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.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
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.
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- ✓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.
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