Synthesis, crystal structure, and catalytic properties of MgCo 6Ge6

Article abstract of DOI:10.1002/chem.200500411

The ternary compound MgCo₆Ge₆ represents a novel member of the RM₆X₆ phases, which contains a graphite-type Ge network, Kagome nets of Co atoms, and Ge₂ dumbbells with an unexpected short Ge-Ge contact in the range of a localized Ge-Ge single bond. The title compound shows a large variety of chemical bonding, which ranges from metallic to multi-center and covalent bonding. The role of polar intermetallic alloys as promising candidates for the application as catalysts for the selective hydrogenation of α,β-unsaturated aldehydes is discussed. MgCo₆Ge₆ possesses a remarkable activity and selectivity for the hydrogenation of cis/trans-citral to geraniol and nerol.

Full text of DOI:10.1002/chem.200500411

DOI: 10.1002/chem.200500411
Synthesis, Crystal Structure, and Catalytic Properties ofMgCo Ge6
6
[
c]
[a]
[a]
[b]
Christine Gieck, Martin Schreyer, Thomas F. Fässler,* Sylwia Cavet, and
[
b]
Peter Claus
Abstract: The ternary compound
which ranges from metallic to multi-
center and covalent bonding. The role
of polar intermetallic alloys as promis-
ing candidates for the application as
catalysts for the selective hydrogena-
tion of a,b-unsaturated aldehydes is
discussed. MgCo Ge possesses a re-
MgCo Ge represents a novel member
6
6
of the RM X phases, which contains a
6
6
graphite-type Ge network, KagomØ
6
6
nets of Co atoms, and Ge dumbbells
markable activity and selectivity for
the hydrogenation of cis/trans-citral to
geraniol and nerol.
2
Keywords: crystal structure · hetero-
with an unexpected short GeÀGe con-
tact in the range of a localized GeÀGe
single bond. The title compound shows
a large variety of chemical bonding,
geneous catalysis
·
host–guest
systems · intermetallic compounds ·
selective hydrogenation
Introduction
arrangement of the atoms relative to each other. To test the
intrinsic properties, catalytic reactions, for example, hydro-
genations, have been performed by using the bulk samples
instead of supported systems. To enhance the polarity, which
is a key factor in controlling the selectivity in the hydroge-
nation of polar functional groups such as C=O in the pres-
ence of an olefinic group, an electropositive metal such as
Mg was added in the present study to an equimolar mixture
of Ge and Co metal.
While the electronic properties of intermetallic compounds
have been studied extensively with respect to stability, elec-
tric conductivity, thermoelectric properties, and work func-
tions, their potential as catalysts remains rather unex-
plored, and only a few attempts have been made to corre-
late the intrinsic properties of well-defined alloys with their
catalytic activity. Up to now no systematic studies on the
suitability of intermetallic compounds for catalytic uses have
been carried out. There are at least two variables that may
have an influence on the catalytic properties of an alloy: 1)
the nature of the individual metals and their combination in
the alloy, as already known for many examples of bimetallic
catalysts, and 2) the structure type and thus the geometric
[
1]
[3]
[2]
In general, the intramolecular selectivity of the hydroge-
nation of a,b-unsaturated aldehydes, that is, hydrogenation
of the C=C group versus the C=O group, can be controlled
by the nature of the individual metal, the presence of a
second metal (bimetallic catalysts), electron-donating ligand
effects by the support material of the catalyst (e.g. through
basic zeolites and graphite), steric constraints in the metal
environment, strong metal–support interactions (SMSI), se-
lective poisoning, substituents on the C=C bond, solvents,
[3–5]
and pressure effects.
