Cyclopentadienyl cation, molecular orbitals

In the case of the cyclopentadienyl cation, there are only two electrons in the e" tt molecular orbitals that are degenerate in Dji, symmetry. The MO heatments... 

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)... 

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. 

A molecule with An n electrons in the ring, with the molecular orbitals made up from An p orbitals, does not show this extra stabilisation. Molecules in this class that have been made include cyclobutadiene 1.13 (n = 1), the cyclopentadienyl cation 1.14, cyclooctatetraene 1.15 andpentalene 1.16 (n 2), [12]annulene 1.17 (n = 3) and [16]annulene 1.18 (n A). 

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. 

The distribution of electrons in the rr molecular orbitals of (a) benzene, (b) the cyclopentadienyl anion, (c) the cyclopentadienyl cation, and (d) cyclobutadiene. 

The relative energy levels of the five tt molecular orbitals of the cyclopentadienyl system are similar to those in benzene. That is, there is a single lowest-energy MO, above which the orbitals come in degenerate pairs. Draw a diagram like that in Figure 15.5, and tell which of the five orbitals are occupied in the cation, radical, and anion. 

Among the odd-membered rings, aromatic ions are readily prepared. Cyclopentadiene is deprotonated by alkoxide bases while cycloheptatriene is not, even with stronger bases. On the other hand, bromocycloheptatriene is ionic while 5-bromocyclopentadiene is not. Tripropylcyclopropenyl perchlorate exists largely as the carbocation in aqueous acetonitrile at pH 7 [4]. Electron configurations for the cyclopentadienyl anion, benzene, and the cycloheptatrienyl cation are shown in Figure 5.9. For simplicity, the molecular orbitals are represented by horizontal lines. 

---