Dodecahedra, distorted

Four jS-ketoenolate ligands about a metal ion produce 8-coordinate complexes in which an anti-prismatic or a dodecahedral arrangement of oxygens about the metal ions is found. The Zr(AA)4 complex is known crystallographically (33) to have the D2 symmetry of the Archimedean square anti-prism. Hafnium complexes also are similar, as is the form of Th(AA)4 and Pu(AA)4 (16). The a form of Th(AA)4 and that of Ce(AA)4 and U(AA)4 may be considered somewhat distorted dodecahedra... 

In tetra-n-butyl ammonium salts hydrates (type IV structure), the host lattice is formed by the association of slightly distorted dodecahedra sharing faces forming groups... 

The smaller cavities are distorted pentagon dodecahedra (distance of oxygen atoms to center varies between 3.77 and 3.95 A) their average free diameter is only about 5.0 A. The larger cavities are almost spherical the oxygen atoms lie at the vertices of hexadeca-hedra and their free diameter is 6.7 A. The distances have been calculated on the basis of a unit cell edge of 17.40 A as found for propane hydrate. 

With an increase in size of the active metals, the interlayer interstitials between the triacontahedral and the penultimate icosidodecahedral shells appear to be occupied by smaller electronegative components, with variable occupancies. These interlayer interstitials are actually the centers of cubes and correspond to the Wyckoff 8c (1/4 V4 A) special position in 1/1 ACs. Strictly speaking, occupation at this site means that the structure is no longer YCd6-type but, for convenience, they are still referred to as Tsai-type phases. According to Piao and coworkers [94], occupation of these cube centers has strong correlation with the orientations of the innermost tetrahedra and distortions of the dodecahedra. 

As noted in 2.11, ligands forming high-symmetry coordination polyhedra (i.e., regular octahedra, tetrahedra, cubes and dodecahedra) about central transition metal ions are rare. Such highly idealized coordinations, nevertheless, do exist in the periclase (octahedra), cubic perovskite (octahedra, dodecahedra) and spinel (tetrahedra) structures. The more important rock-forming oxide and silicate minerals provide, instead, low-symmetry coordination environments. These include trigonally distorted octahedra in the corundum, spinel and gar-... 

Molecule C2o is the smallest one from all the fullerenes [9] and has the form of dodecahedra (point symmetry group Yh). We consider here only polymerized structures (clusters) which are formed by the pairs of bridge like bonds directed along molecules second order axes. The clusters formation is accompanied by the distortion of the geometry of molecules that leads as sequence to decreasing the symmetry both molecule and cluster (for example the symmetry group of cluster (C2o)s is only D2h). 

The complex (NFLOsCeF H2O exists as a dimer in which the dodecahedra share an edge [82], Each cerium atom is surrounded by eight fluorine atoms to form a distorted dodecahedron. The two dodecahedra are joined along an edge to form the centrosymmetric [Ce2Fi4]6 ion as shown in Fig. 5.17. 

The tetrakis(dialkylphosphido) complexes, An(PPP)4 (An = Th, U) PPP = P(CH2CH2P(CH3)2), were prepared by reacting AnC with four equivalents of the lithium or potassium salt of the PPP tetra-anion. The stmcture of these compounds shows triangulated dodecahedra distorted toward bicapped trigonal prisms. These complexes represent one of the first actinide systems containing exclusively metal-phosphorus bonds. These complexes are known to undergo insertion reactions as seen in actinide amido compounds. ... 

Hydrates of Ar, Kr, and Xe were first synthesized by Villard in 1896 [141]. They were further studied, as well as hydrates of krypton and xenon, by de Forcrand [142]. Several structures for noble gas hydrates are known [143-146]. All the hydrate structures are different from that of ordinary hexagonal ice. In the two fundamental structures, the water molecules form pentagonal dodecahedra which are stacked with different degrees of distortion from their ideally regular forms [146]. The two types of structures are shown in Fig. 26a and 26b [140]. One structure contains 46 water molecules in the unit cell with 2 small and 6 larger cavities. The other structure has 136 water molecules in the unit cell with 16 small and 8 larger cavities. The formation of the two fundamental types of hydrates depends mainly on the size of the guest species. More detailed data for the two principal clathrate hydrate structures are available from the literature [147]. 

Garnets are semiprecious gems with the chemical composition Ca3Al2Si30i2. The crystal structure is cubic and is made up of three building blocks tetrahedra, octahedra, and dodecahedra (distorted cubes). 

G and E denote guest and host atoms, correspondingly. G — guest atoms situated inside pentagonal dodecahedra, G , G , and G — guest atoms situated inside larger 24-, 26-, and 28-vertex polyhedra Distorted polyhedra... 

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