Pi-Acceptor Interactions

Because the overlap for these orbitals is smaller than the a overlap described in the previous section, Cq- The other tt interactions are weaker than this reference interaction, with the magnitudes depending on the degree of overlap between the orbitals. Table 10-12 gives values for ligands at the same angles as in Table 10-11. 

FIGURE 10-22 Energies of d Orbitals in Octahedral Complexes Sigma-Donor and Pi-Acceptor Ligands. H- Ae. Metal r 

In an octahedral complex with six 7r-acceptor ligands, the and orbitals do not engage in pi interactions with the ligands in positions 1 through 6 (their parameters in the table are all zero). However, the and dy orbitals all have total interactions of 4  

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The left side of Figure 12.13 shows what happens if a good pi acceptor is attached to a double bond. The LUMO of the pi acceptor interacts with both the double bond HOMO and LUMO to produce a new ally lie system with a new HOMO and LUMO, which are lowered from those of the double bond. A pi acceptor bonded to a double bond makes the combination more electrophilic by lowering the LUMO of the system. When the energy of the LUMO of the electrophile is lower, then it is softer, and the stabilization resulting from overlap with the average nucleophile s HOMO is much greater (recall Fig. 2.14). 

Khashaba et al. [34] suggested the use of sample spectrophotometric and spectrofluorimetric methods for the determination of miconazole and other antifungal drugs in different pharmaceutical formulations. The spectrophotometric method depend on the interaction between imidazole antifungal drugs as -electron donor with the pi-acceptor 2,3-dichloro-5,6-dicyano-l,4-benzoquinone, in methanol or with p-chloranilic acid in acetonitrile. The produced chromogens obey Beer s law at Amax 460 and 520 nm in the concentration range 22.5-200 and 7.9-280 pg/mL for 2,3-dichloro-5,6-dicyano-l,4-benzoquinone and p-chloranilic acid, respectively. Spectrofluorimetric method is based on the measurement of the native fluorescence of ketoconazole at 375 nm with excitation at 288 nm and/or fluorescence intensity versus concentration is linear for ketoconazole at 49.7-800 ng/mL. The methods... 

Sigma- and pi-type donor acceptor interactions Further details of the leading hp —hM donor-acceptor interactions are gathered in Table 2.5 and Figure 2.21. For each such interaction the table shows the hybrid form of the donor (hp)23 and acceptor (hM) orbitals, the occupancy of the acceptor, and the second-order estimate (cf. Eq. (1.35) or (2.7)) of the donor-acceptor stabilization energy. Let us now discuss some of the trends displayed in Table 2.5. 

Figure 2.21 Leading sigma- (left) and pi-type (right) donor-acceptor interactions of representative TM monofluorides (see Table 2.5).

The idealized Lewis structures are modibed in each case by donor-acceptor interactions involving the filled (Lewis) 7ta and unfilled (non-Lewis) 7ta NBOs of the formal pi bond, 7ra 7tb and7tb 7ta, nb 7ta, or7ta- nb for the three prototypes shown in (3.101). 

To maximize the conjugative interaction with a specified acceptor 7ra, one could progressively polarize the donor pi moiety until its entire amplitude is on the atom adjacent to 7ta. In this limit, the two-center donor reduces to a one-center nonbon-ded orbital nb (lone pair), and the donor-acceptor interaction is of nb 7ta type. 

In the limit that nearest-neighbor contributions dominate, such a one-center nb automatically leads to an approximate 21/2-fold increase in overlap and two-fold increase in interaction energy, compared with a two-center 7tb donor. A corresponding enhancement results when the pi-acceptor is reduced from two-center (7tb ) to one-center (nb ) form, i.e., a valence p-type vacancy. Unlike the intrinsically bidirectional character of conjugation between two pi bonds (7ta->-7tb, 7tb 7ta ), the interactions of a pi bond with a nonbonding center are intrinsically mono-directional and lead to uncompensated transfer of pi charge from one moiety to the other. 

Another such instance of anticooperativity can be seen by comparing 5 with 13. In the latter case, the two n— -nco interactions make competitive use of the same busy nitrogen lone pair, and each such interaction (47.9 kcal mol-1) is thereby weakened relative to the value (59.8 kcal mol-1) in 5. Similarly, the comparison of vinylborane 6 with divinylborane 14 reveals an anticooperative effect involving competition for the ns pi acceptor, with each 7tcc—>-113 interaction in 14 (24.2 kcal mol-1) being slightly weakened compared with its value in 6 (26.2 kcal mol-1). 

To determine the distinctions (if any) between conjugative donor-acceptor interactions involving six and those involving four pi electrons, we now examine the cyclopentadienyl anion 19 and cation 20. 

Aniline and nitrobenzene electrophilic substitution reactivity We briefly consider the effect of pi-donor or pi-acceptor substituents on aromatic conjugation patterns for two representative examples aniline (23) and nitrobenzene (24). The leading NBO interactions between ring and substituent in these species are depicted in Fig. 3.47. 

As a result of these substituent-induced polarizations, the complementary conjugative interactions at each ring site become somewhat imbalanced (so that, e.g., the donor-acceptor interaction from C3—C4 to C5—C(, is 23.1 kcal mol-1, but that in the opposite direction is only 16.4 kcal mol-1). From the polarization pattern in (3.133) one can recognize that excess pi density is accumulated at the ortho (C2, C6) and para (C4) positions, and thus that the reactivity of these sites should increase with respect to electrophilic attack. This is in accord with the well-known o, /(-directing effect of amino substitution in electrophilic aromatic substitution reactions. Although the localized NBO analysis has been carried out for the specific Kckule structure of aniline shown in Fig. 3.40, it is easy to verify that exactly the same physical conclusions are drawn if one starts from the alternative Kekule structure. 

Figure 3.119 Leading donor-acceptor interactions in AI2H6 (left) and Ga2H6 (right), showing overlap contours in the plane of three-center bridge-bonding (the pi plane, top row) and two-center skeletal bonding (the sigma plane, bottom two rows), with associated second-order stabilization energies in parentheses.

As a further illustration of the dependence of n i 7t pi-backbonding interactions on metal and ligand character, we may compare simple NiL complexes of nickel with carbonyl (CO), cyanide (CN-), and isocyanide (NC-) ligands, as shown in Fig. 4.41. This figure shows that the nNi 7rL pi-backbonding interaction decreases appreciably (from 28.5 kcal mol-1 in NiCO to 6.3 kcalmol-1 in NiNC-, estimated by second-order perturbation theory) as the polarity of the 7Tl acceptor shifts unfavorably away from the metal donor orbital. The interaction in NiCO is stronger than that in NiCN- partially due to the shorter Ni—C distance in the... 

The principal intermolecular donor-acceptor interactions of this weakly bound complex are found to be of 7tcc-OBrBr form (3 x 0.20 kcalmol-1), as illustrated in Fig. 5.41(b). A prominent feature of Br2 (and other heavy halogens) is the nearly pure-p character of the bromine bonding hybrid, resulting in a conspicuous backside lobe on the OBrBr antibond (see Fig. 5.41(b)) that is effective in end-on complexation to the pi-donor face of benzene. The unusually small energy separation between donor and acceptor NBOs,... 

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