Cyclopentadienyl rings conformation

In this connection, in order to judge the level of these molecular rearrangements, the solid state X-ray structures of ferrocene and ferrocenium ion could be compared. Unfortunately, the molecular disorder caused by the rotation of the cyclopentadienyl rings in ferrocene means that the comparison procedure is far from simple and, in fact, the first results were interpreted in terms of a staggered conformation of the two cyclopentadienyl rings. It is now believed that the eclipsed conformation is the more stable (with a rotation angle of about 10°).2 However, as the rotational barrier is notably low (about 4 kJ mol-1), the conformation that one observes is probably that imposed by crystal packing forces. 

A similar situation occurred for the ferrocenium ion. Until 1983, on the basis of the available structural data, it was thought that the cyclopentadienyl rings assumed an eclipsed conformation. However, more recent data demonstrate that it assumes a staggered conformation, even if once again the crystal packing forces are considered to be the determining factor.3... 

The relative disposition of the cyclopentadienyl rings is partially staggered (with a rotation angle of about 16°, compared to an angle of 36° expected for an exactly staggered conformation). 

In support of the electrochemical evidence, the molecular structure of the biferrocenium ion in [( -CsF Fe -CsfLOf -CsfLOFe -CsHs)] [I3] shows that both pairs of cyclopentadienyl rings have an eclipsed conformation, Figure 16. Furthermore, the mean Fe-Cp(Centroid) distance is equivalent in both the ferrocenyl units and equal to 1.68 A. Speculatively, this value is intermediate between the values previously observed for ferrocene and ferrocenium ions, thus supporting charge delocalization between the two centres.27... 

Finally, a molecule in which the two ferrocenyl subunits are separated by a C2H4 bridge is considered, namely l,2-bis(ferrocenyl)ethane. Its X-ray structure shows that the mean Fe-Cp(Centroid) distance in both the ferrocenyl subunits is 1.67 A and the cyclopentadienyl rings are slightly rotated (8.5°) with respect to the eclipsed conformation.31... 

It is evident that the cyclopentadienyl rings assume a mutual staggered conformation. 

The cyclopentadienyl rings of chromocene, as for vanadocene, assume a staggered conformation. 

Concerned with the electron diffraction structure, the rotation of the cyclopentadienyl rings appears even faster than in ferrocene, hence their mutual disposition is difficult to establish. An X-ray diffraction study at 101 K has however shown that at this temperature the conformation is staggered.94 The increment in the Ni-C distance (2.18 A) compared to the Co-C distance (2.10 A) in cobaltocene reflects once again the increased population of the antibonding e" orbital (nickelocene has 20 valence electrons and a terminal electronic configuration e a e"2). 

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. 

A textbook example of conformational polymorphism is provided by ferrocene [44], for which one room temperature disordered and two low-temperature ordered crystalline forms are known. At the crystal level they differ in the relative orientation of the cyclopentadienyl rings and in small rotations of the molecules,... 

Substituents in the distal position of the cyclopentadienyl ring in these cyclopentadienyl-fluorenyl catalysts has a profound effect on the polymer product produced. A small substituent like methyl produces a novel material, hemiisotactic polypropylene, in which methine carbons of specific conformation alternate with methine carbons of random conformation. A larger substituent hke r-butyl makes isotactic polypropylene (Figure 6). 

The molecular and crystal structure of (81) is simpler than that of ferrocene as only one polymorph featuring eclipsed conformation of the cyclopentadienyl rings, has been found at ambient and low (100 K) temperature. The larger metal-carbon distance (2.186 A in (81) vs. 2.03 or 2.06 A in ferrocene) corresponds to the larger metal covalent radius see Covalent Radii) and may also be responsible for the fact that an eclipsed conformation is found for the solid-state structure of decamethylruthenocene (82), as opposed to decamethylferrocene where more closely spaced methyl groups impose the staggered Dsd conformation. 

Finally, Fig. 7-50 shows the crystal structure of bis[2-(ferrocenylmethyleneamino)-benzenethiolato]nickel(II) [149]. The square planar geometry of the nickel atom is slightly distorted by the bulkiness of the ligands. One of the ferrocenyl units (B) assumes an eclipsed conformation, whereas the other (D) slightly rotates its cyclopentadienyl rings. 

Substitution of the two diphenylphosphino substituents for thiolate groups affords a series of complexes, the structure of which is typically represented by that of dichloro[l,r-bis(ibutylsulfido)ferrocene]palladium(ii) shown in Fig. 7-53 [10]. The assembly is similar to that of the preceding bis (diphenylphosphino) complexes, except for the eclipsed and less tilted (1.9°) conformation of the cyclopentadienyl rings of the ferrocene group. 

Figure 7-60 shows the structure of the 3-ferrocenylpyridine-dichloroplatinum(ii) complex, [(t/ -C5H5)Fe(t/ -C5H4-m-C5H4N)]2PtCl2 [176]. The coordination around the platinum atom is also square-planar in this case. In both ferrocenyl ligands, the cyclopentadienyl rings assume a nearly eclipsed conformation. 

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