Read When Two Formulas Are (and Aren't) Isomers
Structural isomers share a molecular formula but differ in which atoms sit inside the coordination sphere versus outside it. The trap is recognising when a pair of compounds ISN'T isomeric at all.
Ionisation, Hydrate & Coordination Isomerism is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Not every look-alike pair is isomeric
[Co(SO₄)(NH₃)₅]Br and [Co(SO₄)(NH₃)₅]Cl look like a matched pair, but they have different molecular formulas (Br vs Cl) — so they aren't isomers of each other at all; 'no isomerism' is correct precisely because isomers must share the same formula.
Coordination isomerism: swapping ligands between cation and anion complexes
When BOTH the cation and the anion of a salt are complex ions, coordination isomers arise from swapping which ligands sit on which metal while keeping the overall formula fixed — this is what [Pt(NH₃)₄Cl₂][PtCl₄] enables, giving several valid isomers by redistributing Cl⁻ and NH₃ between the two Pt centres.
Coordination-position isomerism in bridged complexes
In bridged binuclear complexes with two different metal-ligand sets connected by a shared bridge, moving which terminal ligands sit on which metal (while the bridge stays fixed) produces coordination-position isomers — a more specific case than simple coordination isomerism, distinct from linkage or ligand isomerism. Choose D for this comparison.
OlympIQ exam lens
- ✓Isomers must share the same molecular formula — check that first before naming an isomerism type.
- ✓Coordination isomerism needs BOTH cation and anion to be complex ions, with ligands swappable between them.
- ✓Hydrate isomerism swaps water between the coordination sphere and the outer lattice, changing which counter-ion precipitates.
- ✓'No isomerism' is a valid, correct answer when the formulas being compared simply differ.
Before hunting for an isomerism type, verify the two compounds actually share one molecular formula — a changed counter-ion (Br vs Cl) means they're just different compounds, not isomers.
Ready to try the pattern?
100% coverage across the 4 linked questions.
Practise 4 linked questions