StudyOSStudyOS CBSE Study Material, Class 5–12

Coordination Compounds

NCERT Class 12 · Chemistry Based on NCERT Class 12 Chemistry textbook · Free CBSE study kit

Everything you need to ace this chapter, in one place.
Notes · Mind Map · Quiz · Assertion-Reason · Case Studies · Question Bank · Past Papers

Get Full Study Kit →
Studying with friends? Send them this chapter.

Chapter Notes

Overview
This chapter explores the chemistry of coordination compounds, which are complex molecules where a central metal atom is bonded to surrounding ligands. It is a critical part of the syllabus because it bridges inorganic and bio-inorganic chemistry, explaining the structure, bonding, and properties of compounds like chlorophyll, haemoglobin, and vitamin B12. The one big idea a student must walk away with is that the properties of a coordination compound—its geometry, colour, magnetic behaviour, and stability—are determined by the nature of the metal-ligand interaction, as explained by Werner's theory, Valence Bond Theory, and Crystal Field Theory.
Key Concepts (18)
Coordination entity
A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules.
e.g. [CoCl₃(NH₃)₃] is a coordination entity where cobalt is surrounded by three ammonia molecules and three chloride ions.
Central atom/ion
In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it, is called the central atom or ion.
e.g. In [Fe(CN)₆]³⁻, the central atom is Fe³⁺.
Ligand
The ions or molecules bound to the central atom/ion in the coordination entity are called ligands.
e.g. Cl⁻, H₂O, NH₃ are common ligands.
Coordination number
The coordination number (CN) of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded.
e.g. In [PtCl₆]²⁻, the coordination number of Pt is 6.
Coordination sphere
The central atom/ion and the ligands attached to it are enclosed in square bracket and is collectively termed as the coordination sphere.
e.g. In K₄[Fe(CN)₆], the coordination sphere is [Fe(CN)₆]⁴⁻.
Coordination polyhedron
The spatial arrangement of the ligand atoms which are directly attached to the central atom/ion defines a coordination polyhedron about the central atom.
e.g. [Co(NH₃)₆]³⁺ is octahedral, [Ni(CO)₄] is tetrahedral, and [PtCl₄]²⁻ is square planar.
Oxidation number
The oxidation number of the central atom in a complex is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom.
e.g. Oxidation number of copper in [Cu(CN)₄]³⁻ is +1, written as Cu(I).
Homoleptic complex
Complexes in which a metal is bound to only one kind of donor groups are known as homoleptic.
e.g. [Co(NH₃)₆]³⁺ is homoleptic.
Heteroleptic complex
Complexes in which a metal is bound to more than one kind of donor groups are known as heteroleptic.
e.g. [Co(NH₃)₄Cl₂]⁺ is heteroleptic.
Chelate ligand
When a di- or polydentate ligand uses its two or more donor atoms simultaneously to bind a single metal ion, it is said to be a chelate ligand.
e.g. Ethane-1,2-diamine (en) is a didentate chelate ligand.
Ambidentate ligand
Ligand which has two different donor atoms and either of the two ligates in the complex is called ambidentate ligand.
e.g. NO₂⁻ ion can coordinate either through nitrogen or through oxygen.
Spectrochemical series
An experimentally determined series of ligands in order of increasing field strength: I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < edta⁴⁻ < NH₃ < en < CN⁻ < CO.
e.g. CO is a strong field ligand, while I⁻ is a weak field ligand.
Double salt
Double salts dissociate into simple ions completely when dissolved in water.
e.g. Mohr’s salt, FeSO₄.(NH₄)₂SO₄.6H₂O, is a double salt.
Complex ion
Complex ions do not dissociate into simple ions when dissolved in water.
e.g. [Fe(CN)₆]⁴⁻ from K₄[Fe(CN)₆] does not dissociate into Fe²⁺ and CN⁻ ions.
Primary valence
The primary valences are normally ionisable and are satisfied by negative ions.
e.g. In CrCl₃, the primary valence of Cr is 3.
Secondary valence
The secondary valences are non ionisable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to the coordination number and is fixed for a metal.
e.g. In [Co(NH₃)₆]³⁺, the secondary valence of Co is 6.
Counter ion
The ions outside the square bracket in a coordination compound are called counter ions.
e.g. In K₄[Fe(CN)₆], K⁺ is the counter ion.
Denticity
The number of ligating groups in a ligand is called the denticity of the ligand.
e.g. EDTA⁴⁻ is a hexadentate ligand.
Formulas & Equations (7)
Δₒ = Energy of e_g orbitals - Energy of t₂g orbitals
