Polyhedron structure, carbides

The structures of 204-207 have been established in the solid state by single crystal X-ray diffraction. Clusters 205 and 206 are based on the Ru6C octahedral skeleton. The [2.2]paracyclophane ligand in 205 coordinates over a triangular metal face, and in 206 one [2.2]paracyclophane adopts a similar face-capping bonding mode and the other coordinates in a terminal mode to an apical Ru atom. The metal polyhedron in 204 is relatively open in comparison to the octahedron having only nine Ru-Ru bonds. A carbide atom occupies the central cavity and interacts with five of the six Ru atoms. 

On the contrary, a trigonal-prismatic polyhedron arrangement is presented by the cluster [Rh6C(CO)15]2. The skeleton structure of the cluster [Co8C(CO)18]2 is a tetragonal antiprism. The structures of more complex carbide clusters are also discussed, for instance [Re8C(CO)24]2, [Os10C(CO)24]2-, and [OsuC(CO)27]2 [25]. 

Dianion [Co8(CO)i8C)] is isoelcctronic with Rh8(CO)i9C but presents a very different stereochemistry as shown in structure XX (Fig. 8) (13). Its geometry can be described as a deformed square antiprism with a carbide atom in the center of the polyhedron. This structure can be derived from that of the bicapped trigonal prism by stretching the common edge of the two capped square faces, as indicated in Fig. 8 by the dotted line. In such a process the loss of an M—M bond formally generates a square face. The idealized symmetry of a tetragonal antiprism is Did. However,... 

People who make discoveries do not necessarily use the aesthetic side until afterward. Then they recognize it and say, yeah, that s beautiful. Of course, we were not the first to find the boron icosahedron either. That was found by Sevastyanov and Zhdanov in C3B12, boron carbide, in 1941. That was a missed opportunity because nobody thought then that you could make fragments of this polyhedron and arrive at a structure of decaborane. That was not an idea that was around. It s an idea we missed. 

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