# Explanation of the trans-effect (thermodynamic and kinetic) for octahedral transition metal complexes

The thermodynamic trans effect refers to the ground state weakening of the metal-ligand bond in a complex by a ligand trans to it. The kinetic trans effect is the lablisation of a ligand in a complex by the ligand trans to it.

These are commonly known effects, and most of thre resources I find are discussing the trans effect in terms of square planar complexes. The explanation of kinetic trans effect for square planar complexes is simple, the trigonal bipyramidal transition state is stabilised by $$\pi$$-acceptor ligands.

However, the trans effects of both type are known to be present in octahedral complexes as well.[1] The paper however, does not go into much detail about why the trans effect works. The structural trans effect I can understand as due to trans ligands sharing the same metal d-orbital for overlap, which would result in competitive electron donation/withdrawal. However, it isn't clear how the kinetic trans effect would work. The paper mentions that sigma donation helps to stabilise the transition state leading to the dissociation of one ligand; but I don't understand why $$\pi$$-donation would not have the same effect, or why $$\pi$$-acceptors would not have the inverse effect.

So I am looking for an explanation of the trans effect, particularly kinetic trans effect in octahedral complexes. An explanation with orbitals would be very helpful.

Please also feel free to direct me to resources (books, journals) which has these explanations or would be helpful for understanding this.

Reference:

1. B. J. Coe, S. Glenwright, "Trans-effects in octahedral transition metal complexes", Coordination Chemistry Reviews, 2000, 203(1), 5-80