Localized bond schemes

Figure 10. 13C-CP/MAS spectrum at 22.6 MHz (90 MHz 1H) of cyclooctate-traenediiron pentacarbonyl (4) shown along with the localized bonding scheme suggested from x-ray diffraction measurements.

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. 

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. 

LOCALIZED BOND SCHEMES FOR CLOSO BORANES AND CARBORANES... 

Interestingly, these regular tetrahedral systems are readily accommodated by localized bond schemes Four 3c-2e bonds in the tetrahedral faces when there are four skeletal bond pairs, as in B4CI4 and six 2c-2e bonds along the edges of the tetrahedra when there are six skeletal bond pairs, as in P4. 

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.)...

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. 

O Neill and Wade have discussed such localized bonding schemes using two-center B-B bonds and three-center B-B-B bonds with the following basic assumptions ... 

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

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

Two common features of boron hydrides (see Sections 13.5 and 13.11) are that the B atoms are usually attached to more than three atoms and that bridging H atoms are often present. Although a valence bond model has been developed by Lipscomb to deal with the problems of generating localized bonding schemes in boron hydrides, the bonding in these compounds is not readily described in terms of VB... 

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. 

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