The influence of these parameters
[
a] Dr. M. Schreyer, Prof. Dr. T. F. Fässler
Department Chemie, Technische Universität München
Lichtenbergstrasse 4, 85748 Garching (Germany)
Fax.:(+49)89-289-13186
can be also found in the selective hydrogenation of citral in
[6]
numerous studies in the past few years. For example, it has
been shown by Singh and Vannice that Pd is the most active
[6a]
E-mail: thomas.faessler@lrz.tu-muenchen.de
metal for citral hydrogenation and is, in accordance with
[
6o]
[
b] Dipl.-Ing. S. Cavet, Prof. Dr. P. Claus
DFTcalculations,
highly selective to C=C bond hydroge-
[6p,q]
Ernst-Berl-Institut für Technische und Makromolekulare Chemie
Fachbereich Chemie, TU Darmstadt
Petersenstrasse 20, 64287 Darmstadt (Germany)
nation, yielding citronellal.
On the other hand, selectivi-
ty towards the allylic alcohols geraniol and nerol was im-
proved by using bimetallic catalysts, for which Sn seems to
be the second metal of choice for modifying the base metal
as shown, for example, for RhÀSn catalysts. The formation
[
c] Dr. C. Gieck
Present address: Dipartimento di Scienze e Tecnologie Avanzate
Università del Piemonte Orientale
Via Bellini 25/G, 15100 Alessandria (Italy)
of alloys, which depends on the type of support material,
1924
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 1924 – 1930

FULL PAPER
catalyst preparation (by an organometallic route or by im-
pregnation), and pretreatment, and their role in controlling
the intramolecular selectivity has been a controversial issue.
In silica-supported bimetallic RhÀSn catalysts, an alloy-type
Results and Discussion
Crystal structure: The atomic structure was revealed by
single-crystal structure determination. In MgCo Ge , planar
6
6
phase, in which a tin-enriched phase is preferentially located
at the exterior surface of the alloy particles, was reported to
be unselective towards the formation of unsaturated alco-
hols in citral hydrogenation. The preferred activation of the
carbonyl group was then attributed to the interaction with
graphite-type layers (A) of Ge atoms and KagomØ nets (B)
of Co atoms alternate along the z axis (crystallographic c
axis, Figure 1a; Table 1). The resulting columns of vertex-
sharing trigonal bipyramids (Figure 1b) form a three-dimen-
sional Co–Ge network with hexagonal tunnels. These tun-
SnO moieties present in the bimetallic phase formed by sur-
nels are alternately centered by Mg atoms and Ge dumb-
x
2
[6m]
face reconstruction during an induction period.
On the
bells. There are two different graphite-type layers: one, in
which the six-membered rings of Ge1 atoms are centered by
Mg atoms (z=0) and a second one (z(Ge2)=0.5), in which
the centers of the six-membered rings coincide with the cen-
other hand, in studies of citral and crotonaldehyde hydroge-
nation high selectivities towards unsaturated alcohols were
dÀ
d
dÀ
d
dÀ
found when bimetallic sites (e.g. Rh ÀSn , Ru ÀFe , Pt À
d
[7]
Ni ) of polar character, as indicated by XANES, were
ters of gravity of (Ge3) dumbbells, which are also aligned
2
[6l,8a–c]
present in the alloyed phases.
From that it was con-
along the z axis. The Co–KagomØ nets separate the layers
of Ge atoms (z=0.2496). Thus, the crystal structure can be
cluded that surface polarity is a precondition for the adsorp-
tion/activation of the C=O group. Furthermore, it has to be
noted, that a catalyst comprising an intermetallic compound,
Ru Sn , on an inert oxide support is useful for the selective
[11]
derived from the CoSn structure type
by doubling the
c axis of the parent structure and by alternately replacing
interstitial Sn atoms with either one Mg atom or a Ge2
dumbbell.
3
7
hydrogenation of carboxylic acids into the corresponding al-
[9]
dehydes.
