ChemistryCBSE Class 12 · Coordination Compounds

Isomerism in Coordination Compounds: Class 12 Chemistry

Updated 15 October 2026 · Fact-checked

Coordination compounds show two kinds of isomerism. Stereoisomers have the same bonds but different spatial arrangements: geometrical (cis–trans, fac–mer) and optical (non-superimposable mirror images). Structural isomers have different bonds: linkage, coordination, ionisation and solvate isomerism. Octahedral complexes with didentate ligands, such as $[Co(en)_3]^{3+}$, are the usual optically active examples.

Same formula, different positions: in cis the two Cl are neighbours at 90°, in trans they sit opposite at 180°.

Illustration drawn for QuestionBuddy.

Understand Isomerism in Coordination Compounds

Isomers have the same formula but a different arrangement of atoms, and so different properties. In coordination compounds isomerism is common because many ligands can be placed around one metal in several ways.

1. Stereoisomerism (same atoms bonded to each other, different arrangement in space).

*Geometrical isomerism* arises in heteroleptic complexes because ligands can occupy different relative positions. In a square planar complex $[M X_{2} L_{2}]$ , the two X can be on the same side (cis, 90°) or opposite (trans, 180°); $[P t (N H_{3})_{2} C l_{2}]$ exists as cis and trans forms. A square planar $[M A B C D]$ has three geometrical isomers. Tetrahedral complexes do not show geometrical isomerism, because all four positions are adjacent to one another. In octahedral $[M X_{2} L_{4}]$ , such as $[C o (N H_{3})_{4} C l_{2}]^{+}$ , the two Cl can be cis or trans. Octahedral $[M X_{2} (L - L)_{2}]$ , such as $[C o C l_{2} (e n)_{2}]^{+}$ , also has cis and trans forms. Octahedral $[M a_{3} b_{3}]$ , such as $[C o (N H_{3})_{3} (N O_{2})_{3}]$ , has a facial (fac) isomer, with the three identical ligands on one triangular face, and a meridional (mer) isomer, with them around a meridian of the octahedron.

*Optical isomerism.* Optical isomers (enantiomers) are mirror images that cannot be superimposed, like left and right hands. A molecule or ion that lacks a plane of symmetry is chiral. The two forms rotate plane-polarised light equally in opposite directions: dextro (d) to the right and laevo (l) to the left. Optical isomerism is common in octahedral complexes with didentate ligands. $[C o (e n)_{3}]^{3 +}$ has a d and an l form. In $[P tC l_{2} (e n)_{2}]^{2 +}$ and $[C o C l_{2} (e n)_{2}]^{+}$ , only the cis isomer is optically active; the trans isomer has a plane of symmetry. So $[C o C l_{2} (e n)_{2}]^{+}$ has three stereoisomers in all: trans, d-cis and l-cis.

2. Structural isomerism (different bonds).

*Linkage isomerism* occurs with an ambidentate ligand, which can bind through two different atoms. Thiocyanate binds through S (M–SCN) or N (M–NCS); nitrite through N (M–NO₂) or O (M–ONO). NCERT's example: $[C o (N H_{3})_{5} (N O_{2})] C l_{2}$ is red when the ligand is O-bonded (nitrito-O) and yellow when N-bonded (nitrito-N).

*Coordination isomerism* occurs when 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 $[C r (N H_{3})_{6}] [C o (C N)_{6}]$ .

*Ionisation isomerism* occurs when a counter-ion can also act as a ligand, so the isomers give different ions in solution: $[C o (N H_{3})_{5} (S O_{4})] B r$ gives $B r^{-}$ (precipitate with $A g N O_{3}$ ), while $[C o (N H_{3})_{5} B r] S O_{4}$ gives $S O_{4}^{2 -}$ (precipitate with $B a C l_{2}$ ).

*Solvate isomerism* (hydrate isomerism when the solvent is water) depends on whether solvent molecules are bonded to the metal or held as free molecules in the crystal: $[C r (H_{2} O)_{6}] C l_{3}$ (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 ions and so in conductivity.

