Page 273 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: GNH Final Pages
Encyclopedia of Physical Science and Technology EN009M-428 July 18, 2001 1:6

530 Metal Particles and Cluster Compounds

FIGURE 12 Typical surfactants used in preparing inverse micelles.

ligands, have been synthesized and structurally charac- proper formula was reported for the bromine analog. We
terized. The symmetries of many cluster compounds of now know these to contain an octahedron of Ta atoms with
various sizes have been similar to those predicted during extensive metal–metal bonding. However, it was not until
the lowest energy growth process from 1 to 13 atoms. the mid-1940s that X-ray crystallography provided direct
Many unusual geometries have been observed in cluster evidence for such bonding by establishing the presence of
compounds, mostly due to the nature of the stabilizing an octahedral core of Mo atoms.
ligands and formal change of the cluster unit. However, a Crystallographic techniques are of paramount impor-
general trend in geometry is toward that of a polytetrahe- tance to cluster research. Until the collection of crystal-
dral icosahedron. lographic data was aided by computerization the field of
The connection between small metal particles and metal metal cluster chemistry remained relatively unexplored.
cluster compounds is that the latter can be structurally While waiting for technology to provide the means for
characterized and hopefully could serve as good mod- rapid acquisition of structural data, theories of bonding
els for small metal particles. However, large cluster com- in polyhedra were being developed. Many of these poly-
pounds where the M n core is large (n > 20) are extremely hedral bonding theories were developed for boron clus-
difficult to synthesize. It has not been possible to synthe- ter compounds since these were most familiar. These ad-
size a cluster compound large enough such that it would vances in boron cluster chemistry greatly aided the field
be stable with or without ligands (e.g., reversible ligand of metal cluster chemistry; this will become clear in later
exchange with a support surface). sections.
However, great progress is being made, and metal clus- Before proceeding any further we should explicitly de-
ter compounds are in the “spotlight” in the field of chem- fine a metal cluster compound. The term cluster should
istry. Recently clusters such as [Pt 19 (CO) 22 ] 4− have been be reserved for those compounds which contain three or
characterized. The [Pt 38 (CO) 44 ] dimer has been recently more metal atoms and enough metal–metal bonding to
mentioned and interestingly exhibits a ccp structure found constitute a bonded triangle of metal atoms; M 3 triangles
in bulk Pt. are important structural units as they are the basic geomet-
Even larger clusters, such as Au 55 (PPh 3 ) 12 Cl 6 and ric building block for higher nuclearity clusters. Nature’s
Au 18 Ag 20 Cl 14 [P(tolyl) 3 ] 12 have been reported. Indeed choice of this simple building block is far from arbitrary,
there appears to be an extensive series of silver-gold clus- and is it not unique to cluster compounds. Bulk metals,
ters based on linked icosahedra. with their close-packed crystal structures, can be viewed
This is an exciting, developing area that holds great as being constructed from triangular building blocks. The
theoretical and practical promise. In the discussion that dominance of the triangular geometry is due to the fact that
followsthereaderwillbemadefamiliarwiththisrelatively it is this geometry which provides the means of maximum
new area of chemistry. electron delocalization between three metal atoms. Since
The scientific community did not realize the importance it is the delocalization of electrons between atoms which
of cluster compounds for nearly 40 years after their dis- constitutes a bond, the strongest bonding or structural in-
covery. In the early 1900s tantalum halides of the gen- tegrity is achieved when the subunits are of a triangular
eral formula [Ta 6 X 12 ] 2+ were being prepared. In 1907 the form. In order to look more closely at the metal–metal
chloride was incorrectly reported as TaCl 2 . In 1910 the bond we will begin our consideration of metal clusters