The GeÀGe distance of 2.501 Š for the Ge unit corre-
2
Here we report the first results of our studies on the cata-
lytic properties of polar intermetallic compounds in the
system MgÀCoÀGe, which show that new combinations of
sponds to values found for GeÀGe single bonds in various
[
18]
[19]
Zintl-type germanides such as SrGe ,
BaGe P ,
and
2
2 2
[20]
NaRb Ge
whereas the GeÀGe distances within the
7
8,
elements must be considered to obtain new hydrogenation
catalysts. Starting from supported bimetallic RhÀSn, RuÀSn,
and PtÀSn catalysts, which are known to catalyze selective
graphite-type layers are substantially longer (2.928 Š). The
CoÀCo distance of 2.536 Š compares to the CoÀCo distance
in the metal (2.506 Š) and is considerably shorter than relat-
ed distances in intermetallic compounds containing Co (for
[3]
hydrogenation reactions, we tested bulk intermetallic com-
[
10]
pounds in the CoÀSn and MgÀCoÀSn system,
and then
example 2.692 Š in CoSn ).
3
extended our studies to the MgÀCoÀGe system.
To investigate a series of related compounds, we chose a
structure type that allows a variation of both the composi-
tion and the components. The structures of ternary interme-
tallic alloys adopting the RM X structure type can be con-
Electronic structure: The LMTO band structure calculations
(Figure 2a) reveal individual bands with a large dispersion
and a bunch of bands crossing the Fermi level E , which is
F
indicative of a metallic behavior (delocalized states). Beside
6
6
[11]
sidered as a “filled” variant of the CoSn structure,
in
this, several bands run parallel to the energy line at E (A-
F
which additional interstitial atoms are arranged in one-di-
mensional tunnels formed by the host framework. Owing to
the high flexibility of the host lattice, intercalation products
can be obtained for a vast range of guest atoms. MgCo Ge
L-H-A), which is characteristic for nonbonding and covalent
contributions (localized states). As a consequence, the
Fermi level lies at a local maximum of the density of states
(DOS; Figure 2a). An analysis of the partial contributions
to the DOS shows that the states at the Fermi level have
predominantly Co character (Figure 2b). The bulk of the
germanium contributions is located well below the Fermi
level, whereas there are virtually no magnesium contribu-
tions at all. This is consistent with the fact that magnesium
is the most electropositive element in the compound.
The chemical bonding was further analyzed by means of
the electron localization function (ELF). The results of the
calculation are given as a three-dimensional (3D) isosurface
of constant ELF value in Figure 1c–j. Two-dimensional (2D)
intersections are presented as a pixel image, with the density
of pixels reflecting the electron density, and the color of
each pixel showing the ELF value according to the color bar
in Figure 1 g, and as a contour line diagram that is superim-
posed on the pixel image in Figure 1 g–m. In the case of an
all-electron calculation the shell structure of the core elec-
trons is visible in the 2D images (Figure 1 f and 1k–m). For
6
6
[12]
was first mentioned in 1981 by Schuster et al., who exam-
ined a series of compounds of the RM X type (R=Li, Mg,
6
6
Sc, Y, Zr, Hf, U, or rare-earth metal; M=Fe, Co, Ni, Mn,
X=Si, Ge, Sn) in terms of four closely related superstruc-
tures of the parent structure type HfFe Ge . In the last two
6
6
decades, the crystal structures and the magnetic properties
of isotypes containing rare-earth metals have been exam-
[
13]
[14]
[15]
ined.
In the case of HoFe Ge ,
TbFe Sn ,
and
6
6
6
6
[16]
LiFe Ge , superstructures have provided a satisfying de-
6
6
scription of the average crystal structure. The adopted struc-
ture of DyFe Sn turned out to be dependent on the reac-
6
6
[
17]
tion conditions of its synthesis.
microscopy studies of HoFe Ge
High-resolution electron
indicate the presence of
[14]
6
6
submicroscopically twinned microdomains. Up to now none
of the compounds crystallizing in this structure type have
been studied with regard to their catalytic properties.
Chem. Eur. J. 2006, 12, 1924 – 1930
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1925

T. F. Fässler et al.
the calculations using only va-
lence electrons, deep blue
areas appear at the atom site.
Figure 1c represents a 3D
image of the valence electron
ELF with a value of h=0.75.