Reads as: Delta subscript o equals energy of e g orbitals minus energy of t two g orbitals
Variables: Δₒ = crystal field splitting energy in octahedral field (Joules or cm⁻¹); e_g = set of two d orbitals (d_{x²-y²}, d_{z²}); t₂g = set of three d orbitals (d_{xy}, d_{yz}, d_{xz})
Use when: To calculate the energy gap between split d orbitals in an octahedral complex, used to predict colour and magnetic properties.
⚠ Students often confuse Δₒ with Δₜ (tetrahedral splitting) or forget that the e_g set is higher in energy in octahedral complexes.
Δₜ = (4/9) Δₒ
Reads as: Delta subscript t equals four-ninths of delta subscript o
Variables: Δₜ = crystal field splitting energy in tetrahedral field; Δₒ = crystal field splitting energy in octahedral field
Use when: To relate the splitting in tetrahedral complexes to that in octahedral complexes for the same metal and ligands.
⚠ Students forget that tetrahedral splitting is smaller and rarely leads to low-spin configurations.
Energy of e_g orbitals = + (3/5) Δₒ
Reads as: Energy of e g orbitals equals plus three-fifths delta subscript o
Variables: e_g = higher energy d orbitals in octahedral field; Δₒ = octahedral crystal field splitting energy
Use when: To calculate the energy increase of the e_g set relative to the barycenter in octahedral complexes.
⚠ Students often write the fraction as 2/5 instead of 3/5 for e_g.
Energy of t₂g orbitals = - (2/5) Δₒ
Reads as: Energy of t two g orbitals equals minus two-fifths delta subscript o
Variables: t₂g = lower energy d orbitals in octahedral field; Δₒ = octahedral crystal field splitting energy
Use when: To calculate the energy decrease of the t₂g set relative to the barycenter in octahedral complexes.
⚠ Students often forget the negative sign or write the fraction as 3/5.
Magnetic moment (μ) = √[n(n+2)] BM
Reads as: Magnetic moment equals square root of n times n plus two Bohr magnetons
Variables: μ = spin-only magnetic moment (BM, Bohr magneton); n = number of unpaired electrons
Use when: To calculate the magnetic moment of a complex from the number of unpaired electrons, used to determine high-spin or low-spin configuration.
⚠ Students forget to use the spin-only formula or incorrectly calculate n from the d-electron configuration.
[Au(CN)₂]⁻ + Zn → Au + [Zn(CN)₄]²⁻
Reads as: Dicyanoaurate one minus plus zinc gives gold plus tetracyanozincate two minus
Variables: [Au(CN)₂]⁻ = gold cyanide complex; Zn = zinc metal; Au = gold metal; [Zn(CN)₄]²⁻ = zinc cyanide complex
Use when: To describe the extraction of gold from its cyanide complex by displacement with zinc.
⚠ Students often write the wrong stoichiometry or forget that this is a redox reaction.
Ni + 4CO → [Ni(CO)₄] (then decomposed to pure Ni)
Reads as: Nickel plus four carbon monoxide gives tetracarbonylnickel zero, then decomposed to pure nickel
Variables: Ni = impure nickel; CO = carbon monoxide; [Ni(CO)₄] = volatile nickel carbonyl
Use when: To describe the Mond process for purification of nickel via formation and decomposition of nickel carbonyl.
⚠ Students forget that the complex is [Ni(CO)₄] and not [Ni(CO)₆].
How It Works
Werner's Theory of Coordination Compounds
  1. Metals show two types of linkages: primary (ionisable, satisfied by negative ions) and secondary (non-ionisable, satisfied by neutral molecules or negative ions).
  2. The secondary valence is equal to the coordination number and is fixed for a metal.
  3. The ions/groups bound by secondary linkages have characteristic spatial arrangements (coordination polyhedra).
  4. Octahedral, tetrahedral, and square planar geometries are common for transition metals.
💡 Students often confuse primary and secondary valences; remember primary = ionic (ionisable), secondary = covalent (non-ionisable) and equals coordination number.
Naming of Mononuclear Coordination Compounds (IUPAC)
  1. The cation is named first, then the anion.
  2. Ligands are named in alphabetical order before the central atom/ion.
  3. Anionic ligands end in '-o', neutral ligands retain their name except aqua (H₂O), ammine (NH₃), carbonyl (CO), nitrosyl (NO).
  4. Prefixes (di, tri, tetra, etc.) indicate the number of ligands; for ligands with numerical prefixes, use bis, tris, tetrakis.
  5. Oxidation state of the metal is indicated by a Roman numeral in parentheses.
  6. If the complex ion is a cation, the metal is named as the element; if an anion, the metal name ends with '-ate' (e.g., ferrate for Fe).
💡 Students often forget that ligands are named alphabetically regardless of charge, and that 'ammine' (with two m's) is used for NH₃, not 'amine'.
