Pi-Donor Interactions

FIGURE 10-24 Energies of 4 Orbitals in Octahedral Complexes Sigma-Donor and Pi-Donor Ligands. Ao = - Ae.y. Metal s and p 

Using the angular overlap model, determine the sphtting pattern of d orbitals for a tetrahedral complex of formula MX4, where X is a ligand that can act as cr donor and tr donor. 

Determine the energies of the d orbitals predicted by the angular overlap model for a square-planar complex  

Ao=3e - 4e,j. Metal s and p orbitals also contribute to the bonding molecular orbitals.  

Halide ions donate electron density to a metal via orbitals, a sigma interaction the ions also have p and p orbitals that can interact with metal orbitals and donate additional electron density by pi interactions. We will use [MXs] , where X is a hahde ion or other ligand that is simultaneously a a and a tt donor. 

With ligands that behave as both tt acceptors and tt donors (such as CO and CN ), the TT-acceptor nature predominates. Although Tr-donor ligands cause the value of to decrease, the larger effect of the 7r-acceptor ligands causes to increase. The net result of pi-acceptor ligands is an increase in A , mostly because d orbital overlap is generally more effective with tt orbitals than with Tr-donor orbitals. 

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Thus, fluorine is the strongest sigma-withdrawing element known, but is a mild donor (pi base) in pi-type interactions. 

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 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,... 

In the CH3CH=CH2- -NO+ complex, the nitrosyl cation retains the characteristic canted geometry indicative of strong 7tcc-7txo interaction (Fig. 5.46(c)). However, the electrophilic attack shifts toward the terminal C of the pi bond, away from the methyl substituent. Such anti-Markovnikov complexation is, of course, to be expected from the relative polarization of the propylene pi bond toward the terminal C (so that the 7tCc antibond is polarized toward the alkyl pi-donor). 

Horak, J., Maier, N.M., and Lindner, W., Investigations on the chromatographic behavior of hybrid reversed-phase materials containing electron donor-acceptor systems ii. Contribution of pi-pi aromatic interactions, J. Chromatogr. A, 1045, 43, 2004. 

Figure 4.10. (a) Sigma-type donor interaction between the bond and the LUMO, cr, of a polarized a bond (shown as a p orbital). (donor interaction between the a bond and the LUMO (shown as a p orbital), which may be the n orbital of an adjacent Z-type substituent, or cr of a polarized cr bond. 

In tryptophan, the indole residue is attached to the amino acid s side chain at indole s position 3. When the point of attachment was moved to indole position 5, however, macrocycle 13 complexed NaBPli4 in the pi-fashion but benzene rather than pyrrole was clearly the donor group <2002JA10940>. One may conclude that 12 is a less reasonable model than is 13. In fact, both unequivocally confirm the cation-pi interaction. Further, the fact that cation-pi interactions are prominent in both cases means that in the natural environment, the rarest of the 20 common amino acids is a truly versatile pi donor. 

Another control experiment was run to further confirm the significance of the cation-pi interaction in these bibracchial lariat ether model complexes. In this case, a diaza-18-crown-6 derivative was prepared in which a 2-phenylethyl pi-donor sidearm was attached to one nitrogen and a 2-methoxyethyl sigma donor was attached to the other <2002CC1808>. The structure is illustrated as 14, above. The solid-state structure of the 14 KI complex showed the typical apical solvation of the ring bound cation. In this case, however, one apex was solvated in the pi-fashion (benzene) and the other by the oxygen sigma donor. 

Figure 11.3 (o) The interaction of a singly occupied molecular orbital with the lowest unoccupied molecular orbital of a pi acceptor, (b) The interaction of a singly occupied molecular orbital with the highest occupied molecular orbital of a pi donor. 

Similarly, orbital interaction diagrams can also explain the stabilization of a radical center by a pi donor (Fig. 11.3fc). The pi donor has an accessible full orbital, HOMO, close in energy to the orbital bearing the single electron, SOMO. The interaction of the SOMO with a full HOMO destabilizes one electron and stabilizes two for a net stabilization of one electron overall. We have created a pi bond with one electron in the antibonding orbital the pi bond order is thus half (our resonance structures were unable to indicate this partial pi bond with lines and dots). 

The right side of Figure 12.13 shows what happens if a good pi donor is attached to a double bond. The pi donor HOMO interacts with both the double-bond HOMO and LUMO to produce a new ally lie system with a new HOMO and LUMO, which are raised... 

Riskin M., Tel-Vered R., Bourenko T., Granot E., and Willner I., Imprinting of molecular recognition sites through electropolymerization of functionalized An nanoparticles development of an electrochemical TNT sensor based on pi-donor-acceptor interactions, J. Am. Chem. Soc., 130(30), 9726-9733, 2008. 

In ligands such as CO that can interact with metal atoms in several ways, the number of electrons counted is usually based on o- donation. For example, although CO is a tt acceptor and (weak) tt donor, its electron-donating count of 2 is based on its cr donor ability alone. However, the rr-acceptor and rr-donor abilities of ligands have significant effects on the degree to which the 18-electron rule is likely to be obeyed. Linear and cyclic organic pi systems interact with metals in more complicated ways, discussed later in this chapter. 

The inclusion of p-substituted fluorobenzene F-nmr shifts in the basis sets suggests that weakly interacting Y pi electron donor groups are also permitted. 

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