Carboranes electron deficient

R. E. Williams, Coordination number-pattern recognition theory of carborane structures, Adv. Incrg. Chem. Radiochem. 18, 67-142 (1976). R. E. Williams, Chap. 2 in G. A. Olah, K. Wade and R. E. Williams (eds.). Electron Deficient Boron and Carbon Clusters, Wiley, New York, 1991, pp. 11-93. 

But the discovery of the carboranes in the early 1960s revealed that bonding possibilities other than simple o-- or 7r-bonds between B and C centers were necessary to understand the structure of such compounds as 1,5-dicarba-closo-pentaborane(5) [6 in Eq. (1)], which is obtained in low yield in an electrical discharge.6 Ordinary valence conventions cannot account for the bonding of boron to five other atoms, and hence the concept of electron-deficient bonding must be invoked for boron. Although carbon seems to adhere to normal tetravalence, again it should be remembered... 

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). 

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... 

There are a number of heteroatoms that can substitute for either boron or carbon in the carboranes. The groups that are as electron-deficient as BH groups are listed vertically to the left of the center line in Table V, whereas those that are as capable as carbon in donating electrons are listed to the right of the center line. The transition elements for the most part electronically substitute for boron and occupy high-coordination sites, but upon electron demand the transition element may also substitute for carbon and concomitantly occupy low-coordination sites. Several transition element moieties, by contrast, are one more electron deficient than boron and occupy, as would be anticipated, high-coordination positions and require additional electron donors (CH groups) to counter the electronic deficit (XIII-24). 

Por a more general overview, based in part on CNPR theory, which encompasses not only electron-deficient carboranes but also includes... 

Boranes, carboranes, and metallocarboranes are electron-deficient compounds in which B-H-B three-center two-electron (3c-2e) bridged bonds are found. The B-H-B bond results from the overlap of two boron sp3 hybrid orbitals and the Is orbital of the hydrogen atom, which will be discussed in Chapter 13. 

Boranes and carboranes are electron-deficient compounds with interesting molecular geometries and characteristic bonding features. 

The electron-deficiency in carboranes arises because of the fact that the number of valence atomic orbitals used in bonding within the molecule is greater than the number of valence electrons available. This can be clearly illustrated by... 

A15. K. Wade, Electron Deficient Compounds. Nelson, London, 1971. An excellent introduction to the reactions of and bonding in metal alkyls and carboranes. 

Because of the number and complexity of the boranes and their derivatives we shall not attempt to describe all their structures in detail. Apart from B2H6 most of the boranes have boron skeletons which are usually described as icosahedral fragments. (They could equally well be related to the more recently discovered carboranes B C2H +2 since the boron-carbon skeletons in these compounds are highly symmetrical triangulated polyhedra (Table 24.8) which include the icosahedron.) The great theoretical interest of the boranes stems from the fact that they are electron-deficient molecules, that is, there are not sufficient valence electrons to bond together all the atoms by normal electron-pair bonds. In these molecules the number of atomic orbitals (1 for each H and 4 for each B) is greater than the total number of valence electrons ... 

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... 

The carboranes are mixed hydrides of carbon and boron having both carbon and boron atoms in electron deficient skeletal framework. 

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