Crystal Field Splitting in Octahedral Complexes
  1. In an octahedral field, the five degenerate d orbitals split into two sets: t₂g (lower energy: d_{xy}, d_{yz}, d_{xz}) and e_g (higher energy: d_{x²-y²}, d_{z²}).
  2. The energy separation is Δₒ; e_g orbitals are raised by (3/5)Δₒ, t₂g orbitals are lowered by (2/5)Δₒ.
  3. For d⁴ to d⁷ ions, if Δₒ > pairing energy (P), the complex is low-spin (strong field ligand); if Δₒ < P, it is high-spin (weak field ligand).
  4. The spectrochemical series ranks ligands by their field strength: I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < edta⁴⁻ < NH₃ < en < CN⁻ < CO.
💡 Students often forget that the e_g set is higher in energy in octahedral complexes (pointing directly at ligands) and that the 'g' subscript is used only for complexes with a centre of symmetry.
Crystal Field Splitting in Tetrahedral Complexes
  1. In a tetrahedral field, the d orbitals split into two sets: e (lower energy: d_{x²-y²}, d_{z²}) and t₂ (higher energy: d_{xy}, d_{yz}, d_{xz}).
  2. The splitting is inverted compared to octahedral, and Δₜ = (4/9)Δₒ.
  3. Because Δₜ is smaller, low-spin configurations are rarely observed in tetrahedral complexes.
💡 Students often forget that tetrahedral complexes lack a centre of symmetry, so the 'g' subscript is not used, and that the e set is lower in energy.
Synergic Bonding in Metal Carbonyls
  1. The M–C σ bond is formed by donation of a lone pair of electrons from the carbonyl carbon into a vacant orbital of the metal.
  2. The M–C π bond is formed by donation of a pair of electrons from a filled d orbital of the metal into the vacant antibonding π* orbital of carbon monoxide.
  3. This metal-to-ligand back bonding creates a synergic effect that strengthens the bond between CO and the metal.
💡 Students often forget that the π bond involves back donation from metal to ligand, not the other way around.
Colour in Coordination Compounds (d-d Transition)
  1. When white light passes through a coordination compound, a specific wavelength corresponding to the energy difference (Δₒ) between t₂g and e_g orbitals is absorbed.
  2. This excites an electron from the lower energy d orbital to the higher energy d orbital (d-d transition).
  3. The complementary colour of the absorbed light is observed.
  4. For example, [Ti(H₂O)₆]³⁺ absorbs blue-green light and appears violet.
💡 Students often forget that the colour observed is the complementary colour of the light absorbed, not the colour absorbed.
Exam Traps — Don't Make These Mistakes
⚠ Confusing 'ammine' with 'amine'
✗ Students write 'amine' instead of 'ammine' when naming NH₃ as a ligand.
→ In IUPAC nomenclature, 'ammine' (with two m's) is the specific term for NH₃ as a neutral ligand, while 'amine' refers to organic derivatives of ammonia.
✓ Write 'ammine' for NH₃ ligand, e.g., 'hexaamminecobalt(III) chloride'.
⚠ Incorrect order of ligands in naming
✗ Students list ligands in order of charge or alphabetical order of the metal, e.g., 'chloridotriammine'.
→ IUPAC rules require ligands to be named in alphabetical order regardless of charge, and the metal is named last.
✓ Write 'triamminechloridocobalt(III) chloride' (alphabetical order: ammine before chlorido).
⚠ Confusing primary and secondary valences
✗ Students say primary valence equals coordination number.
→ Primary valence is the ionisable charge (oxidation state), while secondary valence is the coordination number.
✓ Write: 'Primary valence is the ionisable charge (e.g., +3 for Co in [Co(NH₃)₆]³⁺), and secondary valence is the coordination number (e.g., 6 for Co).'
⚠ Forgetting that tetrahedral complexes do not show geometrical isomerism
✗ Students draw cis-trans isomers for tetrahedral complexes like [NiCl₂(NH₃)₂].
→ Tetrahedral complexes have all bond angles equal (109.5°), so all positions are equivalent; cis-trans isomerism is not possible.
✓ Write: 'Geometrical isomerism is not possible in tetrahedral complexes because all ligand positions are equivalent.'
⚠ Mistaking Δₒ for Δₜ in calculations
✗ Students use the same Δ value for tetrahedral and octahedral complexes.
→ Δₜ = (4/9)Δₒ; tetrahedral splitting is much smaller, so low-spin configurations are rare.
✓ Write: 'For tetrahedral complexes, Δₜ = (4/9)Δₒ, and low-spin configurations are rarely observed.'
⚠ Incorrectly assigning oxidation states in complex anions
✗ Students write the oxidation state of Fe in [Fe(CN)₆]⁴⁻ as +2 without considering the charge of the complex.
