In simple terms
A friendly intro before the formal notes — no formulas yet.
Complex Shapes: A 3D Puzzle
Just like organic molecules, the 3D arrangement of ligands around a central metal ion can create different isomers. These stereoisomers have the same chemical formula and bonds but differ in their spatial layout, leading to unique properties.
Imagine you have a central orange and you're sticking four yellow sweets and two green sweets into it with cocktail sticks. If you place the two green sweets next to each other, that's one arrangement (cis). If you place them on completely opposite sides of the orange, that's a different arrangement (trans). Now, imagine your left and right hands; they are mirror images but you can't place one perfectly on top of the other. Some complex molecules have this 'handedness' too, which we call optical isomerism.
- 1
Octahedral MX₆: cis/trans isomerism with two identical ligands. | Sim hint: Cis: adjacent; trans: opposite positions.
- 2
Square planar and octahedral optical isomerism possible. | Sim hint: No plane of symmetry in complex.
- 3
Bidentate ligands (en) enable optical isomers in [M(en)₃]ⁿ⁺. | Sim hint: Δ and Λ enantiomers.
- 4
Contrast with organic stereoisomerism — chiral at metal centre rare. | Sim hint: Focus on ligand arrangement.
Explore the concept
Use the live diagram and synced steps — play it or tap a step card to walk through.
Full topic notes
Formal explanation with the rigour you need for the exam.
Geometric (Cis-Trans) Isomerism
Geometric isomerism arises when ligands occupy different positions around the central metal ion. The most common form is cis-trans isomerism. This occurs in square planar and octahedral complexes, but not tetrahedral ones because all positions in a tetrahedron are equivalent.
In square planar complexes of the type [MA₂B₂], such as [Pt(NH₃)₂(Cl)₂], two distinct arrangements are possible. The cis-isomer, cisplatin, has the two chloride ligands next to each other. The trans-isomer, transplatin, has them on opposite sides. This difference has profound biological consequences; cisplatin is a potent chemotherapy drug, while transplatin is inactive.
In octahedral complexes of the type [MA₄B₂], such as [Co(NH₃)₄Cl₂]⁺, the two 'B' ligands can either be adjacent (cis) or opposite (trans). Drawing these requires a 3D representation, often using wedges and dashed lines to show perspective.
Cis: Identical ligands are adjacent to each other (90° apart in square planar and octahedral).
Trans: Identical ligands are opposite each other (180° apart).
Optical Isomerism
Optical isomers, also known as enantiomers, are molecules that are non-superimposable mirror images of each other. This property is called chirality. A chiral molecule will rotate the plane of plane-polarised light, making it 'optically active'. The key requirement for chirality is the absence of a plane of symmetry within the molecule.
In transition metal chemistry, optical isomerism is common in octahedral complexes containing bidentate ligands. A classic example is a complex with three bidentate ligands, such as [Ni(en)₃]²⁺, where 'en' is the bidentate ligand 1,2-diaminoethane. The three 'en' ligands wrap around the central nickel ion to form a shape resembling a propeller. This propeller can be either left-handed or right-handed, giving two enantiomers that are mirror images of each other.
Combined Geometric and Optical Isomerism
Some complexes can exhibit both types of stereoisomerism. A key example is an octahedral complex with two bidentate ligands and two monodentate ligands, such as [Co(en)₂(Cl)₂]⁺. This complex has a cis and a trans geometric isomer. The trans-isomer has a plane of symmetry and is therefore achiral. However, the cis-isomer lacks a plane of symmetry and is chiral. This means the cis-isomer exists as a pair of optical isomers (enantiomers). Therefore, there are three stereoisomers in total for [Co(en)₂(Cl)₂]⁺: the trans-isomer, and the two enantiomers of the cis-isomer.
When drawing 3D isomers, clarity is essential. Use solid lines for bonds in the plane of the paper, solid wedges for bonds coming out towards you, and dashed lines for bonds going away from you. Always label your isomers clearly (e.g., 'cis', 'trans', 'enantiomer 1', 'enantiomer 2'). For bidentate ligands, a simple arc connecting the two coordination sites is usually sufficient and much clearer than drawing the full structure.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
The complex ion [Cr(H₂O)₄Cl₂]⁺ exists as two geometric isomers. Draw the 3D structures of these two isomers and label them as cis and trans.
- 1
First, identify the geometry. With six ligands, it is octahedral. The isomers depend on the relative positions of the two Cl⁻ ligands.
Draw the two optical isomers of the complex ion [Co(en)₃]³⁺. In your diagram, represent the bidentate ligand 1,2-diaminoethane (en) as a simple arc.
- 1
Start by drawing the central Co³⁺ ion and the six coordination bonds in an octahedral arrangement.
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Try to recall each definition before you reveal it.
Quick check
Answer in your head first — then tap to check. No pressure.
Revision flashcards
Flip the card. Test yourself before the exam.
What are stereoisomers?
Compounds with the same structural formula (same atoms bonded to each other) but with a different arrangement of the atoms in space.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
- ✓
Cis: Identical ligands are adjacent to each other (90° apart in square planar and octahedral).
- ✓
Trans: Identical ligands are opposite each other (180° apart).
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Practice Questions
Practice Questions
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do Practice Questions on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.