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Coordination Compounds – Class 12 Chemistry notes, important questions and MCQs

NCERT Class 12 Chemistry, chapter 5. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Coordination compounds have a central metal atom bonded to ligands through coordinate bonds. Werner's theory, IUPAC naming and crystal field theory explain their structure and colour.

Basics

Ligands donate electron pairs: monodentate (NH₃, Cl⁻), bidentate (en, oxalate), polydentate (EDTA, hexadentate).
Coordination number = number of donor atoms bonded; oxidation state from the charge balance.
Werner: primary valency (ionisable) and secondary valency (coordination number).

Bonding and isomerism

Crystal field splitting in octahedral complexes: t₂g and e_g; strong-field ligands (CN⁻, CO) cause pairing (low spin).
Spectrochemical series: I⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < en < CN⁻ < CO.
Isomerism: geometrical (cis/trans), optical, ionisation, linkage (NO₂⁻/ONO⁻), coordination.

Key terms

Ligand: ion or molecule donating a lone pair to the central metal
Chelate: complex formed by a polydentate ligand binding through two or more atoms
Coordination number: number of ligand donor atoms directly bonded to the central metal ion

Formulas

Charge of complex = oxidation state of metal + sum of ligand charges

Important questions with answers

What is a chelate? Give an example of a chelating ligand.2 marks

A complex in which a polydentate ligand binds the metal through two or more donor atoms, forming a ring. Ethylenediamine (en) and EDTA⁴⁻ are chelating ligands.

Explain why [Fe(CN)₆]³⁻ is weakly paramagnetic while [FeF₆]³⁻ is strongly paramagnetic.3 marks

Fe³⁺ is d⁵. CN⁻ is a strong-field ligand: electrons pair in t₂g (t₂g⁵), leaving 1 unpaired electron (d²sp³, inner orbital). F⁻ is weak-field: t₂g³e_g², 5 unpaired electrons (sp³d², outer orbital), so it is strongly paramagnetic.

Explain crystal field splitting in an octahedral complex and why [Ti(H₂O)₆]³⁺ is coloured. Also write the types of isomerism in [Co(NH₃)₄Cl₂]⁺.5 marks

In an octahedral field, ligands approach along the axes, so dx²−y² and dz² (e_g) are raised in energy and dxy, dyz, dxz (t₂g) are lowered; the gap is Δₒ. Ti³⁺ (d¹) has its electron in t₂g; it absorbs green-yellow light (about 498 nm) to jump to e_g (d–d transition), so the complementary purple colour is seen. [Co(NH₃)₄Cl₂]⁺ shows geometrical isomerism: cis (two Cl adjacent) and trans (opposite); neither is optically active.

Quick revision

EDTA is hexadentate.
CN⁻ and CO are strong-field ligands.
Chelates are more stable.

Exam tips

In [Co(NH₃)₆]Cl₃, the coordination number is 6 and Co is +3.
Strong-field ligands (CN⁻, CO) cause pairing and low-spin complexes; weak-field ligands (F⁻, H₂O) give high-spin complexes.

Practice MCQs

Coordination number of Co in [Co(NH₃)₆]³⁺ is
1. 6
2. 3
3. 4
4. 9
Show answer
Answer: (a) 6. Six ligands.
EDTA is a
1. monodentate ligand
2. hexadentate ligand
3. bidentate ligand
4. neutral salt
Show answer
Answer: (b) hexadentate ligand. Six donor atoms.
Oxidation state of Fe in K₄[Fe(CN)₆] is
1. +2
2. +3
3. +4
4. +6
Show answer
Answer: (a) +2. 4 + x − 6 = 0.
Which is a strong-field ligand?
1. I⁻
2. CN⁻
3. H₂O
4. Cl⁻
Show answer
Answer: (b) CN⁻. Top of the series.
The IUPAC name of [Co(NH₃)₆]Cl₃ is
1. hexaamminecobalt(III) chloride
2. cobalt hexaammine chloride
3. hexaamminecobalt(II) chloride
4. triamminecobalt chloride
Show answer
Answer: (a) hexaamminecobalt(III) chloride. Co is +3.
[Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br show
1. linkage isomerism
2. ionisation isomerism
3. optical isomerism
4. geometrical isomerism
Show answer
Answer: (b) ionisation isomerism. Different ions in solution.
[Fe(CN)₆]³⁻ (Fe³⁺, d⁵, strong field) has how many unpaired electrons?
1. 1
2. 3
3. 5
4. 0
Show answer
Answer: (a) 1. Low spin t₂g⁵.
[Ni(CN)₄]²⁻ is
1. tetrahedral and paramagnetic
2. square planar and diamagnetic
3. octahedral
4. linear
Show answer
Answer: (b) square planar and diamagnetic. dsp², strong field.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.