Editing Ferrocene (section)

==Structure and bonding==
[[Mössbauer spectroscopy]] indicates that the iron center in ferrocene should be assigned the +2 oxidation state. Each cyclopentadienyl (Cp) ring should then be allocated a single negative charge. Thus ferrocene could be described as iron(II) bis([[cyclopentadienide]]), {{chem2|Fe(2+)[C5H5(-)]2}}.

Each ring has six π-electrons, which makes them [[Aromaticity|aromatic]] according to [[Hückel's rule]]. These π-electrons are then shared with the metal via covalent bonding. Since Fe<sup>2+</sup> has six ''d''-electrons, the complex attains an [[18-electron rule|18-electron]] configuration, which accounts for its stability. In modern notation, this sandwich structural model of the ferrocene molecule is denoted as {{chem2|Fe(''η''^{5}\-C5H5)2}}, where ''η'' denotes [[hapticity]], the number of atoms through which each ring binds.

The carbon–carbon bond distances around each five-membered ring are all 1.40&nbsp;Å, and all Fe–C bond distances are 2.04&nbsp;Å. The Cp rings rotate with a low barrier about the Cp<sub>(centroid)</sub>–Fe–Cp<sub>(centroid)</sub> axis, as observed by measurements on substituted derivatives of ferrocene using <sup>1</sup>H and <sup>13</sup>C [[nuclear magnetic resonance]] spectroscopy. For example, methylferrocene (CH<sub>3</sub>C<sub>5</sub>H<sub>4</sub>FeC<sub>5</sub>H<sub>5</sub>) exhibits a singlet for the C<sub>5</sub>H<sub>5</sub> ring.<ref>{{cite journal | vauthors = Abel EW, Long NJ, Orrell KG, Osborne AG, Šik V |title = Dynamic NMR studies of ring rotation in substituted ferrocenes and ruthenocenes |journal = [[Journal of Organometallic Chemistry|J. Org. Chem.]] |year = 1991 |volume = 403 |issue=1–2 |pages = 195–208 |doi = 10.1016/0022-328X(91)83100-I}}</ref>

From room temperature down to 164&nbsp;K, [[X-ray crystallography]] yields the monoclinic space group; the cyclopentadienide rings are a staggered conformation, resulting in a centrosymmetric molecule, with [[symmetry group]] D<sub>5d</sub>.<ref name=eiland52/> However, below 110&nbsp;K, ferrocene crystallizes in an orthorhombic crystal lattice in which the Cp rings are ordered and eclipsed, so that the molecule has symmetry group D<sub>5h</sub>.<ref name=seiler/> In the gas phase, [[electron diffraction]]<ref name=haal68/> and computational studies<ref name=coriani/> show that the Cp rings are eclipsed. While ferrocene has no permanent dipole moment at room temperature, between 172.8 and 163.5&nbsp;K the molecule exhibits an "incommensurate modulation", breaking the D<sub>5</sub> symmetry and acquiring an electric dipole.<ref>{{Cite journal |last1=Katrusiak |first1=Andrzej |last2=Rusek |first2=Michalina |last3=Dušek |first3=Michal |last4=Petříček |first4=Václav |last5=Szafrański |first5=Marek |date=2023-04-06 |title=Dipole-Moment Modulation in New Incommensurate Ferrocene |journal=The Journal of Physical Chemistry Letters |language=en |volume=14 |issue=13 |pages=3111–3119 |doi=10.1021/acs.jpclett.3c00215 |issn=1948-7185 |pmc=10084461 |pmid=36951481}}</ref>

In solution, eclipsed D<sub>5h</sub> ferrocene was determined to dominate over the staggered D<sub>5d</sub> conformer, as suggested by both [[Fourier-transform infrared spectroscopy]] and [[Density functional theory|DFT]] calculations.<ref>{{Cite journal |last1=Wang |first1=Feng |last2=Mohammadi |first2=Narges |last3=Best |first3=Stephen P. |last4=Appadoo |first4=Dominique |last5=Chantler |first5=Christopher T. |title=Dominance of eclipsed ferrocene conformer in solutions revealed by the IR spectra between 400 and 500 cm-1 |url=https://linkinghub.elsevier.com/retrieve/pii/S0969806X21002401 |journal=Radiation Physics and Chemistry |year=2021 |language=en |volume=188 |pages=109590 |doi=10.1016/j.radphyschem.2021.109590|bibcode=2021RaPC..18809590W}}</ref>