Electron-pair delocalization

The N-basicity of the commonly used amines (pyrrolidine > piperidine > morpholine) drops by 2-3 orders of magnitude as a consequence of electron pair delocalization in the corresponding enamines. This effect is most pronounced in morpholino enamines (see table below). Furthermore there is a tendency of the five-membered ring to form an energetically favorable exocyclic double bond. This causes a much higher reactivity of pyrroUdino enamines as compared to the piperidino analogues towards electrophiles (G.A. Cook, 1969). 

The primary electronic effect is due to the delocalization of electron pairs between the ether oxygen and the carbonyl group of the ester function as expressed by the resonance structure 2. 2, and 2- Resonance structures 1 and 2 show the delocalization of an electron pair between the carbon and oxygen of the carbonyl group H 2) and resonance structures 2 and 3 show that the ether oxygen has one electron pair delocalized towards the same central carbon (2 3), The primary electronic effect can therefore be viewed as the result of two n-v interactions. The three atoms involved can be considered to be sp3 hybridized and on that basis, the three-dimensional representations 4 and 5 correspond respectively to the Z and E forms of the ester function. 

Various organogermyllithiums R H3 GeLi (R = Ph, Mes etc., n = 1-3) were prepared easily following this general procedure33,43,9. Their stability depends on the nature of the R groups linked to germanium, on the possibility of complexation of the two metal centers and on the possibility of electron-pair delocalization. 

In a long and thorough review of the photoelectron spectra of a large number of non-metal compounds, attention is drawn to their ready interpretation by MO models. Topics such as electron-deficiency (of particular interest in the context of boron compounds), cr- and 7r-interaction, and electron-pair delocalization are also considered.7... 

The presence of a pterin moiety in mononuclear Mo enzymes has stimulated work in this area. Concensus has replaced earlier confusion and controversy concerning the formulations, metal and pterin oxidation states, and electronic descriptions of Mo—pterin complexes. High-valent, monomeric oxo—Mo pterin complexes are diamagnetic yet they are formulated, on the basis of X-ray photoelectron and NMR spectroscopic data, as Mov species This formulation and (partial) representations like (265) and (266) attempt to reflect the extent of electron pair delocalization in these non-innocent (buffered588) systems, not the existence of metal- and pterin-based diradicals.630... 

Our need for more than one Lewis structure to depict the ozone molecule is the result of electron-pair delocalization. In a single, double, or triple bond, each electron pair is attracted by the nuclei of the two bonded atoms, and the electron density is greatest in the region between the nuclei each electron pair is localized. In the resonance hybrid for O3, however, two of the electron pairs (one bonding and one lone pair) are delocalized their density is spread over the entire molecule. In O3, this results in two identical bonds, each consisting of a single bond (the localized electron pair) and a partial bond (the contribution from one of the delocalized electron pairs). We draw the resonance hybrid with a curved dashed line to show the delocalized pairs ... 

Electron delocalization diffuses electron density over a greater volume, which reduces electron-electron repulsions and thus stabilizes the molecule. Resonance is very common, and many molecules (and ions) are best depicted as resonance hybrids. Benzene (CsHe), for example, has two important resonance forms in which alternating single and double bonds have different positions. The actual molecule has six identical carbon-carbon bonds because there are six C—C bonds and three electron pairs delocalized over all six C atoms, often shown as a dashed circle (or simply a circle) ... 

Electron Delocalization Our need for more than one Lewis structure to depict O3 is due to electron-pair delocalization. In a single, double, or triple bond, each electron pair is localized between the bonded atoms. In a resonance hybrid, two of the electron pairs (one bonding and one lone pair) are delocalized their density is spread over a few adjacent atoms. 

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