→ The oxidation state must be calculated: let x be oxidation state of Fe, then x + 6(-1) = -4, so x = +2. This is correct, but students often forget the charge of the complex.
✓ Write: 'Oxidation state of Fe in [Fe(CN)₆]⁴⁻ is +2, calculated as x + 6(-1) = -4.'
Compare & Contrast
Dissociation in water
Double salt
Complex
Double salts dissociate completely into simple ions (e.g., Mohr's salt gives Fe²⁺, NH₄⁺, SO₄²⁻), while complexes do not dissociate into simple ions (e.g., K₄[Fe(CN)₆] gives K⁺ and [Fe(CN)₆]⁴⁻, not Fe²⁺ and CN⁻).
Electron configuration in d orbitals
High-spin complex
Low-spin complex
In high-spin complexes, electrons occupy higher energy e_g orbitals before pairing (Δₒ < P), resulting in more unpaired electrons; in low-spin complexes, electrons pair in t₂g orbitals before occupying e_g (Δₒ > P), resulting in fewer unpaired electrons.
Type of d orbitals used in hybridisation
Inner orbital complex
Outer orbital complex
Inner orbital complexes use (n-1)d orbitals for d²sp³ hybridisation (low-spin), while outer orbital complexes use nd orbitals for sp³d² hybridisation (high-spin).
Nature of isomerism
Geometrical isomerism
Optical isomerism
Geometrical isomerism involves different spatial arrangements of ligands (cis/trans, fac/mer) and does not require chirality; optical isomerism involves non-superimposable mirror images (enantiomers) and requires chirality.
Exam Focus
Marks pattern: Typically 2-3 MCQs (1 mark each), 1-2 short answer questions (2-3 marks), and 1 long answer question (5 marks) on nomenclature, isomerism, or bonding theories.
Most asked: Werner's theory and its postulates · IUPAC nomenclature of coordination compounds · Crystal field theory and d-orbital splitting (octahedral and tetrahedral) · Magnetic properties and spin-only formula · Isomerism (geometrical, optical, linkage, ionisation) · Spectrochemical series · Valence bond theory and hybridisation · Applications of coordination compounds (e.g., extraction of gold, EDTA, cis-platin)
Don't skip: Naming coordination compounds and writing formulas (5 marks); Explaining magnetic behaviour using VBT or CFT (5 marks); Drawing and identifying isomers (geometrical and optical) (3-5 marks); Crystal field splitting diagrams and predicting colour/magnetism (5 marks)
One-Line Revision (read aloud before the exam)
  1. Werner's theory states metals have primary (ionisable) and secondary (non-ionisable, = coordination number) valences.
  2. Coordination number is the number of ligand donor atoms directly bonded to the central metal atom.
  3. Ligands are classified as unidentate, didentate, or polydentate based on the number of donor atoms.
  4. Chelate complexes are more stable than similar complexes with unidentate ligands.
  5. Ambidentate ligands like NO₂⁻ and SCN⁻ can bind through two different donor atoms.
  6. IUPAC naming: ligands in alphabetical order, anionic ligands end in '-o', neutral ligands use 'aqua' for H₂O and 'ammine' for NH₃.
  7. Oxidation state of the metal is indicated by a Roman numeral in parentheses.
  8. Geometrical isomerism occurs in square planar and octahedral complexes, not in tetrahedral.
  9. Optical isomerism requires chirality; cis-[PtCl₂(en)₂]²⁺ is optically active, trans is not.
  10. Linkage isomerism arises from ambidentate ligands; ionisation isomerism from exchange of counter ion with ligand.
  11. Valence Bond Theory uses hybridisation (d²sp³ for inner orbital, sp³d² for outer orbital) to explain geometry and magnetism.
  12. Crystal Field Theory explains d-orbital splitting: in octahedral field, t₂g (lower) and e_g (higher) sets.
  13. Spectrochemical series ranks ligands by field strength: I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < edta⁴⁻ < NH₃ < en < CN⁻ < CO.
  14. High-spin complexes have Δₒ < P; low-spin complexes have Δₒ > P.
  15. Colour in complexes is due to d-d transitions; the observed colour is complementary to the absorbed wavelength.
  16. Metal carbonyls exhibit synergic bonding: σ donation from CO to metal and π back donation from metal to CO.
  17. Coordination compounds are used in extraction of gold ([Au(CN)₂]⁻), purification of nickel ([Ni(CO)₄]), and as catalysts (Wilkinson's catalyst).
  18. EDTA is used in water hardness estimation and treatment of lead poisoning.
  19. Cis-platin [Pt(NH₃)₂Cl₂] is an anticancer drug.
  20. Haemoglobin (Fe), chlorophyll (Mg), and vitamin B₁₂ (Co) are biologically important coordination compounds.

