ChemistryISC Class 12 · Coordination Compounds

Isomerism in Coordination Compounds: ISC Class 12 Chemistry

Updated 15 October 2026 · Fact-checked

Coordination compounds show structural isomerism (ionisation, linkage, coordination and solvate) when bonds differ, and stereoisomerism (geometrical and optical) when the same bonds are arranged differently in space. Ionisation isomers are told apart by tests on the free ions: BaCl₂ for sulphate and AgNO₃ for halide.

Understand Isomerism in Coordination Compounds

Isomers have the same formula but different arrangements of atoms. In coordination compounds there are two main kinds.

A. Structural isomerism (different bonds).
(i) Ionisation isomerism. A ligand inside the coordination sphere and a counter-ion outside it exchange places. The isomers give different ions in solution: $[C o (N H_{3})_{5} C l] S O_{4}$ gives free $S O_{4}^{2 -}$ , while $[C o (N H_{3})_{5} S O_{4}] C l$ gives free Cl⁻. Test: add BaCl₂ solution; a white precipitate of BaSO₄ shows free sulphate. Add AgNO₃ solution; a white precipitate of AgCl (or pale yellow AgBr) shows a free halide. Only ions outside the square brackets react.
(ii) Linkage isomerism. An ambidentate ligand can bind through either of two donor atoms: $N O_{2}^{-}$ through N (nitrito-N) or O (nitrito-O), and $S C N^{-}$ through S (thiocyanato-S) or N (thiocyanato-N, isothiocyanato).
(iii) Coordination isomerism. Both cation and anion are complexes and ligands are exchanged between the two metals: $[C o (N H_{3})_{6}] [C r (C N)_{6}]$ and $[C r (N H_{3})_{6}] [C o (C N)_{6}]$ .
(iv) Solvate (hydrate) isomerism. Water is inside the sphere as a ligand in one isomer and outside as water of crystallisation in another: $[C r (H_{2} O)_{6}] C l_{3}$ (violet) and $[C r (H_{2} O)_{5} C l] C l_{2} \cdot H_{2} O$ (grey-green). They differ in the number of Cl⁻ ions precipitated by AgNO₃ and in conductivity.

B. Stereoisomerism (same bonds, different spatial arrangement).
(i) Geometrical (cis-trans) isomerism. In square planar $M A_{2} B_{2}$ , the two A ligands may be next to each other (cis, 90°) or opposite (trans, 180°): $[P t (N H_{3})_{2} C l_{2}]$ has two isomers. Square planar MABCD has three. Tetrahedral complexes do not show geometrical isomerism, because every position is adjacent to every other. In octahedral $M A_{4} B_{2}$ ( $[C o (N H_{3})_{4} C l_{2}]^{+}$ ), the B ligands are cis or trans. In $M (AA)_{2} B_{2}$ ( $[C o (e n)_{2} C l_{2}]^{+}$ ) the same is true. In $M A_{3} B_{3}$ , the three A ligands may occupy one face of the octahedron (fac) or a meridian, a plane through the metal (mer): $[C o (N H_{3})_{3} (N O_{2})_{3}]$ .
(ii) Optical isomerism. A complex that is not superimposable on its mirror image (chiral) exists as two enantiomers, dextro (d) and laevo (l), which rotate plane-polarised light in opposite directions. It is common in octahedral complexes with bidentate ligands: $[C o (e n)_{3}]^{3 +}$ , $[C r (C_{2} O_{4})_{3}]^{3 -}$ and cis- $[C o (e n)_{2} C l_{2}]^{+}$ are chiral; trans- $[C o (e n)_{2} C l_{2}]^{+}$ has a plane of symmetry and is optically inactive.

Counting stereoisomers: a useful list. $M A_{4} B_{2}$ : 2 (cis, trans; neither chiral). $M A_{3} B_{3}$ : 2 (fac, mer). $M (AA)_{2} B_{2}$ : 3 (trans, and cis as a d/l pair). $M (AA)_{3}$ : 2 (d and l). $M (AA) A_{2} B_{2}$ : 4 (three geometrical arrangements, one of which is chiral).

Werner's theory link. Werner deduced structures from the number of ions in solution (conductivity) and the number of Cl⁻ precipitated by AgNO₃. For example, of the cobalt(III) chloride ammines, CoCl₃·4NH₃ exists as two forms, violet and green, which are the cis and trans isomers of $[C o (N H_{3})_{4} C l_{2}] C l$ . Both give 1 mol of AgCl per mole and two ions in solution.

