C/oso-structures

Scheme I. Examples of tetraphosphorus- or arsenic-based molecules with the adamantane-type core A4B6. Lewis acid base complexes are not shown. Key to superscripts (when no reference is given, consult Ref. 1-6) A unique non-ada-mantane P4(NR) c oso-structure has recently been obtained with R = i-CjH- (1) and has been shown to convert into the thermodynamically more stable adamantane-type isomer upon heating as in P SglS), P4Sg, and P4S10 Ref. 9 in P4S N- (101,- Hn[P404(CH2)2j04in) orAs40(CH2)2 (121,- 05 in (P406)04- S (13) and [P4(NMe)6l04. S (14).

Os atoms held by 7 pairs [ n) 1) pairs], therefore a c/oso structure based on a octahedron... 

A crystal structure analysis of Pt(Me2C2B7H7)(PEt3)2 shows that the polyhedral geometry is almost that of a bicapped (by B and C) square antiprism. The metal atom is in a CBBPt face, adjacent to the B cap. The Pt—C distance is 2.83 A, i.c. there is no interaction between them, and thus the molecule has a nido-structure (14), with an open face. The expected c/oso-structure is not preferred, because of the preference of the Pt atom for square-planar co-ordination and the low polyhedral connectivity of... 

Solution. Each Co atom contributes nine valence electrons and each CO contributes two valence electrons, for a total of 86 valence electrons in the Co (CO) 6 cluster. Using the formulas in Table 19.5, this compound will obey the 14n -I- 2 rule, so that it will have the c/oso structure type. 

NMR spectrum gives 8 = 4.68, 4.46 ppm (1 1 area ratio, C5H5) and 5 = -15.6 ppm (Co-H). A crystal structure determination confirms the electron count as a c/oso-structure with adjacent boron atoms as seen in Fig. 2-72 [10]. 

Figure 6.21 Relation between C2B9Hn and C sHs". In this formalism the c/o.ro-carboranes C2B10H12 are considered as a coordination complex between the pentahapto 6-electron donor C2B9H] - and the acceptor BH (which has 3 vacant orbitals). The o/oso-structure can be regained by capping the open pentagonal face with an equivalent metal acceptor that has 3 vacant orbitals.

Figure 9. Structure of c oso-[(HIrB8H7Cl)(PMe3)2]. Selective 1H- 11B) NMR spectroscopy showed that each B atom except B(8) has one terminal H atom attached and that there is a terminal H atom on lr(l) symmetrically cis to the two P atoms (8).

Table 6 Structures of c/oso-1 -CBgH10 derivatives determined experimentally... 

Table 24 Structures of c/oso-1, 7-C2BioH12 (mefa-carborane) derivatives determined experimentally by X-ray diffraction unless otherwise stated ...

Figure 26 Crystal Structure of the eight-vertex c/oso-2,2-(PPh3)2-2-H-3,6,8-(MeO)3-2,1 -RuCBeH4. Reproduced by permission of the American Chemical Society from Inorg. Chem. 2001, 40, 5318.

Figure 43 Crystal structure of 13-vertex nickelacarborane, 4-dppe-4,1,6-c/oso-NiC2B10H-i2. Reproduced by permission of Elsevier from Inorg. Chim. Acta 2003, 347, 161.

Figure 10. Structure of iso-c oso-[H(PPh3)(PPIi2C6H4)(I-IrB9Hs)] with P-phenyl groups omitted for clarity, except for their ipso-C atoms all H atoms were...

Molecular Orbital Descriptions. In addition to the localized bond descriptions, molecular orbital (MO) descriptions of bonding in boranes and carboranes have been developed (4). Early work on boranes helped develop one of the most widely applicable approximate MO methods, the extended Htckel method. Molecular orbital descriptions are particulady useful for c/oso molecules where localized bond descriptions become cumbersome because of the large number of resonance structures that do not accurately reflect molecular symmetry. Such descriptions show that the highest occupied MO (HOMO) is degenerate for most deltahedral B H molecules, but that a closed shell is obtained for the corresponding [B H l2 anions. After... 

Application of the polyhedral expansion methodology to C2B10H12 leads to supraicosahedral metallacarboranes such as closo-(t C HB) Co C2B1 -H 12 [33340-90-8] (194—199). Further expansion of 13-vertex species or thermal metal transfer reactions leads to the 14-vertex cluster [(T C H )Co]2C2B1 ( H 12 [52649-56-6] and [52649-57-7] (199). Similar 14-vertex species have been obtained from tetracarbaboranes (203) and show unusual structures. The isomeric bimetallic cobaltacarborane complexes c/oso-(r 3 -CpCo)2C2BgH10 (cp = C H ) can be formed by either polyhedral expansion or contraction reactions. Six isomers of this cluster are formed in the thermally-induced intermolecular metal transfer and polyhedral expansion of the 11-vertex Ao-(t b-CbHb)CoC2B8H10. 

---