Coordination Compounds — JEE Main Chemistry Notes & Formulas

1. Chapter at a Glance

Coordination compounds contain a central metal atom or ion bonded to surrounding molecules or ions called ligands. Examples include haemoglobin (Fe), chlorophyll (Mg), vitamin B₁₂ (Co) and cisplatin (Pt, an anticancer drug). This chapter covers Werner's theory, naming, isomerism, valence bond theory (VBT) and crystal field theory (CFT). JEE Main asks one or two questions every year — isomer counting, magnetic behaviour and CFSE are favourites.

What JEE asks most

IUPAC naming and finding oxidation state and coordination number.
Number of geometrical and optical isomers.
Hybridisation, shape and magnetism from VBT.
Crystal field splitting, spectrochemical series and CFSE.

2. Werner's Theory and Basic Terms

Metals show a primary valency (oxidation state, satisfied by negative ions, ionisable) and a secondary valency (coordination number, non-ionisable, directional).
Example: CoCl₃·6NH₃ = [Co(NH₃)₆]Cl₃ gives 3 mol AgCl with AgNO₃; CoCl₃·5NH₃ = [Co(NH₃)₅Cl]Cl₂ gives 2; CoCl₃·4NH₃ = [Co(NH₃)₄Cl₂]Cl gives 1.
Ligand: donor of electron pairs. Unidentate (NH₃, Cl⁻), bidentate (en, C₂O₄²⁻), hexadentate (EDTA⁴⁻). Ambidentate ligands can bind through two different atoms (NO₂⁻/ONO⁻, SCN⁻/NCS⁻).
Chelate: a ring formed by a polydentate ligand; chelates are more stable (chelate effect).
Coordination number: the number of donor atoms bonded to the metal.
Homoleptic: only one kind of ligand ([Co(NH₃)₆]³⁺); heteroleptic: more than one kind.

3. IUPAC Nomenclature

Name the cation first, then the anion.
Inside the complex, name ligands in alphabetical order, then the metal.
Anionic ligands end in "-o" (chlorido, cyanido, oxalato, hydroxido); neutral ligands keep their names except aqua (H₂O), ammine (NH₃), carbonyl (CO), nitrosyl (NO).
Use di, tri, tetra for simple ligands; bis, tris, tetrakis for ligands whose names already contain a prefix (bis(ethane-1,2-diamine)).
Give the oxidation state of the metal in Roman numerals. If the complex is an anion, the metal name ends in "-ate" (ferrate, cuprate, argentate, plumbate, stannate).

Formula	IUPAC name
[Co(NH₃)₅Cl]Cl₂	Pentaamminechloridocobalt(III) chloride
K₄[Fe(CN)₆]	Potassium hexacyanidoferrate(II)
[Pt(NH₃)₂Cl₂]	Diamminedichloridoplatinum(II)
K₃[Al(C₂O₄)₃]	Potassium trioxalatoaluminate(III)
[Ni(CO)₄]	Tetracarbonylnickel(0)

4. Isomerism

Structural isomerism

Ionisation: [Co(NH₃)₅SO₄]Br and [Co(NH₃)₅Br]SO₄ — give different ions in solution.
Hydrate (solvate): [Cr(H₂O)₆]Cl₃ (violet) and [Cr(H₂O)₅Cl]Cl₂·H₂O (green).
Linkage: –NO₂ (nitrito-N) vs –ONO (nitrito-O).
Coordination: exchange of ligands between cationic and anionic complexes, e.g. [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆].

Stereoisomerism

Type	Geometrical isomers	Optical activity
[MA₂B₂] square planar	2 (cis, trans)	None
[MA₄B₂] octahedral	2 (cis, trans)	None
[MA₃B₃] octahedral	2 (fac, mer)	None
[M(AA)₂B₂] octahedral	2 (cis, trans)	cis is chiral → total 3 stereoisomers
[M(AA)₃]	—	2 (d and l)
Tetrahedral [MABCD]	No geometrical	Rarely resolved

5. Valence Bond Theory

The metal uses hybrid orbitals to accept lone pairs from ligands. Strong-field ligands (CN⁻, CO) force pairing of d electrons, giving inner-orbital (d²sp³, low-spin) complexes. Weak-field ligands (F⁻, H₂O) give outer-orbital (sp³d², high-spin) complexes.