Isomerism in Coordination Compounds: formulas and key facts

Ionisation isomer tests: $B a^{2 +} + S O_{4}^{2 -} \to B a S O_{4} ↓ (white); A g^{+} + C l^{-} \to A g C l ↓ (white)$; AgBr is pale yellow; only ions outside the brackets react.
Stereoisomer counts: $M A_{4} B_{2} : 2; M A_{3} B_{3} : 2; M (AA)_{2} B_{2} : 3; M (AA)_{3} : 2; M (AA) A_{2} B_{2} : 4$; Octahedral complexes; includes optical isomers.
Square planar geometrical isomers: $M A_{2} B_{2} : 2 (c i s, t r an s); M A B C D : 3$; Tetrahedral complexes show none.
Ambidentate ligands: $N O_{2}^{-} : M - N O_{2} (nitrito-N), M - O N O (nitrito-O); S C N^{-} : M - S C N, M - N C S$; Basis of linkage isomerism.
Optical activity condition: $chiral ⟺ no plane or centre of symmetry$; trans-[M(AA)₂B₂] has a mirror plane: inactive.

How to solve Isomerism in Coordination Compounds questions

ISC asks you to name the type of isomerism, write or draw an isomer, count isomers, and give a test. Use this order.

1Compare formulae: if the coordination sphere (inside [ ]) differs in which ion or water is inside, it is ionisation or solvate isomerism; if the same ligand binds through a different atom, linkage; if ligands swap between two complex ions, coordination.
2If the formulae are identical including brackets, the isomers are stereoisomers: decide geometry first (square planar, tetrahedral or octahedral).
3For geometrical isomers, place like ligands at 90° (cis) or 180° (trans); for $M A_{3} B_{3}$ , on one face (fac) or a meridian (mer).
4For optical isomers, look for a plane of symmetry; bidentate ligands in a cis arrangement usually make the complex chiral.
5For ionisation isomer tests, name the reagent and the precipitate with its colour: BaCl₂ gives white BaSO₄; AgNO₃ gives white AgCl or pale yellow AgBr.
6Write IUPAC names with ligands in alphabetical order, 'ammine' with double m, and the oxidation state in Roman numerals.

Isomerism in Coordination Compounds in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Precipitating chloride identifies a hydrate isomer

Suppose a laboratory store has an old bottle labelled 'CrCl₃·6H₂O' ( $M = 266.5$ g mol⁻¹) whose crystals look grey-green rather than violet. A technician dissolves 2.665 g of it in water and adds excess AgNO₃ solution. The dried white precipitate weighs 2.87 g. (AgCl = 143.5 g mol⁻¹)

How many chloride ions per formula unit are outside the coordination sphere? Write the formula of the complex and name the type of isomerism shown.

Try it first, then show the solutionSolution

Moles of complex $= \frac{2.665}{266.5} = 0.0100$ mol.
Moles of AgCl $= \frac{2.87}{143.5} = 0.0200$ mol, so 2 Cl⁻ per formula unit are free.
The third Cl and five water molecules are inside the sphere (coordination number 6): $[C r (H_{2} O)_{5} C l] C l_{2} \cdot H_{2} O$ .
It is a solvate (hydrate) isomer of violet $[C r (H_{2} O)_{6}] C l_{3}$ , which would give 3 mol of AgCl per mole.

Answer: Two free Cl⁻ per formula unit; the complex is $[C r (H_{2} O)_{5} C l] C l_{2} \cdot H_{2} O$ , a hydrate (solvate) isomer.

Memory hook: Silver nitrate only counts the chloride outside the brackets.

Tricks to remember Isomerism in Coordination Compounds

Short memory aids. Say each one aloud once, then use it on a question the same day.

In and out, bind about, swap about, wet and dry
Ionisation: ion goes in or out. Linkage: the ligand binds about a different atom. Coordination: ligands swap about between two metals. Solvate: water in (wet sphere) or out (dry sphere).
Use it when: Naming the type of structural isomerism.
Ba for sulphate, Ag for halide
Barium catches Sulphate (white BaSO₄); Silver catches halide (white AgCl, pale yellow AgBr).
Use it when: Chemical tests for ionisation isomers.
Face or meridian
fac: three same ligands make a triangle on one Face; mer: they run round the Middle like a Meridian line.
Use it when: MA₃B₃ isomers.

