Structural and Stereoisomerism in Coordination Compounds Class 12 Chemistry | Coordination Compounds

Understand it

Structural isomerism includes ionisation isomerism, solvate or hydrate isomerism, linkage isomerism, and coordination isomerism.
Ionisation isomers give different ions in solution.
Solvate isomers differ in whether solvent molecules are inside or outside the coordination sphere.
Linkage isomers occur with ambidentate ligands such as NO₂^- or SCN^- that can bind through different donor atoms.
Coordination isomerism occurs when ligands are exchanged between complex cation and complex anion.
Stereoisomerism includes geometrical isomerism and optical isomerism.
Geometrical isomers such as cis and trans differ in relative positions of ligands, while optical isomers are non-superimposable mirror images.

Example

[Co(NH3)5NO2]Cl2 and [Co(NH3)5ONO]Cl2 show linkage isomerism because NO₂^- can coordinate through nitrogen or oxygen. In square planar [Pt(NH3)2Cl2], cis and trans forms are geometrical isomers.

Rule to remember

Key identification rule: same composition with different connectivity gives structural isomerism; same connectivity with different spatial arrangement gives stereoisomerism.

Cis means similar ligands are adjacent; trans means similar ligands are opposite.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Identify the isomerism in [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br.

1In the first compound Br^- is inside the coordination sphere and SO₄^2- is outside.
2In the second, SO₄^2- is inside and Br^- is outside.
3Since ions inside and outside the sphere exchange positions and give different ions in solution, the pair shows ionisation isomerism.

Method you can reuse

1Check whether the formula is the same.
2Compare the contents of the square brackets.
3If an outside ion exchanges with an inside ligand, identify ionisation or solvate isomerism depending on the species.
4If the ligand binds through a different atom, identify linkage.
5If the same ligands occupy different positions, decide cis-trans or optical.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What type of isomerism is shown when NO₂^- binds once through N and once through O in two compounds with the same formula?

Think first, then show answer

It is linkage isomerism because the ambidentate ligand uses different donor atoms.

Common wrong answer

A common wrong answer is calling [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br linkage isomers, even though no ambidentate donor atom change is involved.

Don't mix this up

Students often call any pair with the same formula geometrical isomers. The type depends on what changes: ion outside bracket, solvent position, donor atom, ligand distribution, or spatial arrangement.

Exam tip

Compare the coordination sphere carefully. If the donor atom changes, think linkage. If ions inside and outside exchange, think ionisation. If positions around the same metal change, think geometrical or optical.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions ask students to identify the type of isomerism, draw cis-trans forms, match examples with isomerism type, or explain why a given pair is or is not isomeric.

1
Enough for 1 mark
Isomerism in coordination compounds occurs when compounds have the same overall composition but differ in the arrangement of ligands, ions, donor atoms, or spatial positions around the metal.
2
Add for 2 marks
Example: [Co(NH3)5NO2]Cl2 and [Co(NH3)5ONO]Cl2 show linkage isomerism because NO₂^- can coordinate through nitrogen or oxygen. In square planar [Pt(NH3)2Cl2], cis and trans forms are geometrical isomers.
3
Add for 3 marks
Structural isomerism includes ionisation isomerism, solvate or hydrate isomerism, linkage isomerism, and coordination isomerism.

Use the concept graph around this idea instead of revising it in isolation.

Werner's Theory, Coordination Sphere and Basic Terms

Werner's theory states that a metal in a coordination compound shows primary valence, usually ionisable and related to oxidation state, and secondary valence, non-ionisable and equal to the coordination number. The metal and directly attached ligands form the coordination sphere written inside square brackets.

IUPAC Naming of Coordination Compounds

IUPAC nomenclature of coordination compounds follows a fixed order: name the cation before the anion, name ligands alphabetically before the metal within the coordination entity, and write the oxidation state of the metal in Roman numerals.

Valence Bond Theory, Hybridisation and Geometry

Valence Bond Theory explains bonding in coordination compounds by assuming that the central metal ion uses suitable hybrid orbitals to accept electron pairs donated by ligands, producing definite geometries such as octahedral, square planar, or tetrahedral.

Crystal Field Theory, d-Orbital Splitting and Colour

Crystal Field Theory explains coordination compounds by considering electrostatic interaction between metal d orbitals and ligands, causing the five d orbitals to split into sets of different energy in a ligand field.

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Structural and Stereoisomerism in Coordination Compounds

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Structural and Stereoisomerism in Coordination Compounds

Understand it

See it worked out

Check yourself

Write it in the exam

Related concepts

Practice this concept with focused MCQs

Help improve this page