Interstitial clusters evidence

This structure type evidently requires the simultaneous existence of two differently sized halide types, and thus exists only in mixed hahde systems. Recent investigations revealed this cubic stracture also for cluster phases with other interstitial atoms, i.e., the cation-free Si-centered [(Zr6Si)Cli2-xl2+x] [28]. 

In the penta-, hexa-, and heptanuclear carbonyl hydride clusters, terminal hydrides are observed only in the less crowded species, and again no evidence for the presence of interstitial hydride is found. 

In this subsection we examine the mechanism of the very fast diffusion. In the bulk medium the vacancies and interstitial site play a primary role in accelerating the diffusion. However, these diffusion mechanisms are not relevant in microclusters. It is well known that the vacancies created inside the cluster are immediately pushed to the surface. Indeed in our simulation the creation of vacancies inside the cluster is a very rare event even at the temperature close to the melting temperature. Moreover, we cannot find any evidence that the interstitial deformation takes place inside the cluster, and therefore neither of them is responsible for the rapid diffusion into the cluster. The key feature of the cluster that distinguishes the cluster from the bulk medium is that it is surrounded by the surface beyond which no atoms exist. In other words, the outside of the cluster is occupied by vacancies. As a result, the atoms on the surface move very actively along the surface. Such an active movement along the surface will be responsible for the rapid diffusion in the radial direction of the cluster. We focus our attention to the details of the active diffusive motion along the surface of the cluster, and we present a direct evidence that the surface activity controls the radial diffusion. A direct measure of the surface motion is the diffusion rate of the surface atoms... 

An interstitial H atom located in a tetrahedral environment was first reported in 1982, in the tetracapped octahedral cluster [HOsioC(CO)24] . Evidence came from X-ray studies which showed complete coverage of the metal core by carbonyl ligands very similar to that of [OsioC(CO)24] from which it was prepared. It was therefore assumed that the hydride was in an interstitial site, and because the octahedral cavity was already occupied by a carbido-atom it seemed reasonable to assume that the hydride was sited within a tetrahedral cap. The Os satellite pattern associated with the hydride signal in the H NMR spectrum was entirely as would be expected for a tetrahedrally coordinated hydride, which helped to confirm this proposal. 

It has been mentioned in the introduction that the condensed cluster halides of the rare earth metals based on the MgXi2-type cluster with an interstitial atom (or molecular unit) generally exhibit a defect rocksalt structure. Figure 10 provides clear evidence for this remark. The NaCl subcell in the structure of GdijInCg, marked by strong streaks is only weakly distorted (a = 6.07, b = 6.10, c = 5.92 A, a = y = 90°, p = 91°) by the ordering of I atoms and Cj units and occupation of all voids around the C2 units by Gd atoms. 

Much of the interest in clusters of this type arises from their structural similarity to the fundamental structural element of the superconducting Chevrel phases M MogEg (M = Pb, Sn, Cu, etc.) and more recently discovered rhenium halco halide phases. Although these solid-state compounds are known only for Mo and Re, molecular MgEgLg clusters of Zr, V, Cr, Mo, W, Re, Fe, and Co have been reported (Table 2). The lone examples of octahedral zirconium- and vanadium-chalcogenide clusters are unique in that they also possess interstitial O and S atoms, respectively. Evidence for interstitial FI atoms in the clusters [Cr6Eg(PEt3)g] (E = S, Se) has been presented. ... 

MoFe-Protein contains a second cluster, the M-cluster. [33] This is covalently bonded to cysteine and histidine of the a-suhunit. A homocitrate anion R)-2-Hydroxyhutane-l,2,4-tricarhoxylic acid) is complexed as a bidentate ligand with molybdenum (Fig. 4.5). On the basis of better resolved X-ray structural analysis and quantum mechanical calculations, initially it was thought, a nitrogen atom is located in the centre of the Fe/Mo complex, [34] but more recent results gave evidence for an interstitial carbido ligand [35,36], which is unprecedent in bioinorganic chemistry. 

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