Skeletal electron numbers

The skeletal changes that accompany changes in skeletal electron numbers are reflected in one of the most widely used routes to metallo-carboranes, the so-called polyhedral expansion reaction. This reaction, which has been used to considerable effect by Hawthorne and his co-workers (68-71, 77-79, 82), involves conversion of a neutral cloao-... 

For further examples of the ways in which the reactions of boranes (and related clusters) can be rationalized in terms of changes in their skeletal electron numbers and distribution, see Ref. 173a). 

The next important contribution in this area was made shortly thereafter by Wade [16], who recognized that this structural relationship could be related to the number of valence electrons associated with skeletal bonding in the boranes. Thus deprotonation of all of the bridging hydrogens from the related series of boranes B H 2T B iH(n 1)+4, and Bn 2H( 2)+6 gives the ions B H 2 , B H,- 4-, and B -2H 26-. All of these ions can readily be seen to have the same number of skeletal electron pairs, namely it +1, corresponding to 2it + 2 skeletal electrons. 

Rules for counting the number of skeletal electrons provided by each vertex atom need to be established in order to determine the number of skeletal electrons in polygonal and polyhedral clusters of the post-transition elements. The rules discussed above for polyhedral boranes can be adapted to bare post-transition metal vertices as follows ... 

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 following optimum electron numbers have been defined on the basis of the so-called Polyhedral Skeletal Electron-Pair Theory (PSEPT) . 

In addition to the rules reported, a relation between the total electron count (TEC) and the number of skeletal electrons (SE) may be mentioned. This may be summarized as follows (Wade 1976) ... 

Ralph Rudolph made major contributions to our understanding of the structure and bonding of polyhedral cluster compounds and he had an abiding interest in developing a rationale which would enable the structure of individual compounds to be systematized and related to each other. He independently arrived at a method of counting skeletal electrons which is now generally referred to as Wade s Rules, and this has had a decisive influence on our general perceptions of polyhedral cluster compounds. Related to this was his preoccupation with the problem of heteroatoms such as sulfur, and the number of electrons which such atoms contribute to the heteroborane clusters. 

You will recall that boranes are now classified into various series as summarized in Table I. The formulae and structures can be rationalized on the basis of the number of skeletal electrons available for bonding and each B atom is considered to contribute 2 electrons in addition to the one used to form a terminal B-Hj-bond. Supernumerary H atoms form B-H j-B bridges or comprise the endo-li atom in a 6H2 group they occur in the "open... 

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