Cyclopentadienyl anion molecular orbitals

N t/ l Five cyclopentadienyl molecular orbitals 4f Cyclopentadienyl cation (four 77 electrons) 4f Cyclopentadienyl radical (five 77 electrons) -H- Cyclopentadienyl anion (six 7r electrons)... 

Active Figure 15.11 Energy levels of the five cyclopentadienyl molecular orbitals. Only the six-7r-electron cyclopentadienyl anion has a filled-shell configuration leading to aromaticity. Sign in at www.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

The operation of (d) is seen in cyclopentadiene (14) which is found to have a pKa value of 16 compared with 37 for a simple alkene. This is due to the resultant carbanion, the cyclopentadienyl anion (15), being a 6n electron delocalised system, i.e. a 4n + 2 Hiickel system where n = 1 (cf. p. 18). The 6 electrons can be accommodated in three stabilised n molecular orbitals, like benzene, and the anion thus shows quasi-aromatic stabilisation it is stabilised by aromatisation ... 

A Hiickel molecular orbital calculation for the cyclopentadiene system can be carried out as illustrated in Chapter 5. As is shown in Figure 5.20, the Frost-Musulin diagram places the five molecular orbitals at energies of a + 2/3, a + 0.618/3 (2), and a — 1.618/3 (2). Because the cyclopentadienyl anion has six electrons, only the three lowest energy levels are populated and are the orbitals interacting with those on the iron. Figure 21.15 shows the orbitals of the cyclopentadienyl anion. 

Fio. 6. Molecular orbital and resonance structure representation of cyclopentadienyl anion. 

Using a simple resonance approach, we might incorrectly expect both of the cyclopentadienyl ions to be unusually stable. Shown next are resonance structures that spread the negative charge of the anion and the positive charge of the cation over all five carbon atoms of the ring. With conjugated cyclic systems such as these, the resonance approach is a poor predictor of stability. Hiickel s rule, based on molecular orbital theory, is a much better predictor of stability for these aromatic and antiaromatic systems. 

Energy levels of the five cyclopentadienyl molecular orbitals. Only the ix-ir-electron cyclopentadienyl anion has a filled-shell configuration leading to aromaticity. 

Five (yclopcntadienyl molecular orbitals Cyciopentadienyi cation (four 7T electrons) Cydopeufadienyi radical (five electrons) Cyclopentadienyl anion (six 71 electrons)... 

Five Q clopcDtadienyl molecular orbitals (tyi opentadienyi cation (four fT electrons) CVcIopenfadionyl radical five fx electrons) Cyclopentadienyl anion (.[Pg.596]

The distribution of electrons in the tt molecular orbitals of (a) benzene, (b) the cyclopentadienyl anion, (c) the cyclopentadienyl cation, and (d) cyclobutadiene. The reiative energies of the tt molecular orbitals in a cyclic compound correspond to the relative levels of the vertices. Molecular orbitals below the midpoint of the cyclic structure are bonding, those above the midpoint are antibonding, and those at the midpoint are nonbonding. 

To be classified as aromatic, a compound must have an uninterrupted cyclic cloud of rr electrons that contains an odd number of pairs of tt electrons. An antiaromatic compound has an uninterrupted cyclic cloud of tt electrons with an even number of pairs of tt electrons. Molecular orbital theory shows that aromatic compounds are stable because their bonding orbitals are completely filled, with no electrons in either nonbonding or antibonding orbitals in contrast, antiaromatic compounds are unstable because they either are unable to fill their bonding orbitals or they have a pair of TT electrons left over after the bonding orbitals are filled. As a result of their aromaticity, the cyclopentadienyl anion and the cycloheptatrienyl cation are unusually stable. 

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