JEE Chemistry — Coordination Compounds Chapter Guide

Chapter Overview & Weightage

Coordination compounds is one of those chapters where JEE rewards students who understand the logic, not just memorise rules. You get 1–2 questions in JEE Main almost every year — but they test across different sub-topics, so partial preparation gets punished.

Weightage: 5–7% of Inorganic Chemistry in JEE Main. In JEE Advanced, questions appear as part of larger inorganic questions or paragraph-based sets. NEET also tests this chapter (1–2 questions), usually on IUPAC naming and magnetic properties.

Year	JEE Main Questions	Sub-topic Tested
2024	2	IUPAC naming, CFT splitting
2023	1	Isomerism (geometric)
2022	2	EAN rule, magnetic moment
2021	2	Werner’s theory, VBT
2020	1	IUPAC naming
2019	2	CFT, optical isomerism

The pattern is clear: IUPAC naming and Crystal Field Theory together account for ~60% of all questions from this chapter. Prioritise these two.

Key Concepts You Must Know

Listed in order of exam frequency — spend your time accordingly.

Tier 1 (Most Frequently Tested)

IUPAC naming rules: ligand naming order, oxidation state calculation, use of Greek prefixes
Crystal Field Theory: $\Delta_o$ vs $\Delta_t$ , high spin vs low spin, $d$ electron filling
Magnetic properties: spin-only formula $\mu = \sqrt{n(n+2)}$ BM, predicting from $d^n$ configuration
Geometric isomerism: square planar ( $MA_2B_2$ , $MA_2BC$ ) and octahedral ( $MA_2B_2C_2$ , $MA_3B_3$ )

Tier 2 (Appears Every 2–3 Years)

Werner’s theory: primary vs secondary valency, coordination number
Valence Bond Theory: $sp^3$ , $dsp^2$ , $sp^3d^2$ , $d^2sp^3$ hybridisation
Optical isomerism: conditions for chirality in octahedral complexes, $en$ chelates
Chelate effect and stability of complexes

Tier 3 (Low Frequency but High Difficulty)

Linkage isomerism: ambidentate ligands ( $SCN^-$ , $NO_2^-$ )
Ionisation and solvate isomerism
EAN (Effective Atomic Number) rule
Spectrochemical series: strong field vs weak field ligands

Important Formulas

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

where $n$ = number of unpaired electrons.

When to use: Any question giving you a complex and asking for magnetic moment, or reverse — giving $\mu$ and asking for $d^n$ configuration or nature of ligand (strong/weak field).

For octahedral field:

\text{CFSE} = (-0.4 \times t_{2g} \text{ electrons} + 0.6 \times e_g \text{ electrons}) \times \Delta_o

For tetrahedral field: $\Delta_t = \frac{4}{9}\Delta_o$

When to use: Questions on stability of complexes, colour, and why certain metals prefer octahedral vs tetrahedral geometry.

For $[Co(NH_3)_4Cl_2]^+$ :

x + 4(0) + 2(-1) = +1 \implies x = +3

When to use: Every IUPAC naming question requires oxidation state first — this is step zero.

Square planar $MA_2B_2$ : 2 isomers (cis, trans)
Octahedral $MA_3B_3$ : 2 isomers (fac, mer)
Octahedral $MA_4B_2$ : 2 isomers (cis, trans)
Octahedral $MA_2B_2C_2$ : 5 isomers

When to use: Structure-based questions asking “how many isomers are possible.”

Solved Previous Year Questions

PYQ 1 — IUPAC Naming (JEE Main 2024, Shift 1)

Question: Give the IUPAC name of $[Pt(NH_3)_2Cl(NO_2)]$ .

Solution:

Step 1 — Find oxidation state. Let Pt = $x$ . Ligands: $2(NH_3) = 0$ , $Cl^- = -1$ , $NO_2^- = -1$ . Complex is neutral.

x + 0 - 1 - 1 = 0 \implies x = +2

Step 2 — Name ligands alphabetically (ignore Greek prefixes for alphabetical order).

$Cl^-$ → chlorido
$NH_3$ → ammine (double ‘m’)
$NO_2^-$ → nitrito- $\kappa N$ (bonded through N)

Alphabetical order: ammine, chlorido, nitrito.

Step 3 — Assemble the name.

\text{diamminechloridonitrito-}\kappa N\text{platinum(II)}

Students lose marks by forgetting alphabetical ordering. “Diammine” comes before “chlorido” because we sort by ‘a’ in ammine, not ‘d’ in di. The Greek prefix (di, tri) is ignored for ordering.

PYQ 2 — Magnetic Properties via CFT (JEE Main 2022)

Question: $[CoF_6]^{3-}$ has a magnetic moment of $4.9$ BM. What does this tell us about the nature of $F^-$ as a ligand?

Solution:

$\mu = 4.9$ BM $\approx \sqrt{n(n+2)}$ . For $n = 4$ : $\sqrt{4 \times 6} = \sqrt{24} \approx 4.9$ BM. ✓

So $Co^{3+}$ in this complex has 4 unpaired electrons.

