Rearrangements clusters

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

Tetrahedrane (CR)4 with bulky tert. butyl substituents R has been synthesized by G. Maier and coworkers in 1978 (28). In their already classic investigations, an interesting valence isomerization has been observed on melting, the C4 cluster rearranges to the thermodynamically more stable cyclobutadiene derivative, which on photochemical energy uptake forms again the tetrahedrane (28) ... 

Clearly, the spectroscopic properties of the P clusters in the proteins do not reveal their structural nature. However, extrusion of these clusters from the protein leads to the clear identification of 3-4 Fe S clusters(13.291. Despite the uncertainties inherent in the extrusion procedure (due to possible cluster rearrangement) the extrusion result supports the Dominant Hypothesis, which designates the P centers as Fe S units, albeit highly unusual ones. The P clusters are thought to be involved in electron transfer and storage presumably providing a reservoir of low potential electrons to be used by the M center (FeMo-co) in substrate reduction. 

So far, all efforts to generate, isolate and characterize heterofuUerenes via Kratsch-mer-Huffman vaporization of graphite in the presence of hetero-element-containing compounds such as boron nitride (BN) or cyanogen (CN)2 have failed. An alternative route for the direct formation of heterofuUerenes is cluster rearrangement within exohedral fullerene derivatives such as iminofullerenes and azafuUeroids. The first hints of success by this approach were obtained from mass spectrometry investigations of the cis-l-diazabishomo[60]fullerene 3 [12], the n-butylamine adduct 4 [12] the 1,2-epiminofullerene 5 [11] and the cluster opened ketolactam 6 [2]. 

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). 

The substitution reactions can be accompanied by subsequent reactions. Thus, Ru3(C0)i2 reacts with azobenzene (61) or fluorinated azobenzenes (60) to yield products like [47], and the pyrolysis of Ru3(CO)9L3 complexes leads to reactions similar to those discussed in Chapter 3.4. for the corresponding osmium clusters. Rearrangements and orthometalations were observed (65, 66), and one cluster formulated as [42] was isolated (65). 

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. 

Thus, the general curve of E vs qH, as schematically represented in Figure 5-14, can be constructed without recourse to more complicated cluster rearrangements. This suggests that the tunneling motion is simple and does not involve solvent reorganization or fluctuations. 

Reaction of diynes and Ru4(CO)io(PPh) have been examined.210 In the first stages of this reaction only one triple bond is involved (244). Upon thermolysis, a cluster rearrangement occurs to give a pentagonal bipyrami-dal structure (245), whereas prolonged reaction causes involvement of the second triple bond (246). The compound contains a square pyramidal Ru4P core with the C4 fragment bonded in a ju4-tjW, 3, 3 fashion to the square face. 

Referring to Scheme 6 and starting with the Cia enantiomer, if the Fe moves away from Os towards OS2, it generates the C b enantiomer. Movement of Fe away from Ru in either of the enantiomers and toward both Os atoms gives the Cs isomer. Each time the cluster rearranges, the carbonyls execute the cyclic process about a different Fe-M-M face and hence involve different carbonyl ligands in that process. 

The remainder of this article is largely concerned with describing how some of the above observations can be rationalized using Stone s Tensor Surface Harmonic theory, and with the further imphcations of this model for dynamical processes such as cluster rearrangements. The number of example systems and electron count rationalizations will be kept relatively small in favor of explaining the theoretical foundations that underhe the method. Tables of examples and more detailed analyses of the various cases may be found elsewhere. ... 

C. Dellago, P.G. Bolhuis, and D. Chandler (1998) Efficient Transition Path Sampling Applications to Leimard-Jones cluster rearrangements. J. Chem. Phys., 108, pp. 9236-9245... 

An important feature of these clusters is that their structures are independent of the electron count of the surface polyhedron. Since radial metal-metal bonding predominates, there is a soft potential energy surface for cluster rearrangement. This has been experimentally observed for gold clusters129. Theoretical studies of bulk metals have indicated that alternative close packed (and related) structures are also separated by small energy differences130. ... 

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... 

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