How can the 18 electron rule be used to explain that Fe(η5-C5H5)2 doesn't react with hydrogen but Ni(η5-C5H5)2 does?
Hydrogenation of NiCp2 takes place on the ligand, not the metal, to reduce donor electron count, giving a low-energy closed-shell 18 electron count.

http://en.wikipedia.org/wiki/Nickelocene 
http://en.wikipedia.org/wiki/Ferrocene 
http://en.wikipedia.org/wiki/Hapticity 
http://www.chem.mun.ca/homes/cmkhome/Organometallics_The%20Basics.pdf 
   p. 9, 10.  "η" is "hapticity," the number of ligand atoms contributing pi-electron orbitals to the metal center.

$\ce{Cp^-}$ is aromatic (draw the Frost diagram) when coordinated $\ce{η^5}$ on-face, or  $\ce{η^1}$ non-aromatic as a simple anion coordinated on-vertex.  Saturating one localized double bond leaves cyclic delocalized allyl anion coordinated $\ce{η^3}$.  

In the case of $\ce{TiCp4}$, two Cp anions are face pentahapto, two are vertex monohapto.  If we take a proton or C-13 NMR, will we see discrete protons or carbons for the $\ce{η^1}$-Cp, or will allowed 1,5-shifts scramble all five protons?  All five carbons?  Will it be temperature dependent if it happens?  If it happens, will H and C have different kinetics?