Cyclopentadienyl anion electronic configuration

Figure 11.15 shows the Fliickel MOs of cyclopentadienyl anion. Like benzene and cyclo-heptatrienyl cation, cyclopentadienyl anion has six tt electrons and a closed-shell electron configuration. 

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. 

By sharing the four --electrons of the diene and the six --electrons of the cyclopentadienyl anion, the cobalt(I) atom acquires the electronic configuration of krypton. The formulation of the complex as (XXIV) is supported... 

Ferrocene is only one of a large number of compounds of transition metals with the cyclopentadienyl anion. Other metals that form sandwich-type structures similar to ferrocene include nickel, titanium, cobalt, ruthenium, zirconium, and osmium. The stability of metallocenes varies greatly with the metal and its oxidation state ferrocene, ruthenocene, and osmocene are particularly stable because in each the metal achieves the electronic configuration of an inert gas. Almost the ultimate in resistance to oxidative attack is reached in (C5H5)2Co , cobalticinium ion, which can be recovered from boiling aqua regia (a mixture of concentrated nitric and hydrochloric acids named for its ability to dissolve platinum and gold). In cobalticinium ion, the metal has the 18 outer-shell electrons characteristic of krypton. 

Repeat Problem 16-10 for the cyclopentadienyl ions. Draw one all-bonding MO, then a pair of degenerate MOs, and then a final pair of degenerate MOs. Draw the energy diagram, fill in the electrons, and confirm the electronic configurations of the cyclopentadienyl cation and anion. 

Consider the electronic configuration of the cyclopentadienyl anion (Fig. 10.5). Each carbon, trigonally hybridized, is held by a a bond to two other carbons... 

Figure 29.10. Aromatic compounds with 6 w electrons. Configuration of IT electrons in cyclopentadienyl anion, benzene, and cycloheptatrienyl cation.

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

One of the most famous results of the simple MO approach is the prediction 24> of stability for the cyclopentadienyl anion and cyclohept-atrienylium (tropylium) cation, which has been fully confirmed by the experimental findings 161>. The Ji-electron configuration of these ions is fairly similar to that of benzene, and this explains the results obtained in the first approximation. 

Systems such as cyclopentadienyl anion, benzene and tropylium cation with (4N + 2) 7t-electrons (N=Q, 1, 2,. ..) will thus have a closed shell, that is, four electrons in the degenerate pair of the highest occupied MOs in their ground configuration, and that is especially favourable energetically. This is the basis of the well-known Huckel rule of aromaticity. 

HiickeFs rule also pertains to charged cyclic conjugated systems. The cyclo-propenyl (2 tt electrons), cyclopentadienyl anion (6 tt electrons), and cycloheptatrienyl (tropylium) cation (6 tt electrons) are examples of stabilized systems. We say much more about the relationship between MO configuration and aromaticity in Chapter 9. 

Construct an MO energy diagram for the cyclopentadienyl anion and describe its ground-state electron configuration. 

Ground-state electron configuration of the cyclopentadienyl 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. 

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