There is only
a region of
strongly localized electrons,
which has a maximum at the
center of the Ge3ÀGe3 bond,
indicating its covalent charac-
ter. Figure 1d shows a 2D sec-
tion of the valence-electron
ELF along the Ge3ÀGe3
dumbbell in the bc plane. The
highest maximum of ELF (in
white) is found between the
two Ge atoms, which are sur-
rounded by an orange area of
high electron localization. Fig-
ure 1e and 1 f show the same
section for
a
total-electron
ELF calculation. An ELF iso-
surface of h=0.45 is superim-
posed in green on the structure
(
Figure 1e). The 3D represen-
tation in the area of the Ge3À
Ge3 dumbbell again shows a
pronounced maximum be-
tween the Ge atoms of the
dumbbell. However, in the
case of an all-electron calcula-
tion the ELF maxima around
the Ge atoms opposite the
bond become more pro-
nounced. In the 3D picture
they are visible in the form of
a crown or crest on top of the
Ge3 atoms, in the 2D pixel
image they appear as deep
green areas and can be inter-
preted as lone pairs at the Ge
atoms.
Figure 1. Structure and electronic properties of MgCo
layers are shown in blue, the KagomØ nets in yellow. b) View perpendicular to z axis with special emphasis on
the polyhedra formed by the Co atoms of the KagomØ net, the Ge atoms of the graphite net and the Ge
dumbbells. c) Valence ELF at the Ge dumbbell 3D isosurface of h=0.75. d) Valence ELF at the Ge dumbbell
D plot. e) Total ELF at the Ge dumbbell 3D isosurface of h=0.45. f) Valence ELF at the Ge dumbbell 2D
6 6
Ge . a) View along c direction, the graphite-type Ge
2
The structural similarity of
CoSn and MgCo Ge also sug-
2
2
6
6
2
2
2
gests similar electronic proper-
ties, in particular strong cova-
lent contributions to the bond-
ing between the germanium
atoms within the graphite-like
layers, since theoretical investi-
plot. g) Valence ELF along the 110 plane with isobars between 0.3 to 0.7 in steps of 0.1. h) Valence ELF with
isobars between 0.3 to 0.65, in steps of 0.05 at z=0. i) Valence ELF with isobars between 0.3 to 0.4, in steps of
0
.05 at z=0.25. j) Valence ELF with isobars between 0.3 to 0.65 in steps of 0.05 at z=0. k) Total ELF with iso-
bars between 0.3 to 0.8 in steps of 0.05 at z=0. l) Total ELF with isobars between 0.3 to 0.8 in steps of 0.05 at
z=0.25. m) Total ELF with isobars between 0.3 to 0.8 in steps of 0.05 at z=0.5.
gations by Häußermann et al. showed strong ELF maxima
between the tin atoms within the graphite-like layers in
6 6
Table 1. Bond lengths [Š] for MgCo Ge .
Ge1ÀGe1
Ge2ÀGe2
Ge3ÀGe3
Mg1ÀGe1
Mg1ÀGe3
2.9278(3)
2.9278(3)
2.501(2)
2.9278(4)
2.622(1)
Co1ÀCo1
Mg1ÀCo1
Co1ÀGe1
Co1ÀGe2
Co1ÀGe3
2.5354(4)
3.188(1)
2.4248(5)
2.4297(5)
2.6273(4)
[21]
CoSn, and no maxima between those tin atoms that fill
the hexagonal channels in the KagomØ-type layers. In the
case of MgCo Ge a different situation occurs: The hexago-
6
6
nal channels are filled alternately with Mg atoms and Ge2
1926
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Chem. Eur. J. 2006, 12, 1924 – 1930

Polar Intermetallic Alloy
FULL PAPER
each other, they are separated
by minima. Figure 1i and l
show a similar cut along the
0
01 plane but in this case
through the cobalt atoms of
the KagomØ net. Again, the
valence ELF is given in Fig-
ure 1 h–j, and the total ELF in
Figure 1k–m. Maxima drop off
only at very low ELF values
(
h=0.2), indicating a rather
small covalent contribution to
the bonding between those
atoms.
