Borane and Carborane Cluster Compounds

Partial one-electron MO diagram for [RejClg] focusing 5 on the Re-Re bonding. Those 

Proposed structure of the first pentuply bonded compound, [CrCgH3-2,6-(C6H3-2,6-(CHMe2)2)2] and its orbital interactions, [(a) Reproduced from http //en.wikipedia.org/wiki/Quintuple bond (accessed January 2, 2014) and (b) http //commons.wikimedia.0rg/wiki/File Quintuple bond orbital diagram.png (accessed January 2, 2014).] 

Deltahedral cages having n = 5-12, where n is the number of vertices. [Blatt Communications.] 

Determine the valence electron count by adding three electrons for each B atom and one electron for each H atom. If the borane is negatively charged, then additional electrons will need to be added for the charge on the anion. 

Determine the type of borane cluster from Table 10.20, where n is the number of B atoms. Goso structures have n vertices. Nido structures (from the Greek word for nest) are based on the n + I deltahedron with a single vertex missing. Similarly, the arachno (spider), hypho (net), and klado (branch) boranes 

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Although it may not be surprising that the same set of electron-counting rules can be used to describe satisfactorily such similar compounds as boranes and carboranes, we should examine how far the comparison can be extended. Can Wade s rules, for example, be used effectively on compounds containing metals bonded to boranes or carboranes Can the rules be extended even further to describe the bonding in polyhedral metal clusters ... 

There are some interesting observations too concerning the structures of polyhedral molecules. Very often they are electron deficient in the sense that there are fewer than two electrons for each close contact. The heavy atoms forming the skeleton of the molecule may be either main group atoms (for example in the boranes and carboranes) or transition metal atoms (metal cluster compounds) or both (metallocarboranes). 50 shows the structures expected from Wade s rules for five atom polyhedral molecules with six, seven and eight pairs of skeletal electrons. There are a total of fifteen skeletal orbitals... 

Closo, nido, and arac/moboranes form series with formulae B H2 +4, and B H2 +6 confirming their status as formally subvalent compounds. Boranes and carboranes (in which one or more boron atoms in the cluster have been replaced by a carbon atom) have been extensively researched and described, and will not be discussed further here. In an example of the application of Wade s rules to a heavier main group cluster, Sns uses 10 of the total 22 valence electrons in 5 lone pairs. This allows 12 electrons, or 2n - - 2 for n = 5 for cluster bonding and specifies a closo structure, matching the observed Dsh trigonal bipyramidal structure. 

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 chemical species with spherically shaped structures are found only seldom in the chemistry of main group elements. Some examples of this type of compounds are the boranes and carboranes described above and some post-transitional deltahedral clusters to be described later in this Chapter. 

Boranes and carboranes are comprised of a-aromatic cage-like structures with boron and carbon vertices ranging from small tetrahedral to supra-icosahedral clusters. Despite more than four decades of work with this class of inorganic compound, there have been few reports of their photophysical properties, with a distinct paucity of information regarding luminescence. Carboranes have nevertheless been incorporated into photophysically active metal complexes, usually as ancillary components. But what of boranes, carboranes, and metallacarboranes (where a transition metal atom can either be an integral vertex in the polyhedral skeletal framewoik or an exterior component thereof) as chromophores themselves and their resultant optoelectronic behavior In this arena, there are very few contributors. 

Wade electron counting rules borane-like cluster nomenclature. On initially studying compounds such as boranes (boron hydrides) and carboranes (or carbaboranes boron—carbon hydrides), Wade (1976) proposed a number of rules which have then been extended to several compounds and which relate the number of skeletal electrons with the structure of deltahedral clusters. A polyhedron which has only A-shaped, that is triangular, faces is also called a deltahedron. 

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