Complex	Hybridisation	Shape	Unpaired e⁻	Magnetism
[Ni(CN)₄]²⁻	dsp²	Square planar	0	Diamagnetic
[NiCl₄]²⁻	sp³	Tetrahedral	2	Paramagnetic
[Ni(CO)₄]	sp³	Tetrahedral	0	Diamagnetic
[Co(NH₃)₆]³⁺	d²sp³	Octahedral	0	Diamagnetic
[CoF₆]³⁻	sp³d²	Octahedral	4	Paramagnetic
[Fe(CN)₆]³⁻	d²sp³	Octahedral	1	Paramagnetic

6. Crystal Field Theory

CFT treats ligands as point charges. In an octahedral field, the five d orbitals split into a lower set t₂g (d_xy, d_yz, d_xz) and a higher set e_g (d_x²−y², d_z²). The gap is Δ_o.

Octahedral splitting: each t₂g electron lowers the energy by 0.4Δ_o; each e_g electron raises it by 0.6Δ_o.

If Δ_o > pairing energy P (strong field): electrons pair up in t₂g first (low spin). If Δ_o < P (weak field): electrons occupy e_g before pairing (high spin).
Spectrochemical series (increasing field strength): I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < EDTA⁴⁻ < NH₃ < en < CN⁻ < CO.
Tetrahedral splitting is smaller: Δ_t = (4/9)Δ_o, so tetrahedral complexes are almost always high spin.
Colour: the complex absorbs light of energy Δ and shows the complementary colour. [Ti(H₂O)₆]³⁺ absorbs green-yellow and appears purple. Stronger ligands shift absorption to shorter wavelength.

CFSE (octahedral) = (−0.4 n_t2g + 0.6 n_eg)Δ_o + (extra pairs) P

7. Metal Carbonyls and Applications

In metal carbonyls, CO donates a lone pair to the metal (σ bond) and accepts electron density from filled metal d orbitals into its empty π* orbital (synergic back-bonding). This strengthens the M–C bond and weakens the C–O bond.
Effective atomic number (EAN) = Z − oxidation state + 2 × (coordination number). Many stable carbonyls reach the next noble gas (e.g. [Ni(CO)₄]: 28 + 8 = 36).
Applications: EDTA in estimating water hardness (Ca²⁺, Mg²⁺); Mond process purifies nickel via [Ni(CO)₄]; cisplatin in cancer therapy; photography uses [Ag(S₂O₃)₂]³⁻.

8. Formula Master Table

Quantity	Formula / rule
Oxidation state	Sum of charges = charge of complex
EAN	Z − O.S. + 2 × C.N.
CFSE (octahedral)	(−0.4x + 0.6y)Δ_o
Tetrahedral splitting	Δ_t = (4/9)Δ_o
Magnetic moment	μ = √(n(n+2)) BM
Energy absorbed	Δ = hc/λ

9. Mind Map

Coordination Compounds

Basics

Werner's theory
Ligands, denticity
Chelate effect

Naming & isomerism

IUPAC rules
Structural isomers
cis/trans, fac/mer, optical

Bonding

VBT hybridisation
CFT, Δo vs P
Spectrochemical series

Uses

Carbonyls, back-bonding
EDTA, cisplatin

10. Tips and Tricks

Shortcuts

Ni²⁺ (d⁸) with a strong field ligand in CN 4 → square planar, diamagnetic; with a weak field ligand → tetrahedral, 2 unpaired electrons.
Co³⁺ (d⁶) is low spin with almost every ligand except F⁻.
For AgCl counting, only Cl⁻ outside the square bracket precipitates.
Conductivity of a complex depends on the total number of ions it forms: [Co(NH₃)₆]Cl₃ gives 4 ions.

Mistakes students make

Writing ligands in order of charge instead of alphabetically (prefixes like di, tri are ignored when alphabetising).
Forgetting that trans-[Co(en)₂Cl₂]⁺ has a mirror plane and is optically inactive.
Assuming all octahedral complexes of the same metal have the same colour — ligands change Δ_o.

