the
6
6–
very much reminiscent of the family of [Tl6] clusters, where energy region between –2 eV and –8 eV six very narrow bands
the deviation from a regular octahedral shape can be under- per cluster are found with mainly In(5s) character. Although
stood in terms of the Jahn–Teller theorem.[
2,5]
Thus, the In6 the PDOSs of the silicon and oxygen atoms are situated in the
groups contained in the present compound provide a first ex- same energy interval, no overlap with In(5s) can be noticed
6
–
ample of so called “hypoelectronic” discrete [In6] indium due to the spatial separation, and thus no covalent interaction.
clusters. Against the background of our findings concerning The narrow In(5s) bands represent the inert electron pairs. This
“
hypoelectronic” thallium clusters, we have regarded it worth- indication is supported by the ELF which exhibits six domains
while to perform a thorough quantum chemical analysis on the with high values (η = 0.86, Figure 4) at each indium atom,
electronic structures of the extended solids. pointing outwards of the cluster. (Note that the skeletal bond-
It is apparent from the calculated DOS shown in Figure 3 ing is not visible in the ELF down to low values).
that the Cs In (SiO ) compound is a semiconductor with a The pattern of the indium PDOS matches with that of thal-
2
2
6
4 4
6–
[6]
band gap of 0.5 eV. This is in good agreement with our experi- lium in [Tl ] , contained in Cs Tl O, although in the thal-
6
4
2
Z. Anorg. Allg. Chem. 2011, 834–839
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
837

V. Saltykov, J. Nuss, U. Wedig, D. L. V. K. Prasad, M. Jansen
ARTICLE
Figure 3. Projected DOS for Cs22In
6
(SiO
4
)
4
(left) and Rb
2
In
3
(middle and right panels). The gray broken line indicates the Fermi energy level
F F
(E ), set to 0.0 eV. The s states corresponding to the alkali metals are far above from the E and are not shown here. In the middle and right
2 3
panels the PDOS of In1 and In2 in Rb In are given that represent nonbonding and bonding sites, with respect to intercluster bonds.
Figure 4. ELF domains for In
6
in Cs22In
6
(SiO
4
)
4
(left) and Rb
2
In
3
(middle and right) are shown at η = 0.86 and 0.82 for inert pairs and In2–
quasi two dimensional network of Rb In
In2 σ bonds respectively. The right figure shows the extended view of the 4-exo-σ-bonded closo-In
6
2
3
.
lium compound, the inert pair bands are more separated from no band gap opens at the Fermi level. This explains the, al-
the skeletal bands and are less split (–4 eV to –8 eV). Never- though low, metallic conductivity of Rb In .
2
3
theless, the same interpretation applies to the PDOS of the title
compound.
The Bader partial charges for the individual atoms are speci-
fied in Figure 2. Average values for the other atoms are: ce-
When comparing the indium PDOS in Cs In (SiO ) with sium (0.74), silicon (3.1), oxygen (–1.57), and rubidium (0.66).
22
6
4 4
the one calculated for Rb In , the different indium sites in the Despite small variations of the indium values due to the nu-
2
3
latter compound, the apical In1 and the basal In2, have to be merical integration of the electron density, the partial charges
considered separately (Figure 4). Concerning In1, the PDOS of the exo-bonded (–0.4) and the inert pair bearing indium
has a similar signature as the corresponding PDOS in the title atoms (–0.6) differ significantly.
compound, namely a separation of the In(5p) and In(5s) bands.
In contrast, bands attributed to In2 show a significant hybridi-
Conclusions
By solid state synthesis Cs In (SiO ) , a novel indium clus-
22 6 4 4
zation of 5s and 5p, allowing for intercluster bonding. The
ELF clearly supports this view (Figure 4). At In1 we find an
6–
inert pair type ELF domain (η = 0.86), whereas between two ter compound, has been obtained. It contains “isolated” [In6]
In2 atoms of neighboring In -units, a disynaptic attractor ap- clusters with an approximately octahedral shape, that fail to
6
pears with an ELF domain (η = 0.82) definitely indicating a fulfill Wade’s rules for homoatomic clusters and have to be
[1]
covalent σ bond. An interesting feature of the DOS of Rb In3 regarded “hypoelectronic”. The degeneracy of the partly
2
is the strong contribution of In2(5s) to the low lying conduc- filled frontier orbitals, which would be expected for an ideal
tion bands. They can be regarded as the antibonding counter- octahedron of the electron count given, is lifted by a considera-
parts of the intercluster bonds due to hybridization. However, ble geometric, i.e. by a Jahn–Teller distortion. The same sce-
the splitting of bonding and antibonding states is so small, that nario has been encountered for [Tl6]6– in Rb Tl O .
[2,6]
This
10
6
2
8
38
www.zaac.wiley-vch.de
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 834–839

6 6 4 4
First Isolated “Hypoelectronic” [In (SiO )
]6– Cluster in Insulating Cs22In
[
15] S. C. Sevov, J. D. Corbett, J. Solid State Chem. 1993, 103, 114–
130.
kind of electronic stabilization is in contrast to the situation in
4–
Rb In , where [In ] octahedra are linked two-dimensionally
2
3
6
[
16] G. Brauer, Handbuch der Präparativen Anorganischen Chemie,
Vol. 2; Ferdinand Enke: Stuttgart, Germany, 1978, pp. 938–942.
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electrons qualifies the unit as a closo Wade cluster. This latter [17] G. Brauer, Handbuch der Präparativen Anorganischen Chemie,
bonding mechanism can be traced back to a smaller energetic
separation of the lone-pair and skeleton electrons, i.e. a lower
relativistic stabilization of the 5s vs. 6s states.
Vol. 2; Ferdinand Enke: Stuttgart, Germany, 1978, pp. 953–954.
18] G. M. Sheldrick, SADABS, Bruker AXS Area Detector Scaling
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Acknowledgement
The authors thank to G. Siegle for performing the electric resistivity
measurements.
2005.
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Received: February 14, 2011
Published Online: March 24, 2011
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Z. Anorg. Allg. Chem. 2011, 834–839
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
839