Octahedral hosts

We will now illustrate four octahedral hosts related to the Platonic solids. Three are based upon the cube while one possesses features of both a cube and an octahedron. 

Fig. 9.12 Octahedral hosts, (a) Murakami s cyclophane-based cube, (b) X-ray crystal structure of the [(V06)(RP03)g]+ (R = tBu, OSiMe3) ion, (c) Seeman s DNA-based cube (printed with permission by Professor Nadrian Seeman, for further details see http Wseemanlab4.chem.nyu.edu).

Figure 14. Octahedral hosts, (a) Murakami s cyclophane-based cube, (b) X-ray crystal structure of the [(VOeKRPOjla) (R = OSiMea) ion.

For the alkali metal doped Cgo compounds, charge transfer of one electron per M atom to the Cgo molecule occurs, resulting in M+ ions at the tetrahedral and/or octahedral symmetry interstices of the cubic Cgo host structure. For the composition MaCgg, the resulting metallic crystal has basically the fee structure (see Fig. 2). Within this structure the alkali metal ions can sit on either tetragonal symmetry (1/4,1/4,1/4) sites, which are twice as numerous as the octahedral (l/2,0,0) sites (referenced to a simple cubic coordinate system). The electron-poor alkali metal ions tend to lie adjacent to a C=C double... 

Phase Trigonal prismatic void in 2c Metal host lattice atoms in 12k -f- 6h(l) -1- 6h(2) Icosahedral center in 2a Octahedral void in 6g ... 

The stabilizing influence of B atoms in sites is well known from MJ5M33B, borides (Mn5Si3 type), e.g., in the formation of boride interstitials by filling metal host lattices that develop from binaries. Boron occupancy (octahedral metal... 

Phase Octahedral void I in 16c Octahedral void 11 in 8a Icosahedral center in 16d Metal host lattice atoms in 48f + 32e... 

The [Ni(NCS)f,]4 ion is almost perfectly octahedral, with Ni—N distances of around 209.5 pm and N—Ni—N angles around 89.5°. The Ni—N—C and N—C—S entities are practically linear.438,439 In [Ni(NCS)2L2] where L is a R-substituted pyridine, stereochemistry and spin state depend on the type and positions of R.431 While for 2-Me- and 2-Et-pyridine square planar complexes are observed, other pyridins lead to coordination polymers with pseudo-octahedral Ni11 due to N,S-bridging thiocyanate. Ni11 thiocyanato complexes have been studied quite intensively as hosts for inclusion compounds.440"442... 

The structure of alumina on NiAl(l 1 0) was the subject of a surface X-ray diffraction study by Stierle et al. [46]. The model derived by Stierle et al. from the analysis of the X-ray diffraction data was based on a strongly distorted double layer of hexagonal oxygen ions, where the Al ions are hosted with equal probability on octahedral- and tetrahedral-coordinated sites the resulting film structure was closely related to bulk k-A1203. An attractive feature of Stierle s model was that it provided a natural explanation of the domain structure of the alumina overlayer, which is induced by a periodic row matching between film and substrate lattices. However, as pointed out recently by Kresse et al. [47], this structure model has two bonds with... 

Although the face-centred cubic structure of metals is close packed, it is still possible for atoms which are much smaller than the host metal atoms to fit into interstitial sites inside the structure, while maintaining the essential properties of metals such as electrical conductivity and heat transport. These interstitial sites are of two kinds. The octahedral interstitial sites have six metal atoms at equal distances from the site, and therefore at the apices of a regular octahedron. The tetrahedral interstitial sites have four nearest neighbour metal atoms at the apices of a regular tetrahedron. A smaller atom can just fit into the octahedral site if the radius ratio is... 

Spinels are cubic compounds, but the structure can be described as layers of close-packed oxygen. The sites available both for the transition metals and guests are octahedral and tetrahedral, as in Fig. 7.1(a). Unlike the layered compounds in Fig. 7.5, however, where every second metal layer is empty, the metal atoms of the host occupy octahedral or tetrahedral sites in every layer. 

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