Inspite of some structural
similarities between CoSn and
MgCo Ge , there are consider-
6
6
able differences in their elec-
tronic properties. There is no
covalent bonding between the
Ge atoms of the graphite
layers, whereas the Ge dumb-
2
bells in the hexagonal tunnels
of the KagomØ-type lattice
show a considerable degree of
covalent bonding.
Catalytic activity: The hydro-
genation of citral (3,7-dimeth-
yl-2,6-octadienal) was chosen
as a test reaction. Citral has
three unsaturated bonds in-
cluding conjugated C=C and
C=O groups as well as an iso-
lated C=C bond. Selective hy-
drogenation of this a,b-unsatu-
rated aldehyde leads to indus-
trially important compounds,
for example, for the perfumery
6 6 6 6
Figure 2. a) Band structure and DOS of MgCo Ge . b) Partial projections of the DOS of MgCo Ge .
dumbbells. Figures 1g–1j show a number of two-dimension-
al cross sections of the valence-electron ELF along defined
planes in the structure. Figure 1 g displays a cross section
and flavoring industry. In particular, citronellal is formed by
hydrogenation of the olefinic group which is conjugated to
the C=O group, whereas hydrogenation of the latter yields
unsaturated trans and cis alcohols (geraniol and nerol, re-
spectively). Under certain reaction conditions and depend-
ing on the catalyst type, consecutive hydrogenation of these
intermediates can occur leading to citronellol, dihydrocitro-
nellal, and 3,7-dimethyloctanol.
through the unit cell along the 110 plane. The Ge dumbbell
2
is the only pronounced ELF maximum located on an intera-
tomic bond vector in this slice, and there are no maxima be-
tween two adjacent Ge1 and two adjacent Ge2 atoms of the
two different graphite-like networks. The distance of
2
.928 Š between these atoms is considerably longer than
Whereas binary Co–Ge alloys show only a low citral con-
version (3% at 448 K, 8 MPa), which is obviously independ-
that of a covalent GeÀGe single bond. This is contradictory
[22]
to the situation in CoSn, in which the shortest homoatomic
tin distance is found between the tin atoms of the graphite-
like layers and it is much closer in length to a SnÀSn single
ent of the Co/Ge ratio, the catalytic activity is strongly in-
creased by the introduction of Mg. For MgCo Ge samples,
6
6
a citral conversion of 17% was obtained at a reaction tem-
perature of 448 K, which is considerably higher than the 3%
achieved with binary Co–Ge alloys. Furthermore, the cata-
lyst shows an increased selectivity to give geraniol and nerol
in almost 50% (Table 2), instead of the 3–10% obtained by
using the binary alloys. For comparison, Table 2 also con-
bond. This picture is confirmed by several cross sections per-
pendicular to the c axis given in Figure 1 h–m. The Fig-
ure 1h, k, j, m show sections parallel to the graphite-like
layers formed by Ge2 and Ge1. Although the ELF maxima
between adjacent germanium atoms are oriented towards
Chem. Eur. J. 2006, 12, 1924 – 1930
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1927

T. F. Fässler et al.
Table 2. Catalytic results of citral hydrogenation over MgCo
supported reference catalyst (Ru-Sn/SiO ) at 448 K.
6
Ge
6
and a
nent metal catalysts by cheaper three- or more-component
ones.
