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📌 Illustrated Diagram: Coordination-Compound Analysis

Coordination Compounds – Complete CBSE Class 12 Chemistry Notes

Unit 5 | Coordination Compounds | CBSE 2026–27

1. Basic Vocabulary

2. Werner’s Theory

Werner distinguished primary valency, associated with oxidation state, from secondary valency, associated with coordination number and spatial arrangement.

3. Oxidation State and Coordination Number

Worked Example

In [Co(NH₃)₆]Cl₃, the three chloride ions outside the brackets contribute −3. Therefore the complex ion is +3 and Co is +3 because NH₃ is neutral. Six NH₃ donor atoms are attached, so coordination number = 6.

4. Nomenclature Method

1. Name ligands first.
2. Use prefixes such as di-, tri-, tetra- where appropriate.
3. Name the metal and state oxidation number in Roman numerals.
4. For anionic complexes, use the appropriate modified metal name.

5. Isomerism

Coordination compounds can show structural isomerism such as linkage, ionisation and coordination isomerism, and stereoisomerism such as geometrical and optical isomerism where the geometry permits.

6. Bonding and Crystal Field Theory

Valence Bond Theory helps explain hybridisation in the prescribed examples. Crystal Field Theory explains splitting of d orbitals, magnetic behaviour and colour more effectively in many coordination compounds.

7. Metal Carbonyls

CO forms a strong metal–carbon interaction through σ donation and π back-donation. This synergic bonding is an important feature of metal carbonyl chemistry.

🧠 Complex Analysis Flow

📝 Case-Based Question

Consider [Co(NH₃)₅Cl]Cl₂.

1. Determine the charge on the complex ion.
2. Find the oxidation state of Co.
3. Identify the ligand that is inside the coordination sphere.
4. How many chloride ions are outside the coordination sphere?

🎯 Board Practice

1. Find oxidation state and coordination number from five complex formulae.
2. Name coordination compounds using IUPAC rules.
3. Identify possible geometrical/linkage/ionisation isomerism.
4. Explain magnetic behaviour using unpaired electrons and the prescribed bonding model.

MCQs

1. Coordination number counts donor atoms directly attached to the metal. True.
2. In [Co(NH₃)₆]Cl₃, chloride is a ligand. False.
3. NH₃ is a neutral ligand. True.
4. CO can participate in π back-donation. True.

Deep-Dive: Coordination Chemistry

Basic Structure

A coordination compound contains a central metal atom/ion surrounded by ligands that donate electron pairs. The coordination number counts donor atoms directly bonded to the central metal.

Nomenclature

Identify ligands, arrange ligand names alphabetically, state the metal and oxidation state, and apply the appropriate naming conventions for anionic complexes.

Isomerism

Coordination compounds can show structural and stereoisomerism. Be able to distinguish ionisation, linkage, coordination and geometrical/optical forms where applicable.

Bonding

Use coordination chemistry models to explain geometry, magnetic behaviour and hybridisation where required. Distinguish inner-orbital and outer-orbital descriptions in the prescribed framework.

Worked Example

For [Co(NH₃)₆]Cl₃, the coordination sphere is [Co(NH₃)₆]³⁺, coordination number is 6, NH₃ is a neutral ligand, and cobalt has oxidation state +3.

Common Mistakes

• Counting counter-ions as ligands.
• Confusing coordination number with oxidation state.
• Incorrect alphabetical ordering in names.

Practice

1. Determine oxidation state and coordination number.
2. Name coordination compounds systematically.
3. Write formulae from names.
4. Identify possible isomerism.
5. Explain magnetic behaviour using the prescribed bonding approach.

Worked Application: Coordination Number

In a coordination compound, the coordination number is the number of donor atoms directly bonded to the central metal ion. In [Co(NH₃)₆]Cl₃, six NH₃ molecules coordinate directly with Co, so the coordination number is 6. The three chloride ions are counter-ions outside the coordination sphere.

Structure-Based Practice

1. Identify the coordination entity, central ion, ligands and counter-ions in a complex.
2. Determine the coordination number from a displayed complex formula.

Coordination Compounds — Structure & Nomenclature

Coordination Number

Coordination number counts donor atoms directly attached to the central metal ion. It is not necessarily the same as the number of ligand molecules.

Worked Example

In [Co(NH₃)₆]Cl₃, Co is the central ion, NH₃ is a neutral ligand, the coordination number is 6 and the three chloride ions are counter-ions.

Isomerism

Coordination compounds may exhibit structural and stereoisomerism. Pay particular attention to ionisation, linkage, coordination and geometrical isomerism where prescribed.

Bonding

Use the prescribed valence-bond and crystal-field concepts to explain geometry, hybridisation where applicable, magnetic behaviour and splitting of d-orbitals.

Exam Traps

• Count donor atoms, not simply ligand names, for coordination number.
• Determine oxidation state before naming the complex.
• Distinguish coordination-sphere ions from counter-ions.

CBSE Application Practice

1. Name a coordination compound from its formula.
2. Write the formula from a systematic name.
3. Determine oxidation state and coordination number.
4. Identify possible geometrical or linkage isomers.
5. Predict magnetic behaviour from the electronic configuration.

Worked Structure & Magnetic Example

For [Co(NH₃)₆]Cl₃, the three chloride ions are outside the coordination sphere. The complex ion has charge +3, NH₃ is neutral, so cobalt is in the +3 oxidation state. Six NH₃ donor atoms are directly attached to Co, giving coordination number 6.