Capped square pyramid

C5Me5)Ir Co2(CO)4(/i-CO)B3Fl7 274. Pyrolysis of 274 results in H2 loss and the formation of (ij5-G5Me5)Ir Go2(GO)5B3H5 275 which has a BH-capped square pyramidal structure.154... 

There is, for example H2Os6(CO)i8, which has the same electron count as [Os6(CO)18]2- but, as shown in Fig. 16-13, has a different skeletal arrangement, one that can be regarded as a capped square pyramid. It is, however, also consistent with the electron counting rule. 

Capped nido" Capped square pyramid H20s6(CO))g... 

Figure 10. New pathway for the scrambling of metal vertices in 86-electron octahedral metal clusters. The proposed edge-shared bitetrahedral and capped square pyramidal intermediates (or transition states) both maximize connectivity and preserve the 86-electron count...

Capped nidcf Capped square pyramid H20S6(C0)i8... 

Three possible geometries for a cluster containing six metal atoms are the octahedron, the capped square pyramid, and the bicapped tetrahedron which possess 43, 43, and 42 CVMOs, respectively. The 86 cluster valence electrons of Co6(CO)i6 suggest the bicapped tetrahedron would be an unstable cluster configuration since this would place two electrons in an HLAO. The observed octahedral geometry with its 43 CVMOs can accommodate the 86 cluster valence electrons. 

It is interesting to consider Os6(CO)ig and Ose-(CO)igH2. Emperically these compounds appear very similar yet structurally they differ significantly (Fig. 21). The two additional valence electrons of the dihydride can account for this structural difference. The osmium core of Os6(CO)ig defines a bicapped tetrahedron therefore, 42 CVMOs are available and filled by the 84 cluster valence electrons. The two additional electrons of the dihydride Os6(CO)igH2 would occupy an HLAO unless a structural change in the Ose skeleton occurs. Rearrangement to a capped square pyramid increases the CVMO count by one to a net 43 which is sufficient to accept all 86 cluster valence electrons. 

Os atoms define the vertices of a mono- 22 capped square pyramid. CMe groups are capping, one the square plane, and the other, an adjacent triangular face. 

Fig. 2.25. Types of structures frequently adopted by hexanuclear metal clusters, (a) Octahedron (b) bicapped tetrahedron (c) capped square pyramid (d) edge-sharing bitetrahedron (e) trigonal-prism (f) six-atom-raft triangles (g) six-atom-raft rhomb (h), (i), and (j) edge-bridged bi-diminished bipyramidal or edge-bridged butterfly geometries...

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