Stereochemistry plays a central role in how metal complexes react, because the 3D arrangement of ligands around the metal controls access to the metal center, orbital interactions, and even the stability of intermediates. Here are the main ways stereochemistry influences reactivity:
1. Geometrical Isomerism (cis vs. trans)
- The arrangement of ligands strongly affects which reactions are possible.
- Cisplatin (cis-[PtCl₂(NH₃)₂]) is reactive toward DNA, but the trans isomer is almost inactive because the two chlorides are too far apart to bind adjacent DNA bases.
- In substitution reactions, cis ligands can undergo intramolecular reactions (chelation, ring closure), while trans ligands cannot.
2. Steric Effects
- Bulky ligands may block coordination sites or slow down substitution reactions.
- Example: Octahedral complexes with bulky ligands often undergo slower ligand exchange because incoming ligands can’t easily approach the metal center.
- Steric crowding can also stabilize unusual geometries by preventing rearrangements.
3. Electronic Effects via Geometry
- Different stereochemistries change crystal field splitting and thus the electronic distribution.
- Square planar complexes (like many d⁸ metals) tend to undergo associative substitution more easily than octahedral complexes.
- Geometrical isomers may differ in ligand field stabilization energy (LFSE), making one isomer more stable or reactive.
4. Chirality and Enantioselectivity
- Chiral metal complexes can react differently with enantiomeric substrates.
- In asymmetric catalysis, only one enantiomer of a product is formed preferentially because of the stereochemistry of the metal–ligand complex.
- Example: Chiral Rh, Ru, or Ir complexes are widely used in enantioselective hydrogenation.
5. Ligand Arrangement and Mechanism
- Whether a substitution reaction proceeds by associative (A), dissociative (D), or interchange (I) mechanisms depends partly on stereochemistry.
- In octahedral complexes, ligands cis to the leaving group often influence substitution more strongly than trans ligands (the trans effect in square planar complexes is another classic example).
6. Biological Reactivity
- Many metalloproteins rely on stereochemistry for function:
- Hemoglobin requires the proper octahedral arrangement of Fe²⁺ ligands for reversible O₂ binding.
- Distorted geometries can hinder binding or trigger unwanted side reactions.
In summary:
Stereochemistry affects reactivity of metal complexes by controlling which ligands can react (cis/trans effects), how fast ligands are exchanged (sterics and geometry), the stability of intermediates (LFSE), and the selectivity of reactions (chirality).