2
[
6
a]
[b]
MgCo
6
Ge
RuÀSn/SiO
2
citral conversion [%]
activity [mol/gcat h]
selectivity [%]
17.1
0.004
48.2
0.072
Experimental Section
[
c]
geraniol + nerol
48.3
5.2
5.2
41.3
2.1
4.6
72.4
9.3
8.5
9.8
1.6
4.1
[
d]
6 6
Preparation: MgCo Ge was synthesized by arc melting of the elements
citronellal
[
e]
followed by subsequent annealing of the reaction product. A mixture of
magnesium (Alfa Aesar, 99.8%), cobalt (Chempur, 99.9%), and germa-
nium (Aldrich, 99.999%) in the molar ratio 1:2:3 was fused under an
argon atmosphere in the reaction chamber of an arc melter (Johanna
Otto GmbH, MAM-1), and the regulus was annealed in a programmable
tube furnace at 6508C for 12 h. The product consisted of xenomorphous
crystals with a metallic luster. The purity of the sample was checked by
using a STOE STADI P powder diffractometer with a CuKa source. The
citronellol
[
f]
other products
geraniol/nerol
(
geraniol + nerol)/
(
citronellal + citronellol)
0
À1
[
4
a] mcat =100 mg, c citral =0.58 molL , p 2
H
=7.5MPa, t=150 min. [b] mcat
=
0
À1
65 mg,
c
citral =1.11 molL
,
H
p 2
=8.5 MPa, t=100 min, 300-mL Parr
batch reactor. The Ru–Sn/SiO
2
catalyst was prepared according to CSR
powder diffractogram, which could be indexed by using the theoretical
powder pattern calculated from the single-crystal data of MgCo Ge6,
6
[
3]
(
controlled surface reaction) method. [c] (trans + cis) Product of C=O
[23]
group hydrogenation. [d] Product of conjugated C=C bond hydrogena-
tion. [e] Product of consecutive hydrogenation of geraniol, nerol, and cit-
ronellal. [f] Isopulegol, menthol, and non-identified products.
showed that a nearly single-phase product with small amounts of unreact-
ed Ge was obtained. The crystallinity of the samples was highly depen-
dent on the conditions of the arc-melt process and could be increased by
subsequent thermal treatment. Variations of the conditions of the arc-
melting process also yield a novel phase, which was subsequently charac-
terized as MgCo Ge . Other reactions starting from the same Mg:Co:Ge
tains the catalytic properties of a supported bimetallic refer-
4
6
ence catalyst (Ru–Sn/SiO ) which is commonly used for the
ratio of 2:3:4 showed the formation of mixtures containing MgCo
4 6
Ge ,
2
MgCo Ge , CoGe , and Ge. In all cases a mass loss is observed, which
6
6
2
synthesis of higher unsaturated alcohols. Although the activ-
ity of MgCo Ge is one order of magnitude lower than the
mainly originates from the loss of magnesia during the arc-melting pro-
cess.
6
6
reference catalyst, which contains the active components in
the form of bimetallic nanoparticles in high dispersion on an
oxidic support, the most striking property of the polar inter-
metallic title compound is the formation of the desired unsa-
turated alcohols at comparable levels as indicated, for exam-
ple, by the product ratio of (geraniol + nerol)/(citronellal
In the case of binary samples various ratios of Co and Ge were loaded in
an arc-melting furnace and heated repeatedly. The resulting products
consist of CoGe and CoGe
2
together with unreacted Ge.
Crystal structure determination: The structure of MgCo
6
6
Ge was deter-
mined from single-crystal X-ray diffraction data. A single crystal was se-
lected from the reaction mixture and placed into an oil-filled glass capil-
lary. The measurement was performed on an Oxford Diffraction Xcalibur
3 diffractometer equipped with a monochromatic MoKa source (l=
0.71073 Š). The exposure time was 80 s per frame. The collection of in-
+
citronellol). The results obtained for MgCo Ge clearly
6 6
indicate that the modification of the binary Co–Ge alloy by
introducing the electropositive metal Mg increases the po-
larity which, according to the proposed reaction mecha-
[24]
tensity data was carried out with the CrysAlis program. The final lat-
tice parameters were calculated from all reflections observed in the
actual data collection. A summary of the experimental and crystallo-
graphic data is given in Table 3.
[3]
nism, is a prerequisite for the chemisorption of the polar
C=O group through the oxygen atom and, thus, for control-
ling the intramolecular selectivity towards unsaturated alco-
hols.
The structure was solved by direct methods, which revealed the atomic
[25]
positions, and refined by using the SHELX-97 program package. The
2
final refinements were carried out on F
O
. Atomic scattering factors for
spherical neutral free atoms were taken from standard sources, and
It should be noted that the catalytic activity and selectivi-
ty is highly dependent of the sample preparation. In view of
[
26]
anomalous dispersion corrections were applied.
