text: Group 6 and group 7 transition metal complexes  have been found to be the most prominent in regards to dissociation of the CO cis to ligand X.  CO is a neutral ligand that donates 2 electrons to the complex, and therefore lacks anionic or cationic properties that would affect the electron count of the complex. For transition metal complexes that have the formula , group 6 metals (M, where the oxidation state of the metal is zero) paired with neutral ligand X, and group 7 metals (M, where the oxidation state of the metal is +1),  paired anionic ligands, will create very stable 18 electron complexes. Transition metal complexes have 9 valence orbitals, and 18 electrons will in turn fill these valences shells, creating a very stable complex, which satisfies the 18-electron rule. The cis-labilization of 18 e complexes suggests that dissociation of ligand X in the cis position creates a square pyramidal transition state, which lowers the energy of the  complex, enhancing the rate of reaction.  The scheme below shows the dissociation pathway of a CO ligand in the cis and trans position to the X, followed by the association of ligand Y. This is an example of a dissociative mechanism, where an 18 e complex loses a CO ligand, making a 16 e intermediate, and a final complex of 18 e results from an incoming ligand inserting in place of the CO. This mechanism resembles the S1 mechanism in organic chemistry, and applies to coordination compounds as well.
  Figure 1. Intermediates in the substitution of  complexes.
  If ligands X and Y are neutral donors to the complex:
  M = Group 6 metal (m = 0)
  M = Group 7 metal (m = +1)