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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. == Electron counting in metal carbonyl complexes == 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 SN1 mechanism in organic chemistry, and applies to coordination compounds as well. Figure 1. Intermediates in the substitution of M(CO)5X complexes. If ligands X and Y are neutral donors to the complex: M = Group 6 metal (m = 0) M = Group 7 metal (m = +1) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「cis effect」の詳細全文を読む スポンサード リンク
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