Clusters with nuclearity greater than

The problem presented by the fused cluster B2oHi6 with 76 eve is a more difficult one to analyze. Look at B20H16 and note the four atoms held in common. Take it apart at the points of fusion with each fragment retaining the four common 

Fused metal clusters are much more abundant than fused main-group clusters and Mingos developed an effective approach to their description (Section 3.3.3). His rule is that in fusing two clusters, the fragment formally eliminated must obey normal valence rules. Thus we have  

Note that the common fragment appears to be a 22 eve butterfly fragment (like B4H10) rather than a ring isoelectronic with C4H8. We will encounter this ambiguity for metal clusters as well when cluster fusion involves four common atoms. 

The fused deltahedra need not be connected at four vertices, nor closed, nor of the same size. Thus, the number of fused compounds possible is large and the number of macropolyhedral clusters, as they are sometimes described, is growing. 

Considering the fact that the number of polyaromatic compounds known is large, this result should be no surprise. Of course neither the mno rule nor the cluster-fusion rule by itself gives any insight on the synthesis of the compounds. But they do suggest many such fused systems should be possible to make and that is added justification for seeking their syntheses. 

---

The structural determinations of these large clusters with nuclearities greater than 120 metal atoms proved problematic. For such clusters, the intensities of the reflections dropped off rather sharply above 20 40° (for MoKa radiation), and the structure refinement usually resulted in unsatisfactorily high R factors, with a high residual electron density close to heavy atoms within the cluster molecule. More satisfying R factors can only be obtained if this electron density can be modeled as disordered atoms which, on the other hand, complicates efforts to provide precise estimates of the molecular formulae. These effects may result from a range of factors ... 

Figure 3.2.2 Percentage of F atoms involved in various species observed in the system as a function of the composition "polymer" means a cluster with a Be nuclearity greater than four, whereas F implies that the ion is...

---