MCQs — 10 Questions with Answers

Q1. What is the secondary valence of cobalt in [Co(NH₃)₆]³⁺ 3Cl⁻?

  • A. 6 ✓
  • B. 3
  • C. 9
  • D. 0

Answer: A — Secondary valence equals the coordination number, which is 6 because six ammonia molecules are bonded directly to the cobalt ion.

Q2. How many moles of AgCl will precipitate per mole of [CoCl₂(NH₃)₄]Cl when treated with excess AgNO₃?

  • A. 0
  • B. 1 ✓
  • C. 2
  • D. 3

Answer: B — Only the Cl⁻ ion outside the square bracket (the counter ion) precipitates as AgCl; the two Cl⁻ ions inside the bracket are bonded to cobalt and do not precipitate.

Q3. Which of the following is a heteroleptic coordination compound?

  • A. [Ni(CO)₄]
  • B. [Co(NH₃)₆]³⁺
  • C. [CoCl(NH₃)₅]²⁺ ✓
  • D. [Fe(CN)₆]⁴⁻

Answer: C — Heteroleptic complexes contain different types of ligands; [CoCl(NH₃)₅]²⁺ has both chloride and ammonia ligands, while others have only one type of ligand.

Q4. A coordination compound [PdCl₂(NH₃)₂] gives 0 moles of AgCl with excess AgNO₃. What is the coordination number of Pd?

  • A. 2
  • B. 4 ✓
  • C. 6
  • D. Cannot be determined

Answer: B — Zero AgCl means both chlorides are bonded to Pd (inside the coordination sphere), so the formula is [PdCl₂(NH₃)₂] with coordination number 4 (2 Cl + 2 NH₃).

Q5. Which property distinguishes a double salt from a coordination compound?

  • A. Double salts contain counter ions; complexes do not
  • B. Double salts dissociate completely in solution; complexes do not fully dissociate ✓
  • C. Double salts are coloured; complexes are colourless
  • D. Double salts are ionic; complexes are covalent

Answer: B — Double salts like KCl·MgCl₂·6H₂O dissociate into all component ions, while complexes like K₄[Fe(CN)₆] retain the coordination entity [Fe(CN)₆]⁴⁻ and do not fully dissociate.

Q6. What is the oxidation number of cobalt in [Co(NH₃)₆]Cl₃ if ammonia is neutral?

  • A. +2
  • B. +3 ✓
  • C. +4
  • D. 0

Answer: B — Ammonia is neutral, so 6 × 0 = 0; three chlorides outside are −1 each, giving +3 for cobalt to balance the overall +3 charge on the complex.

Q7. Which of the following statements is NOT correct regarding Werner's theory?

  • A. Primary valence is normally satisfied by anions
  • B. Secondary valence is ionisable and can change with conditions ✓
  • C. Secondary valence equals the coordination number
  • D. Ligands arrange in characteristic spatial patterns for a given coordination number

Answer: B — Secondary valence is non-ionisable and fixed for a metal; primary valence is ionisable, so option B is incorrect.

Q8. Assertion: [CoCl(NH₃)₅]²⁺ is a homoleptic complex. Reason: It contains two different types of ligands.

