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

Metal Particles and Cluster Compounds 547

A distortion from pure cotahedral symmetry creates two sites on a metal surface provides another type of cooper-
types of Pt Pt bonds. Note must be taken of these long ation. The bond activation of surface-adsorbed species is
˚
Pt Pt “bond” distances which average 3.32 and 3.40 A. facilitated by the close proximity of active sites. Metal
This suggests very weak Pt Pt interaction meaning the clusters have, to a lesser extent, this cooperative influence
bridging chloride ligands contribute significantly to the on cluster ligands. The high selectivity and low activity
structural integrity of this cluster. associated with mononuclear complexes is due to the pres-
The lack of late transition-metal halide clusters and the ence of only one active site, whereas surfaces (and clus-
poor bonding which results when they are formed can ters) provide a multitude of these active sites. The active
be easily rationalized. In binary metal halide clusters the sites on a metal surface are often considered to be located
metal atoms are in a high oxidation state. The valence at surface defects. By definition, a surface atom has a low
orbital contraction in these high oxidation state metals is coordination number. The atoms defining the boundary of
much greater for the later transition metals than it is for a surface defect will have an even lower coordination num-
the early ones. The diminished radical extension of the ber. The coordination number of the metal atoms in a small
valence shell inhibits metal–metal bonding. metal cluster is generally low. In this aspect small metal
clusters are more like surface defect sites. Large clusters,
however, may serve as models for the extended surfaces
E. Cluster Compound Reaction Chemistry 2−
of bulk metal. [Rh 13 (CO) 24 -H 3 ] , for example, contains
In this last section we will discuss some of the reaction an Rh 13 core with a hexagonal close-packed arrangement
4−
chemistry which has been observed with cluster com- while [Rh 14 (CO) 25 ] exhibits a body-centered ccp of Rh
pounds. Volumes could be written on this aspect of metal atoms.
clusters alone. We will simply discuss the cluster–surface Os 3 (CO) 12 is a molecule in which three active sites have
analogy as well as demonstrate the variety of reactions in been brought together. The effect of this on the selectiv-
which cluster compounds may participate. In doing so the ity of its reactions is clearly seen. Consider the reaction of
complexity and difficulties of cluster chemistry will surely Os 3 (CO) 12 with triphenylophosphine where the three sites
become apparent. in this one molecule promote intramolecular reactions.
Because the nuclearity and geometry of clusters are Three of the nine reaction products are the simple sub-
so dependent on electron count, gross structural changes stitution products Os 3 (CO) 12−x (PPh 3 ) x (x = 1, 2, 3). The
often occur during the course of a reaction. remaining six oxidative addition products have an assort-
The susceptibility of metal clusters to change their nu- ment of ligands which are quite varied, a few are noted
clearity is shown in the following reactions: below.
HOs 3 (CO) 9 (PPh) 3 )(PPh 2 C 6 H 4 ) (Fig. 40) is produced
100 C
◦
Co 4 (CO) 12 −−→Co 6 (CO) 16 when an ortho positioned C H bond in the disubsti-
◦
100 C tuted precursor, Os 3 (CO) 10 (PPh 3 ) 2 , oxidatively adds to
Ru 4 (CO) 12 −−→Ru 6 (CO) 16
a core osmium atom. This intramolecular reaction uti-
180 C
◦
Ru 3 (CO) 12 −−→Ru 6 (µ-C)(CO) 16 lizes two adjacent Os sites and creates a five-member
Os–P–C–C–Os ring. The hydride bridges these same two
210 C
◦
Os atoms.
Os 3 (CO) 12 −−→Os 6 (CO) 18 + Os 8 (CO) 23
+ Os 7 (CO) 21 + Os 5 (CO) 16
250 C
◦
Os 3 (CO) 12 + Os 5 (CO) 15 C −−→
◦
(products of the 210 C pyrolysis) + Os 8 (CO) 21
Two extremes of metal-mediated reactions are those
that occur on bulk metal sufaces versus those that occur
in mononuclear metal complexes. Each extreme has its
advantages as well as disadvantages. Consider, for exam-
ple, the properties of activity and selectivity, two proper-
ties that are usually inversely related. The metal surface
extreme possesses high activity at the expense of poor se-
lectivity. The high activity on a surface can be attributed
to the cooperative nature of the extended array of metal
FIGURE 40 Structure of HOs 3 (CO) 9 (PPh 3 )(PPh 2 C 6 H 4 ). The
atoms. An electronic cooperation occurs through the band five-member chelate ring formed from the oxidative addition of
structure present in bulk metals. The high density of active CH to the Os 3 core is shown.