Nine-vertex structures

Nonicosahedral structures should be anticipated for seven- and nine-vertex nido species. 

A number of studies published since 1971 confirm several of the initial predictions. Two groups at Indiana University [Siedle et al. 128) and Streib et al. (62)] have revealed that the nine-vertex nido structures B9Hi2 (II-N9) and the isoelectronic C, C-dimethyl derivative of C2B7H11 (IX-N9) do have nonicosahedrcd fragment strnctures (see Figs. 1 and 9 ... 

There is an exception in addition to the expected normal arachno nine-vertex fragment characteristic of w-BqHis (XI-A19) (75, ISO), there is a fragment generated by the removal of a low coordination vertex which is reflected in the structures of both i-ByHis (I-A19) 21) and its isoelectronic analog i-C2B7Hi3 (II-A19) 1S6). 

In these cases, I-, II-, and III-Al 9, conservation of the 3-center bonds would require five (or six) 3-center bonds to be expressed within the skeleton matched with four (or three) 3-center bridge hydrogens, respectively. There are several examples demonstrating that the nine-vertex arachno system is flexible enough to adapt to both electronic alternatives. The structures for i-BgHjgrLB (X-A19) and i-B9Hi4 (A-VI9) are both known in the encumbered crystalline phases and both must accommodate six 3-center bonds within the skeletal framework. By contrast, i-C2B7Hx3 (II-A19) absorbs only five 3-center bonds within the skeletal framework. 

A novel ten-vertex bimetallocarborane, (CsHo C C BeHs, is produced upon the polyhedral expansion of 1,7-C2B6H8 (16). The expected monometallic nine-vertex species, C5H5CoC2B6H8, is also produced in this reaction (see Fig. 4). An X-ray crystal structure of the bimetallic product (61) showed the 2 cobalt atoms to occupy adjacent positions on the two equatorial belts of the bicapped square antiprism. The carbon atoms occupy the low-coordinate caps. Thermal rearrangement (30) produced an isomeric complex in which the metals are separated from each other, but the carbon atoms remain in the low-coordinate sites. 

Structure type HfsCuSrts (Rieger et al., 1965) (Figure 7, Table 12). SG Vb mcm, Z = 2, a = 0.8992, c = 0.7031 run for LasGesS (Guloy artd Corbett, 1993). Four chalcogenides of rare earths with structure type HfsCuSns are known. The atomic parameters of these compoimds are not refined. The Lai atom is situated in 13-vertex polyhedron formed by 6La, 5Ge, and 2S atoms. The La2 atom is surrounded by a bicapped hexagonal prism formed by 8La and 6Ge atoms. The Ge atom is located in a nine-vertex polyhedron formed by 9La atoms. The S atom has octahedral coordination sphere formed by 6La atoms. 

Dicarba-wdo-nonaborane(l 1) is a representative example of a nine-vertex molecular structure with a five-membered open face containing one hydrogen bridge and one BH2 group.1,2... 

The most conspicuous feature that distinguishes the cluster chemistry of Sn and Pb from that of the lighter elements is the availability of a large body of crystallographic data, primarily for highly anionic clusters that co-crystallise with large countercations. A survey of this data shows that deltahedral structures dominate rather than the three-connected architectures found for Si and Ge. Within the nine-vertex family, the series [Co Sn9] , [Ni Sn9]" i3-... 

The iridacarbaborane (CO)(PPh3>2lrCB7Hg has been synthesised and characterised. Partially open structures of formally dbso-nine-vertex clusters have been discussed. ... 

Os5(CO),6 N is found to be 9 for this complex, and the only five-vertex figure with nine edges is a trigonal bipyramid the observed structure has a slightly-distorted trigonal bipyramidal arrangement of Os atoms. 

Answer. With two C-H fragments, nine B-H fragments and a charge of —2, the cluster has (2x3 + 9x2 + 2)12 =13 sep. Thus, the structure must be derived from an icosahedron (12-vertex deltahedron) but there are only 11 fragments. One vertex (all are equivalent in an icosahedron) is left vacant (a ra do-structure). Show for yourself that the eve count of 48 gives the same result. 

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