  • A. Both assertion and reason are correct; reason explains assertion
  • B. Both assertion and reason are correct; reason does not explain assertion
  • C. Assertion is correct; reason is incorrect
  • D. Assertion is incorrect; reason is correct ✓

Answer: D — The assertion is false because [CoCl(NH₃)₅]²⁺ is heteroleptic (contains Cl⁻ and NH₃); the reason is correct—heteroleptic means different ligand types.

Q9. If a coordination compound [Pt(NH₃)₂Cl₂] gives 0 moles of AgCl and the compound exists as cis and trans isomers, what is the geometry around Pt?

  • A. Tetrahedral
  • B. Octahedral
  • C. Square planar ✓
  • D. Linear

Answer: C — Both Cl⁻ are bonded to Pt (giving 0 AgCl), coordination number is 4, and cis-trans isomerism occurs only in square planar geometry for MA₂B₂ type complexes.

Q10. Three compounds A, B, and C are formed with CoCl₃ and NH₃ in different molar ratios. A precipitates 3 mol AgCl, B precipitates 2 mol AgCl, and C precipitates 1 mol AgCl per mole of compound. Arrange in order of increasing moles of NH₃ per mole of compound.

  • A. A < B < C
  • B. C < B < A ✓
  • C. A < C < B
  • D. B < C < A

Answer: B — More free Cl⁻ ions (more AgCl precipitate) means fewer Cl⁻ are coordinated to Co, so more NH₃ must be coordinated: C (1 AgCl, 4 NH₃) < B (2 AgCl, 5 NH₃) < A (3 AgCl, 6 NH₃).

Flashcards

What is a coordination entity?

A central metal atom or ion bonded to a fixed number of ligands (ions or molecules) that acts as a single unit, written within square brackets.

Define secondary valence according to Werner's theory.

The number of groups (ligands) bonded directly to the central metal ion; it equals the coordination number and is fixed for a given metal.

How does the AgNO₃ test distinguish between free and coordinated chloride ions?

Only chloride ions outside the square bracket (counter ions) precipitate as white AgCl; chloride ions inside the bracket bonded to the metal remain in solution.

What is the difference between a double salt and a complex?

Double salts dissociate completely into simple ions in solution, while complexes remain as a single coordination entity and do not fully dissociate.

What is the chelate effect?

Chelating ligands (polydendate) form more stable complexes than monodentate ligands due to increased entropy of release of small solvent molecules.

Define geometric isomerism in coordination compounds.

Isomerism where ligands occupy different spatial positions around the metal (cis-trans in octahedral or square planar complexes), resulting in different properties.

What is optical isomerism in coordination compounds?

Isomerism where two complexes are non-superimposable mirror images (enantiomers) and rotate plane-polarised light in opposite directions.

What is the coordination number of cobalt in [Co(NH₃)₆]³⁺?

Six, because six ammonia molecules are bonded directly to the central cobalt ion.

Name a homoleptic and a heteroleptic coordination compound.

Homoleptic: [Co(NH₃)₆]³⁺ (all same ligand); Heteroleptic: [CoCl(NH₃)₅]²⁺ (different ligands).

What biological molecules are coordination compounds?

Chlorophyll (Mg²⁺ centre), haemoglobin (Fe²⁺ centre), vitamin B₁₂ (Co³⁺ centre).

Important Board Questions

Define the term 'coordination number' and state what secondary valence equals. [2 marks]

Coordination number = total number of ligands bonded to central metal atom. Secondary valence (Werner's term) = coordination number and is fixed for each metal.

A coordination compound CoCl₃·5NH₃ precipitates 2 moles of AgCl per mole with excess AgNO₃ solution. Write the correct structural formula and explain why only 2 Cl⁻ ions precipitate. What is the coordination number of cobalt? [5 marks]

AgNO₃ precipitates only free Cl⁻ outside the bracket (counter ions). If 2 mol AgCl forms, then 2 Cl⁻ are free; the remaining 1 Cl⁻ plus 5 NH₃ (= 6 ligands) are coordinated to Co. Formula: [CoCl(NH₃)₅]²⁺ 2Cl⁻. Coordination number = 6.

