Condensed Clusters with Interstitials

Interstitial atoms frequently occur in discrete dusters but even more so in structures that can be described in terms of condensed clusters. As discussed in Section 5.3 for discrete dusters, there are compounds that allow the reversible insertion of interstitial atoms under equilibrium conditions as well as many compounds that are only stable with the inclusion of interstitial atoms. 

The monohalides of zirconium, ZrQ and ZrBr, are examples of condensed cluster systems which can exist both with and without interstitials. Their structures [150] contain characteristic X-Zr-Zr-X layer units composed of single closepacked X and Zr layers. The units can be discussed in terms of edge condensed Zr octahedra which are surrounded by X atoms above all free faces as in the [M6Xg] cluster. 

An important borderline is reached with the last example. Whatever the actual distribution of valence electrons in 1 2820 may be, the formal ionic limit of [(Tk )2(S )2C (e )2] leaves electrons for (extended) M-M bonding. In contrast, all electrons are used for strong heteroatomic bonds in Gd2Br2C ( [(Gd )2-(Br )2C ]). Here, the stabilization of the clusters by the interstitial atoms has finally led to a destruction of the cluster, at least if M-M bonding is a requirement for using the term cluster . Gd2Br2C is a normal and simple ionic compound just as the isostructural and isoelectronic La2028. [164] 

Gd2Br2C is just one example from the broad condensed cluster chemistry of the rare earth metals which has come to light in recent years. From the above, it is obvious that this chemistry lies on the borderline of M-M bonded cluster compounds and salts. The clusters can exist as any combination of empty or filled and discrete or condensed. Bonding within these clusters ranges from M-M bonded species that may be stabilized by additional strong heteropolar bonding 

5 Discrete and Condensed DansitUm Metal Clusters in Solids 

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Geometrically, the main group element tends to retain a tetrahedral nearest neighbor environment, whereas the transition metal element tends to retain an octahedral environment. As a consequence, transition metal clusters with more than six metal atoms have a tendency toward ligand loss, leading to the formation of condensed clusters (multiple interstitial metal atoms). This leads eventually to close-packed structures that mimic bulk metal structures (see Polynuclear Organometallic Cluster Complexes). On the... 

Figure 5-48. Schematized bonding in systems with discrete and condensed clusters (i) empty metal clusters in an environment of nonmetal atoms, (ii) clusters with interstitial atoms in an environment of nonmetal atoms, and (iii) bare clusters with interstitial atoms (see text).

In the structure of Gd jIivCg, as in other structures with interstitial C2 units, the latter are directed towards those corners of the octahedra which are not involved in the condensation, with each octahedron elongated parallel to this direction and contracted in the equatorial plane. To prevent very short I-I contacts this special orientation of the Cj groups needs a folded chain of clusters which fits very well to the close-packed iodine matrix. In fact, the iodine matrix is only slightly distorted from an ideal ccp structure of the anions. 

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. 

The previous structures, which contain distinct R octahedral clusters, necessarily have R/Z > 6. When R/Z > 4, we can find a finite number of these clusters condensed to give larger molecular units. For rare earth halide systems, only edge-sharing octahedra have been observed, as delineated in sect. 2, in which only two octahedra are involved and the interstitial moiety is a C2 dimer. Satpathy and Andersen (1985) applied the LMTO-ASA method to produce the energy DOS illustrated in fig. 43. Their model examined the interaction between an isolated GdioCljs cluster with two... 

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]. 

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