Octahedral cluster compounds, Group

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. 

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. 

Cobalt and rhodium also form a series of high nuclearity carbido-cluster compounds. Contrasting with carbido compounds of the iron group which have a clear tendency to put the carbide atom in octahedral cavities, carbides of the group 9 often place it in trigonal prismatic cavities. As shown in scheme in Fig. 3.12, the parent compound [Co6(CO)i5C] of a series of encapsulated car-bido-cobalt species may be prepared by the reaction of Co3(CO)9CCl with [Co(CO)4]. The same scheme also describes some carbido-cobalt cluster interconversions. Rhodium carbide clusters are in general similar to the cobalt ones. 

Typical structural motifs for S-, Sc-, and Te-containing iron cluster compounds shown in Scheme 9 (E, E = S, Se, Te cf. Scheme 1) involve the tetrahedral monocapped species with the closed iron triangle 118 and the dicapped triiron species with the open Fcs framework 121, which can also be regarded as a square-pyramidal c2,EE skeleton with an Fcafifi basal plane 121. The protonated forms of 118 (119 and 120) and the dicapped octahedral structure 122 based on an Fc4 square are also known. Butterfly 11 and interstitial structures (14 and 15 Scheme 1) are very rare for non-first-row main group elements, and a rare example of the /44-S butterfly cluster 116 is discussed in Section... 

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