Free $Co^{3+}$ is $d^6$ . In a strong field ligand, all 6 $d$ -electrons pair up in $t_{2g}$ → 0 unpaired (diamagnetic). In a weak field ligand, the $d^6$ configuration fills as: $t_{2g}^4 e_g^2$ → 4 unpaired.

Since we see 4 unpaired electrons, $F^-$ is a weak field ligand — $\Delta_o$ is small, pairing energy wins.

Answer: $F^-$ is a weak field ligand causing high spin configuration.

Memorise the spectrochemical series ends: $I^- < Br^- < Cl^- < F^-$ (weak field) and $CO > CN^- > NO_2^-$ (strong field). Everything else fills in between. You don’t need the full series — just strong and weak field anchors.

PYQ 3 — Geometric Isomerism (JEE Main 2023)

Question: How many geometric isomers are possible for $[Cr(NH_3)_3Cl_3]$ ?

Solution:

This is an octahedral complex of type $MA_3B_3$ .

We need to place 3 $NH_3$ and 3 $Cl$ in the 6 positions around Cr.

Isomer 1 — fac (facial): All three identical ligands occupy one face of the octahedron. Each $NH_3$ is trans to a $Cl$ .

Isomer 2 — mer (meridional): Three identical ligands lie in a plane through the metal. One $NH_3$ is trans to another $NH_3$ .

These are the only two arrangements. Answer: 2 geometric isomers.

“fac” and “mer” prefixes are specific to $MA_3B_3$ octahedral complexes. JEE has asked this distinction directly. Fac = face, all three same ligands on one triangular face. Mer = meridian, three same ligands along a plane.

Difficulty Distribution

Difficulty	% of Questions	Sub-topics
Easy	35%	IUPAC naming (straightforward), magnetic moment calculation
Medium	45%	Geometric isomer count, VBT hybridisation, CFT splitting
Hard	20%	Optical isomerism in octahedral complexes, linkage/ionisation isomerism, combined CFT + magnetic moment

JEE Main stays mostly in Easy–Medium territory. JEE Advanced pushes into the Hard category, especially optical isomerism of chelate complexes.

Expert Strategy

Week 1: Lock down IUPAC naming completely. Write out 10 complex formulas, name them, then reverse — see if you can write formulas from names. This chapter rewards practice, not re-reading.

Week 2: Do all CFT questions from the last 10 years of JEE Main. You’ll notice the same 4–5 question types repeating. The examiners recycle heavily here.

When you see a question about a blue or red complex, reach for CFT first. Colour means $d$ - $d$ transitions, which means $\Delta_o$ is involved. This immediately narrows down what the question is testing.

The single highest-ROI move for this chapter: memorise the $d$ -electron configuration of common transition metal ions cold. $Fe^{3+}$ is $d^5$ , $Co^{3+}$ is $d^6$ , $Ni^{2+}$ is $d^8$ , $Cu^{2+}$ is $d^9$ . If you have to calculate these during the exam, you’re wasting time.

For isomerism questions, draw the structure. Every time. Students who try to reason about isomers without drawing almost always get the count wrong by 1.

Common Traps

Trap 1 — Forgetting the charge on the complex in IUPAC naming. For $[Fe(CN)_6]^{4-}$ , students calculate oxidation state as if the complex is neutral. The $4-$ charge means: $x + 6(-1) = -4$ , so $x = +2$ ( $Fe^{II}$ ), not $+6$ .

Trap 2 — Confusing $d^2sp^3$ with $sp^3d^2$ . Both give octahedral geometry, but they’re different. $d^2sp^3$ uses inner $d$ -orbitals (from $n-1$ shell) — seen in strong field, low spin complexes like $[Co(CN)_6]^{3-}$ . $sp^3d^2$ uses outer $d$ -orbitals ( $n$ shell) — seen in weak field, high spin complexes. JEE directly asks which type of hybridisation a given complex shows.

Trap 3 — Optical isomerism for square planar complexes. Square planar complexes are almost never optically active because they have a plane of symmetry (the molecular plane itself). Students apply octahedral rules and get the wrong answer. Only certain chelate complexes of square planar geometry can show optical activity — this is rarely tested and safely ignorable for JEE Main.

Trap 4 — Tetrahedral CFSE vs Octahedral. $\Delta_t = \frac{4}{9}\Delta_o$ , which means tetrahedral complexes almost never show high spin vs low spin splitting in questions — the $\Delta_t$ is so small that pairing energy always wins and the complex is always high spin. If a question involves a tetrahedral complex and magnetic moment, assume high spin.

Quick check for optical isomerism in octahedral: Does the complex have a $C_2$ axis with no mirror plane? If you see $[Co(en)_3]^{3+}$ or $[Co(en)_2Cl_2]^+$ (cis form), those are your standard optically active examples. Commit these to memory and any variation becomes easy to analyse.