Icosahedral metallated

Fig. 5. Proposed arrangements of the atoms in the first four layers of an alkaline earth metal around a C o molecule the atoms at the icosahedral vertices are drawn in black and one of the triangular faces of atoms has been shaded in each layer. Note the spiral of atoms (dark grey) in the fourth layer.

Figure 6.1 The icosahedron and some of its symmetry elements, (a) An icosahedron has 12 vertices and 20 triangular faces defined by 30 edges, (b) The preferred pentagonal pyramidal coordination polyhedron for 6-coordinate boron in icosahedral structures as it is not possible to generate an infinite three-dimensional lattice on the basis of fivefold symmetry, various distortions, translations and voids occur in the actual crystal structures, (c) The distortion angle 0, which varies from 0° to 25°, for various boron atoms in crystalline boron and metal borides.

Mn(N03)4] , [Fe(N03)4] and [Sn(N03)4], which feature dodecahedral coordination about the metal [Ce(N03)5] in which the 5 bidentate nitrate groups define a trigonal bipyramid leading to tenfold coordination of cerium (Fig. 11.17b) [Ce(N03)6] and [Th(N03)6] , which feature nearly regular icosahedral (p. 141) coordination of the metal by 12 O atoms and many lanthanide and uranyl [U02] complexes. It seems, therefore, that the size of the metal centre is not necessarily a dominant factor. 

The structures are shown in Fig. 26.8c and d and differ in that, whereas the Ir compound consists of a tetrahedron of metal atoms held together solely by M-M bonds, the Rh and Co compounds each incorporate 3 bridging carbonyls. A similar difference was noted in the case of the trinuclear carbonyls of Fe, Ru and Os (p. 1104) and can be explained in a similar way. The M4 tetrahedra of Co and Rh are small enough to be accommodated in an icosahedral array of CO ligands whereas the larger Ir4 tetrahedron forces the adoption of the less dense cube octahedral array of ligands. 

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 ... 

Considering the mode of filling the voids in the metal framework of rj phases with the Ti2Ni type (see Ref. 1) (Table 1), Re3Al2B is the only boride member of this group, with B atoms entering the large icosahedral center in 16d, occupied by metal... 

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

All the active metals in Tsai-type clusters lie on and define only the icosahedral shell. As a result, the global percentages of the active metals in Tsai-type phases are always — 157/, in contrast to that in Bergman-type phases ( 32%, below). 

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