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Elemental Gallium as a Source of Subvalent Gallium Units in GalliumЦRhodium Clusters.

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Cluster Chemistry
Elemental Gallium as a Source of Subvalent
Gallium Units in Gallium–Rhodium Clusters**
Manfred Scheer,* Martin Kaupp,* Alexander V. Virovets,
and Sergei N. Konchenko*
Dedicated to Professor Achim M ller
on the occasion of his 65th birthday
reagents, liquid gallium is a gentler reducing agent, which is
especially favorable for preparing complexes with transitionmetal centers in a medium oxidation state. Herein, we report
the novel use of elemental gallium as reducing agent towards
transition-metal halides to generate subvalent GaCl ligands as
well as the first twofold coordinate gallium atoms which lie
between two transition-metal centers in a multicenter–multiple-bond interaction.
The reaction of [{Cp’’RhCl2}2] (1) (Cp’’ = h5-1,3-C5H3tBu2)
with gallium melt in THF at 50 8C leads, after two weeks, to
the formation of novel gallium containing clusters 2 and 3,
Whereas metals of group 1 and 2 elements are well known as
½fðCp00 RhÞ2 ðm-GaÞg2 ðm 4 ; h2 -Ga2 Cl3 Þ2 2
reducing agents, within group 13 elements only aluminum in
½ðCp00 RhÞ3 ðm 3 -GaClÞðm 3 -Ga3 Cl3 Þðm 3 -OGaCl3 Þ 3
Devarda's alloy has been used to generate transition-metal
carbonyl compounds. Furthermore, thallium has been
employed as a reducing agent with metal clusters, such as
which crystallize as their THF adducts (reaction (1) in
[Co2(CO)8] to form metalates, such as Tl[Co(CO)4][2] and
Scheme 1).
elemental gallium was used to reduce
[Re2(CO)10] to give [Re4(CO)12{m3GaRe(CO)5}4].[3] In the reaction of
[Ru3(CO)12] with Ga2Cl4, elemental gallium functioned as a reducing agent, which
resulted in the generation of GaCl2 and
GaCl ligands.[4] A similar approach was
employed when [Re2(CO)10] was treated
with GaI3 in the presence of gallium.[3] The
use of elemental gallium as a reducing
agent for transition-metal halides is so far
limited to the reduction of TiIV to TiIII
complexes to form [(CpTiF2)3Ga] (Cp =
C5H5).[5] The advantage of elemental gallium as a reducing agent lies in its low
melting point (29.8 8C) making it similar
Scheme 1. Synthesis of compounds 2–4, Cp’’ = h5-1,3-C5H3tBu2, [Rh] = Cp’’Rh.
to the common liquid-metal reductants,
for example, sodium amalgam and K/Na
alloy. However, in comparison to these
After two days reaction a deep blue solution is formed,
afterwards the color changes to red—in a separate experiment this blue reaction mixture was worked up. [{Cp’’RhCl}2]
[*] Prof. Dr. M. Scheer
(4) was isolated in about 65 % yield (Scheme 1, reaction (2)).
Institut f$r Anorganische Chemie
Compound 4 has a dimeric structure similar to the known
der Universit*t Karlsruhe
76128 Karlsruhe (Germany)
complex [{Cp*RhCl}2][6] (Cp* = C5Me5). The X-ray structure
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of 4 shows an RhRh bond length of 2.584(1) D and two
bridging Cl atoms.[7] Attempts to synthesize 2 and 3 starting
Prof. Dr. M. Kaupp
exclusively from 4 and elemental gallium alone failed. Thus,
Institut f$r Anorganische Chemie
the presence of an additional amount of the chloro-rich
Universit*t W$rzburg
complex 1 is essential.
Am Hubland, 97074 W$rzburg (Germany)
The products 2 and 3 are the first examples of compounds
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with RhGa bonds. Moreover, they contain GaCl and other
subvalent gallium species and twofold coordinate Ga atoms,
Dr. S. N. Konchenko, Dr. A. V. Virovets
Institute of Inorganic Chemistry
the formation of which is difficult to speculate upon.
