Editing Metallocene (section)

==Structure==
A structural trend for the series MCp<sub>2</sub> involves the variation of the M-C bonds, which elongate as the valence electron count deviates from 18.<ref>{{cite journal|first1= K. R.|last1= Flower|first2= P. B.|last2= Hitchcock|title= Crystal and molecular structure of chromocene (''η''<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>Cr|journal= [[J. Organomet. Chem.]]|year= 1996|volume= 507|issue= 1–2|pages= 275–277|doi= 10.1016/0022-328X(95)05747-D}} Discusses all metallocene structures available at that time.</ref>
{| class="wikitable"
! M(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> !! ''r''<sub>M–C</sub> (pm) !! Valence electron count
|-
| align="center"|Fe|| 203.3|| 18
|-
| align="center"|Co|| 209.6|| 19
|-
| align="center"|Cr|| 215.1|| 16
|-
| align="center"|Ni|| 218.5|| 20
|-
| align="center"|V|| 226|| 15
|-
|}

In metallocenes of the type (C<sub>5</sub>R<sub>5</sub>)<sub>2</sub>M, the cyclopentadienyl rings rotate with very low barriers. Single crystal [[X-ray diffraction]] studies reveal both [[eclipsed conformation|eclipsed]] or [[Staggered conformation|staggered]] rotamers. For non-substituted metallocenes the energy difference between the staggered and eclipsed conformations is only a few [[kilojoule per mole|kJ/mol]]. Crystals of ferrocene and osmocene exhibit eclipsed conformations at low temperatures, whereas in the related bis(pentamethylcyclopentadienyl) complexes the rings usually crystallize in a staggered conformation, apparently to minimize [[steric effect|steric hindrance]] between the [[methyl group]]s.