Chapter 9 — Coordination Compounds — introduces the fascinating world of metal complexes. From haemoglobin in your blood to cisplatin in cancer treatment, coordination compounds are everywhere! This chapter carries 5-7 marks in Board exams. Focus on IUPAC naming, isomerism, and bonding theories (VBT and CFT).
Key Concepts
Basic Terminology
| Term | Meaning | Example |
|---|---|---|
| Central metal atom/ion | Metal that accepts electron pairs | Cu²⁺ in [Cu(NH₃)₄]²⁺ |
| Ligand | Ion/molecule that donates electron pair to metal | NH₃, Cl⁻, H₂O, CN⁻ |
| Coordination number (CN) | Number of ligand donor atoms bonded to metal | CN = 6 in [Co(NH₃)₆]³⁺ |
| Coordination sphere | Central metal + ligands (written in square brackets) | [Fe(CN)₆]⁴⁻ |
| Counter ion | Ion outside the coordination sphere | K⁺ in K₄[Fe(CN)₆] |
| Denticity | Number of donor atoms in a ligand | en (ethylenediamine) = bidentate |
Types of Ligands
| Type | Donor Atoms | Examples |
|---|---|---|
| Monodentate | 1 | H₂O, NH₃, Cl⁻, CN⁻, NO₂⁻, CO |
| Bidentate | 2 | en (ethylenediamine), ox²⁻ (oxalate, C₂O₄²⁻) |
| Polydentate | Many | EDTA⁴⁻ (hexadentate — 6 donor atoms) |
| Ambidentate | 1 (but 2 possible donor atoms) | NO₂⁻ (N or O), SCN⁻ (S or N) |
IUPAC Nomenclature Rules
- Cation is named first, then anion (even if complex is the anion)
- Ligands are named alphabetically (ignoring prefixes di-, tri-)
- Anionic ligands: suffix -o (chlorido, cyanido, hydroxido, oxalato)
- Neutral ligands: usual name, EXCEPT: H₂O = aqua, NH₃ = ammine, CO = carbonyl, NO = nitrosyl
- Metal in anionic complex: suffix -ate (ferrate, cuprate, cobaltate)
- Oxidation state of metal in Roman numerals in parentheses: (III), (II)
Naming Examples
[Co(NH₃)₆]Cl₃ → Hexaamminecobalt(III) chloride
K₃[Fe(CN)₆] → Potassium hexacyanidoferrate(III)
[CoCl₂(en)₂]Cl → Dichloridobis(ethylenediamine)cobalt(III) chloride
[Pt(NH₃)₂Cl₂] → Diamminedichloridoplatinum(II) (cisplatin — anticancer drug!)
Isomerism in Coordination Compounds
Structural Isomerism
| Type | Description | Example |
|---|---|---|
| Ionisation | Exchange of ligand and counter ion | [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br |
| Linkage | Ambidentate ligand bonds through different atoms | [Co(NH₃)₅ONO]²⁺ vs [Co(NH₃)₅NO₂]²⁺ |
| Coordination | Exchange of ligands between cation and anion | [Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆] |
| Solvate/Hydrate | Water as ligand vs as lattice water | [Cr(H₂O)₆]Cl₃ vs [Cr(H₂O)₅Cl]Cl₂·H₂O |
Stereoisomerism
Geometrical (cis-trans): In square planar (CN=4) and octahedral (CN=6) complexes
- Square planar [MA₂B₂]: cis (same side) and trans (opposite)
- Octahedral [MA₂B₄]: cis and trans
- [MA₃B₃] octahedral: fac (facial — same triangular face) and mer (meridional — same plane)
Optical isomerism: Non-superimposable mirror images (d and l forms)
- Common in octahedral complexes with bidentate ligands: [Co(en)₃]³⁺
- Also in cis-[CoCl₂(en)₂]⁺ (cis shows optical isomerism, trans doesn’t)
Bonding Theories
Valence Bond Theory (VBT)
1. Ligands donate electron pairs to empty hybrid orbitals of metal
2. Inner orbital complex: uses (n−1)d orbitals → diamagnetic/low spin
3. Outer orbital complex: uses nd orbitals → paramagnetic/high spin
Common Hybridisations:
CN=4: sp³ (tetrahedral) or dsp² (square planar)
CN=6: d²sp³ (inner, octahedral) or sp³d² (outer, octahedral)
Crystal Field Theory (CFT)
1. Ligands are treated as point charges that create an electrostatic field
2. This field splits d-orbitals into two sets:
Octahedral: t₂g (dxy, dxz, dyz) ↓ lower energy and eg (dx²-y², dz²) ↑ higher energy
Energy gap = Δ₀ (crystal field splitting energy)
Tetrahedral: e (dx²-y², dz²) ↓ lower and t₂ (dxy, dxz, dyz) ↑ higher
Δt ≈ 4/9 Δ₀ (always smaller → tetrahedral are high spin)
Spectrochemical Series (increasing Δ)
Strong field ligands (CN⁻, CO, NO₂⁻, en, NH₃) → large Δ → low spin complexes
CFSE (Crystal Field Stabilisation Energy)
Where n = number of electrons in each set
Applications of Coordination Compounds
- Haemoglobin: Fe²⁺ coordination compound that carries O₂ in blood
- Chlorophyll: Mg²⁺ coordination compound for photosynthesis
- Vitamin B₁₂: Co³⁺ coordination compound
- Cisplatin: cis-[Pt(NH₃)₂Cl₂] — anticancer drug
- EDTA: Used in water softening, treatment of lead poisoning
- Photography: AgBr dissolved by Na₂[Ag(S₂O₃)₂] (fixing)
Important Definitions
| Term | Definition |
|---|---|
| Coordination Compound | Compound containing a central metal bonded to ligands via coordinate bonds |
| Ligand | Atom/ion/molecule that donates electron pair to central metal |
| Coordination Number | Number of donor atoms directly bonded to the central metal |
| Chelate | Complex containing polydentate ligand forming a ring with the metal |
| Crystal Field Splitting | Splitting of d-orbitals into different energy levels due to ligand field |
| Spectrochemical Series | Arrangement of ligands in order of increasing crystal field splitting |
Solved Examples — NCERT Based
Example 1: IUPAC Naming
Q: Write IUPAC names: (a) [Cr(NH₃)₃(H₂O)₃]Cl₃ (b) [CoCl₂(en)₂]⁺ (c) K₂[PdCl₄]
Solution:
(a) Triamminetriaquachromium(III) chloride
(b) Dichloridobis(ethylenediamine)cobalt(III) ion
(c) Potassium tetrachloridopalladate(II)
Example 2: Finding Oxidation State and CN
Q: In K₃[Fe(CN)₆], find the oxidation state of Fe and coordination number.