Calculations per-
formed at an intermediate stage in which the relative positional occupan-
cies of the interstitial atoms were refined, did not indicate any non-stoi-
chiometry of the compound. The final coordinates and equivalent iso-
tropic temperature factors of all atoms are given in Table 4.
the fact that we cannot exclude that MgCo Ge samples con-
6
6
tain also amorphous materials beside unreacted Ge, the cat-
alytic properties may not only result from the intrinsic prop-
erties of the novel phase MgCo Ge . However, since we
Quantum-chemical calculations: The electronic structure was investigated
by means of the ab initio full-potential linear muffin-tin orbital (LMTO)
6
6
tested various binary products with lower conversion rates,
the introduction of the more electropositive component
must play a crucial role in enhancing the catalytic proper-
ties.
method in the atomic sphere approximation (ASA), using the tight-bind-
[27]
ing (TB) program TB-LMTO-ASA.
The calculations were based on
the local-density approximation (LDA), and the Hedin–Lundquist pa-
[
28]
rameterization was employed for exchange and correlation potentials.
The radii of the muffin-tin spheres and empty spheres were determined
[
29]
according to Jepsen and Andersen.
For the calculations s, p, and
“
down-folded” d partial waves for Ge, s, p, and “down-folded” d partial
Conclusion
waves for Mg, and s, p and d partial waves for Co were used.
A topographical analysis of the electron density distribution using the
electron localization function (ELF), which originally was introduced for
the deduction of the shell structure of atoms from the electron density,
gives further insight into the chemical bonding in intermetallic com-
pounds. ELF values are scaled between 0 and 1, with high ELF values
and local ELF maxima corresponding to areas and centers of localized
electrons, and low ELF values defining the spatial area around these
maxima. The resulting partition of space into areas of high electron local-
The present study shows that a combination of elements of
different electronegativity in the form of a polar intermetal-
lic compound possesses catalytic activity, even though none
of the constituting elements does so. Thus, the investigation
of ternary alloys of various compositions might enable the
substitution of some rather expensive one- or two-compo-
[
30]
1928
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 1924 – 1930

Polar Intermetallic Alloy
FULL PAPER
[
a]
Table 3. Crystal data and structure refinement for MgCo
6
Ge
6
.
Acknowledgements
empirical formula
formula weight
temperature [K]
wavelength [Š]
crystal system
space group
6 6
MgCo Ge
813.43
293(2)
0.71073
hexagonal
This work was supported by the Deutsche Forschungsgemeinschaft (proj-
ect number FA 198/4–1 and CL 168/3–1: Polare Legierungen als Mehr-
komponenten-Katalysatoren). We thank Dr. A. Schier for the revision of
the manuscript and R. Klemens for the synthesis of some of the alloys.
P6/mmm
unit cell dimensions
a [Š]
5.0709(7)
7.745(2)
172.46(5)
1
7.832
39.721
366
0.1ꢂ0.15ꢂ0.2
4.64 to 39.698
c [Š]
volume [Š ]
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Z
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À3
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1
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À1
m [mm
F(000)
]
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2
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index ranges
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reflections collected
independent reflections
completeness to q=27.548
absorption correction
refinement method
2787
248
96.1%
CrysAlis
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=0.029, wR
2
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=0.086
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1
1
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Table 4. Atomic coordinates and equivalent isotropic displacement pa-
2
3
rameters (Š ꢂ10 ) for MgCo
6 6
Ge . U(eq) is defined as one third of the
trace of the orthogonalized Uij tensor.
x
y
z
U(eq)
Mg1
Co1
Ge1
Ge2
Ge3
0
0
0
0
21(1)
16(1)
12(1)
11(1)
13(1)
0.5000
0.6667
0.6667
0
0.2496(1)
0
0.5000
0.3385(1)
0.3333
0.3333
0
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6
6
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Received: April 12, 2005
Revised: September 21, 2005
Published online: December 19, 2005
1930
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 1924 – 1930