Lewis bases HOMO-LUMO interactions

In molecular orbital terms, the donation can be viewed as a HOMO-LUMO interaction (Chapter 9). Double hydride bridges, as found in B,H, also are exhibited by bimetallic species such as the chromium anion formed in Eq. 15.18. The sirmlarnv between these two dibridged species is underscored by the fact that their Lewis acid and Lewis base fragments can be interchanged [HCr(CO)sJ reacts with BH, to form 16... 

FIGURE 6-5 HOMO-LUMO Interactions. (Adapted with permission from W. B. Jensen, The Lewis Acid-Base Concepts, Wiley-Interscience, New York, 1980, Figure 4-6, p. 140. Copyright 1980, John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc.)... 

Four different types of HOMO-LUMO interactions between the reference moiecuie A (HjO) and moiecuie B, where B = Na, Ci , Mg +, or Fj and water is acting as an oxidant, Lewis acid, Lewis base, and... 

As shown in Figure 6.2a, an X substituent, which has a p orbital or other suitable doubly occupied orbital which will interact with the bond, raises the energy of the HOMO and LUMO, thus rendering the olefin more reactive as a Lewis base. Of course, the electrons of the HOMO are also delocalized onto X. The probability of attack by an... 

Answer. The Lewis acidity depends on the interaction energy ( ) from the interaction of the LUMO of the acid with the HOMO of the nucleophile. The interaction is of a type, with the base HOMO (usually a nonbonded p or spn hybrid) interacting end on with the LUMO, which for the methyl cation is a single 2p orbital and for the allyl system is a linear combination of 2p orbitals. The LUMOs of the two systems are shown below. 

Exercise 1.2. In the discussion of Lewis acids and bases in the Appendix, the compound [13] is analyzed as an adduct of the base I- with the acid I2. It probably is not clear how I2, a diatomic that perfectly fits the two-center-two-electron bond model and the eight electron rule, can act as a Lewis acid. Show how a HOMO-LUMO analysis of the acid-base interaction rationalizes the interaction and predicts a linear structure. 

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