11. Important One-liners

EDTA is a hexadentate ligand.
CO is the strongest field ligand in the common spectrochemical series.
Tetrahedral complexes have a smaller crystal field splitting than octahedral ones.
[Ni(CN)₄]²⁻ is square planar and diamagnetic.
Cisplatin is the cis isomer of [Pt(NH₃)₂Cl₂].
Chlorophyll contains magnesium; haemoglobin contains iron.

12. PYQ-pattern Questions with Solutions

These questions are original and modelled on concepts frequently tested in JEE Main.

Q1Easy

The oxidation state and coordination number of the metal in [Co(en)₃]³⁺ are:

+3, 3
+3, 6
+2, 6
0, 6

Show solution

en is neutral, so Co is +3. Each en is bidentate: 3 × 2 = 6 donor atoms.

Answer: (B)

Q2Numerical

1 mol of CoCl₃·5NH₃ is treated with excess AgNO₃ solution. How many moles of AgCl precipitate?

Show solution

Co³⁺ has coordination number 6: five NH₃ and one Cl⁻ inside, so the formula is [Co(NH₃)₅Cl]Cl₂. Two Cl⁻ ions are outside and precipitate as 2 mol AgCl.

Answer: 2

Q3Medium

Which statement about [NiCl₄]²⁻ and [Ni(CN)₄]²⁻ is correct?

Both are square planar
Both are diamagnetic
[NiCl₄]²⁻ is paramagnetic and tetrahedral; [Ni(CN)₄]²⁻ is diamagnetic and square planar
[NiCl₄]²⁻ is diamagnetic

Show solution

Ni²⁺ is d⁸. Cl⁻ is weak field: no pairing, sp³, tetrahedral, 2 unpaired electrons. CN⁻ is strong field: the electrons pair, one 3d orbital is freed, dsp², square planar, 0 unpaired.

Answer: (C)

Q4Medium

The number of geometrical isomers of [Co(NH₃)₃Cl₃] is:

Show solution

An [MA₃B₃] octahedral complex has two geometrical isomers: fac (three identical ligands on one face) and mer (three in a meridian plane).

Answer: (B)

Q5Hard

The total number of stereoisomers of [Co(en)₂Cl₂]⁺ is:

Show solution

Geometrical: cis and trans. The trans form has a mirror plane (achiral). The cis form is chiral and exists as a pair of enantiomers (d and l). Total = 1 (trans) + 2 (cis) = 3.

Answer: (B)

Q6Hard

The CFSE of [Co(NH₃)₆]³⁺ (low spin, ignore pairing energy) is:

−0.4Δo
−1.2Δo
−2.4Δo
0

Show solution

Co³⁺ is d⁶. Low spin: t₂g⁶ e_g⁰. CFSE = 6 × (−0.4Δo) = −2.4Δo.

Answer: (C)

Q7Numerical

Find the effective atomic number (EAN) of iron in [Fe(CN)₆]⁴⁻ (Z of Fe = 26).

Show solution

Fe is +2 (six CN⁻ give −6, complex charge −4). EAN = 26 − 2 + 2 × 6 = 36 (the configuration of krypton).

Answer: 36

Q8Medium

The IUPAC name of K₃[Fe(C₂O₄)₃] is:

Potassium trioxalatoiron(III)
Potassium trioxalatoferrate(III)
Potassium trioxalatoferrate(II)
Tripotassium oxalatoferrate(III)

Show solution

Three C₂O₄²⁻ give −6; the complex ion is −3, so Fe is +3. An anionic complex uses the Latin-based name "ferrate". Name: potassium trioxalatoferrate(III).

Answer: (B)

13. Quick Revision Summary

Remember before the exam

Primary valency = oxidation state (ionisable); secondary = coordination number.
Naming: ligands alphabetical, then metal (+ "ate" for anionic complexes), oxidation state in Roman numerals.
[MA₄B₂] and [MA₂B₂] (square planar): cis/trans; [MA₃B₃]: fac/mer; cis-[M(AA)₂B₂]: optically active.
Strong field (CN⁻, CO): inner orbital, low spin; weak field (F⁻, Cl⁻): outer orbital, high spin.
Octahedral: t₂g at −0.4Δo, e_g at +0.6Δo; Δt = (4/9)Δo.
Synergic bonding in carbonyls; EAN = Z − O.S. + 2 × C.N.

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