Metal cluster rearrangement

The papers reviewed here can be divided into two broad categories (i) those that are fairly non-specific in their choice of data, studying basically all complexes with a common stoichiometry ML , where M = metal, L = a coordinated ligand atom, and rt = 3, 4, 5,..., 12, and (ii) those that set out to examine specific reaction paths - such as metal cluster rearrangements, geared rotations, ring whizzing, etc. - and which are fairly selective in data retrieval. We shall discuss the papers in the above order. 

Discriminating Between Reaction Mechanisms Metal Cluster Rearrangements... 

Complexes involving larger metal clusters, eg, Au or Au2Rh, also undergo piezochromic rearrangements with rather dramatic changes in the absorption spectmm, and well-defined changes in molecular stmcture (6,7). 

In example (ii), a metal-ligand bond is formed at the expense of a single metal-metal bond leading to cluster rearrangement or fragmentation,... 

Rearrangements of clusters, i.e. changes of cluster shape and increase and decrease of the number of cluster metal atoms, have already been mentioned with pyrolysis reactions and heterometallic cluster synthesis in chapter 2.4. Furthermore, cluster rearrangements can occur under conditions which are similar to those used to form simple clusters, e.g. simple redox reactions interconvert four to fifteen atom rhodium clusters (12,14, 280). Hard-base-induced disproportionation reactions lead to many atom clusters of rhenium (17), ruthenium and osmium (233), iron (108), rhodium (22, 88, 277), and iridium (28). And the interaction of metal carbonyl anions and clusters produces bigger clusters of iron (102, 367), ruthenium, and osmium (249). 

Some striking demonstrations of metal-metal bond lability are provided by cluster rearrangements due to protonation. This is the case for some anionic osmium clusters (cf. Section VI). It involves ligand activation for some tetrairon clusters (51-53). Thus, the clusters 9 and 11 open up upon protonation, and compensation for the lost iron - iron bonds in the products 10 and 12 comes from the bonding between one iron atom and a carbonyl oxygen. The relation of these unusual nucleophile-electrophile interactions to cluster-induced CO transformations is obvious. 

NMR all three osmium atoms are chemically equivalent (200). This may involve changing the bridging sulfur atom from a four-electron donor to a two-electron donor in order to gain a third Os-Os bond and an equilateral metal triangle. These ligand induced cluster rearrangements are a further demonstration of metal-metal bond weakness as the reason for cluster mobility. For application-oriented purposes they may be the most important ones. 

A number of mixed-metal clusters which undergo metal core rearrangements have been reported since 1986, and these species are listed in Table XVII. The effect of the monodentate phosphine or phosphite ligands... 

The analogous cluster which contains Ph2AsCH2AsPh2 probably also undergoes an intramolecular metal core rearrangement in solution, but it is not... 

However, only one hydrido ligand resonance is observed in the 1H NMR spectrum of each cluster at temperatures low enough to prevent the metal framework rearrangement process from operating, although for the... 

The data from such EXAFS studies show that in all cases the metal clusters remained intact and are catalytically active. However, as shown, for example, in Figure 41, the clusters did remain bonded to the oxide support, but underwent slight rearrangements to accommodate the reaction intermediates. For example, it was shown that as the concentration of adsorbed reaction intermediates (e.g., n-bonded alkenes, alkyl species) on the clusters increased, the cluster frames expanded, and the clusters flexed away from the support. 

B. F. G. Johnson, and A. Rodgers, Polyhedral Rearrangements and Fragmentation Reactions in Cluster Complexes, in The Chemistry of Metal Cluster Complexes (Eds. D.F. Shriver, H. D. Kaesz, and R. D. Adams, VCH Publishers, New York, 1990, Chap. 6). 

Molecular Dynamics of H FeR CCO)-] and Related Mixed-Metal Clusters. Metal clusters have been shown to undergo a wide variety of fluxional processes in which carbonyls, hydrides, and even the metals themselves undergo rearrangement (25). Mixed-metal clusters are ideally suited for studies of fluxional processes because of the low symmetry which is inherent within their metal framework. In such clusters, the majority of... 

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