Solution:
Let oxidation state of Fe = x
3(+1) + x + 6(−1) = 0 → 3 + x − 6 = 0 → x = +3
Coordination number = number of CN⁻ ligands = 6 (each CN⁻ is monodentate)
Example 3: CFT — Predicting Colour
Q: [Ti(H₂O)₆]³⁺ appears purple. Explain using CFT.
Solution:
Ti³⁺ = [Ar] 3d¹ → one electron in t₂g level. When visible light passes through, the electron absorbs green-yellow light (corresponding to Δ₀) and jumps from t₂g to eg. The transmitted/reflected light (purple, the complement of green-yellow) is what we see. This is a d-d transition.
Example 4: Magnetic Properties from CFT
Q: Predict whether [Fe(CN)₆]⁴⁻ is paramagnetic or diamagnetic.
Solution:
Fe²⁺ = 3d⁶. CN⁻ is a strong field ligand → large Δ₀ → electrons pair up before going to eg.
t₂g⁶ eg⁰ → all electrons paired → diamagnetic (0 unpaired electrons).
Compare: [Fe(H₂O)₆]²⁺ with weak field H₂O → t₂g⁴ eg² → 4 unpaired → paramagnetic.
Important Questions for Board Exams
1 Mark Questions
- What is the coordination number in [Co(NH₃)₆]³⁺?
- What is an ambidentate ligand? Give an example.
- Write the IUPAC name of [Pt(NH₃)₂Cl₂].
- What is the oxidation state of Co in [Co(CN)₆]³⁻?
2 Mark Questions
- What is the difference between a double salt and a coordination compound?
- Explain geometrical isomerism in [Pt(NH₃)₂Cl₂] with diagrams.
- What is meant by chelation? Why are chelate complexes more stable?
- Write the spectrochemical series for 5 common ligands.
3 Mark Questions
- Explain Crystal Field Theory for octahedral complexes. What is crystal field splitting?
- Using VBT, predict the hybridisation and magnetic property of [Fe(CN)₆]⁴⁻ and [FeF₆]³⁻.
- What are the different types of isomerism shown by coordination compounds? Give one example each.
- What is spectrochemical series? How does it help predict magnetic properties?
5 Mark Questions
- What is CFT? Explain crystal field splitting in octahedral and tetrahedral complexes. Why are most tetrahedral complexes high spin?
- Discuss the Werner’s theory of coordination compounds. Write IUPAC names and describe the isomerism of [CoCl₂(en)₂]⁺.
Quick Revision Points
- Ligands: monodentate (1 donor), bidentate (2: en, ox), polydentate (EDTA = 6)
- IUPAC: ligands alphabetical, anionic = -o, neutral = name (H₂O=aqua, NH₃=ammine)
- Structural isomers: ionisation, linkage, coordination, solvate
- Stereoisomers: geometrical (cis-trans, fac-mer) and optical (d, l)
- VBT: sp³ (tetrahedral), dsp² (square planar), d²sp³/sp³d² (octahedral)
- CFT: octahedral splits into t₂g (lower) and eg (higher); Δ₀
- Strong field → large Δ → low spin → paired → diamagnetic
- Weak field → small Δ → high spin → unpaired → paramagnetic
- Spectrochemical series: I⁻ < Br⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < en < CN⁻ < CO
- Cisplatin [cis-Pt(NH₃)₂Cl₂] = anticancer; Hb = Fe²⁺ complex; chlorophyll = Mg²⁺
Chapter Navigation
Previous: The d and f Block Elements Class 12 Notes
Next: Haloalkanes and Haloarenes Class 12 Notes
Related Chapters in Class 12 Chemistry
- The d and f Block Elements Class 12 Notes
- The p-Block Elements Class 12 Notes
- General Principles and Processes of Isolation of Elements Class 12 Notes
Practice What You Learned
Test yourself with our JEE Main Mock Test Set 1 to see how well you’ve mastered the concepts.