Isomerism in Coordination Compounds: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Name the isomerism · 1 mark · MCQBoard level

$[C r (H_{2} O)_{6}] C l_{3}$ and $[C r (H_{2} O)_{5} C l] C l_{2} \cdot H_{2} O$ are examples of:
(a) linkage isomerism
(b) solvate (hydrate) isomerism
(c) coordination isomerism
(d) geometrical isomerism

Try it first, then show the solutionSolution

In one, all six waters are ligands; in the other, one water is outside the sphere and a Cl⁻ has taken its place.
Water inside versus outside the coordination sphere is solvate isomerism.

Answer: (b) solvate (hydrate) isomerism.

Example 2: Write a coordination isomer · 2 marks · Short answerBoard level

Write a coordination isomer of $[C u (N H_{3})_{4}] [P tC l_{4}]$ and name both compounds.

Try it first, then show the solutionSolution

Swap the ligands between the two metals: $[P t (N H_{3})_{4}] [C u C l_{4}]$ (1 mark).
$[C u (N H_{3})_{4}] [P tC l_{4}]$ : tetraamminecopper(II) tetrachloridoplatinate(II); $[P t (N H_{3})_{4}] [C u C l_{4}]$ : tetraammineplatinum(II) tetrachloridocuprate(II) (1 mark).

Answer: $[P t (N H_{3})_{4}] [C u C l_{4}]$ ; names as above.

Example 3: fac and mer isomers · 3 marks · Short answerHarder board level

Describe the geometrical isomers of the octahedral complex $[C o (N H_{3})_{3} (N O_{2})_{3}]$ (nitro ligands bound through N). State whether either is optically active and give the oxidation state of cobalt.

Try it first, then show the solutionSolution

Oxidation state: $x + 0 + 3 (- 1) = 0$ , so Co is +3 (½ mark).
fac isomer: the three NH₃ ligands occupy the corners of one triangular face of the octahedron, and the three NO₂ ligands the opposite face (1 mark).
mer isomer: the three NH₃ ligands lie in one plane through cobalt (a meridian), as do the three NO₂ ligands (1 mark).
Both isomers have planes of symmetry, so neither is optically active (½ mark).

Answer: Two geometrical isomers, fac and mer; neither is optically active; Co is +3.

Example 4: Test and name ionisation isomers · 3 marks · Short answerHarder board level

$[C r (N H_{3})_{5} B r] S O_{4}$ and $[C r (N H_{3})_{5} S O_{4}] B r$ are ionisation isomers. (a) Give their IUPAC names. (b) Describe a chemical test to distinguish them, with observations.

Try it first, then show the solutionSolution

(a) $[C r (N H_{3})_{5} B r] S O_{4}$ : pentaamminebromidochromium(III) sulphate; $[C r (N H_{3})_{5} S O_{4}] B r$ : pentaamminesulphatochromium(III) bromide (1 mark).
(b) Add BaCl₂ solution to each: the first gives a white precipitate of BaSO₄ because its sulphate is free; the second gives none (1 mark).
Add AgNO₃ solution to each: the second gives a pale yellow precipitate of AgBr; the first gives none because its bromide is a ligand (1 mark).

Answer: Names as above. BaCl₂ gives white BaSO₄ only with the first; AgNO₃ gives pale yellow AgBr only with the second.

Example 5: Counting all the stereoisomers · 5 marks · Long answerHOTS / competency

Consider the octahedral complex ion $[C o (e n) (N H_{3})_{2} C l_{2}]^{+}$ (en = ethane-1,2-diamine).
(a) Find the oxidation state and coordination number of cobalt. [1]
(b) Write its IUPAC name. [1]
(c) How many geometrical isomers are possible? Describe them. [2]
(d) Which of them is optically active and how many stereoisomers exist in all? [1]

Try it first, then show the solutionSolution

(a) $x + 0 + 0 + 2 (- 1) = + 1$ , so Co is +3; en gives 2 donor atoms, NH₃ 2 and Cl 2: coordination number 6 (1 mark).
(b) Diamminedichlorido(ethane-1,2-diamine)cobalt(III) ion (ligands in alphabetical order: ammine, chlorido, ethane-1,2-diamine) (1 mark).
(c) The en ligand must span two cis positions. Three arrangements remain for 2 NH₃ and 2 Cl: (i) the two NH₃ trans to each other (Cl ligands cis); (ii) the two Cl trans to each other (NH₃ cis); (iii) both pairs cis (2 marks).
(d) Arrangements (i) and (ii) have a plane of symmetry. Arrangement (iii) has none and exists as a pair of enantiomers. Total: 3 geometrical isomers, 4 stereoisomers (1 mark).

