Trans effect

[1] The analogous cis effect is most often observed in octahedral transition metal complexes.

[1] The discovery of the trans effect is attributed to Ilya Ilich Chernyaev,[3] who recognized it and gave it a name in 1926.

A second substitution by ammonia gives cis-[PtCl2(NH3)2], showing that Cl- has a greater trans effect than NH3.

Ligands with a high trans effect are in general those with high π acidity (as in the case of phosphines) or low-ligand lone-pair–dπ repulsions (as in the case of hydride), which prefer the more π-basic equatorial sites in the intermediate.

The relative importance of the cis and trans influences depends on the formal electron configuration of the metal center, and explanations have been proposed based on the involvement of the atomic orbitals.