It is clear then, that what is certainly a bulky cyclopentadienyl ligand, ^-C₅(CH₂Ph)₅, inter alia by its role in the metallocene chemistry of Ge^II, Sn^II and Pb^II for example, apparently exhibits little or no steric demand upon the structure, notably so in the case of Fe^II, since the substituents are free to rotate into sterically undemanding positions. The effect of the significant steric demand of the ^-C₅Ph₅ ligand, however, can be inferred from inter alia the large metal centroid distance and the uncharacteristically large R factor for [Fe(h-C₅Ph₅)₂]: The Ph groups are relatively inflexible, i.e. rigid with regards to bending out of the cyclopentadienyl ring planes, and their resulting arrangement (whereby the Ph groups are able to interlock in a symmetrical fashion), so as to minimise unfavourable interligand Ph^-Ph interactions prevents bending in the structure. Similarly, steric demand rather than bulk is apparent in the case of 1 and 3: It is the regular, symmetrical positioning of the cyclopentadienyl substituents, specifically the twelve Me groups in the "plane of the sandwich", that result in the trans orientation of the silyl groups when compared to 2 and 4 and consequently increased steric saturation at the metal centre. For the Sn and Pb metallocenes the increased inter-ring separation permits the SiMe₃ substituents of the corresponding metallocenes, [Sn{h-C₅Me₄(SiMe₃)}₂] and [Pb{h-C₅Me₄(SiMe₃)}₂], to rotate out of the plane of the sandwich and a bent metallocene results. For the ^-C₅Me₄(SiMe₂Bu^t) analogues, however, a similar rotation of the SiMe₂Bu^t is prevented as the Bu^t group may not rotate down into the C₅Me₄ plane. Thus the metallocenes bearing the Cp^s ligands always display the Bu^t group in an exo manner and preserve the twelve Me groups in the plane of the sandwich resulting in a sterically more demanding cyclopentadienyl ligand.