Explain the difference between geometric and optical isomerism in coordination compounds. Give one example of each and state how they can be distinguished experimentally. [6 marks]

Geometric isomerism: spatial arrangement of ligands (cis-trans in octahedral/square planar); detected by different colours, melting points, or solubility. Optical isomerism: non-superimposable mirror images (enantiomers); detected by optical rotation of plane-polarised light (d- and l-forms rotate in opposite directions). Example geometric: [Co(NH₃)₄Cl₂]⁺ (cis and trans); Example optical: [Co(en)₃]³⁺ where en = ethylenediamine (forms d and l isomers).

Assertion-Reason Questions for Coordination Compounds

Q1. Assertion (A):

In the compound [Co(NH3)6]Cl3, the coordination number of cobalt is 6 and the primary valence is 3.

Reason (R):

The coordination number equals the number of ligands bonded to the central metal ion, and primary valence is satisfied by the counter ions outside the coordination sphere.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both assertion and reason are true, and the reason correctly explains the assertion. The 6 NH3 molecules determine coordination number 6, and the 3 Cl− counter ions satisfy the primary valence of Co3+. This directly demonstrates Werner's classification of primary and secondary valences.

Q2. Assertion (A):

When [CoCl(NH3)5]Cl2 is treated with excess AgNO3, exactly 2 moles of AgCl precipitate per mole of complex.

Reason (R):

The chloride ion bonded directly to cobalt cannot be precipitated by AgNO3 because it is non-ionisable.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both assertion and reason are true. Only the 2 Cl− counter ions outside the coordination sphere can be precipitated (1:2 electrolyte). The 1 Cl− inside the coordination sphere is part of the non-ionisable coordination entity [CoCl(NH3)5]2+, as postulated by Werner's secondary valence concept.

Q3. Assertion (A):

Double salts and coordination complexes differ in their behaviour in solution because double salts completely dissociate into simple ions while complexes do not.

Reason (R):

Double salts are mechanical mixtures of stable compounds held together by weak electrostatic forces, whereas complexes have covalent bonds between the central metal ion and ligands.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both assertion and reason are true, but the reason does NOT directly explain the assertion in terms of dissociation behaviour. The assertion focuses on solution behaviour (dissociation vs non-dissociation), while the reason explains bonding type. Although bonding type affects dissociation, the reason doesn't explicitly connect this to the observed dissociation difference.

Unlock more Assertion-Reason questions in Premium — unlock →

Case-Based Questions (CBQ) for Coordination Compounds

Case Study:

A chemist prepared a series of coordination compounds of cobalt(III) with ammonia and chloride to study Werner's theory. The compounds were treated with excess silver nitrate solution to determine which chloride ions could be precipitated as AgCl. The yellow compound gave 3 moles of AgCl, the purple gave 2 moles, and two green compounds (one with and one without heating) gave 1 mole each. The conductivity of solutions indicated different numbers of free ions. These observations led to different structural formulations for each compound despite having similar empirical formulas.

Q1. Which of the following correctly represents the yellow coordination compound based on the observation that 3 moles of AgCl are precipitated?

  • A. [Co(NH3)6]Cl3 ✓
  • B. [CoCl(NH3)5]Cl2
  • C. [CoCl2(NH3)4]Cl
  • D. [CoCl3(NH3)3]

Answer: A

Q2. The purple compound gave 2 moles of AgCl precipitate. This indicates that in the coordination entity:

  • A. Both chloride ions are bonded to cobalt
  • B. One chloride ion is bonded to cobalt and one is outside as a counter ion ✓
  • C. All chloride ions are outside the coordination sphere as counter ions
  • D. No chloride ions are bonded to cobalt

Answer: B

Q3. Two different compounds with identical empirical formula CoCl3·4NH3 show different colours (green and violet) and different properties. This phenomenon is explained by:

  • A. Ionisation isomerism due to different counter ions
  • B. Geometric (cis-trans) isomerism due to different spatial arrangements of ligands ✓
  • C. Linkage isomerism due to different coordination modes of ammonia
  • D. Optical isomerism due to the presence of chiral centres

Answer: B

Q4. The coordination compound [CoCl(NH3)5]2+ is formed when one ammonia molecule is replaced by one chloride ion in the yellow compound. The secondary valence of cobalt in this compound is:

  • A. 5
  • B. 7
  • C. 6 ✓
  • D. 3

Answer: C

Unlock more case studies in Premium — unlock →

Previous Year Questions (PYQ) for Coordination Compounds

Real CBSE board exam questions from past papers — free to attempt.

