Closo-Boranes polyhedra

No. of Skeletal Bonding Pairs Basic Polyhedron closo- Borane nido- Borane arachno- Borane hypho- Borane... 

The structures of boranes can be grouped into several classifications. If the structure contains a complete polyhedron of boron atoms, it is referred to as a closo borane (closo comes from a Greek word meaning closed ). If the structure has one boron atom missing from a comer of the polyhedron, the structure is referred to as a nido borane (nido comes from a Latin word for nest ). In this type of structure, a polyhedron having n comers has (n - 1) comers that are occupied by boron atoms. A borane in which two comers are unoccupied is referred to as an arachno structure (arachno comes from a Greek word for web ). Other types of boranes have structures that are classified in different ways, but they are less numerous and will not be described. 

Another structural class of the boron hydrides is the so-called quasi-closo boranes. They are related to the closo boranes by removing a framework atom from the latter and adding in its stead a pair of electrons. Thus one of the polyhedron framework sites is taken by an electron pair. 

Similar descriptions of bonding can be derived for other closo boranes. In each case, one particularly useful similarity can be found there is one more framework bonding pair than the number of comers in the polyhedron. The extra framework bonding pair is in a totally symmetric orbital (like the Aig orbital in resulting from... 

In addition, it is sometimes useful to relate the total valence electron count in boranes to the structural type. In closo boranes, the total number of valence electron pairs is equal to the sum of the number of vertices in the polyhedron (each vertex has a boron-hydrogen bonding pair) and the number of framework bond pairs. For example, in there are 26 valence electrons, or 13 pairs (= 2n + 1, as mentioned previously). Six of these pairs are involved in bonding to the hydrogens (one per boron), and seven pairs are involved in framework bonding. The polyhedron of the closo structure is the parent polyhedron for the other structural types. Table 15-8 summarizes electron counts and classifications for several examples of boranes. 

A nido (nestlike) boron hydride is derived from a closo borane by the removal of one skeleton atom. If the starting closo borane is not a regular polyhedron, then the atom removed is the one at a vertex with the highest connectivity. An arachno (weblike) boron hydride is derived from a closo... 

Closo-boranes Closed polyhedron of n boron atoms 4n + 2... 

Boranes of the general formula B Hn2 and isoelectronic carboranes such as CB iHk- and C2B 2H have closo structures, in which n skeletal B and C atoms are located at the vertices of a polyhedron bounded by trianglular faces (deltahedron). For the BMH 2 cfo.vo-boranes... 

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. 

Atoms in Cluster Vertices in Parent Polyhedron Framework Electron Pairs Valence Electrons (Boranes) Closo Nido Arachno Examples Valence Electrons (Transitlon-Metal Clusters) Closo Nido Arachno Examples ... 

In addition, there is a wide range of borane derivatives containing atoms of other elements, such as carbon and phosphorus. Borohydrlde ions of the type BeHe also exist. Boranes and borohydrlde ions are classified according to their structure. Those with a complete polyhedron are said to have a closo-structure. Those in which the polyhedron is incomplete by loss of one vertex have a nido-structure (from the Greek for nest ). Those with open structures by removal of two ot more vertices have an arachno- structure (from the Greek for spider ). See also Wade s rules. 

The best (and historically the first) understood are boron clusters, which class comprises boranes (poly-nuclear boron hydrides), carboranes (boranes with one or two B atoms replaces with C), and metallo-carboranes with a metal atom incorporated into the cluster. Their structures have been rationalized by Wade in a set of rules [238]. The structure of a boron cluster depends on the numbers of its skeleton atoms (u) and of skeleton electron pairs (SEPs)(p) available for bonding these atoms. If p = a- -l,the cluster is a closed deltahedron, i.e. a polyhedron having only triangular faces (closo structure). If p = a + 2, it adopts the form of a higher-order polyhedron with one... 

The localisation of this e set on the open face of the polyhedron has two important consequences firstly, they can be stabilised by overlap with the 1 5 orbitals of protons leading to the nido boranes B H +4, and secondly they can be used as donor orbitals to transition metals to form closo metalloboranes. 

The number of vertices of the polyhedron is ng = S/2 - 1, for example 14/2 - 1 = 6 in the above cases. If one, resp. 2 BH (or Fe(CO)3, etc.) vertices are removed from a borane, carborane or metalloacarborane, one jumps from a closo structure to a nido, resp. arachno structure. For example, the mV/c>-7,8-C2B9Hii " carborane is obtained to removing a BH vertex from a closo-1,2-C2BioHi2 it is analogous to the cyclopentadienyl ligand and forms with Fe a dianionic sandwich, analogous to ferrocene. 

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