Cluster trigonal bipyramid

Multicenter bonding is the key to understanding carboranes. The series [CB H i and C2B H 2 (Schemes 3.2-44) contain mainly polyhedral clusters (trigonal bipyramids 15 and 39, octahedral [CBsHg]- and C2B4H6, icosahedra [CB11 H,2 and C2B10H12) which are three-dimensional a aromatics, however, the... 

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).

Mixed-metal clusters are also accessible from Co2(CO)8. For example, the following reaction (Equation (2)) leads to a purple-black Au-Fe-Co cluster with a trigonal bipyramidal array of metals, via a long-known mixed Co-Fe red-brown colored cluster intermediate.79... 

In multi-component liquids, stabilization of the liquid is revealed by the formation of eutectics where the freezing temperature is suppressed. In such liquids, the atomic species (say A and B) are not distributed at random. There are more associated AB pairs (or other clusters) than expected for a random distribution. As a result in binary metal-metalloid alloys, such as Fe-B, the low melting-point eutectics occur at preferential compositions. The most common of these is at about 17 at. % B, or an atom ratio of one B for five Fe atoms (Gilman, 1978). This suggests that clusters of metal atoms surrounding metalloid atoms form (trigonal bipyramids). These probably share corners, edges, and faces. 

Perhaps the most fruitful of these studies was the radiolysis of HCo(C0)4 in a Kr matrix (61,62). Free radicals detected in the irradiated material corresponded to processes of H-Co fission, electron capture, H-atom additions and clustering. Initial examination at 77 K or lower temperatures revealed the presence of two radicals, Co(C0)4 and HCo(C0)4 , having similar geometries (IV and V) and electronic structures. Both have practically all of the unpaired spin-density confined to nuclei located on the three-fold axis, in Co 3dz2, C 2s or H Is orbitals. Under certain conditions, a radical product of hydrogen-atom addition, H2Co(C0)3, was observed this species is believed to have a distorted trigonal bipyramidal structure in which the H-atoms occupy apical positions. 

The cluster Sng 4 has a monocapped square antiprismatic geometry. On the other hand, the cluster SM5 ]2, 68b, has a trigonal bipyramidal geometry, as shown by X-ray crystallography107. 

A crystalline compound [Na(crypt)]+2Pbs]2 has been prepared183 in which [Pb5]2 , 94, is a trigonal bipyramidal cluster the structure of which has been characterized by X-ray diffraction, with Pb-Pb distances of 3.00-3.22 A [crypt = N(CH2CH2OCH2CH20CH2CH2)3N]. See Section I.B for analogous tin compounds. 

If one applies the same procedure to Figure 1.10B, an iron-sulfur cluster often used as a model for those in biological systems, the same magic number of 60 would be obtained. Cluster magic numbers would occur as 48 e for a triangular clusters, 60 e for tetrahedral, 72 e for trigonal bipyramidal, 74 e for square pyramidal, 86 e for octahedral, 90 e for trigonal prisms, and 120 e for cubic structures. 

For five-metal clusters, two "parent polyhedra may be considered, viz., the trigonal bipyramid and square-based pyramid. 

Agreement is poor for small clusters, but this does not present a fair test—the smallest structure type considered is that derived from a trigonal bipyramid having an S value of 6. However, the predictive power for clusters of four to six atoms, for which the rules perform best, is only moderate. Even in that region, correspondence with the actual... 

Similarly, the classical tetrahedrane D with six 2c2e bonds may be classified as a nido-cluster having 2n + 4 SE. It is derived from the doso-structure (trigonal-bipyramid), with one apical vertex missing. 

The polyhedra in Fig. 1 thus represent suitable shapes for cluster species with n skeletal atoms (each of which can furnish three AO s for use in skeletal bonding) and with (n + 1) skeletal bond pairs. Since it is the cluster symmetry that determines the number of bonding MO s, the same polyhedra can serve as the basis for the structures of a whole range of isoelectronic species, including neutral carboranes of formula C2B 2Hn, bismuth clusters, such as the trigonal-bipyramidal Bis " ",... 

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