Tetracap

The centered 10-vertex polyhedra are of particular interest since the shapes of the outer 10-vertex polyhedron depends on the interstitial atom and the electron count. In fact, four very different 10-vertex polyhedra (Fig. 7) have all been shown to form stable isolable species containing interstitial transition or post-transition metal atoms. These polyhedra include structures with three-, four-, or fivefold symmetry. Thus for the ions M Inio ° (M = Ni, Pd, Pt) found in the intermetal-lics KioInioM, the Injo polyhedron is a Cgv tetracapped trigonal prism [91]. 

Clusters with nudearity higher than eight are more common for ruthenium [369, 380, 399, 403, 404] and osmium [399, 405-414]. They include species based on a M10 tetracapped C-centered octahedron. One Os-Os bond of the capping tetrahedra is bridged by a gold atom, as in [ P Ph 3 Me [[ (Ph 3 P) AuOS 0(p.6-C) (CO)241... 

Fig. 41. Rh15C2(CO)jg, 33, as in its H30+ salt (75). Bonds to the carbon atoms (shaded circles) and the interstitial rhodium atom are omitted. The cluster is most easily visualized as a tetracapped pentagonal bipyramid. Two octahedral cavities with a common vertex are provided by the capped square faces and the interstitial rhodium atom. The carbon atoms, which are remote from one another, occupy these octahedral cavities, with Rh-C distances ranging from 1.93 to 2.12 A (mean 2.04 A). Rh-Rh distances range from 2.738 to 3.332 A (mean 2.87 A).

Formal octacoordination ( 4 + 4 )229 in a silicon complex was recently demonstrated by Corriu s group, who prepared 201 (equation 53). The Si-N distances (Table 27) are long relative to those common in penta- and hexacoordinate compounds, but are in the same range as those found in the heptacoordinate compounds discussed here. Also, the geometry resembles the heptacoordinate complexes, in that it is basically a tetrahedron with dimethylamino donor groups pointing toward the center, in what may be termed a tetracapped tetrahedron . 

A similar tetracapped tetrahedral geometry was recently found by X-ray crystallography in bis[2,4,6-tris(trifluoromethyl)phenyl]-fluorosilane (202)230 and -difluorosilane (203)231,... 

Closely related are [Bi4Co9(CO)16]2 and [Bi8Co14(CO)2o]2-, the metal cores of which are given as 62A/62B and 63. They may be viewed as tetracapped, tetragonally distorted M8 arrays that also possess interstitial metal atoms. Compound 63 consists of two of the 62 metal frameworks fused about one of the Co4 faces. Compared to 59, 62 is seen to be missing two of the ju4-E groups. In addition, the Co-Co bonds parallel to the C4 axis of the molecule have been broken and have been replaced with four Bi-Coimerstitial interactions. The result is that 61 can be viewed as a cuboc-tahedron, which is more apparent if the compound is viewed from a different perspective (62B). This also emphasizes the close-packed nature of the metal array. 

IR Yes Osjo tetracapped octahedron with an interstitial C atom and Au triply bridging a Os3 face of one cap 193... 

For the beautiful tetracapped octahedral Os cluster, [OsioC(CO)24]2- with an interstitial C atom in the octahedral core, shown in Figure 3.10, the predicted eve count is 14(6) + 2 + 4(12) = 134, which agrees with that of the observed stoichiometry. It s a little bit harder to count the sep but give it a try. Each tetrahedral cap consists of an Os(CO)3 fragment and the other six fragments are Os(CO)2 so we have (4x2 + 6x0 + 4 + 2)/2 = 7 appropriate for an octahedron. If you look ahead in Chapter 6 (Exercise 6.1), you will find that this trigonal bipyramidal ten-atom core can be excised from a cubic close-packed metal lattice (ABC layers). [OsioC(CO)24]2- can be considered a nano-sized metal particle stabilized by the ligands in the same manner as Ni atoms are stabilized when removed from Ni metal by CO as Ni(CO)4 in the Mond process. 

Answer. A tetrahedral metal cluster is expected to exhibit a eve of 60 (sep of six) whereas if viewed as a tetracapped tetrahedron the sep remains six but the eve is 68 (tetra-M-capped M4 tetrahedron is 60 + 4 x 12 = 108 from which we subtract 4x10 electrons in the change from four M to four E). A M8 cube should have a... 

The cluster ResIn4(CO)32 has the structure shown below (tetracapped tetrahedron). Justify its geometry based on the isolobal analogy and one form of the electron-counting rules. 

The Os7(CO)2i cluster provides a simple illustration. The TEC is given by (7x8) + (21 x 2), namely, 98. Subtracting 7 x 12 = 84 and dividing by 2, we obtain S = 7, which implicates a six-vertex central polyhedron. This is consistent with there being a closo six-atom deltahedron (namely, an octahedron) capped on one face—which is, indeed, the actual structure of Os7(CO)2i. Correct results are similarly obtained for [Os8(CO)22]2-, which is a bicapped octahedron (16-XX) and [Osi0C(CO)24]2-, which is a tetracapped octahedron. For the latter, a capsule summary of the analysis is as follows ... 

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