Trans Effect - Kinetic Trans Effect

Kinetic Trans Effect

The intensity of the trans effect (as measured by the increase in rate of substitution of the trans ligand) follows this sequence:

F−, H₂O, OH− < NH₃ < py < Cl− < Br− < I−, SCN−, NO₂−, SC(NH₂)₂, Ph− < SO₃2− < PR₃, AsR₃, SR₂, CH₃− < H−, NO, CO, CN−, C₂H₄

The classic example of the trans effect is the synthesis of cisplatin. Starting from PtCl₄2−, the first NH₃ ligand is added to any of the four equivalent positions at random, but the second NH₃ is added cis to the first one, because Cl− has a larger trans effect than NH₃:

If, on the other hand, one starts from Pt(NH₃)₄2+, the trans product is obtained instead:

The trans effect in square complexes can be explained in terms of an addition/elimination mechanism that goes through a trigonal bipyramidal intermediate. 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–d_π repulsions (as in the case of hydride), which prefer the more π-basic equatorial sites in the intermediate. The second equatorial position is occupied by the incoming ligand; due to the principle of microscopic reversibility, the departing ligand must also leave from an equatorial position. The third and final equatorial site is occupied by the trans ligand, so the net result is that the kinetically favored product is the one in which the ligand trans to the one with the largest trans effect is eliminated.