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Twelve One-Electron Ligands Coordinating One Metal Center Structure and Bonding of [Mo(ZnCH3)9(ZnCp.200802811.pdf)3]

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Communications
DOI: 10.1002/anie.200802811
Cluster Compounds
Twelve One-Electron Ligands Coordinating One Metal Center:
Structure and Bonding of [Mo(ZnCH3)9(ZnCp*)3]**
Thomas Cadenbach, Timo Bollermann, Christian Gemel, Israel Fernandez,
Moritz von Hopffgarten, Gernot Frenking,* and Roland A. Fischer*
The highest coordination number for metal complexes of
monodentate ligands has been nine since the days of Alfred
Werner.[1] The term “complex” refers to a molecule [MLm]
that features a central metal atom M that bonds to ligator
atoms E of ligands L by donor–acceptor interactions to yield a
core structure MEn.[2] Metal atoms can also become captured
inside an electron-precise cage En. These compounds obey the
Wade–Mingos rules and are referred to as endohedral clusters
M@En, typically with n > 9. Examples for the latter are the
recently synthesized [M@Pb10]2 and [M@Pb12]2 (M = Ni, Pd,
Pt).[3] Herein we describe the synthesis of an unprecedented
molecule containing a MoZn12 core, which offers a novel
linkage between coordination compounds and cluster molecules (Figure 1).[4]
At first glance, the icosahedral structure of the title
molecule [MoZn12Me9Cp*3] (1; Me = CH3, Cp* = C5Me5) is
reminiscent of the endohedral clusters described above.[3] The
actual bonding situation is, however, intriguingly different.
Quantum chemical analysis revealed a unique situation best
described as a perfectly sd5-hybridized molybdenum atom
that engages in six MoZn two-electron-three-center bonds.
There are six high-lying valence molecular orbitals (MOs)
occupied by 12 electrons that can clearly be identified as Mo
Zn bonding. Another six electrons are delocalized over the
Zn cage, evoking only weak ZnZn interactions (Figures 2
and 3). Before discussing these aspects in detail, we briefly
report the synthesis and the analytical and structural properties of 1.
[*] T. Cadenbach, T. Bollermann, Dr. C. Gemel, Prof. Dr. R. A. Fischer
Inorganic Chemistry II—Organometallics & Materials
Faculty of Chemistry and Biochemistry, Ruhr University Bochum
44870 Bochum (Germany)
Fax: (+ 49) 234-321-4174
E-mail: roland.fischer@rub.de
Dr. I. Fernandez
Dpto. Qu;mica Org<nica, Facultad de Qu;mica
Universidad Complutense de Madrid
28040 Madrid (Spain)
M. von Hopffgarten, Prof. Dr. G. Frenking
Department of Chemistry, Philipps-University Marburg
35032 Marburg (Germany)
E-mail: frenking@chemie.uni-marburg.de
[**] Organo Group 13 Complexes of d-Block Elements, Part 52 (Part 51:
G. Prabusankar, A. Kempter, C. Gemel, R. A. Fischer, Angew. Chem.
2008, 120, 7344–7347; Angew. Chem. Int. Ed. 2008, 47, 7234–7237).
T.C. is grateful for a stipend from the Fonds of the Chemical Industry
(Germany) and for support by the Ruhr University Research School.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802811.
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The title compound [MoZn12Me9Cp*3] (1) was reproducibly obtained in 82 % yield by the treatment of [Mo(GaCp*)6] (2) with 14 equivalents of ZnMe2 in toluene at
110 8C over a period of 2 h. Two mixed Ga–Zn compounds
[MoZn4Ga4Me4Cp*4] (3) and [MoZn8Ga2Me6Cp*4] (4) are
intermediates of this reaction and were isolated in nearly
quantitative yields using 4 and 8 equivalents of ZnMe2
(Scheme 1).
Scheme 1. Generation of compounds 1–4.
