Coordination Compounds Class 12 Notes for IIT JEE & NEET

eSaral ›Coordination Compounds Class 12 Notes for IIT JEE & NEET

Chapter Overview and Exam Weightage 

Coordination Compounds is one of the most concept-rich chapters in Class 12 Chemistry. It connects inorganic chemistry, bonding theory, and even biological chemistry — making it intellectually satisfying and exam-relevant at the same time.

Why This Chapter Matters for JEE and NEET

💡 Expert Tip by eSaral Chemistry Faculty (IITian, Kota): "Coordination Compounds is a chapter where students who understand the logic of IUPAC rules score every single mark in nomenclature questions. Don't memorise names — understand the rules. Name any complex by following the same 6 steps every time and you will never go wrong."

Coordination Compounds Class 12 Notes

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

Werner's Theory of Coordination Compounds 

Alfred Werner (Nobel Prize, 1913) proposed the first systematic theory of coordination compounds in 1893.

Key Postulates of Werner's Theory

1. Metals in coordination compounds show two types of valency:
2. • Primary valency (ionisable) — satisfied by negative ions, shown by oxidation state
   • Secondary valency (non-ionisable) — satisfied by neutral molecules or negative ions directly attached to the metal; equal to the coordination number
2. Secondary valencies are directed in space around the central metal atom, giving complexes their definite geometry.
3. Primary valencies are non-directional; secondary valencies are directional.

Example: [Co(NH₃)₆]Cl₃

• Central metal: Co
• Primary valency: 3 (satisfied by 3 Cl⁻ ions)
• Secondary valency / Coordination number: 6 (satisfied by 6 NH₃ molecules)
• Geometry: Octahedral

Werner proved his theory by studying ionisation in solution. [Co(NH₃)₆]Cl₃ gives 4 ions in solution (1 complex cation + 3 Cl⁻), while [Co(NH₃)₄Cl₂]Cl gives only 2 ions — confirming which Cl⁻ ions are inside vs outside the coordination sphere.

Key Terminology: Ligands, Central Metal, Coordination Number

Central Metal Atom / Ion

The metal atom or ion at the centre of a complex that accepts electron pairs from ligands. Examples: Fe²⁺, Co³⁺, Pt²⁺, Cu²⁺.

Ligands

Ligands are ions or molecules that donate one or more lone pairs of electrons to the central metal.

Chelate complexes are formed when bidentate or polydentate ligands bind to the same metal atom, forming a ring. They are more stable than non-chelate complexes (chelate effect).

Coordination Number

The total number of coordinate bonds formed between the central metal and the ligands. Common coordination numbers: 2, 4, 6.

• CN = 2 → Linear (e.g., [Ag(NH₃)₂]⁺)
• CN = 4 → Tetrahedral or Square Planar
• CN = 6 → Octahedral (most common)

IUPAC Nomenclature of Coordination Compounds 

How to Name a Coordination Compound — 6-Step Rule

1. Name the cation first, then the anion (same as ionic compounds)
2. Within the complex ion: name ligands alphabetically before the central metal
3. Anionic ligands end in -o (e.g., Cl⁻ = chlorido, CN⁻ = cyanido, OH⁻ = hydroxido)
4. Neutral ligands use their molecule name (exceptions: H₂O = aqua, NH₃ = ammine, CO = carbonyl, NO = nitrosyl)
5. Number of ligands shown by Greek prefixes: di, tri, tetra, penta, hexa (use bis, tris, tetrakis for complex ligands)
6. Oxidation state of metal shown in Roman numerals in parentheses immediately after the metal name

Worked Examples

💡 Expert Tip by eSaral Chemistry Faculty (IITian, Kota): "In JEE and NEET, IUPAC nomenclature questions give you the formula and ask for the name — or give you the name and ask for the formula. Practise both directions. The most common error is alphabetical order of ligands (en comes before aqua, but ammine comes before chlorido). Always list ligands A–Z."

Isomerism in Coordination Compounds

Coordination compounds show two main types of isomerism — structural and stereoisomerism.

Structural Isomerism

Stereoisomerism

Geometrical (cis-trans) isomerism:

• Square planar MA₂B₂ type: cis (same groups adjacent) vs trans (same groups opposite)
• Octahedral MA₄B₂ type: cis (B groups at 90°) vs trans (B groups at 180°)

Optical isomerism:

• Complexes with no plane of symmetry are optically active
• [Co(en)₃]³⁺ shows optical isomerism (d and l forms)
• Cis-[CoCl₂(en)₂]⁺ shows optical isomerism; trans form does not

Bonding in Coordination Compounds: VBT and CFT 

Valence Bond Theory (VBT)

VBT explains the geometry and magnetic properties of complexes in terms of hybridisation of the central metal's atomic orbitals.

Outer orbital complexes (sp³d² hybridisation) use 4s, 4p, and 4d orbitals → high spin, paramagnetic.

Inner orbital complexes (d²sp³ hybridisation) use 3d, 4s, and 4p orbitals → low spin, often diamagnetic.

Crystal Field Theory (CFT)

CFT treats ligands as point charges and explains d-orbital splitting when ligands approach the metal.

In an octahedral field: d orbitals split into two sets:

• t₂g (d_xy, d_xz, d_yz) — lower energy
• e_g (d_z², d_x²-y²) — higher energy
• Energy gap = Δ_o (crystal field splitting energy)

Strong field ligands (large Δ_o) → low spin complexes: CO > CN⁻ > NO₂⁻ > en > NH₃

Weak field ligands (small Δ_o) → high spin complexes: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O

Spectrochemical Series (partial, weak to strong): I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO

In a tetrahedral field: Splitting is 4/9 of Δ_o (i.e., Δ_t = 4/9 Δ_o) — always weaker → tetrahedral complexes are almost always high spin.

Colour, Magnetic Properties, and Stability of Complexes 

Why Are Coordination Compounds Coloured?

Colour arises from d–d transitions: electrons absorb visible light to jump from t₂g to e_g orbitals. The colour observed is the complementary colour of the light absorbed.

• [Ti(H₂O)₆]³⁺ absorbs green light → appears violet/purple
• Complexes with d⁰ (like Sc³⁺) or d¹⁰ (like Zn²⁺) have no d–d transitions → are colourless

Magnetic Properties

• Paramagnetic: Contains unpaired electrons → attracted by a magnetic field
• Diamagnetic: All electrons paired → repelled by a magnetic field

Magnetic moment formula:

μ=n(n+2) BM\mu = \sqrt{n(n+2)} \text{ BM}μ=n(n+2)​ BM

Where n = number of unpaired electrons. (BM = Bohr Magneton)

Stability — EAN Rule (Effective Atomic Number)

Many stable complexes obey the 18-electron rule (EAN rule): the total electron count around the metal equals 18 (noble gas configuration).

Biological and Industrial Importance

Biological Importance

Industrial and Medical Applications

• Wilkinson's catalyst [RhCl(PPh₃)₃] — used in hydrogenation reactions
• Cisplatin [Pt(NH₃)₂Cl₂] — anticancer drug (cis isomer is active; trans isomer is inactive)
• EDTA complexes — used in water treatment and as chelating agents in medicine
• Cyanide complexes — used in silver and gold extraction (hydrometallurgy)
• Photographic processing — Na₂S₂O₃ dissolves AgBr by forming a stable complex

Important Facts 

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