Bicapped octahedron

Figure 25.11 Metal frameworks of some high-nuclearity binary carbonyl and carbonylate clusters of osmium (a) Os5(CO)i6 (trigonal bipyramid) (b) Os6(CO)ig (bicapped tetrahedron, or capped trigonal bipyramid) (c) [Os6(CO)ig] (octahedron) (d) Os7(CO)2i (capped octahedron) (e) [Osg(CO)22] (bicapped octahedron) (f) [Osi7(CO)36] (3 shaded atoms cap an Osu trigonal bipyramid).

An unusual type of deltahedral cluster found in organoindium chemistry but not in deltahedral borane chemistry is the eight-vertex In8[Si(CMe3)3]6 (see Chapter 2.3.4.2) its structure is based on a bicapped octahedron with idealized D34 sym-... 

We must conclude that the marked electrochemical quasireversibility is due to the remarkable geometrical reorganization from the bicapped tetrahedron of [Os6(CO)18] to the octahedron of [Os6(CO)i8]2 occurring upon the two electron addition. 

Octacoordination is often encountered in lanthanide complexes. The preferred poly-hedra for eight coordination expected on the basis of interligand repulsivities are square antiprism (D ), dodecahedron with triangular faces (Z)2d), bicapped octahedron (D3(i), truncated octahedron (Z)2ft), 4,4-bicapped trigonal prism (C2v), distorted cube (C2v), and cube (0/,). The most commonly observed polyhedra for this coordination number are, however, the square antiprism and the dodecahedron. 

FBP = pentagonal bipyramid CTP = C , monocapped trigonal prism CO = monocapped octahedron DD = dodecahedron SAP = square antiprism BTP = bicapped trigonal prism. 

Fig. 43. [Re,C(CO)24p, 35, as in its Et4N+ salt (77). The Re, polyhedron comprises a trans-bicapped octahedron, with the carbide carbon at the center of the octahedral cavity (mean Re-C = 2.12 A). Re- Re bond lengths average 2.993 A within the octahedron and 2.970 A for bonds to the capping atoms. There are three terminal carbonyls per metal atom, and the anion has overall D d symmetry.

When the number of metal atoms in a cluster increases, the geometries of the clusters become more complex, and some are often structurally better described in terms of capped or decapped polyhedra and condensed polyhedra. For example, the first and second clusters listed in Table 19.4.3 are a capped octahedron and a bicapped octahedron, respectively. Consequently, capping or decapping with a transition-metal fragment to a deltapolyhedral cluster leads to an increase or decrease in the cluster valence electron count of 12. When a transition-metal atom caps a triangular face of the cluster, it forms three M-M bonds with the vertex atoms, so according to the 18-electron rule, the cluster needs an additional 18 - 6 = 12 electrons. The parent octahedron of [Os6(CO)is]2- has g = 86, the monocapped octahedron Osy(CO)2i has g = 98, and the bicapped octahedron [Oss(CO)22]2- hasg = 110. 

A large number of boron compounds are known in which the boron atoms are arranged in the form of some sort of polyhedron (octahedron, square antiprism, bicapped square antiprism, icosahedron, etc.). In these structures, the boron atoms are most often bonded to four, five, or six other atoms. The most common structure of this type is the icosahedron shown by the species Bi2Hi22. This structure can be considered as a B12 icosahedron with a hydrogen... 

D, dodecahedron A, square antiprism BCTP, bicapped trigonal prism BCO, bicapped octahedron DTC, N,N-diethyldithiocarbamate HFA, hexafluoroacetylacetonato FOD, l,l>l>2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octane dionato NTA, nitrilotriacetato acac, acetylacetonato PCN, phthalocyaninato DMP, 2,6-dimethyl-4-pyrone BA, benzoyacetonato pip, piperidine DPM, 2,2,6,6-tetramethylheptane-3,5-dionato py, pyridine phen, 1,10-phenanthroline trop, troplonato. 

The dihydride Os7(CO)2oH2 also exists in two isomeric forms, which, in this case, are formed by two very different routes (Scheme 14). Protonation of [Os7(CO)2o] gives the brown isomer (118). Only the red isomer, synthesized via the addition of Os(CO)4H2 to Osg(CO),g(MeCN)2, has been structurally characterized (118). Instead of the monocapped octahedron predicted by Wade s Rules and observed for the isoelectronic Os7(CO)2i, it has a structure based on the Osg(CO)ig bicapped tetrahedral metal atoms arrangement, with an edge bridged by an Os(CO)4 unit. These two species do not interconvert. Deprotonation of both isomers yields the same dianion and the red species, once deprotonated, even singly, cannot be reformed (474). 

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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