Cyclopentadienyl electrostatics

Section 14 14 Transition metal complexes that contain one or more organic ligands offer a rich variety of structural types and reactivity Organic ligands can be bonded to a metal by a ct bond or through its it system Metallocenes are transition metal complexes m which one or more of the ligands is a cyclopentadienyl ring Ferrocene was the first metallocene synthesized Its electrostatic potential map opens this chapter... 

Examine the molecular model of ferrocene on Learning By Modeling Does ferrocene have a dipole moment Would you expect the cyclopentadienyl nngs of ferrocene to be more reactive toward nucleophiles or electrophiles Where is the region of highest electrostatic potential... 

Electrostatic potential map for cyclopentadienyl anion shows most negatively-charged regions (in red) and less negatively-charged regions (in blue). 

Describe the similarities and differences in geometries, charge distributions and electrostatic potential maps for cyclopentadienyl sodium, cyclopentadiene and cyclopentadienyl anion. 

Viewed as a group, aU of the lanthanide cyclopentadienyl complexes show the behavior expected if the metal-ring interaction were purely electrostatic in nature. 

The CC bond distances in cyclopentadienyl anion, C5H5, are all equal, because the anion is aromatic (see Chapter 12, Problem 10). Electrophiles that interact electrostatically with the anion, such as Na+, interact equally with all five carbons, and do not disturb the anion s aromatic character. On the other hand, electrophiles that make covalent bonds, such as H+, might interact more strongly with one particular carbon and destroy the aromaticity of the ring. 

The principle aim of the reported studies was to model structures, conformational equilibria, and fluxionality. Parameters for the model involving interactionless dummy atoms were fitted to infrared spectra and allowed for the structures of metallocenes (M = Fe(H), Ru(II), Os(II), V(U), Cr(II), Cofll), Co(ni), Fe(III), Ni(II)) and analogues with substituted cyclopentadienyl rings (Fig. 13.3) to be accurately reproduced 981. The preferred conformation and the calculated barrier for cyclopentadienyl ring rotation in ferrocene were also found to agree well with the experimentally determined data (Table 13.1). This is not surprising since the relevant experimental data were used in the parameterization procedure. However, the parameters were shown to be self-consistent and transferable (except for the torsional parameters which are dependent on the metal center). An important conclusion was that the preference for an eclipsed conformation of metallocenes is the result of electronic effects. Van der Waals and electrostatic terms were similar for the eclipsed and staggered conformation and the van der Waals interactions were attractive 981. It is important to note, however, that these conclusions are to some extent dependent on the parameterization scheme, and particularly on the parameters used for the nonbonded interactions. 

If one of the cyclopentadienyl rings of ferrocene is replaced with an arene moiety, the Fe(II) complex becomes cationic and hence gains the potential for electrostatic interaction with anions. This concept was first explored by Beer and co-workers using the simple amido[CpFe(arene)]+, 30 [25]. Simi-... 

Cyclopentadienyllithium. The tt charge in the aromatic cyclopentadienyl anion is distributed equally to all five carbon atoms. A lithium counterion should thus electrostatically favor a central location (Csv, 26a) over the tt face. The same conclusion is reached on the basis of MO considerations (46). The six interstitial electron interactions involving the three cyclopentadienyl TT orbitals and those of corresponding symmetry on lithium (one of these is shown in 26b) also favor structure 26a. 

Active Figure 15.5 An orbital view of the aromatic cyclopentadienyl anion, showing the cyclic conjugation and six tt 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). Sign in at www.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

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