Carboranes structural rules

Almost all of the ido-carboranes have bridge hydrogens, and the organization of their structures is much more complicated than closo-carborane structures because the bridge hydrogens (rule 2) apparently... 

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

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. 

The rules in overall decreasing order of importance essentially state that the ideal structures for carboranes will be based on most spherical deltahedra (rule 1) the BE hydrogens will tend to be placed in the lowest possible coordination environments (rule 2) when elements to the right of boron in the periodic table are incorporated into the deltahedron or deltahedral fragment, they will tend to preempt low-coordination sites (e.g., carbon) or, if electron-deficient, high coordination sites (rule 3) and, lastly, boron will eschew seven-coordinate BH or six-coordinate... 

In discussing the various types and classes of carboranes 45, 78,123, 131, 147, 147a, 158, 164) in the following sections, the relationship of structure and rule are brought into focus. 

When carborane analogs of B gH x 1 are discovered, it would be expected that an isomer of CB4H10 such as IV-A15 (violating only rule 3s) would be more stable than the isomer III-A15 which contains two undesirable 6 6-bridge hydrogens. Near the completion of this manuscript, Matteson and Mattschei reported (91) a CB Hxo isomer for which they suggest the IV-A15 structure. 

The next development in the understanding of structure and bonding in the Zintl ions recognized their relationship to the polyhedral boranes and the isoelectronic carboranes. Then the Wade-Mingos rules [13-16], which were developed to understand the structure and bonding in polyhedral boranes, could be extended to isovalent Zintl ions and related post-transition element clusters. 

The electron counting rules of Wade (S3), Williams (117), and Rudolph (118) can serve as a useful concept to explain structure and bonding in a variety of systems which at first glance are very different Zintl phases, boranes and carboranes, transition metal n complexes and carbonyl clusters, nonclassical carbocations, and also n complexes of main-group elements. According to... 

For polyhedral clusters (sometimes called deltahedral, because the faces are all triangles resembling the Greek letter delta) the ancestor of all electron counting schemes is the correlation proposed by Wade between borane (or carborane) cages and metal carbonyl cages. Wade first drew attention to the similarity of a M(CO)3 unit and a BH (or CH) unit, a relationship that we would now call isolobality (Section 1-6). He then proposed that the 2n + 2 rule for closo boranes (Chapter 5) would also apply to closo metal cluster species such as [Os CO) ]2, and that 2n + 4 and 2n + 6 electron counts would, similarly, be appropriate for stable M clusters with nido and arachno structures. Hydrogen atoms are assumed to contribute one electron each, an interstitial carbon atom four electrons, and so on. 

Closo, nido, and arachno boranes form series with formulae B H2 +4, and B H2 +6 coirfirming their status as formally subvalent componnds. 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, 805 nses 10 of the total 22 valence electrons in 5 lone pairs. This allows 12 electrons, or 2n 4- 2 for n = 5 for cluster bonding and specifies a closo structure, matching the observed Z>3h trigonal bipyramidal structure. 

In Chapter 15 we observed that the 18-eleciron rule was adequate for predicting stabilities of small organometallic clusters. In this chapter we have seen that Wade s rules allow us to make predictions about borune structures based on the number of framework electrons. These rules al.so are adequate for most carboranes, metallacarboranes, and other heteroboranes.Furthermore, organometallic clusters that are not derived from boranes can be dealt with in a similar fashion. More sophisticated extensions are required for complex larger clusters. 9... 

The carboranes conform to the electronic rules given above for boranes and are known in closo. nido, and aracitiw structures. When applying the formulas to the carboranes, each C—H group should be regarded as donating three electrons to the framework count. Some carboranes provide interesting examples of the possible horizontal transformations of Fig, 16.50 mentioned above. For example - - ... 

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... 

To the structural generalizations on carboranes (p. 185) can be added the rule that, in metallocarboranes, the M atom tends to adopt a vertex with high coordination number M occupancy of a low CN vertex is not precluded, particularly in kinetically controlled syntheses, but isomerization to more stable configurations usually results in the migration of M to high CN vertices. 

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