Answer: (a) +3, CN 6. (b) Diamminedichlorido(ethane-1,2-diamine)cobalt(III). (c) Three geometrical isomers. (d) The all-cis isomer is optically active; 4 stereoisomers in all.

Where students lose marks in Isomerism in Coordination Compounds

The slips examiners see most often, and what to write instead.

Mistake: Using the wrong reagent to detect an ionisation isomer
ISC examiners reported AgNO₃ used where BaCl₂ was needed (2025), and wrong reagents such as BaSO₄ or AgCl and wrong observations (2024).
Do this: To show free sulphate, add BaCl₂ (white BaSO₄). To show free chloride, add AgNO₃ (white AgCl). Name the reagent, the precipitate and its colour.
Usually costs: 1 to 2 marks.
Mistake: Misspelling ligand names
Examiners noted 'tetramine', 'triamine' and 'ethylenediammine' (2024, 2025).
Wrong working
tetraminedichloridocobalt(III) chloride
Right working
tetraamminedichloridocobalt(III) chloride
Do this: NH₃ as a ligand is 'ammine' (double m); keep the double a in 'tetraammine'; en is 'ethane-1,2-diamine' (or ethylenediamine).
Usually costs: 1 mark per name.
Mistake: Wrong number of ligands inside the sphere
Examiners reported [Co(NH₃)₄]Cl₃ or isomer drawings with three Cl ligands for CoCl₃·4NH₃ (2024).
Do this: Use the AgCl or conductivity data: one free Cl⁻ means two Cl are ligands, $[C o (N H_{3})_{4} C l_{2}] C l$ ; the cis and trans isomers each have exactly two Cl on Co.
Usually costs: Up to 3 marks across parts.
Mistake: Adding 'ion' to the name of a neutral salt or a wrong oxidation state
Examiners noted 'ion' written after names of whole compounds and wrong Roman numerals.
Do this: Write 'ion' only for a charged complex on its own. Work out the oxidation state from the charges before naming.
Usually costs: ½ to 1 mark.

Exam tips

For any isomer question, first write the formula with square brackets; it shows which ions are free.
When drawing cis and trans isomers, draw the octahedron clearly and keep the other ligands in the same places so the only change is the position of the two like ligands.
Say 'non-superimposable mirror images' for optical isomers.
In naming, list ligands alphabetically ignoring numerical prefixes (diammine is under 'a').
In test questions, give both the positive and the negative result for each isomer.