Siberian Division of RAS
Probably, in the presence of an excess of liquid gallium, an
Acad. Lavrentyev str. 3, Novosibirsk 630090 (Russia)
equilibrium mixture of different subvalent gallium chlorides,
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which mainly includes GaCl, is generated. This GaCl interacts
with reduced forms of the rhodium complex, leading to the
[**] This work was comprehensively supported by the Deutsche
formation of 2 and 3. Insertion of gallium into the RhCl
Forschungsgemeinschaft and the Fonds der Chemischen Industrie.
can also play a role in the formation of the products.
S. N. K. is grateful to the DAAD and the DFG and the RFBR(03–03–
The red cubes of 2 and the light-brown blocks of 3[8] are
32374). The authors thank Degussa AG for the gift of precious
sparingly soluble in toluene and readily soluble in THF. In the
Angew. Chem. Int. Ed. 2003, 42, 5083 –5086
DOI: 10.1002/anie.200351226
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
H NMR spectra the Cp’’ ligands are magnetically equivalent.
In the mass spectra (EI and FI) of 2 and 3 peaks of molecular
ions could not be detected and even fragment peaks are
difficult to assign. The products were characterized by singlecrystal X-ray diffraction. Complex 2 crystallizes in the triclinic
space group P1̄ with two independent molecules (A and B) in
the asymmetric unit, which have similar structural features.[7]
In Figure 1 the molecular structure of the molecule A is
Figure 2. Isosurface of the electron localization function (ELF = 0.37)
for 2’, cut open along the equatorial mirror plane. The torii represent
Rh-Ga-Rh multicenter multiple bonding along the long edges,
Rh-Ga-Rh three-center attractors are shown for one of the Rh2Ga2
Figure 1. Molecular structure of 2 (molecule A). Selected bond
lengths [D] and angles [8]: Rh1-Ga3 2.346(1), Rh2’-Ga3 2.340(1), Rh1Ga1 2.438(1), Rh1-Ga2 2.442(1), Rh···Rh 3.069(1), Ga1-Cl1 2.219(2),
Ga1-Cl2’ 2.445(2), Ga2’-Cl2’ 2.363(2), Ga2’-Cl3’ 2.215(2), Rh2-Ga1
2.418(1), Rh2-Ga2 2.408(1); Rh1-Ga3-Rh2’ 174.40(4), Ga2-Cl2-Ga1’
115.65(8), Cl3-Ga2-Cl2 94.01(8), Rh1-Ga1-Rh2 78.42(4), Rh1-Ga2-Rh2
depicted: four Cp’’Rh units are connected by two bridging
Ga2Cl3 ligands (folding angle of the Rh2Ga2 planes along the
Rh···Rh axis is 18.758(A) and 18.438(B)) and also by twocoordinate Ga atoms. The latter Rh-Ga-Rh units are almost
linear (A: 174.40(4)8 and B: 174.61(4)8) with very short Rh
Ga bonds (A: 2.346(1) D and 2.340(1); B: 2.343(1) and
2.346(1)),[9] which suggest multiple bonding in these moieties.
Density-functional calculations[10] on the model complexes 2’ and 3’ containing Cp ligands[11] have been used to
obtain insight into the bonding in the two new cluster
compounds. The DFT(B3LYP) optimized structures agree
well with the X-ray structure data for 2 and 3. For a better
understanding of bonding modes, the electron localization
function (ELF)[10h] was analyzed for both molecules.
