MHT-CET Chemistry · Coordination Compounds

Isomerism in Coordination Compounds: Structural and Geometric

Coordination isomers share a formula but differ either in which atoms are bonded to the metal — structural isomers: ionisation, solvate, linkage and coordination — or only in how the same ligands are arranged in space — stereoisomers: geometric (cis and trans) and optical.

All Coordination Compounds notes MHT-CET Chemistry strategy

Why this matters

6 PYQs, none HARD. Five are structural: name the isomerism of a given pair (ionisation for the sulphato/bromo pair, solvate for the chromium hydrates, asked twice) or pick the ligand that can form linkage isomers (nitro, SCN⁻). One asks which complex is of the MA₂BC type that shows cis and trans forms. Two cards.

Concept 1 of 2

The Four Structural Isomerisms

Intuition

Ask what has swapped places. If an anion inside the bracket has traded places with the counter ion outside, the pair are IONISATION isomers — they give different ions in water ([Co(NH₃)₅Br]SO₄ gives SO₄²⁻, its isomer gives Br⁻). If a water molecule has moved between the sphere and the crystal, they are SOLVATE (hydrate) isomers. If one ambidentate ligand binds through a different atom — NO₂ through N or O, SCN through S or N — they are LINKAGE isomers. If a complex cation and a complex anion have exchanged ligands between their metals, they are COORDINATION isomers.

Definition

Ionisation: $[\text{Co(NH}_3)_5\text{SO}_4]\text{Br}$ and $[\text{Co(NH}_3)_5\text{Br}]\text{SO}_4$ — the first gives Br⁻ (AgBr with AgNO₃), the second SO₄²⁻ (BaSO₄ with BaCl₂).
Solvate (hydrate): $[\text{Cr(H}_2\text{O)}_6]\text{Cl}_3$ (violet), $[\text{Cr(H}_2\text{O)}_5\text{Cl}]\text{Cl}_2\cdot\text{H}_2\text{O}$ , $[\text{Cr(H}_2\text{O)}_4\text{Cl}_2]\text{Cl}\cdot2\text{H}_2\text{O}$ .
Linkage: needs an ambidentate ligand — NO₂⁻ (nitro/nitrito), SCN⁻ (thiocyanato/isothiocyanato), CN⁻. $[\text{Fe(H}_2\text{O)}_5\text{SCN}]^{2+}$ and $[\text{Fe(H}_2\text{O)}_5\text{NCS}]^{2+}$ . Aqua, ammine, halo and oxalato cannot.
Coordination: $[\text{Co(NH}_3)_6][\text{Cr(CN)}_6]$ and $[\text{Cr(NH}_3)_6][\text{Co(CN)}_6]$ — both ions must be complex.

Isomerism	What differs	Example pair
Ionisation	Anion inside ↔ anion outside	[Co(NH₃)₅Br]SO₄ / [Co(NH₃)₅SO₄]Br
Solvate (hydrate)	Water inside ↔ water of crystallisation	[Cr(H₂O)₆]Cl₃ / [Cr(H₂O)₅Cl]Cl₂·H₂O
Linkage	Donor atom of an ambidentate ligand	–NO₂ / –ONO; –SCN / –NCS Only an ambidentate ligand can do this.
Coordination	Ligands swapped between cation and anion metals	[Co(NH₃)₆][Cr(CN)₆] / [Cr(NH₃)₆][Co(CN)₆]

Find what moved: an anion, a water, a donor atom, or a whole set of ligands.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Coordination CompoundsMODERATE

Identify the type of isomerism exhibited by [Co(NH

_3

)

_5

Br]SO

_4

and [Co(NH

_3

)

_5

_4

]Br.

[Q97 · 9th May Shift 2 · 2023]

Calling a water swap 'ionisation'

In the chromium hydrates a chloride also moves, but what defines the pair is the WATER moving between sphere and crystal — solvate isomerism. Ionisation isomers exchange two different anions and contain the same water.

