Octahedral clusters zirconium

The versatility of transition-metal-ligand combinations in stabilizing localized, partially localized and delocalized cluster bonding is evident in the three examples discussed to this point. But we still have no analog of a late metal-carbonyl cluster that follows the main-group counting paradigm for an octahedral shape. Centered zirconium chloride clusters provide an example of an octahedral cluster with seven... 

For Nb or Ta clusters MgLig units are formed when the ligands are Cl or Br for niobium, and Cl, Br or I for the tantalum. In contrast, the niobium iodides have the MeLi4 unit, owing to the important steric effect of the iodine, as discussed later. Such MeLis units can also be prepared from valence-electron-poor transition metals such as zirconium, but in this case an interstitial element located at the center of the octahedral cluster is necessary to stabilize the cluster. ... 

K.5 Halide-supported octahedral clusters of zirconium structural and bonding questions... 

This concept, adding electrons via the incorporation of electropositive elements outside and inside the cluster, becomes more and more important when the number of valence electrons decreases. Zirconium, yttrium and the rare-earth elements associated therewith are well known to form octahedral clusters but virtually never without an interstitial. In box 3 elements that form halide clusters are printed in bold type, and the rare-earth elements (in the box) that seem to form clusters most easily are indicated by shading. 

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.

Much of the interest in clusters of this type arises from their structural similarity to the fundamental structural element of the superconducting Chevrel phases M MogEg (M = Pb, Sn, Cu, etc.) and more recently discovered rhenium halco halide phases. Although these solid-state compounds are known only for Mo and Re, molecular MgEgLg clusters of Zr, V, Cr, Mo, W, Re, Fe, and Co have been reported (Table 2). The lone examples of octahedral zirconium- and vanadium-chalcogenide clusters are unique in that they also possess interstitial O and S atoms, respectively. Evidence for interstitial FI atoms in the clusters [Cr6Eg(PEt3)g] (E = S, Se) has been presented. ... 

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. 

The surprising existence of zirconium cluster phases centered by the 3d metals Cr, Mn, Fe, Co or Ni requires further consideration of both M.O. expectations and the nature of the Zr-Z bonding. The former are relatively straightforward, as shown in Figure 7 for the charge-consistent result of iron in an idealized octahedral... 

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