Cyclopentadienide anion aromaticity

According to a semiempirical study by Malar, the different polyphospholide anions have 86—101% aromaticities of that of the cyclopentadienide anion. Chesnut and Quin reported on the basis of GIAO NMR calculations using a triple-valence quality basis set that the phospholide anion s aromaticity is 63% that of the cyclopentadienide anion. The aromatic stabilization energy (ASE) obtained by Schley-er et al. from eq 2 (X = P ) was 90% that of the cyclopentadienide anion. 

The divergent hyperfine patterns of the two ions can be ascribed to their 6ti aromatic substructures, cyclopentadienide anion, and cycloheptatrienylium cation, respectively. 

The tropylium ring is planar with equal C—C distances. Like benzene and the cyclopentadienide anion, the tropvlium cation is an aromatic, six-electron species. 

The crucial structural feature which underlies the aromatic character of benzenoid compounds is of course the cyclic delocalised system of six n-electrons. Other carbocyclic systems similarly possessing this aromatic sextet of electrons include, for example, the ion C5Hf formed from cyclopentadiene under basic conditions. The cyclopentadienide anion is centrosymmetrical and strongly resonance stabilised, and is usually represented as in (7). The analogous cycloheptatrienylium (tropylium) cation (8), with an aromatic sextet delocalised over a symmetrical seven-membered ring, is also demonstrably aromatic in character. The stable, condensed, bicyclic hydrocarbon azulene (Ci0H8) possesses marked aromatic character it is usually represented by the covalent structure (9). The fact that the molecule has a finite dipole moment, however, suggests that the ionic form (10) [a combination of (7) and (8)] must contribute to the overall hybrid structure. 

The process is an acid-base reaction in which cyclopentadiene transfers a proton to amide ion (the base) to give the aromatic cyclopentadienide anion. The sodium ion (Na+) has been omitted from the equation. 

Cyclopentadienide (Cp) 1 is well known as one of the most frequently used ligands in organometallic chemistry. In addition, the cyclopentadienide anion 1 has always been quoted as a classic example of Hiickel aromaticity, to demonstrate along with benzene and the cydoheptatrienyl cation the validity of the (4n + 2) -electron rule. In contrast, a simple and stable cyclopentadienyl cation of the type 1+ remains to be elusive [5]. With the highly unstable neutral cyclobutadiene and the cydoheptatrienyl anion, 1+ shares the character-... 

One should note that we do not put barriers between organic and inorganic cyclic structures. We have on one hand aromatic organic homocycles such as benzene, cyclopentadienide anions, cyclooctatetrae-nide dianions and tropylium cations, and on the other hand aromatic inorganic homocycles, and then we gradually fuse them into aromatic heterocycles. The only restriction we observe is for discussing only... 

An energy difference for the isodesmic reaction [Eq. (28)] indicates a quantitative measure of the extent of electron delocalization (6-31G ) (62). The energy difference for the C, structure of the silacyclopentadienide anion is only 2.2 kcal/mol, which is much smaller than 73.4 kcal/mol for the carbon analog, a planar cyclopentadienide anion (C2 ). On the basis of these calculations, it is concluded that the ground-state structure of the silacyclopentadienide anion has only ca. 3% of the resonance stabilization exhibited by the cyclopentadienide anion. However, the previous calculation (3-21G//STO-2G) on the 2 planar structure shows that the resonance energy is 23 kcal/mol, which means ca. 25% as aromatic as the carbon analog (65). 

Certain charged compounds are aromatic also. The electron-deficient C atom in the tropylium and cyclopropenium ions is sp2-hybridized and has an empty p orbital. The tropylium ion has a cyclic array of seven p orbitals containing three pairs of electrons, and the cyclopropenium ion has a cyclic array of three p orbitals containing one pair of electrons therefore, both ions are aromatic. (Note that cyclopropene itself is nonaromatic, because it doesn t have a cyclic array of p orbitals ) Similarly, the lone-pair-bearing C atom in the cyclopentadienide anion is sp2-hybridized so that the lone pair can be used in resonance as a result, the cyclopentadienide anion has a cyclic array of five p orbitals containing three pairs of electrons, and it is aromatic, too. 

Cyclopentadienide anion is an aromatic anion. It has six ir electrons delocalized over a completely conjugated planar monocyclic array of five 5p -hybridized carbon atoms. 

Cyclopentadiene is only a slightly weaker acid than water. The eqnilibrium for its deprotonation is more favorable than for other hydrocarbons becanse cyclopentadienide anion is aromatic. The contrast is striking when we compare this eqnilibrium with that for loss of a proton from cycloheptatriene. 

Resonance stractures can be written that show delocahzation of the negative charge over all of its seven carbons nevertheless, because cycloheptatrienide anion contains eight tt electrons, it is not aromatic. The equilibrium constant for formation from the parent hydrocarbon is more favorable by 10 ° (20 pA a units) for the aromatic cyclopentadienide anion than for the nonaromatic cycloheptatrienide anion. 

Like benzene, cyclopentadienide anion is an aromatic six rr-electron system (Section 11.21) and bonds to transition metals in a similar way. Ferrocene is the best known example, with a structure aptly described as a sandwich. 

The first of the ir-organometallic compounds to be characterized was ferrocene. This compound is a neutral molecule that is derived from two cyclopentadienyl anions and iron(II). Two convenient methods of preparation have been described both involve reaction of the aromatic cyclopentadienide anion with Fe(II). Related... 

Pentafluorophenyl-substituted cyclopentadienide anions are useful ligands for transition metals because tbe CgFs substituent is not only bigbly electron withdrawing but also relatively stable.Up to four CsFs substituents may be attached to cyclopentadiene (or up to two to indene ) by one-pot nucleophilic aromatic substitution reactions of cyclopentadienyl or indenyl anions with hexafluoroben-zene. The title diene was also reported as the major organic product of the photolysis of Cp2Ti(CgF5)2. ... 

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