Ferrocene cyclopentadienyl ring orientation

Metallocenes (but only with unsubstituted cyclopentadienyl rings) also form thiourea inclusion compounds. X-ray diffraction and Mdssbauer data on the ferrocene inclusion compound show that at 295 K the guest molecules are orientationally disordered at the sites of 32 symmetry. A phase change at 162 K generates a more ordered structure78). 

We have seen how a substituent on one cyclopentadienyl ring in ferrocene markedly affects subsequent ring substitution reactions. While certain transformations of this type are at least partially determined by steric factors, it seems certain that resonance and inductive interactions also play a very important role in determining orientation. Electronic transmissions both within and across the cyclopentadienyl rings are not fully understood, since the precise nature of the bonding in ferrocene is still uncertain. 

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,... 

Very soon we could deduce that electrophilic substitution rules are transformed when the benzene nucleus is replaced by the ferrocene nucleus. Actually, substituents of the first kind (ortho and para orientants) direct an electrophile to the same cyclopentadienyl ring while those of the second kind (meta orientants) direct electrophiles to the other. We showed this to be true for ricochet substitutions as well. However, the first/second type boundary, as determined for ferrocene, differs from that found in the benzene series, the electron-accepting type embracing an essentially greater number of substituents in the former case (see below). 

The best-known cyclopentadienyl complex is the sandwich compound ferrocene, (r -Cp)2Fe it is a diamagnetic, orange solid (mp 393 K) which obeys the 18-electron rule (structure 23.25). In the gas phase, the two cyclopentadienyl rings are eclipsed (23.57) but the solid exists in several phases in which the rings are co-parallel but in different orientations. 

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