Similarly, [MoZn12Et10Cp*2] (5) is formed from 1 and
excess ZnEt2, which points to the accessibility of a truly
homoleptic compound [Mo(ZnR)12] under the proper conditions. The reactions involve reduction of ZnII to ZnI with
concomitant release of Me2GaCp* and GaMe3 as the most
important GaIII-containing byproducts and of different fulvalene species as well as migration of Cp* from gallium to zinc
and methyl or ethyl groups from zinc to gallium. The
obviously involved redox reactions are not very selective
with respect to the variety of side products, suggesting a
radical mechanism. Accordingly, the resulting monovalent
zinc fragments ZnMe, ZnEt, and ZnCp* are trapped by
bonding to the Mo0 center (Figure 1). The driving force and
the high selectivity of formation of 1 from 2 via the mixed Ga–
Zn intermediates 3 and 4 is certainly connected to the
favorable oxidation of GaI to GaIII combined with the rather
similar electronic and steric properties of the monovalent
GaR and ZnR ligands at a transition-metal center. Since a
selection of precursors [Ma(ER)b] similar to 2 is available,[5]
quite a number of analogues to 1 should be accessible with
many transition metals. For example, [PtCd8Me4Cp*4] (6)
with an unusual octacoordinated platinum center has been
derived from [Pt(GaCp*)4] and excess CdMe2. All new
compounds 1–6 have been analytically and structurally
characterized (see the Supporting Information). Our discussion below will focus on the key compound 1.
2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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The single crystal X-ray structure determination of 1[4]
reveals an almost perfect icosahedral environment of the Mo
atom surrounded by twelve ZnR ligands. The nine methyl and
three Cp* substituents are distributed such that the h5coordinated, bulky Cp* groups occupy positions as far as
possible from each other, thus resulting in overall C3
symmetry (Figure 1). The ZnC bond lengths of the ZnR
Figure 1. X-ray crystal structure of 1; thermal ellipsoids are shown at
the 50 % probability level, hydrogen atoms are omitted for clarity.[4]
C gray, Zn green, Mo red. The solid lines between the zinc atoms do
not indicate strong covalent ZnZn interactions but should guide the
eye to easily recognize the icosahedral coordination of the central
molybdenum atom. Selected interatomic distances [H]: Mo–Zn
2.637(1)–2.677(1) (average 2.651), Zn–Zn 2.724(1)–2.853(1) (average
2.788), Zn–CH3 1.960(10)–1.968(10) (average 1.965), Zn–Cp*(centroid) 1.951–1.959 (average 1.956).
moieties are within the expected range. Note that full
elemental analysis of 1 rules out the presence of gallium
and confirms the MoZn12 composition. The 1H and 13C NMR
spectra of 1 in solution agree with the solid-state structure and
display chemically equivalent Cp* units and the expected
three sets of methyl groups. Temperature-dependent measurements (25–70 8C) show a static structure without exchange
of the Cp* and methyl positions. Thus, the formula of 1 can be
written as [Mo(ZnMe)9(ZnCp*)3]. The MoZn separations
are nearly equal, with 2.672(8)–2.677(13) G for MoZnCp*
and 2.636(9)–2.648(13) G for MoZnMe. These values match
the related MoZn contacts of 2.638(3)–2.683(3) G in the
close-packed intermetallic solid-state compound MoZn20.44,
which also exhibits icosahedral MoZn12 units.[6] Two examples
of dinuclear MoZn complexes are known; these contain
MoZn bonds of 2.711(1)[7] and 2.793(3) G.[8] The ZnZn
separations of 1 (2.724(2)–2.853(2) G) are in line with the
corresponding values in MoZn20.44 (2.748(3)–2.790(3) G).[6]
However, the ZnZn single bond in the dimer
[Cp*ZnZnCp*] (2.305(3) G) is much shorter.[9]
Over the years, we have studied the complexes [M(ER)n]
(M = transition metal; E = Al, Ga, In; R = bulky substituent),
in which the central atom M is coordinated by the metal
atoms E as ligators rather than by nonmetal atoms, as is
usually the case in coordination chemistry.[5] The new
octahedral, closed-shell, 18-electron complex [Mo(GaCp*)6]
Angew. Chem. Int. Ed. 2008, 47, 9150 –9154
(2) is related to the classic metal hexcarbonyls [M(CO)6] (M =
Cr, Mo, W) by the isolobal analogy of CO with the carbenoid
GaI species GaCp*. The MoGa6 core of 2 is held together by
fairly strong, polar MoGa donor–acceptor bonds (see the
Supporting Information). From this view point of coordination chemistry, one two-electron GaCp* ligand is equivalent
