Cis effect

In inorganic chemistry, the cis effect is defined as the labilization (making unstable) of CO ligands that are cis to other ligands. CO is a well-known strong pi-accepting ligand in organometallic chemistry that will labilize in the cis position when adjacent to ligands due to steric and electronic effects. The system most often studied for the cis effect is an octahedral complex M(CO)₅X where X is the ligand that will labilize a CO ligand cis to it. Unlike trans effect, where this property is most often observed in 4-coordinate square planar complexes, the cis effect is observed in 6-coordinate octahedral transition metal complexes. It has been determined that ligands that are weak sigma donors and non-pi acceptors seem to have the strongest cis-labilizing effects. Therefore, the cis effect has the opposite trend of the trans-effect, which effectively labilizes ligands that are trans to strong pi accepting and sigma donating ligands.

Group 6 and group 7 transition metal complexes (M(CO)₅X) 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 M(CO)₅X, group 6 metals (M⁰,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 M(CO)₄X 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 S_N1 mechanism in organic chemistry, and applies to coordination compounds as well.

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