text: Some complexes have more than 18 electrons. Examples:
  * Cobaltocene (19 e)
  * Nickelocene (20 e)
  * The hexaaquacopper(II) ion [Cu(HO)] (21 e)
  *TM(CO) (TM = Sc, Y) (20 e)
  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.
  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.
  The 20-electron systems TM(CO) (TM = Sc, Y) have a cubic (O) equilibrium geometry and a singlet (A) electronic ground state. There is one occupied valence MO with a symmetry, which is formed only by ligand orbitals without a contribution from the metal AOs. But the adducts TM(CO)  (TM=Sc, Y) fulfill the 18-electron rule when one considers only those valence electrons, which occupy metal–ligand bonding orbitals.