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H3 Chemistry 9813 · Study focus: H3 Chemistry: Identify cis/trans isomerism in square-planar and octahedral complexes

H3 Chemistry: Identify cis/trans isomerism in square-planar and octahedral complexes

Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.

Your success criteria

  • Identify cis/trans isomerism in square-planar and octahedral complexes
  • Use named chemical evidence.
  • Transfer the governing reason to an unfamiliar case.
Diagnose this objective

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.

Explore this H3 topic and lesson sequence.

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°.

  1. 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.

  2. 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.

H3 Chemistry: Identify cis/trans isomerism in square-planar and octahedral complexes: move from the evidence or givens, through the governing Chemistry idea, to a conclusion that stays inside the selected course boundary.
H3 Chemistry: Identify cis/trans isomerism in square-planar and octahedral complexes evidence representation. The same coordination formula names two isomers, so ligand positions and metal-centred angles must be explicit in square-planar and octahedral diagrams.
cis-square-planar MA₂B₂cis-square-planar MA₂B₂BBAAMB–M–B = 90°trans-square-planar MA₂B₂trans-square-planar MA₂B₂BABAMB–M–B = 180°cis-octahedral MA₄B₂cis-octahedral MA₄B₂BBAAAAMB–M–B = 90°trans-octahedral MA₄B₂trans-octahedral MA₄B₂BBAAAAMB–M–B = 180°

Text alternative: Each diagram position is also named by axis, adjacent/opposite relation and numerical angle; formulae and charges are written in full.

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.

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.
Open the feedback checkpoint after attempting
  • Award square-planar geometry, a 90° Cl–Pt–Cl angle and the cis descriptor.

Read an octahedral axis

In an octahedral complex, a top ligand and a bottom ligand on the same axis are 180° apart.

If those positions are both H2O in [Co(NH3)4(H2O)2]2+, the complex is trans.

  • Classify the cobalt complex when its water ligands occupy opposite axial sites.
Open the feedback checkpoint after attempting
  • Credit trans-[Co(NH3)4(H2O)2]2+ and a 180° H2O–Co–H2O angle.

Start the diagnostic and follow its feedback

Transfer to a new MA4B2 ion

[Cr(NH3)4Cl2]+ is octahedral: chromium is +3 because four NH3 are neutral and two chloride ligands contribute −2 overall.

Two adjacent chloride ligands form the cis isomer; two opposite chloride ligands form the trans isomer.

  • Draw and name both cis and trans forms of [Cr(NH3)4Cl2]+.
Open the feedback checkpoint after attempting
  • Show octahedral coordination with Cl–Cr–Cl angles of 90° and 180°, respectively, and retain the + charge.

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.

  • Repair: ‘Any complex with two ligand types must have cis and trans forms.’
Open the feedback checkpoint after attempting
  • 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.

  • List the evidence required to distinguish the two [Pt(NH3)2Cl2] isomers.
Open the feedback checkpoint after attempting
  • Require square-planar geometry, chloride positions, 90° versus 180° separation and the corresponding cis/trans names.