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

514 Metal Particles and Cluster Compounds

particles can be surrounded and stabilized by ligands to Note the correspondence of heat of formation with bond
yield new chemical compounds. These ligands can be in- length presented in the table above.
organic, such as halide ions, or organic such as alkenes, The atoms pack differently for the metallic elements.
alkynes, and carbon monoxide. As of 1999 the known There are three common packing geometries as shown in
well-characterized metal cluster compounds are of the Fig. 2. For two layers the packing is the same, the second
formM x L y (M =metalatom,L =ligandmolecule),where layer of atoms resting in the declivities of the other. This
x = 2–55. is the densest arrangement. When the third layer is added
two possibilities arise. The atoms can be placed directly
over the first layer giving hexagonal symmetry (hexago-
I. METALLIC ELEMENTS
nal close packed, hcp) or slightly displaced with respect to
the first layer to give cubic symmetry (cubic close packed,
Humans have used and shaped metals since the
ccp). A third less common packing arrangement is body
Mesopotamian era (3500–1500 B.C.). The first metals
centered cubic (bcc) where each atom has only eight in-
to be fabricated were gold, copper, and tin. Gold has
stead of twelve nearest neighbors, although there are six
been a fascination for over 4000 years, especially in the
next nearest neighbors that are only 15% farther away.
period 1000–1600 A.D. when alchemists tried to produce
The bcc arrangement is 92% as dense as the ccp and hcp
pure gold by transmutation of base metals. Much was
arrangements. Of course, some metals can exist in more
learned about chemical processing of metals because of
than one geometrical arrangement (e.g., Ca, Sr, Ti, Zr,
the alchemists’ dreams. In addition, chemical processing
Hf, Fe, and Co). The important metals Rh, Ir, Ni, Pd, Pt,
and shaping of iron, beginning in 1500 B.C., yielded much
Cu, Ag, and Au take on ccp geometry while V, Nb, Ta,
more information about metals.
Cr, Mo, and W prefer bcc. Most other metals prefer hcp
As the science of chemistry slowly grew out of alchemy
geometries.
and all the elements were painstakingly discovered and
As more and more atoms pack together to form large
systematized,itbecameapparentthatmuchofourmaterial 4 25
metal particles (say 10 –10 atoms) the typical metallic
universe is made up of metals or chemical compounds of
properties are assumed. Long-range effects are observed
metals. The periodic chart displays more than 70 elements
such as ferromagnetism, reflectivity, conductivity, and
that are considered metals. Some of these substances are
malleability.
hard and brittle whereas some are soft, malleable, and low
When growth stops there naturally must be edges, cor-
melting. The extreme of course is mercury which melts
ners, steps, and kinks formed on the particle surface as
◦
at minus −39 C (bp = 357 C), whereas tungsten melts at
◦
shown in Fig. 3. Metal atoms that find themselves in
3410 C (bp = 5660 C). These physical properties reveal
◦
◦
such positions are much more chemically reactive, due
the strength of the bonding interaction between the indi-
to their accessibility and their coordinative unsaturation.
vidual metal atoms. Another property that is an even better
These sites are believed to be very important in some cat-
measure of the bonding strength between metal atoms is
alytic processes. Likewise many catalysis experts believe
the heat of formation, which is roughly a measure of the
that only certain geometrical arrangements of metal atoms
energy needed to vaporize the element (to break all chem-
are capable of carrying out a specific chemical (catalytic)
ical bonds). Figure 1 shows a periodic chart listing mp,
transformation of an adsorbed molecule (the so-called en-
bp (atmospheric pressure), and heat of formation of the
semble effect). We will deal with these subjects in more
elements.
detail later.
Osmium Iridium Platinum Gold Mercury II. METAL PARTICLES
Distance 2.6754 2.714 2.746 2.8841 3.005 A. Atoms
between
atoms ( ˚ A) The smallest metal particle is a single atom. In recent
Heat of 187 160 135 88 15 years atoms of metallic elements have attracted a great
formation deal of interest. First, their formation by evaporation of
(kcal)
the bulk element has been studied extensively. It is pos-
sible to evaporate metals that vaporize under vacuum at
Also shown in Fig. 1 are the bond lengths between indi- temperatures below 2000 C by using electrical resistive
◦
vidual atoms of the elements. This is another crude mea- heating of ceramic crucibles (such as aluminium oxide
sure of the strength of bonding for elements in the same coating on a tungsten wire basket). The metals listed be-
row, although many things must be taken into account. low all evaporate nicely using this method. All but Sn, As,