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

540 Metal Particles and Cluster Compounds

FIGURE 27 Polyhedra cluster cores may be defined by both metal and ligand atoms. The M 3 C 2 trigonal bipyramid
in (a) Fe 3 (CO) 9 C 2 Et 2 , the M 4 C 2 octahedron in (b) Co 4 (CO) 10 C 2 Et 2 , and the M 3 C 2 square based pyramid of (c)
Os 3 (CO) 10 C 2 Ph 2 are depicted here.

equatorial vertices are four coordinate. Six skeletal elec- the hopes that we may realize practical applications for
tron pairs would be necessary to give this five-vertex, these compounds.
closo geometry. Consider Fe 3 (CO) 9 C 2 Et 2 , one axial (low- There are numerous ligands known to stabilize clusters,
coordinate) site is occupied by a carbon atom rather than and many of these ligands are capable of coordinating to
the alternative of having both carbon atoms occupying the the cluster in a variety of ways. To specify the manner
x
four coordinate equatorial sites. in which a ligand is bound, the qualifiers µ x and η are
When an M 3 C 2 unit constitutes the five vertices of a used. When a ligand is bound to more than one metal atom
cluster with seven skeletal bond pairs a nido structure the symbol µ x identifies the number of metal atoms x to
should be observed. This situation is observed in Os 3 which the ligand is bound. For example, a ligand which
(CO) 10 C 2 Ph 2 . The Os 3 C 2 core forms a squared-based bridges the edge of a polyhedron is interacting with two
pyramid which has the high coordinate apex occupied by metal atoms and is designated as µ 2 (a nonsubscripted µ
an Os atom, again demonstrating the aversion carbon has implies µ 2 ). A ligand that caps the triangular face of a
x
to high-coordinate vertices. polyhedron would be designed as µ 3 . The symbol η is
Alkynes in metal cluster compounds are perhaps more used to indicate how many atoms of a ligand are coordi-
commonly considered to be a four-electron-donating ent- nated to the metal center. In [Fe 4 -(CO) 13 H] , for exam-
−
ityratherthantwoseparatethree-electrondonors.Thisdif- ple, one CO interacts with all four Fe atoms using both
ference being due to the fact that the PSEP theory is based C and O (Fig. 20). The formula is more explicitly ex-
2
onadelocalizedbondingmodelwhereasthefour-electron- pressed as [Fe 4 (µ 4 ,η -CO)(CO) 12 (µ 2 -H)] . The (µ 2 -H)
−
donating models are based upon two-electron–two-center indicates that the hydride bridges two Fe atoms. When the
5
bonding schemes. In the four-electron-donating models cyclopentadienyl ligand is designated as (η -C 5 H 5 ) this
the four π electrons in C C are reassigned to M C σ indicates that all five carbon atoms of the aromatic ring
and π bonds, one of each type originating from each car- are interacting with the metal center.
bon. A single C C remains keeping the “alkyne” unit Speciesthataresuitedparticularlywellforclusterbond-
intact. ing are unsaturated molecules or molecular fragments. By
virtue of their unsaturation these ligands have an abun-
dance of electron density which can be made available to
D. Ligands
the cluster. In addition to their electron-donating ability
The discussion of ligands in metal cluster compounds has many unsaturated ligands impart further stabilization by
so far been quite limited. It is the goal of this section to acceptingelectrondensitybackfromthemetalcenter.This
elucidate some of the specifics of metal–ligand bonding in synergistic or cooperative bonding interaction is known as
cluster compounds. Ligands serve many important func- backbonding. Alkenes, alkynes, nitrosyl, and carbonyl lig-
tions in cluster compounds, and their importance must not ands are among those ligands capable of participating in
be overlooked. First, it is the ligand field which is respon- this type of bonding. We will use the metal–carbonyl bond
sible for stabilizing a bare cluster or particle. As demon- to outline the principles of this bonding.
strated earlier, metal clusters require an electron count Carbon monoxide can bind to a metal center in a variety
which is greater than the metals can themselves provide. of ways. When only the carbon atom of CO is interacting
The ligands, by supplying these electrons, stabilize the with a metal center it is acting as a two-electron donor re-
cluster. Second, the variation and manipulation of ligands gardless of the number of metals to which it is bound. That
provide an interesting chemistry through which we can is, terminal doubly bridging (µ 2 ) and triply bridging (µ 3 )
learn more about metal clusters. These reactions also fuel carbonyls all donate two electrons. The donation occurs