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Extending the Metal ClusterЦMetal Surface Analogy.

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Extending the Metal Cluster-Metal Surface Analogy
By Bruce C. Gates*
By the early 1970s, the chemistry of metal clusters+ompounds with metal-metal bonds stabilized by ligands such as
carbonyls-was emerging from its infancy, as described in
the seminal work of Chini."] In 1975 Muetterties['] proposed analogies between metal clusters and metal surfaces
and speculated that molecular metal clusters would be found
to be catalysts with novel properties. A central argument was
that clusters have reactivities different from those of complexes with a single metal atom because they have neighboring metal centers, and the unique metal-ligand bonding patterns in clusters could be expected to facilitate reactions like
those occurring on metal surfaces.
In 1975, the western world had just undergone one of its
periodic oil supply crises, and a result of this was a surge in
interest in catalysis related to production of synthetic fuels;
research on the chemistry of catalytic hydrogenation of CO to
give hydrocarbons (Fischer-Tropsch chemistry) went through
one of its periodic spikes of activity. Simultaneously, metal
cluster chemistry became a "hot" topic, with much of the
research motivated by the prospect of finding novel catalysts
for CO hydrogenation, among other reactions.
A few years later, Muetterties et al.[31published a detailed,
quantitative assessment of the analogies in structure and
bonding between metal clusters and metal surfaces, but reactivity and catalysis were largely missing from the assessment.
A consensus emerged, articulated by ErtIJ4I that the clustersurface analogy, although imperfect,['I includes structure
and bonding but does not extend to reactivity and catalysis.
Surface scientists, building from this analogy, have now
come to rely primarily on comparisons of vibrational spectra
of adsorbed species with those of molecular metal clusters
characterized by crystallography as the basis for determining
structures of adsorbates on metal surfaces.16]Thus the merging of organometallic chemistry and surface chemistry is well
The anticipated evidence of catalysis by metal clusters,
however, has not emerged,"] and the few examples of cluster
catalysis are neither very well understood nor of practical
importance; the prospects are limited by the lack of stability
of most metal clusters. One consequence of the lack of practical successes has been a marked decrease in the attention
paid to research on metal clusters, although progress had
continued to be rapid."'
Now a pair of recent papers['0.
illustrates that the analogy between metal carbonyl clusters and metal surfaces with
adsorbed CO ligands extends beyond structure and bonding:
The reactivity of a CO molecule bridging metal atoms in a
metal cluster reported by Chisholm et aI.["' is analogous to
that of a C O molecule bridging metal atoms on a metal
surface, reported by Yates et aI.[lo1The bridging C O dissociates into carbido and oxido ligands (Scheme 1). This step is
[*] Prof. Dr. B. C . Gates
Department of Chemical Engineering
University of California
Davis. CA 95616 (USA)
Scheme 1. Dissociation of a bridging CO molecule
important in the Fischer-Tropsch reaction catalyzed by
metal surfaces.
Both the CO bound to alkoxidotetratungsten [W,(OR),,]
clusters['01 and the CO adsorbed on a sparsely covered
Mo(l10) surface react with dissociation to give carbido and
0x0 ligands bonded to neighboring metal centers. The evidence for this reactivity in molecular metal clusters is based
on N M R spectra and crystallographic data for a family of
clusters, including [W,(p,-C)(O)(OiPr), J, the skeleton of
which is shown in Figure
The carbido ligand is bonded
to four tungsten atoms in a butterfly arrangement; the 0x0
ligand (06) is bonded to two tungsten atoms. The species on
the Mo(lI0) surface were characterized by vibrational (electron energy loss) spectroscopy, and indications of reactivities
of variously bonded CO species were obtained by temperature-programmed desorption (thermal desorption) with
analysis of the desorbed products by mass spectrometry." ' I
Fig, 1. The W,(C)(O),, core of the [W,(pc,-C)(0)(OiPr),,] cluster. O(6) is the
ligand (from ref. [lo]).
The analogy between the reactivity of CO in the cluster
and that of CO on the surface is thus clearly evident, confirming what had been supposed by Muetterties.[I2] However, there are differences between the reactivity in the cluster and on the surface. Chisholm et al."ol identified a cluster
with a bridging ($,p,-CO) group. This structural feature is
most pertinent to the cleavage of a CO ligand; however, they
found no direct evidence of the reaction of CO with the
coordinatively unsaturated tungsten clusters to give
[W,(OR),,(CO)] intermediates, and they could only speculate that the C O may initially bind to one of the W atoms of
the cluster framework. The results imply that the bonding of
C O to the cluster must be the rate-determining step in the CO
dissociation. As the temperature was raised, indications of
carbido ligands in the cluster appeared at 243 K.
