Cyclopentadienyl ion

Another interesting compound that shows the different bonding abilities of cyclopentadiene is Ti(C5H5)4. In this compound, two of the cyclopentadienyl ions are coordinated as r/5-Csl ls as in... 

The overlap of orbitals on Fe with the molecular orbitals on the cyclopentadienyl ion. 

As a second illustrative aspect of the boron/carbon connection, we consider the dicarbollide ion, C2B9H112-, which can be pictured as derived from Bi2Hi22- by removing one BH(t) vertex and replacing two adjacent borons of the exposed pentagonal face by carbon atoms, as shown in Fig. 3.116. This ion exhibits remarkable electronic mimicry of the cyclopentadienyl ion (C5H5-, Section 3.3.2), forming similar coordination and sandwich complexes with many metallic species.162... 

Whatever the initial or preferred stmcture, the [CvHv]" ions dissociate by loss of ethine, C2H2, to yield the cyclopentadienyl ion, [CsHs]", m/z 51, which fragments to yield [CsHs]", m/z 39, upon repeated ethine loss ... 

Two species in Figure 6-7 are not molecules but ions. Aromaticity is not restricted to molecules. The cyclopentadienyl ion and tropylium ion (cyclo-heptatrienyl ion) are aromatic species where n = 1. 

An interesting feature is the ability to form a 67t-electron anion that is isoelectronic with the cyclopentadienyl ion. The cation that is produced by hydride substraction contains a 47t-electron system. 

Such reactions are easy in cationic derivatives for example, in the 1,2-dithiolylium series (699), substituted cyclopentadienyl ion gives fulvene derivatives (700) (66AHC(7)39). 2-Methylthio groups in 1,3-dithiolylium ions are substituted by primary amines or secondary amines (80AHC(27)l5l), and similar reactions are known for 2-alkylthiothiazoles. 

Using a simple resonance approach, we might incorrectly expect both of the cyclopentadienyl ions to be unusually stable. Shown next are resonance structures that spread the negative charge of the anion and the positive charge of the cation over all five carbon atoms of the ring. With conjugated cyclic systems such as these, the resonance approach is a poor predictor of stability. Hiickel s rule, based on molecular orbital theory, is a much better predictor of stability for these aromatic and antiaromatic systems. 

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. 

The gas-phase reactions of lanthanide (Ln+ = La+-Lu+, except Pm+) cations with iron pentacarbonyl, Fe(CO)s, and with ferrocene, Cp2Fe, have been studied by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR/ MS). In the case of Fe(GO)s, the observed primary products were of the type LnFe(CO)x+ (Ln = La, Ce, Pr, Nd, Gd, Tb v = 3 Ln = Ho, Er, Lu x = 3 and 4 Ln = Sm, Eu, Dy, Tm, Yb x = 4), and evidence was obtained for the presence of direct Ln-Fe bonds in these species. With Cp2Fe the majority of the Ln+ cations reacted by metal exchange, yielding Ln bis(cyclopentadienyl) ions Cp2Ln+, while the less reactive Ln+ cations formed the adduct ions LnFeCp2+.851... 

Another important class of compounds are the so-called sandwich compounds, featuring a metal bound (or sandwiched ) between two flat aromatic anions, the best known of which is the cyclopentadienyl ion (Gdis-). These compounds can be prepared by reactions such as (6.46), performed in a nonaqueous solvent such as diethyl ether. 

Qualitatively, the energies of the MOs for Cp can be determined on the basis of the number of nodal planes they possess. The 02" MO has no nodal planes and will be the lowest in energy. The e " MOs have one nodal plane and will lie somewhere near the middle of the MO diagram, while the 62" MOs have two nodal planes and will have the highest energy. Notice that if you connected the energies of the five pi MOs with lines, the shape will match that of the cyclopentadienyl ion itself if it... 

However, if Y is the cyclopentadienyl ion or malonic ester anion, contraction of the ring takes place, leading to the formation of a neutral arene complex (11.18). 

Increasing delocalization, as in the cyclohexadienyl cation, is further stabilizing. This structure is simply protonated benzene, and it serves as a model for the intermediate in electrophilic aromatic substitution (see Section 10.18). Similarly, benzyl ion is quite stable for a formally 1° ion. Aromaticity effects are clear, as in the much greater stability of the six it electron tropylium ion vs. the four tt electron cyclopentadienyl ion (see Section 2.4.1 for a discussion of aromaticity). 

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