Antiprisms bicapped

A large number of boron compounds are known in which the boron atoms are arranged in the form of some sort of polyhedron (octahedron, square antiprism, bicapped square antiprism, icosahedron, etc.). In these structures, the boron atoms are most often bonded to four, five, or six other atoms. The most common structure of this type is the icosahedron shown by the species Bi2Hi22. This structure can be considered as a B12 icosahedron with a hydrogen... 

Hexagonal antiprism Bicapped pentagonal prism Anticuboctahedron... 

Several carboxylates, both simple salts and complex anions, have been prepared often as a means of precipitating the An ion from solution or, as in the case of simple oxalates, in order to prepare the dioxides by thermal decomposition. In K4[Th(C204)4].4Fl20 the anion is known to have a 10-coordinate, bicapped square antipris-matic structure (Fig. 31.8b). -diketonates are precipitated from aqueous solutions of An and the ligand by addition of alkali, and nearly all are sublimable under vacuum. [An(acac)4], (An = Th, U, Np, Pu) are apparently dimorphic but both structures are based on an 8-coordinate, distorted square antiprism. 

Pr )2(PPh3)7](PFg)2-3249 and [Au, i(PPlioIV1 e)i(l]3, 3250 The structure of the mixed-ligands clusters corresponds to a C3v metal skeletal framework and the homoleptic to a centered bicapped square antiprism (Figure 32). Other stoichiometries are known as, for example, [AunCl2(PPh3)8]Cl.3251... 

Bicapped Square Antiprism. TlSn93 has the (1)(1) + (9)(2) + 3 = 22 = 2n + 2 skeletal electrons required for an n = 10 vertex globally delocalized D4( deltahedron (cf. the bicapped square antiprism in Figure 1-7) analogous to that found in the BioHio2- anion [84]. 

Octacoordination is often encountered in lanthanide complexes. The preferred poly-hedra for eight coordination expected on the basis of interligand repulsivities are square antiprism (D ), dodecahedron with triangular faces (Z)2d), bicapped octahedron (D3(i), truncated octahedron (Z)2ft), 4,4-bicapped trigonal prism (C2v), distorted cube (C2v), and cube (0/,). The most commonly observed polyhedra for this coordination number are, however, the square antiprism and the dodecahedron. 

A polyhedron based on bicapped dodecahedron possibly exists in decacoordi-nated [Ce(NOs)5]2- anion, which has been isolated (275) as triphenylethylphos-phonium salt (C2/c). The bicapped dodecahedron is a more appropriate description for the polyhedron than the bicapped square antiprism. However, Al-Karaghouli and Wood 215) from aesthetic point of view preferred to call the geometry of the coordination polyhedron as being trigonal bipyramid. 

Fig. 33. Skeletons of the gold clusters (Au(AuL)8] and [Au(AuL)io] compared with the icosahedron and the bicapped Archimedean antiprism. Central atoms are shaded peripheral atoms are unshaded.

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