Identify cis/trans isomerism in square-planar and octahedral complexes
Square-planar complexes have four coplanar ligand positions, whereas octahedral complexes have six positions directed to the vertices of an octahedron.
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The core idea
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Learning objectives
- Identify cis/trans isomerism in square-planar and octahedral complexes
Place ligands in the coordination geometry
Square-planar complexes have four coplanar ligand positions, whereas octahedral complexes have six positions directed to the vertices of an octahedron.
Cis/trans classification depends on the positions of a specified identical ligand pair, not merely on the ligand count in the formula.
Adjacent and opposite ligand pairs
In a square-planar MA2B2 complex, cis has the A ligands adjacent at 90° and trans has them opposite at 180°.
In an octahedral MA4B2 complex, cis has the B ligands at 90° and trans has them at 180°.
Construct before naming
For [Pt(NH₃)₂Cl₂], place Pt at the centre of a square and occupy four vertices; adjacent chlorides define cisplatin, while opposite chlorides define transplatin.
For [Co(NH₃)₄(H₂O)₂]²⁺, place two water ligands among six octahedral sites; their 90° or 180° separation fixes cis or trans while the four ammonia ligands fill the remaining sites.
The formula tells you which ligands are present but not where they sit. Draw the correct coordination shape first, mark the specified identical pair, and measure their relationship as adjacent 90° or opposite 180° positions.
Name the platinum isomer
A square-planar drawing has chloride ligands at the top and right positions around Pt, separated by 90°.
The remaining left and bottom positions contain NH₃, giving cis-[Pt(NH₃)₂Cl₂] with adjacent chlorides.
Name a square-planar [Pt(NH₃)₂Cl₂] drawing with adjacent chlorides.
Check your answer
A strong answer gives square-planar geometry, a 90° Cl–Pt–Cl angle and the cis descriptor.
Read an octahedral axis
Supplied structure: octahedral [Co(NH₃)₄(H₂O)₂]²⁺ with one H₂O at the top axial site and the other at the bottom axial site.
Find the H₂O–Co–H₂O angle from the octahedral positions, not from how far apart the ligands look on the page, then classify the complex.
Classify the cobalt complex when its water ligands occupy opposite axial sites.
Check your answer
Top and bottom axial sites lie on the same axis, so the H₂O–Co–H₂O angle is 180°. The complex is trans-[Co(NH₃)₄(H₂O)₂]²⁺.
Transfer to a new MA4B2 ion
Supplied ion: octahedral [Cr(NH₃)₄Cl₂]⁺, with neutral NH₃ ligands and Cl⁻ ligands.
Check the metal's oxidation state from the ligand charges, then place the two Cl ligands in every distinct way and use the Cl–Cr–Cl angle to name each arrangement.
Draw and name both cis and trans forms of [Cr(NH₃)₄Cl₂]⁺.
Check your answer
Chromium is +3 (four neutral NH₃, two Cl⁻, overall +1). Adjacent Cl ligands (Cl–Cr–Cl 90°) give cis-[Cr(NH₃)₄Cl₂]⁺; opposite Cl ligands (180°) give trans-[Cr(NH₃)₄Cl₂]⁺. Keep the + charge on both.
Formulae do not encode positions
The formula [Pt(NH₃)₂Cl₂] is shared by both geometric isomers, so a spatial drawing or positional description is required.
Tetrahedral MA2B2 has all ligand–metal–ligand angles equivalent and therefore does not form a cis/trans pair analogous to square planar MA2B2.
Better reasoning: ‘Any complex with two ligand types must have cis and trans forms.’
Check your answer
Reject it; geometry and positional nonequivalence must first permit adjacent and opposite arrangements.
Check geometry, pair and angle
State coordination geometry, identify the repeated ligand pair, measure its 90° or 180° relationship and then attach cis or trans to the complete formula.
After geometric isomerism, test octahedral chelate arrangements for non-superimposable mirror images.
A complete answer names the geometry, identifies the ligand pair being compared and quotes 90° for cis or 180° for trans. This also explains why the formula alone cannot distinguish the isomers.
List the evidence required to distinguish the two [Pt(NH₃)₂Cl₂] isomers.
Check your answer
Your answer should include square-planar geometry, chloride positions, 90° versus 180° separation and the corresponding cis/trans names.
Identify cis/trans isomerism in square-planar and octahedral complexes scientific representation
Each diagram position is also named by axis, adjacent/opposite relation and numerical angle; formulae and charges are written in full.
About 5 minutes
Text alternative: Each diagram position is also named by axis, adjacent/opposite relation and numerical angle; formulae and charges are written in full.