Isomerism in Coordination Compounds: formulas and key facts

Square planar geometrical isomers: $[M A_{2} B_{2}] : 2 (c i s, t r an s); [M A B C D] : 3; [M A_{3} B] : none$; Tetrahedral complexes: no geometrical isomers.
Octahedral geometrical isomers: $[M A_{4} B_{2}] : 2 (c i s, t r an s); [M A_{3} B_{3}] : 2 (f a c, m er); [M A_{2} B_{2} C_{2}] : 5$; $[M A_{2} B_{2} C_{2}]$ : one of the five (all-cis) is also chiral, giving 6 stereoisomers.
Complexes with didentate ligands: $[M (AA)_{3}] : d + l; [M (AA)_{2} B_{2}] : t r an s + d - c i s + l - c i s = 3$; Example: $[C o (e n)_{3}]^{3 +}$ ; $[C o C l_{2} (e n)_{2}]^{+}$ .
Structural isomers: quick tests: $ionisation: different ions (AgNO_{3} or BaCl_{2} test); solvate: different conductivity$; Linkage: same formula, ambidentate ligand bonded through different atoms.

How to solve Isomerism in Coordination Compounds questions

Board questions ask you to name the type of isomerism shown by a pair, to draw (or describe) isomers, or to count them. Decide first whether the bonding is the same (stereo) or different (structural).

1Compare the two formulas. If different ions are outside the brackets, it is ionisation or solvate isomerism; if both ions are complexes with swapped ligands, coordination isomerism; if an ambidentate ligand changes donor atom, linkage isomerism.
2If the formula is identical and only positions differ, it is stereoisomerism: decide the geometry (square planar, tetrahedral, octahedral) first.
3For geometrical isomers, place identical ligands cis (adjacent) or trans (opposite); for $[M A_{3} B_{3}]$ use fac and mer.
4For optical isomerism, look for a plane of symmetry. Trans isomers and complexes like $[M A_{4} B_{2}]$ have one (inactive). Complexes with two or three didentate ligands in cis arrangement usually lack one (chiral).
5To count stereoisomers, list geometrical isomers, then double any that are chiral.
6For distinguishing tests, say which ion each isomer releases and the reagent that detects it.
7When drawing, use an octahedron with numbered positions 1–6 (1 and 6 trans) to avoid duplicates.

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.

Cis and trans isomers differ in properties

Suppose a pharmaceutical chemist prepares the square planar complex $[P t (N H_{3})_{2} C l_{2}]$ and obtains two samples with different properties. Only one of them, the cis isomer, is wanted, because it is the form used as an anticancer drug.

What type of isomerism is this? Can the two samples be distinguished by optical rotation? Suggest one physical property that differs and explain why.

Try it first, then show the solutionSolution

Both have the same formula and the same bonds; only the positions of the Cl atoms differ: geometrical (cis–trans) isomerism.
Both square planar isomers have a plane of symmetry (the molecular plane), so neither is chiral; optical rotation cannot distinguish them.
In the cis isomer the two Pt–Cl bond dipoles point to the same side and add up; in the trans isomer they are opposite and cancel.
So the cis isomer is polar (non-zero dipole moment) and the trans isomer is non-polar.

Answer: Geometrical isomerism; optical rotation does not help (both achiral); dipole moment (polarity) differs: cis is polar, trans is non-polar.

Memory hook: Cis means 'same side': same-side dipoles add, so the cis isomer is polar.

Tricks to remember Isomerism in Coordination Compounds

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

LCIS for structural isomers
Linkage, Coordination, Ionisation, Solvate: 'Lovely Cats In Sunlight'.
Use it when: Listing the four structural types.
Trans is a mirror of itself
Trans isomers have a plane of symmetry, so they are never optically active; look for chirality only in cis forms.
Use it when: Optical isomerism of $[M X_{2} (AA)_{2}]$ .
Tetrahedral: no cis, no trans
In a tetrahedron every corner touches every other corner, so there is no 'opposite'.
Use it when: Why tetrahedral complexes lack geometrical isomers.

Isomerism in Coordination Compounds: solved examples

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

Example 1: Spot the optically active ion · 1 mark · MCQBoard level

Which of the following shows optical isomerism?
(a) $[C o (N H_{3})_{6}]^{3 +}$ (b) trans- $[C o C l_{2} (e n)_{2}]^{+}$ (c) $[C o (e n)_{3}]^{3 +}$ (d) $[P t (N H_{3})_{2} C l_{2}]$

Try it first, then show the solutionSolution

Three didentate en ligands wrap around Co like a propeller with no plane of symmetry, so mirror images do not superimpose.
The others all have at least one plane of symmetry.

