1 Molecular Clusters

General Introduction

F. Albert Cotton

When I was asked to write a general introduction to this book, at first I was hesi- tant. It was not my natural modesty, but a sense of the vastness of the subject that made me pause. When I introduced the term “metal atom cluster” in 1964”’ to designate “a finite group of metal atoms that are held together mainly, or at least to a significant extent, by bonds directly between the metal atoms, even though some nonmetal atoms may also be intimately associated with the clusterLz1,” the number of compounds that were known to satisfy this definition was small. I would estimate that there were no more than a dozen well-characterized examples of such com- pounds and perhaps fewer. Today I would hesitate to guess how many thousand cluster compounds there are, for fear of underestimating, and the range of types is now great. Over the past thirty years, growth in the field has been exponential.

Perhaps even more than in other areas of inorganic chemistry, little if any of this growth could have occurred were it not for the enabling technique of X-ray crys- tallography. Unlike Werner complexes or certain other areas of chemistry (much of organic chemistry, for example), there are no simple structural prototypes to which the majority of clusters could be a priori assigned. For much of the field, structural knowledge, which of course is indispensable in understanding the chemistry, has to be obtained on a case-by-case basis by X-ray crystallography.

As an historically interesting illustration of the impossibility of making progress in metal atom cluster chemistry without X-ray crystallographic structural data, I would draw attention to the case of the now largely forgotten chemist Kurt Lind- ner. In the 1920s Lindner and his students at Berlin University carried out many studies on the chemistry of the compounds of Mo”, W”, and Ta”, and in 1927 Lindnerl3I published a summary account of all their work in which he attempted to assign structures that he concocted by fusing together tetrahedra and/or octahedra via p 2 - and p,-halide ligands. These structures, of course, bear no resemblance to reality.

A few more words about the definition of a cluster are appropriate. At the time the term was proposed, there had been occasional use of the word “cage”. This

Metal Clusters in Chemistry Volume 1 :

Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby

Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999

4 I Molecular Clusters

1 2 3

term has to be (and has been) rejected for several good reasons. The word cage suggests the ideas of containment and encapsulating of something, but this is not characteristic, nor even possible, for many clusters. Clearly a triangle of metal atoms, being two dimensional, or even some three-dimensional clusters, such as butterflies, cannot surround anything and small closo-clusters such as tetrahedra do not have sufficient space inside to do so. If one wishes to consider a pair of bonded metal atoms to be a logical part of the cluster family, the term cage would clearly be inappropriate. Of course, there are some metal atom clusters that really do encap- sulate non-metal atoms (H, C, N usually), but this is a special characteristic of some rather than a general characteristic of all clusters.

Another important point concerning the definition of a metal atom cluster com- pound is the requirement that there is significant direct bonding between the metal atoms. In the absence of this there is no justification for using a special word (clus- ter) since the molecule or ion would be simply the kind of polynuclear Werner complex already well known at the turn of the 19th Century. Such a species is little more than the sum of its parts, apart from some weak, chemically insignificant, magnetic interactions. To illustrate, a molecule such as 1 is not a metal atom cluster compound, whereas 2, with its direct M-M bond, is. There are, of course, some ambiguous cases, such as the iron-sulfur clusters, 3. Whether there is significant “bonding directly between the metal atoms” of such clusters, at the Fe-Fe distances concerned, 2.6-2.7 A, is a moot point. Indeed, the word “cluster” is often used more broadly to designate aggregates in which any direct metal-to-metal bonding is unlikely. As so often happens in chemical nomenclature, and indeed in linguistic questions more generally, borderline areas arise, grow, and require elasticity in ter- minology. I cannot help saying that I do not think a compound with only metal to ligand contacts is appropriately called a “metal cluster”. But it happens.

Despite its spectacular present, it should be remembered that metal atom cluster chemistry also has a very early origin. The earliest point to which I have been able to trace it is the period 1857-1861 when a Swedish chemist, Christian Bloomstrand, discovered the dichloride and dibromide of molybdenum. 14] Based on the significant observation that these contained two kinds of halogen, one precipitable by silver ion and the other not, in a 1 : 2 ratio, he concluded that the minimum acceptable molecular formula for these compounds was M03X6, and that this consisted of a

General Introduction 5

4 5

non-dissociating M O ~ X ~ ~ + core and two dissociable X- ions. It was not until many years later that suggestions were made by Werner”’ and WeinlandL6] as to the structures of these “dihalides.” Their suggestions are shown as 4 and 5. Both were based on the assumption that metal atoms could be joined together only by bridging ligands, direct metal-metal bonding being a non-existent concept in that period. The later suggestions of Lindner, which have already been mentioned, also assumed that the compounds are trinuclear, which we now know to be untrue.

Other early discoveries in the field were reported in the period 1907-1913.”~s~91 There were the [ M ~ X I ~ J X ~ . ~ H ~ O compounds, where M = Nb, Ta and X = C1, or Br. Even at this early time Chapin was able to show that “Ta6C114.7H20” in solu- tion dissociated into a TagC112~+ core and two C1- ions. Much later, X-ray work supplied structural details. ‘‘‘9’

One of the most impressive contributions to the early development of metal atom cluster chemistry was the work of C. Brosset,’12’ a Swedish crystallographer who determined the structures of several compounds derived from “MoC12” and showed that they were based on a M06Clg4+ core, consisting of an octahedron of directly bonded Mo atoms (each to four others) with a p3-C1 atom lying above each trian- gular face. It is remarkable that these complex structures were resolved as long ago as the early 1940s. For a number of years these octahedral Mg clusters remained the largest ones known.