to two ZnCp* or ZnMe ligands providing one electron each,
which results in 18 valence electrons for 1 (as for 3 and 4), as
expected for Mo0 complexes. From the view point of classical
cluster chemistry, the bonding in a deltahedral closo cluster
requires 2n + 2 delocalized valence electrons, where n is the
number of the vertices. For an icosahedral structure, 26
valence electrons are required, and taking into account either
lone pairs or covalent bonds at the vertices as well, the total
number of electrons is 50. This situation is typical for
endohedral Zintl clusters such as [Pt@Pb12]2, in which the
central closed-shell Pt atom acts as a template for the
electron-precise Pb122 cluster and not as an electron donor
or acceptor.[3, 10] A similar electron-counting exercise for 1
results in a total of only 42 electrons. Accordingly, 1 may be
compared with hypoelectronic clusters[11] such as Al13 [12] or
In117 [13] , which have 40 total electrons each and 16 and 18
valence electrons, respectively, in accordance with the Jellium
model.[14] Also, metalloid clusters pioneered by SchnIckel
and co-workers, such as [Ga19R6] (R = C(SiMe3)3), can be
considered.[15] This particular example contains a Ga13 core
and six face-bridging two-electron GaR ligands with an
electron count of 52. It is well accepted that structure and
bonding of such metalloid cluster species cannot be understood by a single electron-counting rule; rather, each case
must be studied individually.[16, 17] A quantum chemical
analysis of the electronic structure of Al13 clearly shows
that the bonding situation is very different from the chemical
bonding in the present molecule 1, discussed below.[18]
Zinc is a rare element in metal-cluster chemistry, although
it forms a multitude of alloys and intermetallic solid-state
compounds. In zinc-rich transition-metal phases MaZnb (M =
transition metal, b > a), for example, the zinc atoms form
complex homonuclear networks with high coordination
numbers in which the transition-metal atoms are encapsulated icosahedrally by zinc atoms.[6] Interestingly, this situation had no parallel in the molecular chemistry of zinc, until
now. Likewise, homoleptic zinc clusters Znn do not exist,
except a tetrahedral Zn4 unit hosted in zeolite X.[19] The
atomic closed-shell s2 electron configuration, together with
the large valence s/p separation of 4 eV, strongly disfavors
delocalized ZnZn bonding in non-extended, comparably
small molecular species. Note that the heteroleptic cluster
[Zn9Bi11]5 [17] with an icosahedral M13 core Zn@(Zn8Bi4) is
well described as an electron-precise Zintl cluster, in analogy
to [Pt@Pb12]2, and thus evidently differs from 1. These
considerations raise the key question of the importance of the
tangential ZnZn interactions within the Zn12 cage relative to
the radial MoZn interactions.
We carried out DFT calculations[20, 21] at the RI-BP86/
TZVPP level[22] to elucidate the bonding situation in 1. The
optimization of the structure of 1 and the model compounds
[Mo(ZnMe)12] (1 M) and [Mo(ZnH)12] (1 H) gave excellent
agreement between the calculated and experimental param-
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eters for 1 and only minor deviations for 1 M and 1 H (see the
Supporting Information). The Ih point group with identical
MoZn and ZnZn separations is retained for 1 H. Therefore,
the electronic structure of the parent molecule 1 H was
analyzed to shed light on the bonding situation in the
substituted analogues 1 M and 1. Compound 1 H has 81
doubly occupied valence orbitals. The fragment orbital
analysis of ADF[22] shows that only the HOMO1 and the
HOMO2 have significant contributions from the molybdenum 5s and 4d valence atomic orbitals (AOs). The lower-lying
orbitals of 1 H are combinations of the 3d10 closed shell of the
zinc atoms and the ZnH bonding orbitals. There are 18
electrons for MoZn and ZnZn bonds, of which six electrons
come from the Mo atom and 12 electrons from the Zn atoms.
Inspection of the three highest-lying valence orbitals of 1 H
reveals an unusual combination of metal–ligand (MoZnH)
and intra-cage (ZnZn) bonding. Figure 2 displays an orbital
Figure 2. MO correlation diagram for [Mo(ZnH)12] (1 H).
interaction diagram for the MoZn and ZnZn bonds of 1 H,
which shows that the second-highest-lying (HOMO1)
orbital of the molecule is a quintuply degenerate hg MO.
