Skeletal bonding electrons

B. Significance of the Numbers of Skeletal Bonding Electron Pairs... 

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 remaining valence shell electrons (b pairs) are regarded as skeletal bonding electrons. 

The Number of Skeletal Bonding Electrons (v + x — 2) That Main Group Cluster Units Can Contribute... 

Estimated, assuming that the skeletal bonding electrons are distributed equally among the skeletal atoms. 

Polymetallic anions, prepared by dissolution of alloys of the alkali and post-transition metals in amine solvents (often with a complexand for the alkali metal cation), have been characterized in crystalline and solution phases. Clusters TlSng3, Ge92 (with 20 skeletal bonding electrons), Sn93- (21 skeletal e) and Bi95+ (22 skeletal e) possess a tricapped trigonal prismatic structure, symmetry D3A, with variations of dimensional detail which correlate with the electron population.291 292 This structure is a ctoso-deltahedron, and with 20 (2h + 2) skeletal electrons can be construed to be three-dimensionally aromatic.292 The 22e clusters M94 (M = Ge, Sn, Pb) occur as the C4v monocapped square antiprism, a nido polyhedron. 

Similarly we can show that in case of nido boranes with n atoms we should have (2n + 4) skeletal bonding electrons and in case of arachno boranes with n atoms we must have (In + 6) skeletal bonding electrons. 

For example, B5H9 (nidoborane) has 14 skeletal bonding electrons. Wade s rules are applicable to "naked" clusters formed by / -block elements and so in case of [Pb5]2- having 22 valence electrons we expected 10 used lane pairs and 12 skeletal bonding electrons, i.e. a closo structure. 

On similar considerations [Sn9]4- which has 22 skeletal bonding electrons should possess nido structures. 

Borane Clusters Containing 6 to 13 Skeletal-Bonding Electron Pairs (N) 182... 

Table 1 shows examples of closo-, nido-, arachno- and hypho-boianes containing 6 through 13 skeletal bonding electron pairs. Where neutral borane example are not available, ionic species or heteroboranes are cited. 

This section presents the structure of the boron hybrides and is arranged in accordance with the relationship defined by Wade s Rules and expressed by Williams and Rudolph. Thus for the boranes containing six or more pairs of skeletal bonding electrons, the relationship between the structures of the closo-, nido-, arachno-and hypho-species is described. In cases where the parent borane does not exist, examples from heteroboranes with the correctly predicted structure based on Williams coordination number pattern recognition theory (CNPR) of borane structures will be described 70. Treated separately will be mono- and diborane species and also species with more than 12 boron atoms. Although there have been several reviews on the structures of the boranes in recent years none have used the current approach89. ... 

The structural relationship between the closo-, nido- and arachno-species containing seven skeletal bonding electron pairs is shown in Fig. 13. 

K. Wade, The Structural Significance of the Number of Skeletal Bonding Electron-pairs in Carboranes, the Higher Borane Anions, and Various Transition-metal Carbonyl Cluster Compounds, Chem. Comm. 1971, 792-793. 

A second structural feature common to metal clusters, but very rare with main-group clusters, is face-capping. Examples of face-capped clusters are shown in Figure 3.4. Look at a couple. You should be able to see that each can be considered to be formed from a primary cluster by capping a triangular face. Mingos showed that the number of skeletal bonding electron pairs associated with a capped cluster is the same as the number associated with the primary cluster. Thus, a capped octahedral cluster like Os7(CO)2i has seven sep, i.e., 7 Os(CO)3 = V2(7 x 2) = 7, which is the same... 

To underline the structural and bonding relationship between these mixed metal-carbon cluster species and carboranes, we list the formulae of representative examples in Table 4.1, classified according to the numbers of skeletal bonding electrons they contain and thus according to their structural type (closo, nido, or arachno ). 

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