Skeletal MO

Fig. 19. Sketch of (a) the f2g surface skeletal MOs of [B6H6]2 and (b) the lowest lying surface MO of l,6-[(OC)3Fe]2B4H4 2. Arrows represent the vectorial character of the 2p functions on the individual boron atoms.

Figure 3,11. Frontier MOs of square pyramidal B5H9 derived from the skeletal MOs of octahedral by replacement of a unit by a square arrangement of...

All of the clusters of this type (see Table 8 and Fig. 14 for examples) possess a single interstitial atom and are characterised by (12 ns + 18 + x) cluster valence electrons. These clusters generally have 3 extra occupied skeletal MO s (i.e. x = 6), which are exclusively involved in tangential bonding in the external cluster polyhedron. 

The pattern of skeletal MO s characteristic of a general 3-connected cluster [EnHn] (where E is a main group element) is contrasted in Fig. 24 with that for a general deltahedral cluster of the same nuclearity. The figure shows that, while deltahedra gener-... 

Table 9. Bonding and non-bonding skeletal MO s of some 4-connected and related capped 3-connected polyhedral clusters...

It is therefore, apparent that, despite the fact that the vertices of a 3-connected polyhedron may all lie on the surface of a single sphere, the arrangement of these vertices represents a poor coverage of the sphere s surface. Thus, these polyhedra do not emulate the sphere sufficiently well for their skeletal MO s to be good approximations to the particle on a sphere solutions. In particular, the low number of close atom-atom contacts (i.e. cluster edges) is insufficient to enable a distinction to be made, in all cases, of the bonding characteristics of the even (L") and odd parity (L31) tangential orbitals. 

Fig. 24. Schematic comparison of the skeletal MO pattern of deltahedral and 3-connected [EnH ] molecules (where E is a main group atom)...

Figure 5.13 Construction of the skeletal MOs of pyramidal borane by the ring-cap interaction.

CF bonding or with F atom lone pairs, again superimposed on a basic set of skeletal MOs for the central OC2 moiety, analogous to those found for H2O. 

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