The HOMO1 is easily recognized as the bonding combination of the five valence 4d orbitals of Mo with the valence 4p
orbitals of Zn and the 1s functions of H. There is only a small
contribution of the Zn 4s AO, which slightly polarizes the Zn
4p AO towards the Mo center. The Mulliken breakdown of
the AO coefficients for the hg MOs gives contributions of 37.5
(Mo), 24.9 (Zn), and 30.6 % (H). The next-lower-lying
occupied ag MO (HOMO2) is the bonding combination of
the MO 5s AO mainly with the Zn 4p orbitals and the H 1s
functions (Figure 2). The shape of the molecular orbitals
suggests that the six valence electrons of the Mo atom are
engaged in six electron-sharing bonds with six valence
electrons of the (ZnH)12 fragment. Since each ZnH group
possesses one unpaired electron, the question arises as to the
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interactions of the remaining six electrons of the (ZnH)12 unit
in the diamagnetic (closed-shell) molecule.
Inspection of the HOMO of 1 H, which is a triply
degenerate (t1u) orbital in which each component has two
HZnZnH bonding contributions, answers this question. The
six unpaired electrons of the ZnH moieties that are not
engaged in MoZn bonding yield three electron pairs that are
delocalized over the (ZnH)12 cage. This means that for each
Zn atom there is only 1=4 electron pair for ZnZn bonding.
The MO correlation diagram, which uses the calculated
eigenvalues of the orbitals, indicates that the stabilization of
the triply degenerate t1u orbital of (ZnH)12 by interactions
with the empty Mo 4p functions is negligible. The results of an
energy decomposition analysis (EDA)[23] are in agreement
with this assessment. The total orbital interactions between
Mo (5s14d5) and (ZnH)12 in the electronic septet state amount
to 403.2 kcal mol1, which come mainly from the hg orbitals
(288.0 kcal mol1, 71.5 %) and the ag orbitals (96.4 kcal mol1, 23.9 %), while the
ZnZn bonding t1u orbitals contribute
only 18.3 kcal mol1 (4.5 %) to the orbital
term. The full table with the EDA results
is given in the Supporting Information.
Although the electron count of 1 H
suggests an 18-electron configuration for
the Mo atom, the actual electronic structure clearly shows that there are only 12
electrons in the Mo valence shell.[24] The
interaction of the Mo atom with the
(ZnH)12 cage may thus be interpreted as
six sd5 hybridized Mo orbitals in a valence
bond model. Intuitively it might be
assumed that sd5 hybridization yields
octahedral symmetry for the six bonds,
but this is not correct. It was shown a long
time ago[25] that sd5 hybrids do not favor
Oh symmetry for compounds MR6 (M =
transition metal), but rather a lower
symmetry (C3v or C5v) is predicted for
six-coordinated molecules with MR s
bonds. Figure 3 b shows the orientation of
the sd5 hybrid orbitals of a transition
metal and the corresponding icosahedron. It becomes obvious
that the lobes of the sd5 hybrid orbitals are ideally suited for
orbital interactions with twelve atoms placed at the vertices of
an icosahedral frame like the Zn atoms of (ZnH12). Unlike in
CH4, in which each of the four sp3 hybridized orbitals is
associated with one electron pair, the twelve lobes of the sd5
hybridized orbitals of the Mo center in 1 H each carry one half
of an electron pair. In other words, the MoZn12 core is held
together by six three-center-two-electron bonds stretching
across the diagonals of the icosahedron.
The electronic structure of 1 H was also analyzed with the
atoms in molecules (AIM) method developed by Bader.[26]
Figure 3 a shows the chemical bonds of 1 H given by AIM.
There are bond paths for twelve MoZn bonds and twelve
ZnH bonds but there are no ZnZn bond paths! A figure
showing the Laplacian 521(r) of 1 H in a plane containing the
Mo atom and four ZnH ligands is given in the Supporting
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Figure 3. a) Chemical bonds in 1 H given by the AIM analysis. The red
dots give the bond critical points, and the black lines give the bond
paths. Mo orange, Zn blue. b) Depiction of sd5 hybridization.
Information. The bonding scenario suggested by the AIM
analysis perfectly agrees with the sd5 hybridization of the
valence orbitals of the Mo atom (Figure 3 b). We want to
emphasize that the analysis of the MOs (Figure 2), which
points to weak ZnZn bonding interactions, and the AIM
analysis do not contradict but rather complement each other.
