Delocalized pi bonding

There is a special class of hydrocarbons called the aromatic compounds. The aromatics are characterized by their six carbon rings and their alternating double and single bonds. The simplest of these compounds is a compound called benzene. Because the double and single bonds alternate, there are two resonance structures (thanks to delocalized pi bonds) that are illustrated in Figure 11.6. 

In Figure 7.10, the sigma bonds shown in Figure 7.9 are now shown as just lines. This is to make the delocalized pi bonds clear. 

Delocalized pi bond that contains a total of 6 electrons, t from each carbon atom... 

FIGURE 13.44 Delocalized Pi Bonding in Carbene Complexes. E designates a highly electronegative heteroatom, such as 0, N, or S. 

Another species in which delocalized pi orbitals play an important role is benzene, QHg. There are 30 valence electrons in the molecule, 24 of which are required to form the sigma H H bond framework ... 

The structures of aromatic hydrocarbons, like benzene (C6H6), involve sp2 hybridized carbon atoms with pi bonds whose electrons are delocalized over the entire benzene ring. 

The geometries in Figs. 4.86 and 4.87 suggest an important distinction in the multicenter hapticity character of ligand attachment to the metal atom. Hapticity refers to the number of atoms in a ligand that are coordinated to the metal. In the Ir+ diammine complex (Fig. 4.86(a)), the metal attaches to each of two nN donor lone pairs in simple monohapto (one-center, q1) fashion. However, in the Ir+ complexes with HCCH or CML the metal attaches to the face of the pi bond or three-center allylic pi system in dihapto (two-center, r 2) or trihapto (three-center, q3) fashion, respectively. The hapticity label q" therefore conveniently denotes the delocalized n -center character of the donated electron pair(s) and the geometry of the resulting coordination complex. 

Molecular fluorescence involves the emission of radiation as excited electrons return to the ground state. The wavelengths of the radiation emitted are different from those absorbed and are useful in the identification of a molecule. The intensity of the emitted radiation can be used in quantitative methods and the wavelength of maximum emission can be used qualitatively. A considerable number of compounds demonstrate fluorescence and it provides the basis of a very sensitive method of quantitation. Fluorescent compounds often contain multiple conjugated bond systems with the associated delocalized pi electrons, and the presence of electron-donating groups, such as amine and hydroxyl, increase the possibility of fluorescence. Most molecules that fluoresce have rigid, planar structures. 

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