Cluster-stabilized reduced halides

Owing to the ample contributions of the groups of Corbett and Simon, one must have the impression that the so-called reduced halide chemistry of the rare-earth elements is that of interstitially stabilized [R Z] clusters. It is certainly dominated by these units when condensed metal clusters are considered. This chemistry has been reviewed several times (Corbett 1992, Simon 1995, Meyer 1988). 

Additional areas of lanthanide halide chemistry that have been reviewed include the synthesis, phase studies, and structures of complex lanthanide halides - compounds formed between one or more group 1 cation and lanthanide element halides (Meyer 1982). Halides in combination with lanthanide elements in the II, III, and IV oxidation states were considered with the chemistry of the heavier halides being emphasized. More recently the reduced ternary lanthanide halides (Meyer 1983) and the reduced halides of the lanthanide elements were reviewed (Meyer 1988). The latter review considered lanthanides in which the formal oxidation state of the cation was 2 and included hydride halides, oxide halides, mixed-valence ternary halides, and reduced halide clusters. Corbett et al. (1987) discussed the structures and some bonding aspects of highly reduced lanthanide halides and compounds stabilized by a second-period element bound within each cluster, e.g., SC7CIJ2B, SC5CI5B, YjCljC. 

Strictly speaking, a catalyst is some species directly involved in the catalytic cycle and, in the reactions discussed here, these species are usually low-valent, coordinatively unsaturated transition metal complexes. Metal halides, e-.g., chloroplatinic acid, PdCl, etc., although often claimed as catalysts are more properly catalyst precursors, since in the presence of silyl hydrides the metal halides are reduced. If no stabilizing ligands, e.g., olefins, phosphines, etc. are present, the reduction normally proceeds to a finely divided form of the metal or to insoluble metal silyl/hydride clusters which may act as heterogeneous catalysts. 

Many reduced (metal-rich) halides of group 4 (especially Zr) and the rare earth metals have been prepared. Most of these compounds are stabilized, by the metals forming Mg octahedral or other clusters having strong metal-metal bonds. The reactions to form these clusters are slow. Other nonmetals, especially oxygen, are undesirable impurities that may form more stable phases. Therefore the reactions are carried out with stoichiometric mixtures of pure halide and metal in degassed Ta or Nb tubes that have been loaded in an inert atmosphere and arc-welded shut. The welded ampule is then sealed in a protective quartz tube and heated to a temperature adequate to achieve a reaction in a week or more ( >600°C) . Yields may be small in some cases individual single crystals are produced as evidence of synthesis of a new material with metal-metal bonds. 

The comproportionation route (Corbett, 1983a, 1991) is widely used and is very efficient when pure phases are desired, especially when the phase relationships are known or can be anticipated. It led to a great variety of reduced rare-earth halides, binary, ternary, and higher, simple, and complex salts, and such that incorporate metal clusters interstitially stabilized by a non-metal atom or by a (transition) metal atom, for example,... 

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