Dodecahedron, triangulated

Fig. 8.4. Coordination geometries for seven- and eight-coordination (a) pentagonal bipyramid, (b) capped trigonal prism, (c) capped octahedron, (d) cube, (e) square antiprism and (/) triangulated dodecahedron. Central atom-ligand bonds are shown by full lines dashed lines indicate the edges of the polyhedral figure.

The solid state structure of [c/oio-B8H8]2 is very similar to that of an idealized triangulated dodecahedron and is shov/n in Fig. 19118). Bond distances not related by molecular symmetry are given in Table 6. 

There are three especially important idealized structures the cube (Oh), the square antiprism (D ), and the triangulated dodecahedron (DM). All three are depicted in Fig. 1-4, which also shows how each of the latter two can be obtained by distortions of the cube. The cube rarely occurs in discrete complexes, although it is found in various solid arrays (e.g., the CsCl structure). Since each of the other two structures, which can be so easily obtained from it, allow the same close metal-ligand contacts while alleviating the ligand-ligand repulsions, their energetic superiority over the cube is understandable. 

A relatively few species are known in which the polyhedron is, at least approximately, a triangulated dodecahedron (Fig. l-4b). These are the boron species BgHl-, B6C2H8, and B8C18. 

The octanuclear lanthanide hydroxide cluster core was first identified in Er8( X4-0)( X3-OH)i2(THD)s (THD = 2,2,6,6-tetramethyUieptane-3,5-dionate) [25, 87]. It is a triangulated dodecahedron with an interstitial oxo group each of its triangular faces is capped by a IX3-OH group. The same cluster core has also been found in [Eu8(p-6-0)((i,3-0H)i2( X2-0Tf)i6(0Tf)2] (Eigure 6.40) [99] and [Eu8( X4-0)((i,3-0H)i2(DME)8(Se3)(Se4)3(Se5)2] [105]. That the same cluster core is obtained with different ancillary ligands suggests the prevalence of this motif in the lanthanide hydroxide coordination chemistry. 

The triangulated dodecahedron (Fig. 3.7(a)) is the third dodecahedron we have encountered, the others being the pentagonal and rhombic dodecahedra (with 5-gon and 4-gon faces respectively). 

The adherence to close-packed structural arrangements lends support to the idea that these compounds can be used as models for metal surface chemistry—with respect to chemisorbed species and their mobility and reactions of substrates on these surfaces. It also indicates a marked deviation from the behavior of boranes and their derivatives. Structures based upon some polyhedra favored by boron, such as the pentagonal bipyramid, triangulated dodecahedron, and especially the icosahedron, are absent so far in metal-carbonyl cluster chemistry. In this connection, it has been mentioned that [M(CO)3],g compounds should be the closest analogs to On skeletal electron counting... 

The two most important eight-coordinate geometries for metal complexes are the square (or Archimedian) antiprism (Dad) and the triangulated dodecahedron (D2d). As shown in the following figure, both polyhedra can be derived by distortion of the cube (Oh) in different ways. 

A second class of clusters with 4-alkyne units is [Ru4(7t4-2 ) 2 -C2Rz)z(At-CO),c(CO)n J ( = 1 or 2) compounds that have a /it4-alkyne bound on either side of an RU4 unit that is best described as a tetrahedrally distorted square with four RuRu bonds. The RU4C4 frame is a triangulated dodecahedron (Figure 4) that is consistent with nine skeletal electron... 

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