Localized bond schemes for

LOCALIZED BOND SCHEMES FOR CLOSO BORANES AND CARBORANES... 

Figure 3.10. Localized bond schemes for square pyramidal and pentagonal pyramidal carboranes, showing only one of the several possible canonical forms in each case. (2,4-C2B4H8 and 2,3,5-6363117 incorporate CHB bridge hydrogens. Relative energies, jei in kcal per mole.)...

Localized Bonding Schemes for Bonding in Polyhedral Boranes 1119... 

Closo dusters with unusual electron numbers localized bond schemes for... 

The localized bonding scheme for Ti(H20)e is obtained by first constructing six equivalent hybrid orbitals that are octahedrally directed. We use the six cr valence orbitals of Ti for this purpose 4s, 4px, 4py, and Apz. Thus we want to construct six d sp hybrid orbitals, each with one-third d character, one-sixth s character, and one-half p character. 

Fig. 4. Localized two- and three-center bond schemes for the boranes B4H10, BbHii, and BeHio.

In the hands of Lipscomb and others 76,143, 145a, 153,189, 190), such localized bond schemes, particularly when obtained via self-consistent field (SCF) molecular orbital (MO) treatments, have proved particularly valuable for rationalizing the shapes and interatomic distances and estimating the charge distribution in many higher boranes. 

Fig. 5. An apparently satisfactory localized skeletal bond scheme for a hypothetical icosahedral anion Bi2Hij (two-center B—B bonds are represented by full lines, three center BBB bonds by triangles with exclusively broken-line edges).

However, used qualitatively, localized bond schemes are less satisfactory for rationalizing the shapes of such relatively symmetrical species as the closo anions B H , for most of which it is not possible to find a single localized bond arrangement that appears appropriate for the symmetry. For example, the equations of balance for the anion require its... 

MO treatments are preferred in such cases. The manner in which these same cations, (CsHs) and (CeMce) can be treated in MO terms is worth illustrating here for the purpose of comparison with the localized bond schemes just discussed, and also to underline the relationship between these pyramidal systems and normal aromatic ring systems. 

Fig. 12.21 The structures of some representative aluminium-nitrogen cluster compounds. Localized bonding schemes are appropriate for each cage (see problem 12.17 at the end of the chapter).

An electron-deficient species possesses fewer valence electrons than are required for a localized bonding scheme. 

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