Condensed cluster halides

K.2 Coordination chemistry in the solid state cluster and condensed cluster halides of the early transition metals... 

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. 

As illustrated in the previous section, the metal-rich rare earth metal halides and their interstitial derivatives provide a vast collection of compounds that transcends the structural chemistry of both molecules and extended solids. On the one hand, these substances can be considered as connected or condensed clusters of the MgXi2-or MgXg-type, which may contain interstitial species. On the other hand, many of them can be derived from the structures of simple salts, e.g. NaCl or La20jS. 

We will then move on to super-central-particle complexes with tri-, tetra-, penta- and hexanuclear monomeric clusters and a few oligomers. We will not discuss the so-called metal-rich halides with condensed clusters, mostly derived from R6(Z) clusters sharing common R edges. This has already been done in breadth in chapter 100 of this Handbook (Simon et al. 1991a). 

Just as for group 5, 6, and 7 ( -CsF MCU species, Fehlner has shown that BH3-THF or Li[BH4] react with group 8 and 9 cyclopentadienyl metal halides to result in metallaborane clusters, many of them having a metal boron ratio of 1 3 and 1 4, and much of the synthetic chemistry and reactivity shows close connections with the earlier transition metals. The main difference between the early and later transition metallaboranes that result is that the latter are generally electron precise cluster species, while as has been shown, the former often adopt condensed structures. Indeed, as has been pointed out by King, many of the later transition metallaborane clusters that result from these syntheses have structures closely related to binary boranes and, in some cases, metal carbonyl clusters such as H2Os6(CO)18.159... 

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]. 

There are several interesting families of inorganic mixed-valence compounds that we have not discussed here (see Yvon, 1979 McCarley, 1982). For example, there are metal-cluster compounds such as the Chevrel phases, M,jMo6X8(X = S or Se) and condensed metal-cluster chain compounds such as TlMojScj, TijTe, NaMo O and M PtjO. TTF halides and TTF-TCNQ complexes (Section 1.9) constitute molecular mixed-valent systems in which the mixed valency is associated with an entire molecule the charge on TTF in such compounds is nonintegral. The structure of TTF-Br(, 79 and... 

Thus, reactions in metal-containing films are of interest from the standpoint of producing small metal clusters and studying the specific features of their formation and interaction with organic components. In the following sections, these questions will be surveyed as applied to the classical reaction of magnesium with organic halides in films of their co-condensates. 

The same composition of the magnesium cluster adducts with halides and hydrocarbons confirm the above conclusion that the composition of primary condensation products is determined by the competition between... 

Nevertheless, the range of reactions catalyzed by cluster organomagne-sium halides is rather limited. Catalytic applications of organopolymagne-sium hydrides, which have become accessible due to the interaction of magnesium with hydrocarbons in the films of their co-condensates, are considerably more diverse. 

A growing chemistry of niobium oxide clusters continues to emerge. The reduced niobates and the NbO suboxide are also built up from condensation of Me units through shared vertices, namely, Nb60i2. Although the inner halides within the [MeXi2] + core are relatively inert to substitution, Nbe and Tae oxohalides, such as [Nb6Cli2-mOm]"+ m = 1 —4) and... 

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