Answer: (c) $[C o (e n)_{3}]^{3 +}$ .

Example 2: Name the type · 2 marks · Very short answerBoard level

Name the type of isomerism shown by each pair: (a) $[C o (N H_{3})_{5} (S O_{4})] B r$ and $[C o (N H_{3})_{5} B r] S O_{4}$ (b) $[C o (N H_{3})_{5} (N O_{2})] C l_{2}$ and $[C o (N H_{3})_{5} (O N O)] C l_{2}$ .

Try it first, then show the solutionSolution

(a) The bromide and sulphate swap between ligand and counter-ion positions, giving different ions in solution: ionisation isomerism (1 mark).
(b) The ambidentate nitrite binds through N in one and through O in the other: linkage isomerism (1 mark).

Answer: (a) Ionisation isomerism. (b) Linkage isomerism.

Example 3: Isomers of [CoCl₂(en)₂]⁺ · 3 marks · Short answerHarder board level

Describe all the stereoisomers of $[C o C l_{2} (e n)_{2}]^{+}$ . Which are optically active? How many stereoisomers are there in total?

Try it first, then show the solutionSolution

Geometrical isomers: trans (two Cl opposite, the two en in the plane between them) and cis (two Cl adjacent) (1 mark).
The trans isomer has a plane of symmetry (through Co and the four N atoms), so it is optically inactive (½ mark).
The cis isomer has no plane of symmetry; its mirror image cannot be superimposed, so it exists as d and l forms (1 mark).
Total: trans + d-cis + l-cis = 3 stereoisomers (½ mark).

Answer: Trans (inactive) and cis (a d/l pair): three stereoisomers; only the cis forms are optically active.

Example 4: fac, mer and other geometries · 3 marks · Short answerHarder board level

(a) Explain fac and mer isomerism using $[C o (N H_{3})_{3} (N O_{2})_{3}]$ . (b) Why do tetrahedral complexes not show geometrical isomerism? (c) How many geometrical isomers does a square planar complex $[P t (N H_{3}) (B r) (C l) (p y)]$ have?

Try it first, then show the solutionSolution

(a) In fac, the three $N H_{3}$ (and the three $N O_{2}$ ) occupy the three corners of one face of the octahedron, all mutually cis. In mer, they lie around a meridian: two of them are trans to each other (1 mark).
(b) In a tetrahedron all four positions are adjacent (equivalent relative to each other), so no cis/trans distinction exists (1 mark).
(c) For $[M A B C D]$ , fix A; it can be trans to B, to C or to D: three isomers (1 mark).

Answer: (a) fac: identical ligands on one face; mer: on a meridian. (b) All positions adjacent. (c) Three.

Example 5: Telling structural isomers apart · 5 marks · Long answerHOTS / competency

(a) How would you distinguish $[C o (N H_{3})_{5} (S O_{4})] B r$ from $[C o (N H_{3})_{5} B r] S O_{4}$ by simple tests? Give their IUPAC names. (b) Give a pair of coordination isomers formed from $C o^{3 +}$ , $C r^{3 +}$ , $N H_{3}$ and $C N^{-}$ . (c) $[C r (H_{2} O)_{6}] C l_{3}$ and $[C r (H_{2} O)_{5} C l] C l_{2} \cdot H_{2} O$ are dissolved at equal molarity. Which has the higher molar conductivity, and why? What type of isomers are they?

Try it first, then show the solutionSolution

(a) $[C o (N H_{3})_{5} (S O_{4})] B r$ releases $B r^{-}$ : a pale yellow precipitate with $A g N O_{3}$ , none with $B a C l_{2}$ . $[C o (N H_{3})_{5} B r] S O_{4}$ releases $S O_{4}^{2 -}$ : a white precipitate with $B a C l_{2}$ , none with $A g N O_{3}$ (1½ marks).
Names: pentaamminesulphatocobalt(III) bromide and pentaamminebromidocobalt(III) sulphate (1 mark).
(b) $[C o (N H_{3})_{6}] [C r (C N)_{6}]$ and $[C r (N H_{3})_{6}] [C o (C N)_{6}]$ (1 mark).
(c) $[C r (H_{2} O)_{6}] C l_{3}$ gives 4 ions per formula unit; $[C r (H_{2} O)_{5} C l] C l_{2} \cdot H_{2} O$ gives 3. More ions give higher molar conductivity, so the hexaaqua compound conducts better (1 mark).
They are solvate (hydrate) isomers (½ mark).

Answer: (a) $A g N O_{3}$ test for $B r^{-}$ , $B a C l_{2}$ test for $S O_{4}^{2 -}$ ; pentaamminesulphatocobalt(III) bromide and pentaamminebromidocobalt(III) sulphate. (b) $[C o (N H_{3})_{6}] [C r (C N)_{6}]$ / $[C r (N H_{3})_{6}] [C o (C N)_{6}]$ . (c) $[C r (H_{2} O)_{6}] C l_{3}$ (4 ions vs 3); hydrate isomers.

Where students lose marks in Isomerism in Coordination Compounds

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

Mistake: Claiming geometrical isomers for tetrahedral complexes
Do this: Only square planar and octahedral complexes show cis–trans isomerism.
Usually costs: 1 mark.
Mistake: Calling the trans isomer optically active
Students assume every complex with en is chiral.
Do this: Check for a plane of symmetry; trans- $[M X_{2} (AA)_{2}]$ has one.
Usually costs: 1 mark.
Mistake: Confusing ionisation and solvate isomerism
Do this: Ionisation: a different anion is inside/outside the sphere. Solvate: water (or solvent) moves between ligand and lattice positions.
Usually costs: 1 mark.
Mistake: Drawing the same isomer twice
A rotated drawing looks different.
Do this: Describe each isomer by which ligands are trans to each other; two drawings with the same trans pairs (and same handedness) are identical.
Usually costs: 1 mark in counting questions.

Exam tips

When asked to draw isomers, label the geometry and write cis/trans or fac/mer under each drawing.
For optical isomers, draw the two structures as mirror images with a dotted mirror line.
Use NCERT's examples: $[C o (N H_{3})_{5} (N O_{2})] C l_{2}$ (linkage), $[C o (N H_{3})_{6}] [C r (C N)_{6}]$ (coordination), $[C o (N H_{3})_{5} (S O_{4})] B r$ (ionisation), $[C r (H_{2} O)_{6}] C l_{3}$ (solvate).
In 'distinguish' answers, name the reagent and the observation for each isomer.
State the total number of stereoisomers clearly when a question says 'how many isomers'.

