Overview
This lecture covers stereoisomerism in coordination compounds, focusing on geometric (cis/trans) and optical isomers, their identification, and related exam-style problems.
Geometric Isomers (Cis/Trans)
- Geometric isomers arise when identical ligands can be arranged on the same (cis) or opposite (trans) sides of a central metal ion.
- Geometric isomerism typically occurs in compounds with coordination numbers of 4 (square planar) or 6 (octahedral).
- At least two identical ligands must be present for cis/trans isomerism.
- In square planar geometry, cis isomers have identical ligands on the same side; trans isomers have them across from each other.
- In octahedral geometry, cis isomers have identical ligands next to each other; trans isomers have them opposite on the same plane.
- To determine if a compound can have cis/trans isomers, check for coordination numbers of 4 or 6 and the presence of at least two identical ligands.
Optical Isomers (Enantiomers)
- Optical isomers (enantiomers) are mirror image structures that are nonsuperimposable and occur in chiral environments.
- A compound is optically active if it rotates plane-polarized light, due to its asymmetric (chiral) arrangement.
- Most commonly found in octahedral complexes with bidentate or polydentate ligands creating "loops" or rings.
- To confirm optical isomerism: 1) Draw the mirror image, 2) Rotate by 180°, 3) Check nonsuperimposability and for chirality (asymmetry).
- Chiral (asymmetric) molecules cannot be divided into equal halves; symmetry implies no optical activity.
- Optical isomerism is relevant to pharmaceuticals, where different enantiomers can have distinct biological effects.
Problem-Solving Strategies
- For geometric isomerism: Verify coordination number and count of identical ligands.
- For optical isomerism: Check for asymmetry and nonsuperimposable mirror images.
- Use elimination to narrow down answer choices, ensuring chirality and correct structure.
Key Terms & Definitions
- Stereoisomer — Molecules with same formula and bonding but different spatial arrangements.
- Geometric isomer (cis/trans) — Isomers differing by position of identical ligands around a central metal.
- Optical isomer (enantiomer) — Mirror-image, nonsuperimposable isomers; display optical activity.
- Chirality — Property of a molecule that makes it non-superimposable on its mirror image.
- Coordination number — Number of ligand atoms attached to the central metal ion.
- Bidentate ligand — Ligand that attaches to the central metal at two points.
Action Items / Next Steps
- Practice drawing and identifying geometric and optical isomers.
- Prepare for upcoming lecture on bonding in complex ions: review valence bond theory, hybridization, and crystal field theory.
- Bring questions about isomer identification to the next class.