C/oso-Borane anions

All these molecules contain 86 electrons associated with the valence shells of the 6 metal atoms (the core carbon atoms of the carbides are considered to contribute all their valence shell electrons to these clusters). In the hydride H2Rufl(CO)i8, for example, the metal atoms supply 48 electrons, the carbonyl ligands 36, and the hydrogen atoms 2. Formally, this hydride maj be regarded as derived from the anion [Ru(CO)3]g , which, in turn, can be shown to be formally related to the c(oso-borane anion RgHe " as follows (199, 200). 

Boranes are extremely reactive compounds and several are spontaneously flammable in air. Arachno-horanes tend to be more reactive (and less stable to thermal decomposition) than nWo-boranes and reactivity also diminishes with increasing mol wt. C/oso-borane anions are exceptionally stable and their general chemical behaviour has suggested the term three-dimensional aromaticity . 

The diagrams also indicate why neutral c/oso-boranes B Hn4-2 are unknown since the 2 anionic charges are effectively located in the low-lying inwaidly directed orbital which has no overlap with protons outside the cluster (e.g. above the edges or faces of the B oct edron). Replacement of the 6 H, by 6 further B builds up the basic three-dimensional network of hexaborides MBs (P- ISO) just as replacement of the 4 H in CH4 begins to build up the diamond lattice. 

Carboranes. The accumulation of charge observed in borane anions discussed above may also be afforded by changing some boron atoms by other elements with more valence electrons than boron. The best known examples of this kind of isoelectronic compound are the carboranes. The most important series of carboranes is that with molecular formulas C2B 2Hn, the dicarba-c/oso-boranes, where n is any integer from 5 to 12. These neutral species are isoelectronic with the borane anions BnH . The structures of these compounds are closed deltahedra with terminal hydrogen atoms attached to each carbon or... 

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... 

The very important anion [7,8-C2B9Hi2]" was first obtained by alkaline solvolysis of 1,2-dicarba-ctoso-dodecaborane(12), l,2-C2BioHi2. Later, various modifications of the original procedure were described. This anion represents a basic substance for preparing a large number of metallocarbaborane sandwich compounds as well as for the syntheses of several nido- and c/oso[Pg.231]

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