Drill 4 more on the four structural isomerisms

Concept 2 of 2

Geometric (cis–trans) Isomerism and the MA₂BC Type

Intuition

Write the complex as M with letters for its kinds of ligand. In a square planar complex, MA₂B₂ has two forms — the two A's side by side (cis) or opposite (trans) — and MA₂BC does too: the two identical A's are either adjacent or across. MA₄ or MA₃B cannot be arranged in two ways. A tetrahedral complex never shows cis–trans because every position is adjacent to every other. In octahedral MA₄B₂ the two B's are cis or trans, and MA₃B₃ gives fac and mer. Optical isomers need a complex with no mirror plane — [Co(en)₃]³⁺ and cis-[Co(en)₂Cl₂]⁺ are chiral; the trans form is not.

Definition

Square planar: $\text{MA}_2\text{B}_2$ (cisplatin and transplatin, $[\text{Pt(NH}_3)_2\text{Cl}_2]$ ) → 2; $\text{MA}_2\text{BC}$ ( $[\text{Pt(NH}_3)(\text{H}_2\text{O})\text{Cl}_2]$ ) → 2; $\text{MABCD}$ → 3. $\text{MA}_4$ , $\text{MA}_3\text{B}$ → none.
Tetrahedral: no geometric isomers.
Octahedral: $\text{MA}_4\text{B}_2$ ( $[\text{Co(NH}_3)_4\text{Cl}_2]^+$ ) → cis and trans; $\text{MA}_3\text{B}_3$ → fac and mer; $[\text{M(AA)}_2\text{B}_2]$ ( $[\text{Co(en)}_2\text{Cl}_2]^+$ ) → cis and trans, and the cis form is also optically active.
Geometric isomers are diastereoisomers: not mirror images, with different physical properties (cisplatin is the anticancer drug, transplatin is not).

Square planar geometric isomers

\text{MA}_4, \text{MA}_3\text{B}: 0;\quad \text{MA}_2\text{B}_2, \text{MA}_2\text{BC}: 2 \ (\textit{cis}, \textit{trans});\quad \text{MABCD}: 3

Worked example

Classify [Pt(NH₃)₂ClBr] by type and say how many geometric isomers it has.

Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 2Coordination CompoundsMODERATE

Which from following complexes is an example of

MA_{2}BC

type of distereoisomers?

[Q82 · 19 April Shift I · 2025]

Reading MA₂B₂ as MA₂BC

Pt(NH₃)₂Cl₂ also has cis and trans forms, but it is MA₂B₂ — two pairs of identical ligands. MA₂BC needs one pair and two DIFFERENT single ligands.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Geometric (cis–trans) Isomerism and the MA₂BC Type
Square planar geometric isomers
$\text{MA}_4, \text{MA}_3\text{B}: 0;\quad \text{MA}_2\text{B}_2, \text{MA}_2\text{BC}: 2 \ (\textit{cis}, \textit{trans});\quad \text{MABCD}: 3$

Reference tables (1)

The Four Structural Isomerisms4 rows

Isomerism	What differs	Example pair
Ionisation	Anion inside ↔ anion outside	[Co(NH₃)₅Br]SO₄ / [Co(NH₃)₅SO₄]Br
Solvate (hydrate)	Water inside ↔ water of crystallisation	[Cr(H₂O)₆]Cl₃ / [Cr(H₂O)₅Cl]Cl₂·H₂O
Linkage	Donor atom of an ambidentate ligand	–NO₂ / –ONO; –SCN / –NCS Only an ambidentate ligand can do this.
Coordination	Ligands swapped between cation and anion metals	[Co(NH₃)₆][Cr(CN)₆] / [Cr(NH₃)₆][Co(CN)₆]

Find what moved: an anion, a water, a donor atom, or a whole set of ligands.

Watch out for (2)

Calling a water swap 'ionisation'
→ The Four Structural Isomerisms
Reading MA₂B₂ as MA₂BC
→ Geometric (cis–trans) Isomerism and the MA₂BC Type

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