Boranes classification

Arachno Clusters (2 n + 6 Systems). In comparison to the number of known closo and nido boranes and heteroboranes, there are rdatively fewer arachno species. Pardy because of the lack of a large number of structures on which to base empirical rules, arachno structures appear to be less predictable than their closo and nido counterparts. For example, there are two isomeric forms of B9H15, one with the arachno [19465-30-6] framework shown in Figure 2 (33), the other with a framework more reminiscent of that shown for the nine-atom nido classification (34). Structures of arachno molecules involve the presence of even more extra hydrogens or other electron-donating heteroatoms than nido molecules. Typical examples are given in Table 1. 

This survey presents a historical review of the classification of boranes a description of the modern approach follows, and then a survey of the structures of the known boranes completes the presentation. 

Another type of reaction used in the classification by Edwards and Parry was the evolution of molecular hydrogen by members of the BnHn+6 series to yield the corresponding member of the BnHn+4 series. The former species are clearly less stable than the latter. For example, B5HU forms B5H9 as one of its major decomposition products however, this property has not been used with success to distinguish the two classes of boranes. 

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. 

DOT CLASSIFICATION 4.1 Label Flammable Solid, Poison SAFETY PROFILE Poison by inhalation, ingestion, skin contact, and intraperitoneal routes. Ignites in O2 at 100°C. Forms impact-sensitive explosive mixtures with ethers (e.g., dioxane) and halocarbons (e.g., carbon tetrachloride). Incompatible with dimethyl sulfoxide. When heated to decomposition it emits toxic fumes of boron oxides. See also BORON COMPOUNDS and BORANES. 

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. 

Carboranes may be classified by structural type using the same method described previously for boranes. Because a carbon atom has the same number of valence electrons as a boron atom plus a hydrogen atom, formally each C should be converted to BH in the classification scheme. For example, for a carborane having the formula C2B8H)q,... 

As we have seen previously, the number of electrons involved in framework bonding in boranes is related to the classification of the structure as close, nido, arachno, hypho, or klado. Rearranging this equation gives... 

The structural classification and bonding in boranes is described in Topic C7 especially striking are the anions [BWHn] with closed polyhedral structures. Boranes with heteroatoms can also be... 

Since this article was submitted to the publisher, two papers concerned with classification of the reactions of the boranes in relation to their structures have appeared (83a, 86a). 

As a matter of classification, these single bond complexes are divided into three groups according to the number of electrons formally donated by the borane ligand to the metal. The zero-electron donor (or electron pair acceptor) complex is exemplified by the compound Na[(OC)sMn BH3] (J), the one-electron donor by the compound l,2-(CH3)2-3-[(C5H5)Fe(CO)2]-BioC2H9 (2), and the two-electron donor by the compound (CH3)4N[7,8-B9H,oCHPCr(CO)5] (3). 

Valence electron counts provide a convenient basis to classify these structural types. Various schemes for relating electron counts to structures have been proposed most are based on rules formulated by Wade. Wade s classification scheme is summarized in Table 15.7. It is remarkable that the pairs of framework bonding electrons solely depend on the number of corners of the parent polyhedron. The challenge is identifying the parent polyhedron on the basis of the borane formula. An example of a counting scheme to conveniently deduce the number of framework bonding pairs (and therefore the parent polyhedron) is presented later in this section. 

Classification Borane Empirical C5H13BO Formula (C2Hs)2BOCH3... 

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