Interstitial atoms in clusters

Clusters with interstitial atoms are an interesting class of polynuclear complexes. Several examples of compounds with interstitial atoms (H, C, Ge, N, P, S, etc.) have been reported. These compounds exhibit special behavior because the presence of main-group atoms in the cluster cage influences the chemical reactivity and stability of the system and its geometric structure. In the following section we consider some of these peculiarities, related both to the stability and to the electronic properties of clusters with interstitial elements. 

two trends can be clearly identified by comparing the B-, C-, and N-centered Au clusters.  

these two trends work in opposite directions - their combined effect results in maximum stability for the C-centered Aug cluster, the only octahedral structure which has been synthesized so far. The stability differences among the three structures are, however, not so large that the formation of the B- and N-centered compounds is excluded. 

It is interesting to note that cluster stability is reduced if Au is substituted by Cu, forming the hypothetical isoelectronic cluster compound [(R3PCu)6C] +. Most 

This result emphasizes the role of interstitial atoms in determining the electronic structure and the properties of these materials. The NigCe core of the cluster can be regarded as a microscopic fragment of a nickel carbide. The interstitial main group atoms form strong bonds with the surroundings these bonds may be viewed as initial step in the formation of a different phase (carbides, nitrides, etc.) within the cluster complex. 

5 Interstitial atoms in clusters. As the size of clusters increases (and also that of their central cavity) the insertion of atoms becomes easier and easier. In particular for 12-atom clusters having a cubo-octahedral structure, the insertion of an atom having the same radius as that of the peripheral atoms is possible. Notice that this arrangement can be compared with those of the metallic cubic and hexagonal, close-packed structures. 

Typical examples are carbon interstitial carbonyl clusters such as the octahedral Co6C(CO) -2, the trigonal prismatic Co6C(CO)i52- and its isoelectronic (mononegative) nitrogen interstitial Co6N(CO)15 or the icosahedral clusterNi12Ge(CO)222 with the interstitial atom (Ge or Sn) in the centre of aNi icosahedron. 

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Cluster unit with an interstitial atom in compounds such as ZrgCCl14 and ThgFeBr15... 

Of course, valence electron concentration is not only related to the metal atoms but also to the number and valence of the ligands. Ligand deficiency creates vacant coordination sites at metal atoms and results in cluster condensation, which is the fusion of clusters via short M-M contacts into larger units ranging from zero- to three-dimensional. The chemistry of metal-rich halides of rare earth metals comprises both principles, incorporation of interstitial atoms and cluster condensation, with a vast number of examples [22, 23]. 

To determine the existence of stable steady state, a model [109] was studied of the destmction of clusters in the case vp = 700. At the initial instant of time 10 uniformly distributed clusters of 300 vacancies each, were put into the crystal , and interstitial atoms in the intervals between them (Uq 10). Then pairs of randomly distributed defects of different types were created in the crystal . The newly generated defects break up the orginally existing clusters and the concentration of defects declines to a steady-state value. The values of Uo were obtained by averaging a region of the curve of length 2.5 x 104 events of defect creation. The result unambiguously implies the existence of stable steady state in the problem of accumulation of point defects and in the problem of breakup of clusters. 

There is much interest in transition-metal carbonyl clusters containing interstitial (or semi-interstitial) atoms in view of the fact that insertion of the encapsulated atom inside the metallic cage increases the number of valence electrons but leaves the molecular geometry essentially unperturbed. The clusters are generally anionic, and the most common interstitial heteroatoms are carbon, nitrogen, and phosphorus. Some representative examples are displayed in Fig. 19.4.3. 

A. Sironi, /. Chem. Soc., Dalton Trans., 173 (1993). Influence of Interstitial Atoms in Transition-Metal Carbonyl Clusters. 

The solid-state clusters are distinguished from liquid ones by the rigidity of their structure. As a defining criterion for rigidity we shall choose the stability of point defects, i.e., vacancies and interstitial atoms, in the body of the cluster. 

Many metal carbonyl clusters have interstitial atoms or groups located in the eenter of the polyhedron. Such interstitial atoms may be a light atom sueh as boron, carbon, or nitrogen a post-transition element such as germanium, tin, or antimony or a transition metal. Interstitial atoms most frequently provide all of their valence electrons as skeletal electrons since all of their valence orbitals are neeessarily internal orbitals because of the location of the interstitial atom in the center of the polyhedron. Exceptions to this rule may occur when some of the valence electrons of the interstitial atom occupy orbitals of symmetries which cannot mix with any of the molecular orbitals arising from the polyhedral skeletal bonding. 

The Role of Interstitial Atoms in Transition Metal Carbonyl Clusters... 

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