Propenyl system delocalization

The resonance formulations for the three 2-propenyl species indicate that it is mainly the two terminal carbons that accommodate the charges in the ions or the odd electron in the radical. The molecular-orbital picture is consistent with this view The three structures differ only in the number of electrons present in molecular orbital tt2, which possesses a node passing through the central carbon therefore, very little of the electron excess or deficiency will show up at this position. The electrostatic potential maps of the three 2-propenyl systems show their delocalized nature. (The cation and anion have been rendered at an attenuated scale to tone down the otherwise overwhelming intensity of color). To some extent, especially in the cation and anion, you can also discern the relatively greater charge density at the termini. Remember that these maps take into account all electrons in all orbitals, a and tt. 

Although uncommon, stable silylenes, are known. There is one that is stable in solution at low temperatures, 2,4,6-Triisopropylphenyl)-l,2,3-tri-rert-butylcyclo-propenyl)silylene and in solid state, Si[Me2Si(t-BuN)2]2 , the first stable n complex, CpJSi the four-coordinate Si[CH(PMe2)2] , Si(r-BuNCH2)2 , and Si[c>-(N(CH2CMe3)2 Ph]2 (with either H or Me at the 4-position are stable. Electron delocalization within the supporting ligand is a useful, but not necessary, component in the stability on Si (II) systems ... 

Thus the SHM predicts that aU three propenyl spedes will be lower in energy than if the n electrons were localized in the formal double bond and (for the radical and anion) in one p orbital. Because this lower energy is associated with the ability of the electrons to spread or be delocalized over the whole n system, what we have called (stab) is often denoted as the delocalization energy, and designated Ed-Note that Er (or Ep) is always some multiple of p (or is zero). Since electron delocalization can be indicated by the familiar resonance symbolism the Fluckel delocalization energy is often equated with resonance energy, and designated Er. The accord between calculated delocalization and the ability to draw resonance structures is not perfect, as indicated by the next example. 

Allyl (propenyl cation) (Section 13.3) The carbocation formally related to propene by removal of a proton from its methyl group. The two contributing resonance structures of the delocalized carbocation each include a positive charge on a carbon adjacent to the double bond, such that a p orbital on each of the three carbons overlaps to delocalize positive charge to each end of the allyl system. 

OVERLAP OF THREE ADJACENT p ORBITALS ELECTRON DELOCALIZATION IN THE 2-PROPENYL (ALLYL) SYSTEM... 

Resonance Representation of Delocalization in the 2-Propenyl (Allyl) System... 

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