Donor orbitals

The first step in constructing a molecular orbital picture of a chemical reaction is to decide which orbitals are most likely to serve as the electron donor and electron acceptor orbitals. It should be obvious that the electron donor orbital must be drawn from the set of occupied orbitals, and the electron acceptor orbital must be an unoccupied orbital, but there are many orbitals in each set to choose from. 

Orbital energy is usually the deciding factor. The chemical reactions that we observe are the ones that proceed quickly, and such reactions typically have small energy barriers. Therefore, chemical reactivity should be associated with the donor-acceptor orbital combination that requires the smallest energy input for electron movement. The best combination is typically the one involving the HOMO as the donor orbital and the LUMO as the acceptor orbital. The HOMO and LUMO are collectively referred to as the frontier orbitals , and most chemical reactions involve electron movement between them. 

Another way to assess nucleophilic reactivity is to examii the shape of the nucleophile s electron-donor orbital (th is the highest-occupied molecular orbital or HOMC Examine the shape of each anion s HOMO. At which ato would an electrophile, like methyl bromide, find the be orbital overlap (Note This would involve overlap of tl the HOMO of the nucleophile and the lowest-unoccupif molecular orbital or LUMO of CH3Br.) Draw all of tl products that might result from an Sn2 reaction wi CHaBr at these atoms. 

Backside attack may be favored in order to facilitate transfer of nonbonding electrons from the nucleophile into the electrophile s lowest-unoccupied molecular orbital (LUMO). Efficient electron transfer requires maximal overlap of the LUMO and the donor orbital (usually a nonbonded electron pair on the nucleophile). Examine the LUMO of methyl bromide. How would a nucleophile have to approach in order to obtain the best overlap Is your answer more consistent with preferential backside or frontside attack ... 

Some electrophile-nucleophile reactions are guided more by orbital interactions than by electrostatics. The key interaction involves the donor orbital on the nucleophile, i.e., the highest-occupied molecular orbital (HOMO). Examine the HOMO of enamine, silyl enol ether, lithium enolate and enol. Which atom is most nucleophilic, i.e., which site would produce the best orbital overlap with an electrophile ... 

CH3I should approach the enolate from the direction that simultaneously allows its optimum overlap with the electron-donor orbital on the enolate (this is the highest-occupied molecular orbital or HOMO), and minimizes its steric repulsion with the enolate. Examine the HOMO of enolate A. Is it more heavily concentrated on the same side of the six-membered ring as the bridgehead methyl group, on the opposite side, or is it equally concentrated on the two sides A map of the HOMO on the electron density surface (a HOMO map ) provides a clearer indication, as this also provides a measure of steric requirements. Identify the direction of attack that maximizes orbital overlap and minimizes steric repulsion, and predict the major product of each reaction. Do your predictions agree with the thermodynamic preferences Repeat your analysis for enolate B, leading to product B1 nd product B2. 

One way to anticipate the favored product is to consider the shape of naphthalene s best electron-donor orbital, the highest-occupied molecular orbital (HOMO). Display the HOMO in naphthalene and identify the sites most suitable for electrophilic attack. Which substitution product is predicted by an orbital-control mechanism Ts this the experimental result ... 

Finally, examine the highest-occupied molecular orbital (HOMO) of phenoxide anion. Is the HOMO the best electron-donor orbital Is the orbital localized primarily on oxygen or on carbon Is the observed product consistent with orbital control Explain your answers. 

Accordingly, the CO moiety acquires negative charge. The consequent exigencies of the electroneutrality principle are then met by the CO group donating this charge back to the metal via its now expanded <7-donor orbital ... 

Fig. 3. Top n and

Figure 1.3 The two-electron stabilizing interaction between a filled donor orbital (pi<(]) and an unfilled acceptor orbital corresponding to perturbation...

What are the expected hybrids for transition-metal bonding In analogy with the treatment of Section 2.4, we expect that the pF ligand donor orbital can interact with a general spM hybrid mixture of valence s, p, d orbitals of the form (cf. Eq. (2.3))... 

Because the amplitude of a two-center donor orbital crAc is generally maximal near the bond midpoint (rather than at the ends, where only one of the two hybrids contributes), the preferred ubc hb geometry is generally T-shaped,... 

Examples involving d -type acceptor orbitals or 6-type donor orbitals will be considered in Chapter 4.) Figures 3.98-3.100 show the three-center NBOs and occupancies for each of the prototype species (3.239a)-(3.239c). 

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... 

In this case the formal donor orbital is the two-center aim, so the complex is formally of type (5.43b). The optimized structure of BH4 - -HOH is shown in Fig. 5.17, together with the dominant cum cron donor-acceptor interaction. 

Because the H-end of the cta i i antibond encloses no electronic core, the incoming ns donor orbital can overlap with practically the entire hydride end of the antibond, rather than merely the outer annular region that lies beyond the nodal boundary of the core region. 

More generally, we can recognize that an acceptor orbital of unusual size or shape may demand an unusual Lewis base to offer a suitable matching donor orbital. The CT complexes formed by a monomer therefore provide a direct reflection of the shapes, sizes, and energies of its filled and unfilled valence orbitals. The rich diversity of donor-acceptor chemistry can be largely attributed to the richly variegated forms of donor and acceptor orbitals, which is consistent with the strongly quantum-mechanical character of donor-acceptor phenomena. 

This simple valence-bond rationale, involving a resonance hybrid of forms 1 and 2, appears to explain many of the physical data available for sulfoxide complexes. It appears that S-bonding does not involve such a major internal rearrangement of the molecule as one may initially expect and is almost certainly a result of the increased orbital diffuseness on passing from oxygen to sulfur. Thus with typical hard acids (see ref. 437), orbital overlap will be most favorable with the less diffuse donor orbital of oxygen. In the case of typical soft metals, this overlap is less favorable due to the orbital diffuseness of the soft acid, and so coordination via sulfur occurs, where the orbital diffuseness of the donor and acceptor are more evenly matched. 

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