Figure 2 shows an ELF = 0.37 isosurface for 2’, cut open
along the equatorial mirror plane (isosurface = surface of the
same ELF). Two three-center Rh-Ga-Rh attractors can be
clearly seen on each of the Rh2Ga2 faces, with similar ELF
values at the attractor (ELF = 0.406) as found for
[Rh6(CO)16][12]). In addition, however, ELF indicates multiple
bonding for the almost linear Rh-Ga-Rh arrangements of the
long edges of the cluster. Two types of p-bonding attractors
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
with ELF = 0.501 are found for each of the GaRh bonds
along the edge, which merge to a torus at an ELF value of
approximately 0.429. The orientation of the attractors suggests more Rh=Ga=Rh p-bonding contributions perpendicular to the Rh4Ga2 plane of 2’ than within the plane. This type
of polar multiple bond is also confirmed by inspection of the
canonical molecular orbitals. As the torii above the mirror
plane (not shown) are identical to those shown below the
plane, the two pp-type orbitals on the central gallium atoms
must be shared in a three-center bonding fashion by both
rhodium partners. At ELF = 0.447, the p-bonding domains
merge with those of four additional two-center Ga–Rh
bonding attractors (ELF = 0.481), which are located slightly
below and outside the respective Rh2Ga2 faces (not shown),
somewhat closer to Rh than to Ga. This arrangement is
consistent with a general multicenter bonding situation.
The molecular structure of 3 (Figure 3)[7] reveals a
distorted[13] Rh3Ga3 trigonal antiprism in which each Rh
atom is coordinated by a Cp’’ ligand and each Ga atom by a Cl
ligand. Additionally, the Rh3 triangle is capped by a GaCl
moiety, and the Ga3 face is capped by an OGaCl3 unit. Both
RhRh (2.964(1) D) and GaGa (2.823(2) D) bond lengths in
3 lie close to the upper limit of the mean values found for
corresponding single bonds, but still in the accepted range.[14]
However, the evidently high coordination number of the
atoms Ga1 shows that the Ga3 moiety and its connectivity to
the Rh3 triangle can only be described in terms of a
multicenter bonding in a metal cluster arrangement. This is
also evident from simple electron counting. The multicenter
bonding situation in 3 is further supported by the ELF analysis
of the model complex 3’. Several attractors related to covalent
bonding within the cluster were found. 1) six two-center
attractors (ELF = 0.425) connecting each Ga1 atom to two Rh
atoms, and the three types of three-center attractors: 2) 3 N
Angew. Chem. Int. Ed. 2003, 42, 5083 –5086
to subvalent Ga and GaCl ligands incorporated in unusual
bonding situations. Our bonding analyses clearly confirm
three-center-type bonding comparable to polyhedral boron
clusters, but also significant charge alternation Ga(d+)/Rh(d)
in both 2 and 3. In addition, the linear Rh-Ga-Rh bridges in 2
feature multicenter–multiple-bonding.
Experimental Section
Figure 3. Molecular structure of 3. Selected bond lengths [D] and
angles [8]: Rh-Rh’ 2.964(1), Rh-Ga1 2.426(1), Rh-Ga1’’ 2.458(1), Ga1Ga1’ 2.823(2), Rh-Ga2 2.437(2), Ga2-Cl2 2.164(6), Ga1-Cl1 2.151(3),
Ga1-O 2.055(9), Ga3-O 1.909(15), Ga3-Cl3 2.173(4); Rh-Rh’-Rh’’ 60.0,
Ga1-Rh-Ga1’’ 70.61(6), Rh-Ga1-Rh’’ 74.71(5), Rh-Ga2-Rh’ 74.88(6),
Ga1-O-Ga1’ 86.7(5), Ga3-O-Ga1 127.5(3).
Rh-Ga1-Rh (ELF = 0.473), 3) 3 N Ga1-Rh-Ga1 (ELF =
0.459), and 4) 3 N Rh-Ga2-Rh (ELF = 0.411). At a lower
ELF value of approximately 0.36, the domains of the multicenter-bonding attractors merge to the bands across the
surface of the cluster, very similar to findings for polyhedral
boron hydrides or halides.[15] GaCl, RhCp, and GaO
bonds are indicated by the ELF plot (not shown)as being
significantly ionic.
