Cydopentadienyl anion aromaticity

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. 

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. 

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. 

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. 

Anti-aromaticity was predicted by the Hiickel approach for conjugated cyclic planar structures with 4n 7i electrons due to the presence of two electrons in antibonding orbitals, such as in the cydopropenyl anion, cydobutadiene, and the cydopentadienyl cation (n = 1), and in the cydoheptatrienyl anion and cydooctatetraene (n = 2). It has been argued that a simple definition of an anti-aromatic molecule is one for which the 1H NMR shifts reveal a paramagnetic ring current, but the subject is controversial. The power of the Hiickel theory indeed resides not only in the aromatic stabilization of cydic 4n + 2 electron systems, but also in the destabilization of those with An electrons [22, 27, 42]. 

The general synthesis for this class of metallocene compounds is illustrated in Scheme 32 for the preparation of 110. Ferrocene undergoes ri -cydopentadienyl ligand replacement with the aromatic hydrocarbon in the presence of aluminum, aluminum chloride, and titanium tetrachloride to afford the desired Ti -cyclopentadienyl(isopropylbenzene)iron (II) chloride. Anion exchange with KPFg in aqueous solution gives the PFe" salt 110. 

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