Cluster face-capped deltahedron

The Os7(CO)2i cluster provides a simple illustration. The TEC is given by (7x8) + (21 x 2), namely, 98. Subtracting 7 x 12 = 84 and dividing by 2, we obtain S = 7, which implicates a six-vertex central polyhedron. This is consistent with there being a closo six-atom deltahedron (namely, an octahedron) capped on one face—which is, indeed, the actual structure of Os7(CO)2i. Correct results are similarly obtained for [Os8(CO)22]2-, which is a bicapped octahedron (16-XX) and [Osi0C(CO)24]2-, which is a tetracapped octahedron. For the latter, a capsule summary of the analysis is as follows ... 

Figure 2.6.1 The two known anionic clusters of tin (a) the nine-atom Sng Snf. Both clusters are deltahedral, although the nine-atom species has an open square face (the bottom). It is derived from the complete ten-atom deltahedron, a bicapped square antiprism, by removal of one of the capping vertices. The five-atom cluster is a trigonal bipyramid...

When 0s3(C0)j2 is heated in a sealed tube to 190 C, Os (CO)ig is formed, together with OSg, Os and OSg species (Fig. 11.11). The pentaosmium cluster 0s5(C0)jg and the anion Os5(CO)j which is derived from it by reduction possess trigonal bipyramidal structures (p.e.c. 72). This figure can be viewed either as a doso-deltahedron (14t -h 2) or as a capped tetrahedron (2 tetrahedra sharing one face p.e.c. 60 + 60 — 48). 085(00)3 reacts with carbon monoxide to form 085(00)35 which has the fascinating bow-tie arrangement of five metal atoms all lying in a plane. 

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