Fragment charges obtained from natural population
analyses are shown in Table 1. For both 2’ and 3’, the data
clearly show pronounced charge alternation within the
A solution of [Cp’’RhCl2]2 (1) (0.3 g, 0.4 mmol) in THF (15 mL) and
an excess of elemental Ga (0.7 g, 10 mmol) was thoroughly degassed
in a Schlenk flask equipped with a teflon stop-cock and then heated
under reduced pressure at 50 8C. After two days the mixture became
deep blue indicating formation of 4. Workup at this stage by filtration
to remove the excess gallium and removal of the solvent from the
filtrate until crystallization began, followed by storage the solution at
25 8C led to the isolation of 165 mg of 4 as brown crystals (65 %
yield). If the reaction is allowed to go further, over the following
12 days, the color of solution changed to red indicating complete
transformation of 4. Then the resulting solution was filtered and the
filtrate reduced in volume to a viscous fluid (about 2 mL). After a
week at 8 8C 0.1 g (51 % based on Rh content) of crystals of two
different types were formed and separated manually under a microscope in a glovebox: red cubes of 2·2.5 THF ( 60 mg, 31 %) and lightbrown blocks of 3·THF ( 40 mg, 20 %).
2: Rh:Ga ratio (total reflection X-ray fluorescence analysis
(TRFA)): found 4:5.7, calcd: 4:6. IR (Nujol): ~
n = 836(m), 589(m, br),
398(s), 384(m), 334(w) cm1, calcd for 2’: 840, 596, 400, 392, 332 cm1
(most intensive peaks). 1H ([D8]THF, 298 K, 250 mHz): d = 1.23
(18 H, CH3), 5.34 (2 H, CH), 5.74 ppm (1 H, CH).3: Rh:Ga ratio
(TRFA): found: 3:4.8, calcd: 3:5. IR (Nujol): ~
n = 836(m), 351(s),
326(w), 289(w), 226(m) cm1, calcd for 3’: 837, 352, 327, 291, 227 cm1
(most intensive peaks)). 1H ([D8]THF, 298 K, 250 mHz): d = 1.23
(18 H, CH3), 5.21 (2 H, CH), 5.64 ppm (1 H, CH).4: 1H ([D8]THF,
298 K, 250 mHz): d = 1.19 (18 H, CH3), 5.35 (2 H, CH), 5.55 ppm (1 H,
CH); MS (EI, 70 eV): 630 [M+], 315 [{M/2}+].
Received: February 19, 2003
Revised: June 18, 2003 [Z51226]
Keywords: cluster compounds · electron localization functions ·
gallium · insertions · rhodium
Table 1: Natural population analysis partial charges for 2’ and 3’.
Q(Ga bridge)
0.481 (4 J )
+ 0.278 (4 J )
+ 0.135 (2 J )
+ 0.825 (2 J )
+ 0.518 (3 J )
0.163 (3 J )
+ 0.311
cluster, with significant negative charge on the RhCp fragments and significant positive charge on the GaCl containing
fragments (with significantly polar GaCl and GaO bonds,
and with GaRh bonds that are also notably negatively
polarized towards the transition-metal center). Note also, that
at the computational level used (B3LYP-DFT), appreciable
HOMO–LUMO gaps are found of approximately 2.76 eV for
2’ and approximately 2.65 eV for 3’. This result suggests a
stable closed-shell situation and is consistent with a diamagnetic nature of both clusters.
This work shows that the use of gallium metal as reducing
agent towards transition-metal halides opens up a novel route
Angew. Chem. Int. Ed. 2003, 42, 5083 –5086
[1] W. A. Herrmann, K. Sfele, C. E. Zybill in Synthetic Methods of
Organometallic Chemistry, Vol. 7 (Ed.: W. A. Herrmann),
Thieme, Stuttgart, 1997, p. 13.
[2] S. E. Pedersen, W. R. Robinson, D. P. Schussler, J. Organomet.
