Low-lying LUMO

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

Early work established that S4N4 forms di-adducts with alkenes such as norbornene or norbomadiene. Subsequently, structural and spectroscopic studies established that cycloaddition occurs in a 1,3-S,S"-fashion. The regiochemistry of addition can be rationalized in frontier orbital terms the interaction of the alkene HOMO with the low-lying LUMO of S4N4 exerts kinetic control. Consistently, only electron-rich alkenes add to S4N4. 

JOM(400)149 96BSCF33]. The complex 164 is the first known t] -phospholyl species. The tungsten atoms have a coordination number of 9, and the carbon atoms of the phospholyl ring are coplanar. The phosphorus atom deviates from the plane of carbon atoms by 0.015 nm. The basic difference between the Ti -cyclopentadienyl and ri -phospholyl complexes is the existence of a low-lying LUMO localized mainly at the phosphorus atom. 

The palladium catalyst is essential in this reaction, as was shown in control experiments to make sure that this was not a direct nucleophilic addition of the amine to the electron-poor (regarding the low lying LUMO ) cyclobutadiene ligand. A series of amino-substituted cyclobutadiene complexes have been synthesized by this methodology [29]. 

C with BCP (3) most likely for the preferred interaction of the high lying HOMO of BCP with the low lying LUMO of ozone. 

Another interesting feature of Si-Si bonds is the low energy level of the LUMO. The LUMO levels decrease with increasing chain length of polysilanes (Fig. 10). Because of the low lying LUMO, polysilanes can be reduced by both... 

While PAE-type polymers normally have relatively large bandgaps of about 2.1-2.6 eV [85-91], diethynylsilole-based polymers (described by Tamao et al.) show significantly narrower bandgaps (up to 1.8 eV [92]). A silacyclopentadi-ene ring is conspicuous by its low-lying LUMO level due to the conju-... 

Figure 9. HOMO s and low lying LUMO s as calculated by HF/6-31GV/3-21C. Orbital energies in eV.

Cycloadditions of ketenes and alkenes have been shown to have synthetic utility for the preparation of cyclobutanones.101 The stereoselectivity of ketene-alkene cycloaddition can be analyzed in terms of the Woodward-Hoffmann rules.102 To be an allowed process, the [2n + 2n] cycloaddition must be suprafacial in one component and antarafacial in the other. An alternative description of the transition state is a [2ns + (2ns + 2ns)] addition.103 Figure 6.6 illustrates these transition states. The ketene, utilizing its low-lying LUMO, is the antarafacial component and interacts with the HOMO of the alkene. The stereoselectivity of ketene cycloadditions can be rationalized in terms of steric effects in this transition state. Minimization of interaction between the substituents R and R leads to a cyclobutanone in which these substituents are cis. This is the... 

EHMO calculations performed on the reactants, products, and possible intermediates of the reactions shown in Sch. 15 supported the proposed mechanism [58]. Thus, steric and electronic factors combine to make 23 a good substrate-binding site it has an uncongested M02S2 face and a low-lying LUMO available to hold an incoming nucleophile. The proposed mechanism remains valid if the CO substrate is replaced by other two-electron donor ligands such as isocyanide [56] or cyanide [59]. 

It is well-known that [60]fullerene is a fairly electronegative system having a low-lying LUMO. Therefore, [60]fullerene behaves like an electron-poor conjugated polyolefin... 

All C60 adducts have low-lying LUMOs that can easily be populated by electrochemical methods. For C60 itself, six reduction couples have been observed by cyclic voltammetry (CV) or square-wave voltammetry (SWV), and as many as four reduction couples have been found for many organometallics (9,84). Most of the studies have been performed in thf or acetonitrile at lower temperatures, which increases the size of the potential window. Table VII lists the half-wave potentials for some metal complexes, and Fig. 7 shows the cyclic voltammogram for [Co(NO)(PPh3)2(i72-C60)]. 

It turns out that small M S V units with delocalized ground state MOs have low-lying LUMOs which are related to the conducting and semiconducting properties of metal sulfides. We will discuss some general properties of these, from a different point of view, in the following section. 

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