Practice questions on Isomerism in Coordination Compounds

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which of the following complexes shows optical isomerism?
(a) $[C o (N H_{3})_{6}]^{3 +}$
(b) trans- $[C o (N H_{3})_{4} C l_{2}]^{+}$
(c) $[C r (C_{2} O_{4})_{3}]^{3 -}$
(d) $[P t (N H_{3})_{2} C l_{2}]$
Hint
Look for three bidentate ligands.
Answer
(c) $[C r (C_{2} O_{4})_{3}]^{3 -}$ , an $M (AA)_{3}$ complex with no plane of symmetry.
Q2 · 2 marks · Short answerBoard level
What is linkage isomerism? Illustrate it with a pair of complexes containing the thiocyanate ligand.
Hint
SCN⁻ can bind through S or N.
Answer
Isomerism shown when an ambidentate ligand binds through different donor atoms. Example: $[C o (N H_{3})_{5} (S C N)]^{2 +}$ (bound through S, thiocyanato-S) and $[C o (N H_{3})_{5} (N C S)]^{2 +}$ (bound through N, thiocyanato-N).
Q3 · 2 marks · Short answerHarder board level
How many geometrical isomers are possible for the square planar complex $[P t (N H_{3}) (p y) C l B r]$ (py = pyridine)? Explain briefly.
Hint
Fix one ligand and see which can be trans to it.
Answer
Three. Fix NH₃; the ligand trans to it can be py, Cl or Br, and each choice gives a different isomer (MABCD type).
Q4 · 3 marks · Short answerHarder board level
Explain, with one example each, solvate isomerism and coordination isomerism. How does ionisation isomerism differ from solvate isomerism?
Hint
What moves in and out of the sphere in each?
Answer
Solvate: water is a ligand in one isomer and water of crystallisation in the other, e.g. $[C r (H_{2} O)_{6}] C l_{3}$ and $[C r (H_{2} O)_{5} C l] C l_{2} \cdot H_{2} O$ . Coordination: ligands exchanged between cationic and anionic complexes, e.g. $[C o (N H_{3})_{6}] [C r (C N)_{6}]$ and $[C r (N H_{3})_{6}] [C o (C N)_{6}]$ . In ionisation isomerism an anion (not water) swaps between the sphere and the outside, so the isomers give different ions in solution.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): $[C o (N H_{3})_{4} C l_{2}]^{+}$ exists in two geometrical forms.
Reason (R): In an octahedral $M A_{4} B_{2}$ complex the two B ligands can be adjacent (cis) or opposite (trans).
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Picture the two Cl ligands at 90° and 180°.
Answer
(a).
Q6 · 3 marks · Short answerHarder board level
Why does the square planar complex $[P d (N H_{3})_{2} B r_{2}]$ show geometrical isomerism while the tetrahedral complex $[Z n (N H_{3})_{2} C l_{2}]$ does not?
Hint
In which shape are some positions opposite each other?
Answer
In a square planar complex, two positions can be adjacent (90°) or opposite (180°), so the two NH₃ ligands can be cis or trans: two isomers. In a tetrahedron every position is adjacent to all the others (all angles 109.5°), so any arrangement of two A and two B ligands is the same; no geometrical isomers.
Q7 · 3 marks · Short answerHOTS / competency
How many stereoisomers does the octahedral ion $[C r (C_{2} O_{4})_{2} (H_{2} O)_{2}]^{-}$ have? Identify which are optically active.
Hint
It is of the $M (AA)_{2} B_{2}$ type.
Answer
Three: the trans isomer (water ligands opposite; has a plane of symmetry, optically inactive) and the cis isomer, which is chiral and exists as a pair of enantiomers (d and l).
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a student repeats Werner's experiments on two platinum(IV) ammine chlorides, A = PtCl₄·5NH₃ and B = PtCl₄·3NH₃. Both have coordination number 6. With excess AgNO₃, 1 mol of A gives 3 mol of AgCl and 1 mol of B gives 1 mol of AgCl.
(i) Write the Werner formulae of A and B and the number of ions each gives in solution. [1]
(ii) Which of them shows geometrical isomerism? Name the isomers. [1]
(iii) Name the complex cation of B. [1]
Hint
Ions precipitated by AgNO₃ are outside the brackets.
Answer
(i) A: $[P t (N H_{3})_{5} C l] C l_{3}$ , 4 ions; B: $[P t (N H_{3})_{3} C l_{3}] C l$ , 2 ions. (ii) B: its cation is of the $M A_{3} B_{3}$ type and has fac and mer isomers (A's cation, $M A_{5} B$ , has none). (iii) Triamminetrichloridoplatinum(IV) ion.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Name two ambidentate ligands.Show answer
$N O_{2}^{-}$ and $S C N^{-}$ .
2.How many geometrical isomers does $M A_{3} B_{3}$ have?Show answer
Two: fac and mer.
3.Which isomer of $[C o (e n)_{2} C l_{2}]^{+}$ is optically active?Show answer
The cis isomer.
4.Why do tetrahedral complexes not show geometrical isomerism?Show answer
All four positions are adjacent to each other.
5.Reagent to show free sulphate in an ionisation isomer?Show answer
BaCl₂ solution: white precipitate of BaSO₄.
6.Give a pair of coordination isomers.Show answer
$[C o (N H_{3})_{6}] [C r (C N)_{6}]$ and $[C r (N H_{3})_{6}] [C o (C N)_{6}]$ .

Isomerism in Coordination Compounds: frequently asked questions

What are the types of isomerism in coordination compounds?

Structural: ionisation, linkage, coordination and solvate isomerism. Stereoisomerism: geometrical (cis-trans, fac-mer) and optical.

How do you test for ionisation isomers?

Test the free ions: BaCl₂ gives white BaSO₄ with a free sulphate ion; AgNO₃ gives white AgCl or pale yellow AgBr with a free halide ion.

What is the difference between fac and mer isomers?

In fac, three identical ligands occupy one triangular face of the octahedron; in mer, they lie in a plane through the metal.

Why is trans-[Co(en)₂Cl₂]⁺ optically inactive?

It has a plane of symmetry, so its mirror image can be superimposed on it.

Do square planar complexes show optical isomerism?

Usually not, because the plane of the complex is itself a plane of symmetry.