Borane and Carborane Anions

Highly Fluorinated closo-Borane and -Carborane Anions... 

Bowser, ).R. et al. (1979) Nickel-boron hybrid clusters. Synthesis of polyhedral nickelaboranes and nick-elacarboranes from small borane and carborane anions. Crystal structure of (.eta.5-C5H5)4Ni4B4H4./. Am. Chem. Soc., 101 (21), 6229-6236. 

In the area of synthesis, the structural and theoretical framework contributed by Lipscomb provided a foundation for the monumental synthetic work of M. F. Hawthorne and his students, who transformed boron cluster chemistry first by their synthesis of the polyhedral borane anions BioHio and Bi2Hi2 (30) and later by the synthesis of metallacarboranes (31). In our own laboratory, considerations of charge distribution in borane anions (14) prompted us to explore reactions of transition metal ions with small borane and carborane anions (32). This, in turn, led to a number of advances including the first closed polyhedral metallaboranes (33), the discovery of metal-induced oxidative cage fusion (34), and the synthesis of B12H16, the only neutral Bi2 hydride, via metal-promoted fusion of BeHQ (35)... 

Ivanov, S. V., Lupinetti, A. J., Solntsev, K. A., and Shauss, S. H. 1998a. Fluorination of deltahedral closo-borane and -carborane anions with N-fluoro reagents. J. Fluorine Ghem., 89, 65-72. 

Neutral carboranes and boranes react with transition-metal complexes forming metallocarboranes or metalloboranes, respectively. However, most metallocarboranes and metalloboranes are prepared from transition-metal halides and anionic carborane and borane species ( 6.5.3.4) or by reacting metal atoms and neutral boranes and carboranes. These reactions are oxidative addition reactions ( 6.5.3.3). 

The polyhedral boranes and carboranes discussed above may be regarded as boron clusters in which the single external orbital of each vertex atom helps to bind an external hydrogen or other monovalent atom or group. Post-transition main group elements are known to form clusters without external ligands bound to the vertex atoms. Such species are called bare metal clusters for convenience. Anionic bare metal clusters were first observed by Zintl and co-workers in the 1930s [2-5], The first evidence for anionic clusters of post-transition metals such as tin, lead, antimony, and bismuth was obtained by potentiometric titrations with alkali metals in liquid ammonia. Consequently, such anionic post-transition metal clusters are often called Zintl phases. 

The positions of the hydrogen atoms in boranes and carboranes are worth noting. In doso-borane anions the n BH units are aligned... 

The structures of nido- and amcAwo-boranes and carboranes can be rationalized in a similar manner to those of the closo species if one considers the hypothetical anions, and B Hn , from which they... 

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... 

Many boranes and carboranes can act as very effective polyhapto ligands to form metallaboranes and metallacarboranes. Metallaboranes are borane cages containing one or more metal atoms in the skeletal framework. Metallacarboranes have both metal and carbon atoms in the cage skeleton. In contrast to the metallaboranes, syntheses of metallacarboranes via low- or room-temperature metal insertion into carborane anions in solution are more controllable, usually occurring at a well-defined C2BK open face to yield a single isomer. 

Entries in the second column of Figure 3.1 and in Figures 3.3 and 3.4 show the deltahedral fragment skeletal shapes most commonly encountered among boranes and carboranes of formulae B H +4, CB iH +3, C2B 2H +2, and so on, all of which can be regarded as derived from hypothetical anions [B H ]4 by protonation and/or isoelectronic replacement of B by C. Their molecular formulae are sometimes written as CxB H +4, where x + y = n, the total number of skeletal atoms. The skeletal structures of these nido species. 

Borane or carborane anions and Si halides form the Si—boron bond in >50% yield. ... 

The most versatile route to Ge—boron-containing species is from Ge halides and boron hydride anions, products embracing both oxidation states of Ge, inorganic and mono-, di-, and triorganogermyl derivatives, as well as borane and carborane substrates (Table 1) (cf. 5.3.7.3.2 for the analogous Si-based reactions). 

The occurrence of stable neutral. binary carbonyls is restricted to the central area of the d block (Table 19.3), where there are low-lying vacant metal orbitals to accept o-donated lone-pairs and also filled d orbitals for jr back donation. Outside this area carbonyls are either very unstable (e.g, Cu, Ag, p. 1199), or anionic, or require additional ligands besides CO for stabilization. As with boranes and carboranes (p, 181), CO can be replaced by isoelectronic equivalents such as 2e , H , 2H or L. Mean bond dissociation energies Z>(M-CO)/kJ mol increase in the sequence Cr(CO)fi 109, MoCCO>6 151. W(CO)6 176, and in the sequence Mn2(CO)so lOO, Fe(CO)s 121, Co2(CO)g 138. NiCCO)4 147. 

Boranes and carboranes have structures in which their skeletal B- or C-atoms form triangular-faced polyhedra. There are basically three structural types, namely the closo- (an euphonious modification of the Greek clovo = cage, i. e., a complete or closed polyhedron), the nido (from Latin nest-like ) and the arachno- (from Greek cob-web ) structure. Each of these three types is adopted by cluster compounds of specific atomic ratios. c/o o-Structures occur in borane dianions B H , in car-borane anions (CB iH ) , and carboranes (C2B 2H ). Each skeletal atom has a single H-atom terminally attached by a bond directed outwards, away from the polyhedron center (see the example of BioHio in Fig. 3-1 below). Wo-Structures are adopted by boranes B H +4 and their related carboranes CB iH +3, C2H 2H +2 etc., and amc/z/2o-structures by boranes B H +6 and related carboranes CB iH +5, C2B 2H +4 etc. In other words, carboranes have the general formula [(CH) (BH) Hc] , where the sum a + c + x) is equal to 2 for a closo-structure, 4 for a /do-structure, and 6 for an amc/z o-structure. 

The fundamental assumption in the boranes and carboranes-metal clusters analogy that considers each unit of the cluster contributing only three s-p hybrid orbitals to form the metal cluster skeleton appears to be not totally confirmed by the theoretical treatment analyzed above. This shows that is not possible to define a precise set of molecular orbitals involved exclusively with the bonding in the metal network. However, the number and the symmetry of the antibonding molecular orbitals in [Co6(CO)i4] (11 orbitals with symmetries Ti , T2u, and Eg) with a strong s-p character are qualitatively analogous with the equivalent orbitals calculated for the anion [B6H6] . 

---