The advent of the first metal carbonyl cluster, Fe?(C0)12, was also long ago.“” Here again there was long uncertainty about its structure, resolved only by the work of Dahl in 1966,[141 who was also responsible for clarifying the nature of the analogous Ru.,(C0)12 and O S ~ ( C O ) ~ ~ . [ ~ ~ ] During the same period of time Chini and coworkers[’”] prepared the first four-atom metal carbonyl cluster, Coq(C0)12, whose Rh and Ir analogues were soon to follow, and in 1963 Dahl showed the true formula and structure of the first six-atom cluster compounds of the carbonyl type,

Also in the 1960s the [Re3C112I3- cluster was discovered, and the name cluster was generally adopted. Thus, by about 1970, the stage had been set for the explo- sive development that has occurred over the past quarter of a century. The field is now so broad that an all-inclusive catalogue may no longer be feasible. We can,

Rh6 (co) 1 6.

6 1 Molecular Clusters

however, mention a number of the major classes, as they are represented in the present contribution to the secondary literature.

The two “classical” types of metal atom cluster chemistry that already existed (embryonically) some thirty years ago have both prospered and grown enormously. One of these encompasses clusters with the metal atoms in low to medium oxidation states together with halide, oxygen or chalcogenide ions. In addition to MogX8, Nb6X12, Re3X9 and their derivatives, there are now M03 and W3 based clusters, Chevrel phases, Re6 based clusters, Nb3 and Nb4 based clusters, and others. The other classical type embraces those containing metal atoms in very low (ca. 0) oxi- dation states, in which the ligands are most commonly CO. For this class some useful if not rigorous relationships between structures (closo, nido, arachno) and electron counts have been developed, beginning with “Wade’s rules”.

To this second class a new subdivision has been added, namely, those into which main group elements have been incorporated. Sometimes they are encapsulated (C, N ) or they may be part of the polyhedron, for example in RCCo3(C0)9, where the C and Co atoms together define a tetrahedral cluster. Hydrido clusters and the corresponding anionic clusters have also become very numerous.

Of course, metal atom clusters have become an important part of transition metal organometallic chemistry. Thus, isolobal species such as v6-C6H6Mo and Mo( CO)3 can be substituted for one another.

Mixed metal clusters have also become very numerous and well known. These range from simple cases, where obvious isoelectronic replacements are made, as exemplified by C04( CO) 12 and RhCo3 (CO) 12, to very large and complex species such as [Ni9Pt3(C0)21HI3-. There is no end in sight to the possibilities for more mixed metal clusters.

A recent innovation is the creation of very large clusters that have solid cores of close-packed metal atoms. An example is one containing a Pd33Ni9 core, covered with CO and PR3 molecules. The Pd atoms form a trigonal stack of hexagonally close packed atoms, with the Ni atoms at exterior corners. Another unusually in- teresting example[’71 is a compound containing the anion [A177 {N(SiMe3)2)20l2-. This is not only a very large cluster but is an opening into the realm of main group metals.

Another more recent development is the occurrence of clusters that are integral components of extended solid state structures. The Chevrel phases, already men- tioned, are one example but there are numerous others, of which in some the clus- ters have a hetero atom in the center of the octahedron, e.g., ZrgNC115. Recently, it has been shown that the individual clusters can often be extracted and dissolved in suitable solvents.

A major new sub-field of cluster chemistry is that of nanoparticles of metals in which there are no intentionally appended ligands. These are not molecular and thus neither homodisperse nor crystalline.

Interest in all these newer types of clusters is often driven by their potential in terms of heterogeneous catalysis, and the unusual electrical, magnetic and spectro-

General Introduction 7

scopic properties they may possess. Therefore, the chemistry of metal clusters is a subject of practical as well as scientific interest. It is thus timely that this wide- ranging survey of the field should appear. The editors are to be congratulated for having assembled a group of authoritative reviewers, able to write as experts on the many areas of the subject. These 77 contributions constitute a remarkable encyclo- pedia on a fascinating, active and important subject.

References

[I] F. A. Cotton, Inorg. Chem. 1964, 3, 1217. [2] This definition was later slightly modified to read “. . . . held together entively, mainly or at least

to a significant extent ....” C’ F. A. Cotton, Quurt. Reu. 1966, 20, 389; F. A. Cotton, McGruw-Hill Yeurbook of Science and Technology, 1966.

131 K. Lindner, Zeits. Anorg. Allg. Chem. 1927, 162, 203. 141 C. W. Bloomstrand, J . Prukt. Chem 1857, 71, 449; 1859, 77, 88; 1861, 82, 433. [ 5 ] A. Werner, “Neuere Anschuuungen uuf dem Gebiete der anorganischen Chemie,” Braunschweig,

[6] R. Weinland, “Einjuhrung in der Chemie der Komplexuerbindungen,” 2”d Ed., 1924. [7] M. C. Chabrie, Comptes Rend. 1907, 144, 804. [8] W. A. Chapin, J. Am. Chem. Soc. 1910, 32, 323. [9] H. S. Harned, J. Am. Chem. SOC. 1913, 35, 1078.

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[ 131 J. Dewar and H. 0. Jones, Proc. Roy. Soc. (London), 1905, A76, 558; 1907, A79, 66. 1141 C. H. Wei and L. F. Dahl, J. Am. Chem. Soc. 1966,88, 1821; 1969, 91, 1351. [15] E. R. Corey and L. F. Dahl, J. Am. Chem. Soc. 1961, 83, 2203. [16] P. Chini, V. Albano and S. Martinengo, J. Organomet. Chem. 1969, 16, 471. [I71 E. Ecker, E. Weckert and H. Schnockel, Nature 1997,387, 379.

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