Octahedral cluster compounds, Group metals

The structural relationship between the molecular and solid-state compounds has been a hot issue in inorganic chemistry for some time (25-27). The extrusion (or excision) from preformed solid-state cluster compounds is one of the major synthetic methods of the preparation of cluster complexes (26). Use of cluster complexes as precursors to solid-state cluster compounds is the reverse reaction of excision. Both reactions utilize the structural similarity of the metal cluster units. The basic cluster units of polyhedra (deltahedra) or raft structures are triangles, and both molecular and solid-state clusters with octahedral, tetrahedral, and rhomboidal cores have been reported. Similarity of other properties such as electronic structures based on the cluster units is also important. The present review is concerned with the syntheses and structures of the cluster complexes of the group 6 metals and with their relationships to solid-state chemistry. 

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. 

In this section, we introduce the first group of cluster compounds of the heavier -block metals in which the external ligands are halides. Octahedral Mg frameworks are present in most of these clusters, but, in contrast to similar group 5 and 6 species (Sections 23.6 and 23.7), most zirconium clusters are stabihzed by an interstitial atom, e.g. Be, B, C or N. 

When one or more of the vertices of a carborane are replaced by metal (M) atoms or metaUo (ML ) groups, a metaUocarborane is formed. Transition metal atoms in these cluster compounds are considered to be pseudo-octahedrally coordinated with valence shells having 18 electrons, corresponding to noble gas configuration. When this assumption is combined with Wades 2 + 2 rules, we can readily arrive at a general formula for the number of valence electrons C for the skeleton of a metaUocarborane ... 

Hexanuclear Organometallic Clusters. Although the octahedral arrangement of metal atoms is the most representative one for hexanuclear cluster compounds, a number of other geometries have also been observed. The structural characteristics of a selected group of hexanuclear compounds are briefly described in Table 2.6. The different types of structures found for these clusters are illustrated schematically in Fig. 2.25. 

Table IV lists specific examples of compounds related through this form of dimensional reduction, By far, the majority of these are zirconium chloride and iodide phases, in which case lower main group and even transition metals have been found to incorporate as interstitial atoms. A few analogues are known with hafnium (135), and very recently it has been shown that nitrogen can be substituted for carbon in tungsten chloride clusters adopting the centered trigonal-prismatic geometry (see Fig. 2) (32). It is hoped that a variability similar to that exposed for the octahedral zirconium clusters will be attainable for such trigonal-prismatic cluster phases.

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