Octadecahedron

Fig. 39. Skeletal structures of the carbo cation (CgMegH) and the boat and chair forms of cyclohexane compared with the octadecahedron of (BuHu) -.

Octadecahedron C2B8HioML [ML, = CoCp, IrH(PPh3)2] [Co(riCC2B8Hio)2] C2B7H9(CoCp)2 [CB9H10C0CP]-... 

Idealized structures up to nine-coordination are summarized in Figure 4.21. These do not represent all of the shapes met, since, apart from all these idealized structures, it is necessary to remember that bond angle and bond length distortions of these structures can occur some of the shapes resulting from these effects are themselves common enough to be represented as named shapes, and we have discussed some examples of these earlier. Further, beyond nine-coordination, an array of additional shapes can be found, of which perhaps the best known are the bicapped square antiprism (for ten-coordination), the octadecahedron (for eleven-coordination) and the icosahedron (for twelve-coordination). Clearly, the options are extensive, so it may be time to find out what directs a complex to take a particular shape. 

Figure 3.2. Polyhredra of closo clusters (a) trigonal bipyramid (I>3h), (b) octahedron (O ), (c) pentagonal bipyramid (/>5h), (d) dodecahedron (i>2dX (e) tricapped trigonal prism (Z>3h), (0 bicapped Archimedean antiprism (g) octadecahedron (C2v), (h) icosahedron (7 ).

Trigonal bipyramid Octahedron Pentagonal bipyramid Dodecahedron Tricapped trigonal prism Bicapped square antiprism Octadecahedron Icosahedron... 

Probable structure, octadecahedron, is not definitively stabilised. Dianion is highly fiuxional in solution. 

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