Inorganic — kinetic directing effect
The trans effect in square-planar substitution
How the The trans effect in square-planar substitution works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: K₂[PtCl₄], NH₃ (cisplatin route); [Pt(NH₃)₄]²⁺, Cl⁻ (transplatin route). Conditions: Water, 25–50 °C. Bonds formed: Pt–N (or Pt–Cl) at the site trans to the strong-trans-effect ligand; bonds broken: Pt–Cl trans to Cl. Stereochemistry: Ligand order of addition controls cis vs trans. Rate and selectivity: Trans-effect series: CN⁻, CO, C₂H₄ > PR₃ > H⁻ > CH₃⁻ > I⁻ > Cl⁻ > NH₃ > H₂O.
At a glance
Step 1: Associative attack by NH₃
In an associative substitution at square-planar Pt(II), the incoming nucleophile attacks from the same plane as an existing ligand positioned trans to the leaving group.
Step 2: Cl trans to Cl is the labile one
Certain ligands (CO, CN⁻, alkenes, phosphines — all strong π-acceptors) withdraw electron density through the metal specifically along the trans axis, weakening the bond to whatever sits opposite them and stabilizing the five-coordinate transition state.
Step 3: Order of addition sets cis vs trans
This makes substitution happen selectively at the position trans to the strong-trans-effect ligand — chemists exploit this directing effect to control exactly which ligand gets replaced, most famously in the stepwise synthesis of cisplatin.
Abbreviations and terms
- CO
- Carbon monoxide (carbonyl ligand). σ-donor / strong π-acceptor ligand; also a C1 building block.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
How it compares with related reactions
| The trans effect in square-planar substitution | The chelate effect in ligand binding | Associative ligand substitution at a square-planar Pt(II) complex | |
|---|---|---|---|
| Reagents | K₂[PtCl₄], NH₃ (cisplatin route); [Pt(NH₃)₄]²⁺, Cl⁻ (transplatin route) | [Ni(H₂O)₆]²⁺, ethylenediamine (en) vs 2 NH₃ | K₂[PtCl₄] + NH₃ (aq) |
| Conditions | Water, 25–50 °C | Water, 25 °C | H₂O, 25 °C |
| Stereochemistry | Ligand order of addition controls cis vs trans | Ring closure gives a puckered five-membered ring | Trigonal-bipyramidal intermediate; retention of geometry |
| Rate / selectivity | Trans-effect series: CN⁻, CO, C₂H₄ > PR₃ > H⁻ > CH₃⁻ > I⁻ > Cl⁻ > NH₃ > H₂O | log K₁(en) ≈ 7.5 vs log β₂(NH₃) ≈ 5.0 for Ni²⁺: entropy-driven | Rate = k₁[Pt] + k₂[Pt][NH₃] (associative; entering-ligand dependent) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the The trans effect in square-planar substitution?
K₂[PtCl₄], NH₃ (cisplatin route); [Pt(NH₃)₄]²⁺, Cl⁻ (transplatin route). Conditions: Water, 25–50 °C.
Which bonds are formed and which are broken?
Formed: Pt–N (or Pt–Cl) at the site trans to the strong-trans-effect ligand. Broken: Pt–Cl trans to Cl.
What is the stereochemical outcome?
Ligand order of addition controls cis vs trans
What controls the rate or the selectivity?
Trans-effect series: CN⁻, CO, C₂H₄ > PR₃ > H⁻ > CH₃⁻ > I⁻ > Cl⁻ > NH₃ > H₂O
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam