Structures of boranes

W. N. Lipscomb (Harvard) studies on the structure of boranes illuminating problems of chemical bonding. 

Fig. 5.7. Crystal structure of borane complex of a,a-diphenylprolinol oxazaborolidine catalysts. Reproduced from...

William Lipscomb United States structure of borane and bonding... 

The problems outlined in the previous section can be avoided if, instead of allocating the skeletal bonding electron pairs to localized bonds, one simply compares their number with the number of skeletal bonding MO s (199). The closo, nido, and arachno structures of boranes and carboranes can then be seen to reflect the numbers of skeletal bond pairs that are available to hold their skeletal atoms together. 

The triangular-faced polyhedra shown in Fig. 1 form the bases for the structures of boranes and carboranes. 

Fig. 16.47 Molecular structures of boranes related to (a) decaborane l4) formed...

Complex 26 is among the first examples of intermolecular coordination of a single B-H bond in a neutral borane to a transition metal, and these species can be regarded as models for alkane coordination. The crystal structures of borane complexes such as 26 are of interest to compare with those for other octahedral a complexes, particularly... 

Historically, a scheme of skeletal electron-counting was developed to rationalize the structures of boranes and their derivatives, to which the following Wade s rules are applicable. 

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. 

The electronic structures of borane clusters were first successfully described using localized three-center- and two-center-two-electron bonds. These treatments have been replaced by the cluster electron-counting rule based on MO methods hence, why bother with the three-center bond model in a book about clusters Let s consider why there is value in a more localized approach. 

Deficiency without deficiency. Let us turn now to the electronic structure of boranes. Usually they are classified with electron-deficient compounds because the number of possible two-centre interactions exceeds the number of valence-electron pairs. 

Alternatively, one might explore close-packed structures of boranes. Consider, for example, the difference between deltahedral and tetra-capped octahedral BioHio structures (Fig. 4). The former is stable as the dianion (11 cluster pairs) while, based on Extended Htickel calculations (60), the latter is slightly more stable for the +6 cation (7 cluster pairs). The capping principle predicts 7 cluster pairs for capped octahedra (61). A structurally characterized transition metal analogue of the tetra-... 

Fig. 16.47 Molecular structures of boranes related to [Bi2H,2] (a) decaborane(l4) formed by removal of atoms B, and B(, (b) hexaborane(IO). Note that the pentagonal pyramid is an apical fragment of an icosahedron, (c) Octaborane(l2) related to [B 2H 2] by removal of B, B. B5. Bft. [From Muetterties, E. L. The Chemistry of Boron and Its Compounds, Wiley New York, 1967. Reproduced with permi.ssion.]...

In Section 12.11, we introduced Wade s rules to rationalize the structures of borane clusters. This method of counting electrons can be extended to simple organometallic clusters by making use of the isolobal relationship between cluster fragments. 

Wade (1976) derived a set of guidelines, the so called Wade s rules , for relating the structures of boranes to their composition. They are based on the results of MO calculations and pertain to boranes of the type B H ", where m>n and x > 0. For m = n, Wade s rules predict that the cteo-structure is preferred. Analogous rules apply to the nido- and arachno-iypQS. We will not discuss these two types, because only cteo-structures have been found for diazoboranes. 

The structures of boranes can be classified according to the following scheme ... 

Fig. 2.15. Structure of borane molecules B2H BgH,p small circles (after Wells (1962)).

The PSEP method [217, 225-227] is derived from rules that had originally been developed to ratio ize the structures of borane compounds. [215, 216] The borane anions are so-called closo compounds and exhilnt deltahedral... 

The bonding and structures of boranes are of great interest as they are different from all other hydrides. As there are not enough valency electrons to form conventional two electron bonds between all of the adjacent pairs of atoms, these compounds are electron deficient compounds. 

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