Isomerism

CHEIsomerism

Coordination compounds show structural isomerism (ionisation, hydrate, linkage, coordination) and stereoisomerism (geometrical cis/trans and optical). ISC questions ask you to name the isomerism type, draw isomers, decide optical activity, and give a distinguishing test, so learn both the definitions and the diagnostic reactions.

Ionisation isomerism: isomers give different ions in solution, e.g. $[Co(NH_3)_5Br]SO_4$ (gives $SO_4^{2-}$ , tested with $BaCl_2 \rightarrow$ white $BaSO_4$ ) vs $[Co(NH_3)_5SO_4]Br$ (gives $Br^-$ , tested with $AgNO_3 \rightarrow$ pale-yellow $AgBr$ ).
Hydrate isomerism: $CrCl_3\cdot 6H_2O$ exists as $[Cr(H_2O)_6]Cl_3$ (violet, 3 ionisable $Cl^-$ ), $[Cr(H_2O)_5Cl]Cl_2\cdot H_2O$ (blue-green, 2 $Cl^-$ ) and $[Cr(H_2O)_4Cl_2]Cl\cdot 2H_2O$ (green, 1 $Cl^-$ ); distinguish by the amount of $AgCl$ precipitated with $AgNO_3$ .
Linkage isomerism arises with ambidentate ligands: nitrito-N $[Co(NH_3)_5NO_2]Cl_2$ (M–N bond) vs nitrito-O $[Co(NH_3)_5ONO]Cl_2$ (M–O bond).
Coordination isomerism: ligands swap between cation and anion complexes, e.g. $[Cr(NH_3)_6][Co(CN)_6]$ vs $[Co(NH_3)_6][Cr(CN)_6]$ .
Square-planar $MA_2B_2$ has cis and trans geometrical isomers: cis- $[Pt(NH_3)_2Cl_2]$ (cisplatin) is the anticancer drug; the trans form is inactive.
Octahedral $[MA_3B_3]$ gives two geometrical isomers — facial (fac) and meridional (mer); octahedral $[MA_4B_2]$ and $[M(AA)_2B_2]$ give cis and trans.
Optical isomerism needs a non-superimposable mirror image (no plane/centre of symmetry): $[Co(en)_3]^{3+}$ exists as two enantiomers ( $d$ and $l$ ); cis- $[Co(en)_2Cl_2]^+$ is optically active but the trans form is not.
Structural isomerism = same formula, different connectivity (ionisation, hydrate/solvate, linkage, coordination); stereoisomerism = same connectivity, different spatial arrangement (geometrical and optical).
Geometrical isomerism is shown by square-planar $MA_2B_2$ , $MABCD$ and octahedral $MA_4B_2$ , $MA_3B_3$ , $M(AA)_2B_2$ types; it is NOT shown by tetrahedral complexes (all four positions are mutually adjacent) or by $MA_5B$ / $MA_6$ types.
Optical isomers (enantiomers) are non-superimposable mirror images that rotate plane-polarised light equally but oppositely; a 1:1 mix (racemate) is optically inactive by external compensation.
Test for symmetry: a complex with a plane or centre of symmetry is optically inactive. trans- $[Co(en)_2Cl_2]^+$ has a plane of symmetry (inactive), whereas the cis form lacks one (active); $[Co(en)_3]^{3+}$ has no such symmetry and is chiral.
Number of isomers to quote: $[Pt(NH_3)_2Cl_2]$ (square planar) gives 2 (cis, trans); $[CoCl_3(NH_3)_3]$ (octahedral) gives 2 (fac, mer, both inactive); $[Co(en)_2Cl_2]^+$ gives 3 stereoisomers in all (trans inactive + cis pair of enantiomers).

Where the marks go

Claiming tetrahedral complexes show cis–trans isomerism — they cannot, because all four corners are mutually adjacent; only square-planar and suitable octahedral complexes do.
Calling the trans isomer of $[Co(en)_2Cl_2]^+$ or $[Pt(NH_3)_2Cl_2]$ optically active — the trans form has a plane of symmetry and is optically inactive.
Confusing ionisation isomerism with hydrate isomerism: hydrate isomers specifically exchange $H_2O$ between inside and outside the coordination sphere.
Forgetting that both fac and mer isomers of $[MA_3B_3]$ are achiral (optically inactive) — examiners often test whether you wrongly assign optical activity here.
Mixing up linkage isomerism (one ambidentate ligand, different donor atom) with coordination isomerism (ligand interchange between two complex ions).

How the board asks it

Structure / namingstructural isomerism: ionisation isomers give different ions in solution
Name the type of isomerism shown by the pair $[Co(NH_3)_5Br]SO_4$ and $[Co(NH_3)_5SO_4]Br$ , and give one chemical test to distinguish between them.
Structure / naminggeometrical and optical isomerism of octahedral $M(AA)_2B_2$ complexes
Draw the structures of all the stereoisomers of $[Co(en)_2Cl_2]^+$ and label each as cis, trans, optically active or optically inactive.
Give reasonsa plane or centre of symmetry makes a complex optically inactive
Account for the fact that cis- $[Co(en)_2Cl_2]^+$ is optically active whereas the trans isomer is optically inactive.
Distinguishhydrate isomers release different numbers of ionisable $Cl^-$
The compound $CrCl_3\cdot 6H_2O$ exists as $[Cr(H_2O)_6]Cl_3$ , $[Cr(H_2O)_5Cl]Cl_2\cdot H_2O$ and $[Cr(H_2O)_4Cl_2]Cl\cdot 2H_2O$ . Name this type of isomerism and explain how these can be distinguished using $AgNO_3$ solution.
Predict the productcounting geometrical isomers of square-planar $MA_2B_2$ and octahedral $MA_3B_3$
State the number of geometrical isomers possible for $[Pt(NH_3)_2Cl_2]$ and for octahedral $[Co(NH_3)_3Cl_3]$ , and name each isomer.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.