CBSE 2026 · Set 56(B) · 1 mark

The secondary valency of Co in [Co(en)3]2 (SO4)3 is :

  • 12
  • 2
  • 3
  • 6

CBSE 2026 · Set 56(B) · 2 marks

Define the following terms : (a) Ambidentate ligand (b) Crystal field splitting energy

CBSE 2026 · Set 56(B) · 3 marks

Give reasons : (i) Why are low spin tetrahedral complexes not formed ? (ii) Why is [Co(en)3]3+ complex more stable than [Co(NH3)6]3+ ? (iii) Why are geometrical isomers not possible for tetrahedral complexes having two different types of unidentate ligands coordinated to the central metal ion ?

CBSE 2026 · Set 56/1/1 · 1 mark

The secondary valency of Co in the complex [Co(NH3)5(NO2)]2+ is

  • 5
  • 1
  • 4
  • 6

CBSE 2026 · Set 56/1/1 · 2 marks

Write IUPAC names of the following coordination compounds: (i) [Ag(NH3)2][Ag(CN)2] (ii) K3[Fe(C2O4)3]

CBSE 2026 · Set 56/1/1 · 4 marks

The Valence Bond Theory (VBT) explains the formation, magnetic behaviour and geometry of coordination compounds. The Crystal Field Theory (CFT) of coordination compounds is based on the effect of different crystal fields (provided by the ligands taken as point charges), on the degeneracy of d-orbital energies of the central metal atom/ion. The splitting of the d-orbitals provides different electronic arrangements in strong and weak crystal fields.

In octahedral crystal field, energies of which d-orbitals will be raised when ligands approach the central metal atom/ion? Give reason in support of your answer.

CBSE 2026 · Set 56/1/2 · 1 mark

The secondary valency of Pt in [Pt(en)2Cl2]²⁺ is

  • 6
  • 5
  • 4
  • 2

CBSE 2026 · Set 56/1/2 · 4 marks

The Valence Bond Theory (VBT) explains the formation, magnetic behaviour and geometry of coordination compounds. The Crystal Field Theory (CFT) of coordination compounds is based on the effect of different crystal fields (provided by the ligands taken as point charges), on the degeneracy of d-orbital energies of the central metal atom/ion. The splitting of the d-orbitals provides different electronic arrangements in strong and weak crystal fields.

(a) In octahedral crystal field, energies of which d-orbitals will be raised when ligands approach the central metal atom/ion? Give reason in support of your answer. (2 marks)

CBSE 2026 · Set 56/1/3 · 1 mark

The secondary valency of Co in [Co(en)3]3+ is

  • 4
  • 3
  • 5
  • 6

CBSE 2026 · Set 56/1/3 · 2 marks

(a) Write IUPAC names of the following coordination compounds : (i) [Ag(NH 3)2] [Ag(CN)2] (ii) K 3[Fe(C2O4)3]

CBSE 2026 · Set 56/1/3 · 4 marks

The Valence Bond Theory (VBT) explains the formation, magnetic behaviour and geometry of coordination compounds. The Crystal Field Theory (CFT) of coordination compounds is based on the effect of different crystal fields (provided by the ligands taken as point charges), on the degeneracy of d-orbital energies of the central metal atom/ion. The splitting of the d-orbitals provides different electronic arrangements in strong and weak crystal fields.

(a) In octahedral crystal field, energies of which d-orbitals will be raised when ligands approach the central metal atom/ion ? Give reason in support of your answer.

CBSE 2026 · Set 56/2/1 · 1 mark

The correct IUPAC name of the complex [Pt(NH3)2Cl2] is:

  • diamminedichloridoplatinum (IV)
  • diamminedichloridoplatinum (II)
  • dichloridodiammineplatinum (IV)
  • dichloridodiammineplatinum (II)

See all 159 board PYQs for this chapter — open PYQ tab →

Class 12 Chemistry Formula Sheet

Every formula and named reaction from all 10 chapters on one page, checked against the NCERT textbook and the CBSE 2026-27 syllabus. Free PDF download.

View the formula sheet →

Class 12 Chemistry Previous Year Questions, Chapter-wise

All 1241 CBSE board questions from 2022–2026, sorted by chapter and question type. Free PDF download.

See all past board questions →

Next chapterHaloalkanes and Haloarenes →
Found this useful? Share it with your class group.

You've seen the free preview. Open the full interactive study kit — mind map, instant-graded quiz, progress tracking — free, no sign-in required.

Open Full Study Kit — Free →

Premium unlocks the full Question Bank, Daily Practice Papers, Chapter Quiz and Full Mock Papers for every chapter.