Aromatic Bonds Electron Rule

Fenocene has an even more interesting stmcture. A central iron is ir-bonded to two cyclopentadienyl ligands in what is aptly described as a sandwich. It, too, obeys the 18-electron rule. Each cyclopentadienyl ligand contributes five electrons for a total of ten and iron, with an electron configuration of [Ar]45 34i contributes eight. Alternatively, fenocene can be viewed as being derived from Fe " (six valence electrons) and two aromatic cyclopentadienide rings (six electrons each). 

Although the cyclopentadienyls dominate the aromatic chemistry of this group, bis(arene) compounds are also well established. They are able to satisfy the 18-electron rule as the dications, [M(arene)2] " or by the two rings adopting different bonding modes one tj the other tj". ... 

HUckel s 4n -H 2 rule orgchem Aromatic (ring) compounds must have 4n + 2 pi-bonding electrons, where n is a whole number and generally limited to = 0 to 5. When M = 1, for example, there are six pi-electrons, as for benzene. Also known as Huckel s rule. hhk-alz jfor, en plos tu, rul ... 

As we have already seen, delocalization of electrons by conjugation decreases the energy difference between the HOMO and LUMO energy levels, and this leads to a red shift. Alkyl substitution on a conjugated system also leads to a (smaller) red shift, due to the small interaction between the cr-bonded electrons of the alkyl group with the K-bond system. These effects are additive, and the empirical Woodward-Fieser rules were developed to predict the 2max values for dienes (and trienes). Similar sets of rules can be used to predict the A ax values for a,P-unsaturated aldehydes and ketones (enones) and the Amax values for aromatic carbonyl compounds. These rules are summarized in Table 2.4. 

On the basis of Hiickel s An +2) n- electron rule, all of these systems can be expected to be aromatic in nature. They do indeed exhibit varying degrees of aromatic stabilization depending on the nature and position of the heteroatom. They cannot all be represented by conventional classical structures. Structures (la)-(lc) can be represented by classical covalent bonded structures whereas those of the (Id) type form the nonclassical structures in the sense that they can be drawn only as charge-separated systems or biradicals in systems wherein X/Y are sulfur or selenium atoms, d- orbital participation in bonding is conceivable, leading to tetravalent sulfur or selenium. 

It should be remembered that the Hiickel rule involving 4m- -2 7t-electrons (with n — 0, 1, 2,...) is necessary but insufficient, because other factors may interfere with the extent of delocalization. The most obvious ones are the existence of a closed shell of filled bonding electrons, the absence of occupied anti-bonding levels and the absence of non-bonding levels. As a consequence, no classically aromatic three-membered ring with a re-electron sextet is possible. As will be discussed in the section devoted to four-membered rings, steric strain is an important factor. 

The cyclopentadienyl anion is a cyclic and planar anion with two double bonds and a non-bonded electron pair. In this way it resembles pyrrole. The two n bonds contribute four electrons and the lone pair contributes two more, for a total of six. By HiickeTs rule, having six n electrons confers aromaticity. Like the N atom in pyrrole, the negatively charged carbon atom must be sp hybridized, and the nonbonded electron pair must occupy a p orbital for the ring to be completely conjugated. 

The rule fails trivially for all complexes of saturated ligands (PH3 may be considered as unsaturated because of its empty 3d orbitals), even where the 18-electron rule is obeyed, since there is no possibility of back-bonding in these cases. It also fails for (C6H6)2Cr, where the d(xy, x —y ) are used in back-bonding, but the d(z ) (or d(z )—s hybrid) is either totally non-bonding or else feebly antibonding to the lowest aromatic molecular orbital of the rings. 

Recent ab initio calculations by Schleyer and coworkers indicate that 4 is strongly stabilized by interaction of the empty 3p Si orbital with the Jt-orbital of one allylic double bond [5]. This interaction leads to a pyramidalized silicon center and relatively short Si-C(sp ) distances and stabilizes the cation 4 by 15.4 kcal moT compared to trimethylsilylium (at MP2/6-31G //MP2/ 6-3IG ). The bonding situation in 4 is best described in terms of three dimensional aromaticity with the 3p(Si) and the C=C double bond obeying the 4n+2 interstitial electron" rule. Due to the reduced positive charge at silicon, cations such as 4 should be also less reactive towards nucleophiles. A facile synthetic approach to this novel typ of silyl cation is outlined in Scheme 1. 

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