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
Linkage isomerism is possible in complexes containing:
(a) $N H_{3}$ (b) $C l^{-}$ (c) $N O_{2}^{-}$ (d) $H_{2} O$
Hint
Needs an ambidentate ligand.
Answer
(c) $N O_{2}^{-}$ (can bind through N or O).
Q2 · 2 marks · Very short answerBoard level
What is coordination isomerism? Give one example.
Hint
Both ions are complexes.
Answer
It arises from the interchange of ligands between the cationic and anionic complex entities of different metal ions in a compound, e.g. $[C o (N H_{3})_{6}] [C r (C N)_{6}]$ and $[C r (N H_{3})_{6}] [C o (C N)_{6}]$ .
Q3 · 2 marks · Very short answerHarder board level
How many geometrical isomers are possible for (a) $[P t (N H_{3})_{2} C l_{2}]$ and (b) $[C o (N H_{3})_{4} C l_{2}]^{+}$ ? Is either optically active? Give a reason.
Hint
Look for a plane of symmetry.
Answer
(a) 2 (cis and trans). (b) 2 (cis and trans). Neither is optically active: every isomer has a plane of symmetry.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): trans- $[C o C l_{2} (e n)_{2}]^{+}$ is optically inactive.
Reason (R): The trans isomer has a plane of symmetry.
(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
Symmetry plane → superimposable mirror image.
Answer
(a).
Q5 · 3 marks · Short answerHarder board level
$[C r (H_{2} O)_{6}] C l_{3}$ is violet and $[C r (H_{2} O)_{5} C l] C l_{2} \cdot H_{2} O$ is grey-green. Name the type of isomerism and suggest two ways to distinguish them in the laboratory.
Hint
Count free chloride ions and total ions.
Answer
Solvate (hydrate) isomerism. (1) With excess $A g N O_{3}$ , one mole of the violet compound gives 3 mol AgCl, the grey-green gives 2 mol. (2) Molar conductivity: the violet compound (4 ions) conducts more than the grey-green one (3 ions). (Colour itself also distinguishes them.)
Q6 · 3 marks · Short answerHarder board level
Which of the following can show optical isomerism? Explain. (a) $[C r (C_{2} O_{4})_{3}]^{3 -}$ (b) $[C o (N H_{3})_{4} C l_{2}]^{+}$ (c) cis- $[P tC l_{2} (e n)_{2}]^{2 +}$
Hint
Didentate ligands in a propeller or cis arrangement.
Answer
(a) Yes: three didentate oxalates around Cr, no plane of symmetry (d and l forms). (b) No: both cis and trans forms have planes of symmetry. (c) Yes: the cis isomer with two en ligands has no plane of symmetry.
Q7 · 3 marks · Short answerHOTS / competency
How many geometrical isomers does the octahedral ion $[C o (N H_{3})_{2} (H_{2} O)_{2} C l_{2}]^{+}$ have? Which of them is optically active, and what is the total number of stereoisomers?
Hint
Type $[M A_{2} B_{2} C_{2}]$ : count arrangements where each pair is cis or trans.
Answer
Five geometrical isomers: all three pairs trans; one pair trans and the other two cis (three ways, one for each pair being trans); and all three pairs cis. Only the all-cis isomer lacks a plane of symmetry, so it exists as a d/l pair. Total stereoisomers = 6.
Q8 · 4 marks · Case-basedHOTS / competency
Read and answer.
Suppose a student is given two salts with the same empirical formula, $C o B r S O_{4} \cdot 5 N H_{3}$ . Solution X gives a pale yellow precipitate with silver nitrate but nothing with barium chloride. Solution Y gives a white precipitate with barium chloride but nothing with silver nitrate.
(a) Write the structural formulas of X and Y. (1 mark)
(b) Name the type of isomerism. (1 mark)
(c) Give the IUPAC name of Y. (1 mark)
(d) What is the coordination number and oxidation state of cobalt in both? (1 mark)
Hint
The ion that precipitates is outside the bracket.
Answer
(a) X: $[C o (N H_{3})_{5} (S O_{4})] B r$ ; Y: $[C o (N H_{3})_{5} B r] S O_{4}$ . (b) Ionisation isomerism. (c) Pentaamminebromidocobalt(III) sulphate. (d) Coordination number 6; oxidation state +3 in both.

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.Two types of stereoisomerism?Show answer
Geometrical and optical.
2.Four types of structural isomerism?Show answer
Linkage, coordination, ionisation and solvate.
3.Isomers of $[C o (N H_{3})_{3} (N O_{2})_{3}]$ ?Show answer
fac and mer.
4.Number of stereoisomers of $[C o C l_{2} (e n)_{2}]^{+}$ ?Show answer
Three (trans, d-cis, l-cis).
5.Two ambidentate ligands?Show answer
$N O_{2}^{-}$ and $S C N^{-}$ .
6.Geometrical isomers of square planar $[M A B C D]$ ?Show answer
Three.

Isomerism in Coordination Compounds: frequently asked questions

What types of isomerism are shown by coordination compounds?

Stereoisomerism (geometrical and optical) and structural isomerism (linkage, coordination, ionisation and solvate).

Why don't tetrahedral complexes show geometrical isomerism?

All four positions in a tetrahedron are adjacent to one another, so ligands cannot be arranged cis or trans.

Which complexes show optical isomerism?

Chiral complexes without a plane of symmetry, most often octahedral complexes with didentate ligands such as [Co(en)_3]^{3+} and cis-[CoCl_2(en)_2]^+.

What is linkage isomerism?

Isomerism in which an ambidentate ligand is bonded to the metal through different donor atoms, such as M–NO₂ and M–ONO.

How can ionisation isomers be distinguished?

They release different ions in solution, which can be detected by precipitation tests, for example AgNO_3 for halide or BaCl_2 for sulphate.