According to the MO analysis, there are six electron pairs in
orbitals that are directed along the MoZn axes, which gives
1
=2 electron pair for each MoZn contact.
There are three electron pairs in orbitals which have Zn
Zn bonding character. This gives only 1=10 electron pair for
each of the 30 ZnZn contacts, which, according to the AIM
analysis, do not constitute chemical bonds. A similar situation
was found in recent studies of compounds with weak metal–
metal interactions that do not exhibit a bond path in the AIM
calculations.[27] Accordingly, the attractive ZnZn interactions in 1 H are not strong enough to keep an empty (ZnH)12
cage intact that has only 36 electrons in total, 14 electrons
short of the “magic number” of 50 needed for stable, electronprecise icosahedral clusters. Thus, the chemical bonding in 1 H
may be best compared with the bonding situation in MAu12
(M = Mo, W), which exhibit only 18 cluster electrons in total:
twelve unpaired electrons at the ligand Au atoms are
available for bonding interactions with the six valence
electrons of the central Group 6 atom. The molecule WAu12
(Ih) was theoretically predicted in 2002 by PyykkI and
Runeberg.[28] It was experimentally prepared in the gas phase
in the same year by Wang and co-workers.[29] Autschbach
et al.[30] proposed a closed-shell 18-electron bonding situation
for the central tungsten atom with a significant population of
the valence p orbitals. This latter detail, however, is different
for the molybdenum atom of 1 H.
Should the model 1 H and hence the experimental
compound 1 be classified as a cluster or as a coordination
compound? An essential feature of a cluster is “a system of
bonds connecting each atom directly to its neighbors in the
polyhedron”.[31, 32] Accordingly, the pivotal bonding interactions of a cluster En are between the cage atoms E. In contrast,
a coordination compound exhibits donor–acceptor bonds
between the central metal and the ligand atoms of the core
structure MEn. The electronic structure analysis suggests that
1 H is neither a prototypical cluster nor a prototypical
Angew. Chem. Int. Ed. 2008, 47, 9150 –9154
coordination compound. Our discussion shows that there
are ZnZn bonding interactions in 1 H, but they are much less
important than the MoZn bonds. On the other hand, the
MoZn bonds are not typical donor–acceptor bonds but
genuine electron-sharing bonds. From the bonding analysis
using three different partitioning methods (MO correlation,
EDA, and AIM) we conclude that 1 H is best described as a
perfectly sd5-hybridized transition-metal compound in which
all 12 lobes of the hybrid orbitals are used for strong covalent
chemical bonding. The eighteen valence electrons of 1 H
available for MoZn and ZnZn bonding interactions yield
six two-electron-three-center MoZn bonds and three very
weak delocalized ZnZn bonds. The latter bonds just serve to
minimize ligand–ligand repulsion and thus favor the exceptionally high coordination number as compared with usual
transition-metal complexes [MLn] of monodentate ligands L.
In summary, the title compound 1 is the first molecule
with such a unique bonding situation to be synthesized in
substantial quantities and structurally characterized by single
crystal X-ray diffraction analysis.[33] A door is opened into a
new field of metal-rich molecules beyond the Zintl boarder,[10]
filling the space between coordination compounds and
clusters and further linking the chemistry and physics of
molecules to solid-state materials.
Experimental Section
All manipulations were carried out using standard Schlenk and glove
box techniques using dry argon. All solvents were degassed, dried,
and saturated with Ar prior to use.
1: A sample of 2 (0.300 g, 0.226 mmol) was dissolved in toluene
(5 mL) and treated with ZnMe2 in toluene (2 m, 1.58 mL, 14 equiv,
3.168 mmol) at room temperature. The reaction mixture was warmed
to 110 8C for 2 h, whereupon a yellow solution formed. After cooling
to room temperature the solution was filtered, and the solvent was
removed in vacuo. The crude product was redissolved in hexane. The
product crystallized by slow cooling to 30 8C. Yield: 0.263 g (82 %).
1
H NMR (C6D6, 25 8C): d = 2.14 (45 H, C5Me5), 0.24 (9 H, Me), 0.19
(9 H, ZnMe), 0.03 ppm (9 H, Me); 13C NMR (C6D6, 25 8C): d = 113.71
(C5Me5), 23.82 (Me), 23.37 (Me), 22.19 (Me), 11.38 ppm (C5Me5).
