<Li> The hexaaquacopper (II) ion (Cu (H O)) (21 e) </Li> <P> Often, cases where complexes have more than 18 valence electrons are attributed to electrostatic forces--the metal attracts ligands to itself to try to counterbalance its positive charge, and the number of electrons it ends up with is unimportant . In the case of the metallocenes, the chelating nature of the cyclopentadienyl ligand stabilizes its bonding to the metal . Somewhat satisfying are the two following observations: cobaltocene is a strong electron donor, readily forming the 18 - electron cobaltocenium cation; and nickelocene tends to react with substrates to give 18 - electron complexes, e.g. CpNiCl (PR) and free CpH . </P> <P> In the case of nickelocene, the extra two electrons are in orbitals which are weakly metal - carbon antibonding; this is why it often participates in reactions where the M--C bonds are broken and the electron count of the metal changes to 18 . </P>

Explain how the iron in ferrocene follows the 18-electron rule