Chem. 1972, 43, C44; D. P. Schussler, W. R. Robinson, W. F.
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[3] H.-J. Haupt, P. Balsaa, B. Schwab, Z. Anorg. Allg. Chem. 1985,
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[4] G. N. Harakas, B. R. Whittlesey, Inorg. Chem. 1997, 36, 2704 –
[5] F.-Q. Liu, A. KUnzel, A. Herzog, H. W. Roesky, M. Noltemeyer,
R. Fleischer, D. Stalke, Polyhedron 1997, 16, 61 – 65. For the use
of Al in analogous reactions see: F.-Q. Liu, H. Gornitzka, D.
Stalke, H. W. Roesky, Angew. Chem. 1993, 105, 447 – 448;
Angew. Chem. Int. Ed. Engl. 1993, 32, 442 – 443.
[6] a) P. R. Sharp, D. W. Hoard, C. L. Barnes, J. Am. Chem. Soc.
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1993, 32, 612 – 620.
[7] Crystal structure analyses of 2, 3, and 4 were performed on a
STOE IPDS diffractometer with MoKa radiation (l = 0.71073 D)
for 2 and 3 and AgKa radiation (l = 0.56087 D) for 4. The
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
structures were solved by direct methods with the program
SHELXS-97,[16a] and full-matrix least-squares refinement on F2
in SHELXL-97[16b] was performed with anisotropic displacements for non-hydrogen atoms. Hydrogen atoms were located in
idealized positions and refined isotropically according to the
riding model. In each of the compounds 2 and 3 one of the THF
solvate molecules is disordered over two and three positions,
respectively. The C and O atoms of the THF molecules of 2 and 3
were not refined anisotropically. The flack parameter of the
acentric structure of 3 came to value of 0.02(3). 2·2.5 THF:
C62H104Cl6Ga6O2.5Rh4, Mr = 1932.11, crystal dimensions 0.20 N
0.15 N 0.04 mm3, triclinic, space group P1̄ (No. 2), a = 15.761(3),
b = 16.860(2) D, c = 16.896(3) D, a = 61.07(3)8, b = 87.24(3)8,
g = 73.01(3)8, T = 200(1) K, Z = 2, V = 3734.5(13) D3, 1calcd =
1.718 g cm3, m(MoKa) = 3.239 mm1, 10 973 independent reflexes
(Rint = 0.0426, 2qmax = 48.068), 7576 observed with Fo = 4s(Fo),
R1 = 0.0382,
wR2 = 0.0975.
C43H71Cl7Ga5O2Rh3, Mr = 1525.48, crystal dimensions 0.15 N
0.15 N 0.10 mm3, trigonal, space group P31c (No. 159), a = b =
13.684(2) D, c = 17.151(3) D, T = 200(1) K, Z = 2, V =
2781.0(8) D3, 1calcd = 1.822 g cm3, m(MoKa) = 3.619 mm1, 2701
independent reflexes (Rint = 0.0257, 2qmax = 48.148), 1966
observed with Fo = 4s(Fo), 190 parameters, R1 = 0.0469, wR2 =
0.1274. 4: C26H42Cl2Rh2, Mr = 631.32, crystal dimensions 0.20 N
0.10 N 0.03 mm3, monoclinic, space group P2/n (No. 13); a =
14.272(3), b = 6.209(1), c = 15.612(3) D, b = 97.66(3)8, T =
203(2) K,
Z = 2,
V = 1371.1(5) D3,
1calcd = 1.529 g cm3,
m(AgKa) = 0.744 mm1, 2850 independent reflexes (Rint =
0.0544, 2qmax = 41.848), 2247 observed with Fo = 4s(Fo); 142
parameters, R1 = 0.0379, wR2 = 0.1015. CCDC-203124–203126
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+ 44) 1223-336-033; or
[8] The origin of the O atom in the composition of 3 is uncertain. If
the reaction is carry out in THF, in which traces of H2O were
added, an slightly increase of the yield of 3 was obtained,
whereas the amount of 2 remains unchanged. Thus, the
intermediately formed Ga hydroxide species could be responsible for the formation of the m3-OGaCl3 ligand in 3.