Elemental analysis (%) calcd for C39H72Zn12Mo: C 32.95, H 5.10,
Zn 55.20, Mo 6.75; found: C 32.66, H 5.64, Zn 55.01, Mo 6.44; no
gallium was detected.
2: A sample of freshly prepared [Mo(h4-C4H6)3] (0.300 g,
1.162 mmol) was dissolved in toluene (12 mL) in a Fischer–Porter
bottle. After addition of GaCp* (1.666 g, 8.129 mmol), the reaction
mixture was pressurized to 3 bar H2. The orange solution was warmed
to 100 8C, whereupon a red microcrystalline precipitate formed. After
stirring for further 16 h at 80 8C, the reaction mixture was transferred
into a Schlenk tube. The red crystals were isolated by decantation of
the supernatant (cannula technique), washed with a small amount of
n-hexane, and dried in vacuo. Recrystallization from mesitylene gave
well-formed dark red needle-shaped single crystals. Yield: 0.785 g
(51 %). 1H NMR (C6D6, 25 8C): d = 1.96 ppm (90 H, C5Me5);
13
C NMR (C6D6, 25 8C): d = 117.11 (C5Me5), 11.91 ppm (C5Me5);
C,H analysis (%) calcd for C60H90Ga6Mo: C 54.36, H 6.84; found:
C 53.86, H 6.24.
3: Compound 3 was prepared analogously to 1 as described above
using four rather than 14 equiv ZnMe2. Yield: 0.237 g (85 %).
1
H NMR (C6D6, 25 8C): d = 2.07 (60 H, C5Me5), 0.39 ppm (12 H,
Me); 13C NMR (C6D6, 25 8C): d = 109.75 (C5Me5), 34.10 (Me),
11.06 ppm (C5Me5). Elemental analysis (%) calcd for
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C44H72Ga4Zn4Mo: C 42.71, H 5.86, Ga 22.54, Zn 21.14, Mo 7.75;
found: C 42.56, H 5.82, Ga 22.36, Zn 21.25, Mo 7.87.
4: A sample of 2 (0.300 g, 0.226 mmol) in benzene (5 mL) was
treated with ZnMe2 in toluene (2 m, 0.67 mL, 8 equiv, 1.356 mmol) at
room temperature. The reaction mixture was heated at reflux for
30 min, whereupon a yellow microcrystalline precipitate formed. The
precipitate was isolated by decantation of the supernatant (cannula
technique), washed twice with a small amount of n-hexane, and dried
in vacuo. The product was re-crystallized from hot benzene by slow
cooling to room temperature. Yield: 0.261 g (83 %). 1H NMR (C6D6,
25 8C): d = 2.15 (30 H, C5Me5), 2.07 (15 H, C5Me5) 1.91 (15 H, C5Me5),
0.57 (6 H, Me), 0.14 (3 H, Me), 0.02 (6 H, Me), 0.14 ppm (3 H, Me).
C,H,Zn analysis (%) calcd for C46H78Ga2Zn8Mo: C 39.76, H 5.66,
Zn 37.64; found: C 39.99, H 5.94, Zn 37.11.
See the Supporting Information for additional analytical data for
compounds 5 and 6.
Received: June 13, 2008
Published online: October 7, 2008
.
Keywords: cluster compounds · coordination chemistry ·
gallium · molybdenum · zinc
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[18] The AIM analysis of Al13 gives AlAl bond paths in the Al12
cage. The EDA results suggest that the bonding situation is best
described as an endohedral complex in which an Al+ cation is
situated in a Al122 cage. M. von Hopffgarten, G. Frenking,
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systems, are driven both by the bonding contributions to the
center and by the kinetic-energy (nodal-structure) terms in the
ligand subsystem, Ln. The latter imposes a filling order s < p < d,
even for the MLn complex. Then the 18e principle can be right
for the right reasons even without any np contributions at the
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[33] The species MH12 (M = Cr, Mo, W) also possess twelve oneelectron ligands around one metal atom, but the bonding
analysis showed that there are in fact four classical MH
bonds and four nonclassical M(H2) bonds: L. Gagliardi, P.
PyykkI, J. Am. Chem. Soc. 2004, 126, 15014. The tungsten
compound [WH4(H2)4] has recently been observed by FTIR
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2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 9150 –9154
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