Cydopentadienyl anion

According to the Hiickel criteria for aromaticity, a molecule must be cyclic, conjugated (that is, be nearly planar and have ap orbital on each carbon) and have 4n + 2 tt electrons. Nothing in this definition says that the number of p orbitals and the number of nr elections in those orbitals must be the same. In fact, they can he different. The 4n + 2 rule is broadly applicable to many kinds of molecules and ions, not just to neutral hydrocarbons. For example, both the cydopentadienyl anion and the cycloheptatrienyl cation are aromatic. 

A is correct This should be an easy question. Each ring in isolation (cydopentadienyl anion not bonding to the metaJ atom) has a negative chatge giving it 6 pi electrons, which equals 4n + 2 where = 1. 

We can also possible to get aromatic ring. The cydopentadienyl anion and the cycloheptatrienyl cation are both aromatic. Both are cyclic and planar, containing six n electrons, and all the atoms in the ring are sp2 hybridised. 

Fig. (a) Cydopentadienyl anion (b) cycloheptatrienyl cation. Bicyclic and polycyclic systems can also be aromatic. 

When an active metal such as sodium or magnesium reacts with cyclopentadiene (C5H6), a compound is formed that contains the cydopentadienyl anion, C5H5 ... 

Pyrrole, a five-memberecl aromatic heterocycle, has a tt electron arrangement much like that of the cydopentadienyl anion. 

Note that the nitrogen atoms have different roles in pyridine and pyrrole even though both compounds are aromatic. The nitrogen atom in pyridine is in a double bond and therefore contributes only tt electron to the aromatic sextet, just as a carbon atom in benzene does. The nitrogen atom in pyrrole, however, is not in a double bond. Like one of the carbons in the cydopentadienyl anion, the pyrrole nitrogen atom contributes two ir electrons (the lone pair) to the aromatic sextet. 

An orbital view of the aromatic cydopentadienyl anion, showing the cyclic conjugation and six rr electrons in five p orbitals. The electrostatic potential map further indicates that the ion Is symmetrical and that all five carbons are electron-rich (red). 

Similarly, the five-membered heterocycle pyrrole (18) is aromatic, although this molecule obeys Huckel s rule only because the nitrogen atom contributes two electrons to the n-cloud. In this respect, pyrrole is analogous to the cydopentadienyl anion. As a consequence, the nitrogen atom does not retain a lone pair of electrons and pyrrole is not basic. 

Two bridged, ionic 10 n systems directly analogous to the bridged l,6-methano-[10]annulene discussed earlier, have been studied. Thus the cation 61 and anion 62 (see Table 13) bear a similar relationship to 1,6-methano-[10]annulene, 28, as do the tropylium ion and cydopentadienyl anion to benzene. The n.m.r. spectrum of cation 6117>203> is characterized by low-field absorptions of the peripheral protons (t 0.4 to 1.7) and by... 

There has been much discussion over the years regarding the C—C bond distances in the Cp ring. Five equidistant bonds is consistent with the electron delocalization found in the Dsh cydopentadienyl anion. If however, there are two intermediate, one short, and two long C—C bonds, a structure closer to cydopentadiene (Ct) is... 

Cydopentadienyl Anion Here we use a linear anion for our hypothetical transfonnalion ... 

Both calculations and experiments confirm that the cyclic anion has a lower 7r-electron energy than its open-chain counterpart. Therefore the cydopentadienyl anion is classified as aromatic. 

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