In contrast, Yates et al." ' I observed variously bonded CO
ligands on M o metal at fractional surface coverages of less
than 1. Some ligands were conventionally bound, and one of
the CO ligands, characterized by a vco band at 1130 cm-',
was inferred (on the basis of a comparison with the IR spectra of carbonylmetal clusters) to bridge two Mo atoms:
Mo-C--0-Mo. This is a stable intermediate in the dissociation reaction on the surface. At a surface coverage by CO of
about 0.23, the only observed CO species was inferred to be
inclined (vco = 1345 cm- I),with the C atom bonded to a Mo
atom and the 0 atom interacting with a neighboring M o
atom but not bonded to it. This CO was converted to the
bridging species (vco = 1130 cm- '), and a t least some of the
dissociation of adsorbed CO proceeded through this bridged
intermediate. When the surface coverage was 0.17, the adsorbed CO was converted into C and 0 at about 200 to
250 K ; the surface reaction was rate determining.
In summary, the data reveal differences in reactivity of the
cluster and surface, characterized by different rate-determining steps and different rates of reaction-although these are
not determined quantitatively. It is knownt4. that the rate
of a reaction induced by a molecular cluster is usually less
than that induced by a sparsely covered surface of the same
metal. The initial step of bonding of a ligand to a cluster
requires coordinative unsaturation of the cluster. Since most
stable metal clusters are coordinatively saturated, reaction
steps such as dissociation of a ligand or reaction of a pair of
bound ligands with each other to free a coordination site are
prerequisites. In contrast, relatively clean surfaces present
arrays of rigid, stable coordinatively unsaturated sites where
ligands can be readily bonded and activated.
Thus, although the reaction path of a ligand bonded to a
molecular metal cluster and that of a ligand on a metal
surface may sometimes be similar, the catalytic activity of a
molecular metal cluster is usually expected to be less than
that of a sparsely covered surface with a comparable arrangement of metal atoms. However, metal surfaces are usu-
Anfieti. ('hcm. 1171. Ed. Engl. 1993. 32. "0. 2
ally largely covered with adsorbates during catalysis, and
one might still be justifiably tempted to seek similarities between the catalytic properties of such surfaces and those of
metal clusters. If metal cluster-metal surface analogies in
catalysis are to be found, they are expected to be closest
when the metals are dispersed in the form of small particles
("clusters") on solid supports, as was recognized early by
Basset and U ~ O . [ 'This
~ ] is precisely the form of many industrial metal catalysts. Relatively robust carbonylmetal clusters such as [H,Os,(CO),,]- have been observed on surface
suring catalysis of CO hydr~genation,['~l
but their role in
catalysis remains to be assessed. Furthermore, some industrial supported metal catalysts have such small clusters (typically, five or six Pt atoms in clusters supported in L zeolite" used to convert n-hexane into benzene) that they may
be regarded as virtually molecular in character.
German version: Angcir. Chem. 1993, 105, 240
[I] P. Chini, Inorg. Chim. Acro Rer. 1968,2, 31 : Pure Appl. Chrm. 1970. 23.
489; P. Chini. G . Longoni, V. G . Albano. Adv. Orgonomet. Chem. 1976,
14, 285; P. Chini, Guzz. Chim. Itul. 1979, 109, 225.
[2] E. L. Muetterties, Bull. Soc. Chim. Belg. 1975, 84, 959.
[3] E. L. Muetterties, T. N . Rhodin. E. Band. C. Brucker. H. Pretzer. Chem.
Res. 1979. 79, 91
[4] G . Ertl in M e t u l Clusters in C u t u l ~ s i(Eds.:
B. C Gates. L. Guczi, H.
Knozinger), Elsevier, Amsterdam, 1986. p. 577.
[j] M. Moskovits, Arc. Chem. Rrs. 1979, 11. 229.
161 G . A. Somorjai in Perspectii~e.sin Curu/wi.s (Eds.. J. M . Thomas. K. 1.
Zamaraev). Blackwell, London. 1992. p. 147.
171 M. R. Albert. 3. T. Yates, Jr., A SurJuce Srienzisr',s Guide to Orpanomerullir.
ChernDtrj, American Chemical Society. Washington, DC. 1987.
[8] W. L. Gladfelter. K. J. Rosselet in [9], p. 329.
[9] Tile Chenusti:r of Metal Cluster Cornple-yes (Eds.: D. F. Shriver. H . D.
Kaesz, R. D. Adams), VCH, Weinheim. 1990.
[lo] M. H. Chisholm, C. E. Hammond, V. J. Johnston, W. E. Streib. J. C. Huffman, .f Am. Chem. SOc. 1992. 114. 7056.
[ l l ] M. L. Colaianni. J. G. Chen. W. H. Weinberg, J. T. Yates. Jr.. J Am. Chem.
Soc. 1992. 114. 3735
[12] E . L. Muetterties. Science (Washington, DC) 1977, 196. 839.
1131 JLM. Basset, R. Ugo in Aspects qf Homogeneous Curnlwis, I4l. / I / (Ed.
R. Ugo), Reidel, Dordrecht, 1977, p. 137.
[14] H. H. Lamb, B. C. Gates, J. Am. Chem. Suc. 1986, 108, 81; H. H. Lamb,
T. R. Krause. B. C. Gates, Proc. lnt. Congr. Cutal. 9th 1988, 3, 1378.
[15] M. Vaarkamp, J. V. Grondelle. J. T. Miller, D. J. Sajkowski, F. S. Modica,
G . S . Lane. B. C. G a t e s D. C. Koningsberger, C u r d Leu. 1990, 6 , 369.
8.: VCH Verlug.s~e.sell.schuftm h H , W-6940 Wemhc+n. 1993
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