trans effect

In a flat, square-planar complex, each ligand has one partner sitting directly across the square from it — its trans neighbour. It turns out that some ligands are bullies: they make whatever sits opposite them much easier to kick out. The trans effect is this ability of a ligand to speed up substitution of the group trans to itself. It is the steering wheel of square-planar chemistry, and learning to use it is how chemists build a particular isomer on purpose.

The trans effect is a kinetic phenomenon — it is about how fast the trans group leaves, not about how stable the ground-state bond is. Chemists have ranked ligands by it into a rough series, with the strongest trans directors being good pi-acceptors and strong sigma-donors: roughly CN-, CO, and C2H4 at the top, then ligands like H- and NO2-, down through halides Br- and Cl-, and at the weak end amines, ammonia, water, and hydroxide. Two effects combine to produce it. A strong sigma-donor trans ligand competes for the same metal orbital pointed at the trans site, weakening the ground-state bond there (that part overlaps with the trans influence). And a good pi-acceptor stabilizes the five-coordinate associative transition state, where the incoming and leaving groups crowd into the same equatorial plane — lowering the barrier specifically for losing the group trans to it.

Where this becomes spectacular is in rational synthesis, above all the platinum drug cisplatin. To make cis-[Pt(NH3)2Cl2] you start from [PtCl4]2- and add ammonia: because chloride is a stronger trans director than ammonia, the second ammonia goes in cis to the first, giving the cis isomer needed for anticancer activity. To make the inactive trans isomer you reverse the order, starting from [Pt(NH3)4]2+ and adding chloride. Same atoms, different sequence, different drug — a direct, life-or-death payoff of understanding the trans effect.