Bond hybridization

The triaLkoxy(aryloxy)boranes are typically monomeric, soluble in most organic solvents, and dissolve in water with hydrolysis to form boric acid and the corresponding alcohol and phenol. Although the rate of hydrolysis is usually very fast, it is dependent on the bulk of the alkyl or aryl substituent groups bonded to the boron atom. Secondary and tertiary alkyl esters are generally more stable than the primary alkyl esters. The boron atom in these compounds is in a trigonal coplanar state with bond hybridization. A vacantp orbital exists along the threefold axis perpendicular to the BO plane. 

The promotion of electrons will occur if, overall, it leads to a lowering of energy by permitting the formation of more bonds. Hybrid orbitals are constructed on an atom to reproduce the electron arrangement characteristic of the experimentally determined shape of a molecule. 

The lower signal is more complicated, and before we can interpret it exactly we need some background information. The magnitude of one-bond C-C coupling constants depends on bond hybridization (ethane 35, ethene 68, benzene 56, ethyne 172 Hz), while two- and three-bond C-C couplings are very small, often around 2-5 Hz. The second thing we have to remember, and this is a new concept, is that the lines in the multiplets from INADEQUATE spectra often come from different spin systems ... 

The stable carbanions may belong in a special category since their stability is in most cases due to resonance, and the resonance has geometrical requirements that might or might not be the same as those of the bond hybridization of an ordinary carbanion. The central hydrogen of triptycene has none of the acidity of the central hydrogen of triphenylmethane.364... 

Figure 2.7 Percentage p character of Li and F bonding hybrids in ionic-bond formation.

This is again consistent with the essentially pure s character of the bonding hybrids in H2.10... 

In conclusion, sp-hybrid character differs strongly between dialkali neutrals and cations, in sharp distinction to the rather constant s character in H2 versus H2+. A comparison of bonding in Li2 and Li2+ therefore does not merely depend on the number of electrons in the bonding NBO (change in formal bond order), but rather reflects deep differences in the bonding hybrids themselves, particularly with regard to directional p character. 

Both (3.17) and (3.19) are presumed to be expressed in terms of the same hA and hB bonding hybrids,16 and each function provides only an approximation to the true bond pair wavefunction. In particular, in the closed-shell singlet case V ab(NBO) assumes the same spatial dependence for each electron of the pair, and thus neglects electron-correlation effects that could better be represented as... 

Solution For the given hybrids, the angle >h,h between the bonding hybrids to H and H is determined from the directionality theorem... 

A more detailed comparison of the various carbon bonding hybrids in these species is presented in Fig. 3.12. This figure shows that the visual changes in hybrid shape, even for rather large changes in k (e.g., the changes from sp1 -like to sp3-like in the left-hand panels), are rather subtle. Most obvious is the increasing... 

Figure 3.20 Fractional p character fv) in mono-substituted first-row ALH hydrides for (a) A lone-pair hybrids (upper), and (b) A—L bond hybrids (lower), showing trends in dependence on the substituent s electronegativity (xl) for first-row (solid line) and second-row (dotted line) —FH substituents of C (circles), N (squares), O (triangles), and F (plus signs) central atoms.

As examples of the previous section have shown, bond hybrids are sometimes misaligned with respect to the line of centers between nuclei, a condition described as bond bending. Such bending may be considered to represent the influence of factors other than Bent s rule. In this section we examine the origin and characteristics of bond bending. 

Figure 3.21 The bent C—C bond of cyclopropane, showing (a) the ate NBO and (b) the he bonding hybrid (sp3,46), which is oriented 24.3° outside the line of C—C centers.

Figure 3.24 The bent N—H bond of NH3, showing (a) hN and hH NHOs and (b) the ctnh NBO, with the bonding hybrid oriented 3.9° (dashed line) below the line of N—H centers (dotted line).

Figure 3.79 illustrates some aspects of the nonbonded and cross-bonded hybrid interactions in CH4 and SiH4, showing the weakened H H nonbonded interaction in the latter case. As a consequence, the geminal asm—asm stabilization (1.98 kcalmol-1) of SiH4 is considerably stronger than the corresponding och—och stabilization (0.19 kcal mol-1) of CH4. 

The cross-bonded terms cxCy/ ax and caCaFay result from interaction of X and Y with the wrong bonding hybrids on A. The magnitudes of these terms can usually be judged from simple overlap considerations. Unless X and Y are of quite dissimilar electronic character, the two cross-bonded terms are inherently of similar magnitude and therefore tend to cancel one another out. Thus, cross-bonded terms tend to make only minor contributions to geminal delocalization. 

It is noteworthy that Rydberg orbital occupancies on the central atom (rY, final column of Table 3.29) are relatively negligible (0.01-0.03e), showing that d-orbital participation or other expansion of the valence shell is a relatively insignificant feature of hyperbonded species. However, the case of HLiH- is somewhat paradoxical in this respect. The cationic central Li is found to use conventional sp linear hybrids to form the hydride bonds, and thus seems to represent a genuine case of expansion of the valence shell (i.e., to the 2p subshell) to form two bonding hybrids. However, the two hydride bonds are both so strongly polarized toward H (93%) as to have practically no contribution from Li orbitals, so the actual occupancy of extra-valent 2pu orbitals ( 0.03< ) remains quite small in this case. 

---