Cis effect

It has been determined that ligands that are weak sigma donors and non-pi acceptors seem to have the strongest cis-labilizing effects.

This is because these ligands will stabilize the 16 e− intermediate by electron donation from the p-pi lone pair donor orbital.

[4] Note that the strongest labilizing effects come from ligands that are weak sigma donors with virtually no pi-accepting behavior.

Since the cis and trans effects seem to have generally opposite trends, the electronic argument supports both phenomena.

Further evidence for cis labilization of CO can be attributed to the CO ligands being in competition for the dxy, dyz, and dxz orbitals.