вход по аккаунту


Molecular Systems with Perfect Metal Structure.

код для вставкиСкачать
Molecular Systems with Perfect
Metal Structure
By Georg Suss-Fink *
Lord Lewis and his research group at the University of
Cambridge have taken a spectacular step on the road from
molecular clusters to metallic structures. They have synthesized the dianionic transition-metal cluster [Os,,(CO),oJzo.
whose metal framework represents a perfect tetrahedral
cubic-close-packed metal core. This cluster anion, crystallized as its tetrabutylphosphonium salt, is remarkable for the
aesthetics of its molecular structure.
The last two decades have witnessed a very rapid expansion in the chemistry of transition-metal complexes containing more than two metal atoms. E A . Cotton proposed that
such complexes be termed cluster^".^'^ They are oligonuclear systems generally consisting of a framework of metal
atoms held together by intermetallic bonds and surrounded
by a shell of terminal or bridging ligands.
\ I /
-M-M-M-/ I \ /I\ / I
As oligonuclear species, metal clusters occupy a noman's-land between mononuclear metal complexes on the
one side and polynuclear metal surfaces on the other, and
thus between typical homogeneous and typical heterogeneous catalysts. Starting from the notion of P. Chini"1 that
clusters represent molecular metal fragments containing
chemisorbed species on their peripheries, .
Lewis et a1.131and
E. L. Muettertied4' related transition-metal clusters to catalysis: On the one hand, transition-metal clusters can serve as
molecular models for poorly understood heterogeneous catalysts. On the other hand, they might function as unconventional homogeneous catalysts. Furthermore, the pioneering
work of G. Schmidrslon superclusters has awakened hope
that they might allow metal particles of defined size to be
prepared, a possibility of great interest not only for chemistry but also for physics and materials science. The chemistry
of transition-metal clusters has received considerable impulse from these potential applications. A basic understanding of the formation and properties of metal cores is expected
to emerge in this area of research.
The large transition-metal clusters--containing up to 42
metal atoms-observed during the last two decades exhibit
diverse structures, which have been established by X-ray
diffraction techniques.I6] For very large clusters with 55 or
more metal atoms, electron microscopy has proved particularly useful.'51Whereas the structures of smaller clusters can
be explained largely in terms of electronic factors (i.e., they
[*I Prof. Dr. G . Suss-Fink
Institut de Chimie, Universite de Neuchltel
Avenue de Bellevaux 51, CH-2000 Neuch2tel (Switzerland)
obey the noble-gas rule or the skeletal electron pair theory),
the frameworks of larger metal clusters are governed primarily by geometric factors (i.e, their metal atoms are as close
packed as possible).
Now, the research group of Lord Lewis has reported a new
cluster that strikingly illustrates the transition from molecular structures to compact metal structures. The trinuclear
cluster [Os,(CO) ,,,(NCCH,),] decomposes in an unclear
fashion upon vacuum pyrolysis at 300°C and l o L 3Torr.
Extraction of the decomposition residue affords the dianion
[Os,,(CO),,]20, together with other lower-nuclearity clusters. The dianion can be crystallized as its tetrabutylammonium salt.[']
A single-crystalX-ray analysis reveals that the dianion has
a metal framework consisting of 20 cubic-close-packed osmium atoms arranged to form a perfect tetrahedron. The symmetry of the ligand shell, consisting of 40 terminal carbonyl
groups, corresponds to that of the metal framework.
The new cluster [OS,,(CO),,]~~should be compared with
the cluster [HNi,,Pt,(CO),,]50, described in 1985 by G.
Longoni et al.''l (Fig. 1). Whereas the metal lattice of the
Q VCH Verlugsgesellschuft mbH, W-6940 Weinheim, 1991
Fig. 1. Tetrahedral section of a cubic close packing (ccp) in [ O S , , ( C O ) , ~ ] ' ~
(left) and an octahedral section of a ccp in [HNi,,Pt,(CO),,]5e (right).
latter corresponds to an octahedral section of a cubic-closepacked array of metal atoms, the osmium framework of the
former represents a tetrahedral section. It should be noted,
however, that metallic osmium crystallizes as a hexagonalclose-packed structure, whereas the osmium atoms in
[OS,,(CO),,]~~are cubic close packed.
Fig. 2. Gradual
Angew. Chem. Int. Ed. Engl. 30 (1991) No. 1
Still another comparison emerges: In the series of metal
clusters with tetrahedral symmetry, W,OS~(CO)IZI~' and
[ l o ] were the first two clusters containing
two and three osmium atoms, respectively, on the edges of
the tetrahedron. The new cluster [ ~ S 2 0 ( ~ ~ ) 4 0 ] 2 ewhich
four osmium atoms on the edges of a face-centered tetrahefits into this series (Fig' 2)' Thus, gradual tetradron,
hedral growth is possible on going from molecular clusters to
metallic structures.
Angen. Chem. Int. Ed. Engl. 30 lf991) No. I
[l] F. A. Cotton, Quarr. Rev. 20 (1966) 389.
[2] P. Chini, Inorg. Chim. Acta Rev. 2 (1968) 31.
[31 B. E G. Johnson, J. Lewis, Pure A m l . Chem. 44 (1975) 43.
I41 E. L. Muetterties, Bull. SOC. chim. B e k . 84 (1975) 959.
[S] G. Schmid, Nachr. Chem. Tech. Lab. 35 (1987) 249.
[6] D. M. P. Mingos In D. F. Shriver, H. D. Kaesz, R. D. A d a m (Eds.): The
Chemistry of Metal Cluster Complexes, VCH Publishers, New York
[7] A. J. Amaroso, L. H. Gade. B. F. G. Johnson. J. Lewis, P. R. Raithby, W,-T.
Wong, Angew. Chem. I03 (1991) 102; Angew. Chem. Inl. Ed. Engl. 31 (1991)
Verlagsgesellschafr mbH, W-6940 Weinheim, 1991
0570-0833/9l/0t0l-0073 $3.50+.25/0
Без категории
Размер файла
193 Кб
perfect, structure, molecular, metali, system
Пожаловаться на содержимое документа