[9] Compare with other RhGa bond lenghts: in 4 d(Rh1-Ga2) =
2.442(1) D; in 3 d(Rh1-Ga1) = 2.438(1) D, d(Rh1-Ga2) =
2.442(1) D.
[10] a) The DFT calculations employed the B3LYP hybrid functional[10b] in the implementation within the Gaussian 98 program.[10c] The calculations used quasi-relativistic small-core
pseudopotentials (effective-core potentials, ECP) for Rh[10d]
and Ga,[10e] together with a [8s7p6d]/(6s5p3d) valence basis set
for Rh[10d] and a [12s12p9d]/(6s6p4d) basis for Ga.[10e] C, O, and
Cl were treated by ECPs and DZP valence basis sets,[10f] and a
DZ basis[10g] was used for hydrogen. The electronic structure of
the complexes has been studied using the electron localization
function (ELF[10h]) and natural population analyses (NPA[10i] ;
using the built-in NBO module of the Gaussian 98 program[10c]).
The ELF data have been created using the TopMoD suite of
programs[10j] and are graphically displayed using the MOLEKEL
program.[10k] b) A. D. Becke, J. Chem. Phys. 1993, 98, 5648 –
5652; C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785 –
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Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S.
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2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A.
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W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon, E. S.
Replogle, J. A. Pople, Gaussian, Inc., Pittsburgh, PA, 1998; d) D.
Andrae, U. HWußermann, M. Dolg, H. Stoll, H. Preuß, Theor.
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Stoll, M. Dolg, P. Schwerdtfeger, J. Chem. Phys. 1996, 105, 1052 –
1059; f) A. Bergner, M. Dolg, W. KUchle, H. Stoll, H. Preuß,
Mol. Phys. 1993, 80, 1431 – 1441; g) T. H. Dunning, P. J. Hay in
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Chemistry, Vol. 3 (Ed.: H. F. Schaefer III), Plenum, New York,
1977 h) See, for examnple: A. D. Becke, K. E. Edgecombe, J.
Chem. Phys. 1990, 92, 5397 – 5403; A. Savin, R. Nesper, S.
Wengert, T. F. FWssler, Angew. Chem. 1997, 109, 1892 – 1918;
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F. Weinhold, J. Chem. Phys. 1985, 83, 735 – 746; j) S. Noury, X.
Krokidis, F. Fuster, B. Silvi, TopMoD Programmpaket, UniversitY Pierre et Marie Curie, 1997. k) See, for example, S.
Portmann, H. P. LUthi, Chimia 2000, 54, 766 – 770.
The Cp’’ ligand was in both cases replaced by unsubstituted Cp,
and the structures were idealized slightly to make use of point
group symmetry, C3v for 2’ and D2h for 3’.
M. Kaupp, Chem. Ber. 1996, 129, 527 – 533.
There are two different lenghts of the RhGa bonds within the
antiprism: 2.426(1) and 2.458(1) D.
A comparison of structures included in Cambridge Crystallographic Database shows that lengths of RhRh and GaGa
bonds (described as single ones) vary in quite broad ranges:
2.315–3.294 D and 2.324–3.046 D respectively. At that a maximum of distribution for RhRh distances lies between 2.660 and
2.880 D. As to GaGa bonds the distribution does not have an
explicit maximum, and most of the values are represented almost
evenly between 2.367 and 2.720 D.
Compare for example, with: A. Burkhardt, U. Wedig, H. G.
von Schnering, A. Savin, Z. Anorg. Allg. Chem. 1993, 619, 437 –
a) G. M. Sheldrick, SHELXS-97, UniversitWt GZttingen, 1997;
b) G. M. Sheldrick, SHELXL-97, UniversitWt GZttingen, 1997.
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