JEE Chem — Coordination Compounds Deep Dive

Chapter Overview & Weightage

Coordination Compounds is one of the most heavily tested chapters in JEE Main Inorganic Chemistry — typically 2 to 3 questions per paper worth 8 to 12 marks. JEE Advanced also pulls 1 to 2 questions, often demanding deeper application of crystal field theory or isomerism.

The chapter has four big themes: nomenclature, isomerism, crystal field theory (CFT), and magnetic + colour properties.

Year	JEE Main Weightage	JEE Advanced
2024	12 marks	8 marks
2023	8 marks	4 marks
2022	12 marks	8 marks
2021	8 marks	4 marks
2020	12 marks	8 marks

Key Concepts You Must Know

A coordination compound has a central metal ion surrounded by ligands. The metal and ligands together form the coordination sphere.
Coordination number is the number of ligand donor atoms attached to the metal.
Ligands are classified by denticity (number of binding sites): monodentate (Cl⁻, NH₃), bidentate (en, ox²⁻), polydentate (EDTA⁴⁻).
Isomerism: structural (linkage, ionisation, coordination, hydrate) and stereo (geometrical, optical).
Crystal Field Theory explains splitting of d-orbitals into $t_{2g}$ and $e_g$ in octahedral fields, and into $e$ and $t_2$ in tetrahedral.
Spectrochemical series ranks ligands by crystal field strength: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO.

Important Concepts and Rules

Octahedral: d-orbitals split into lower $t_{2g}$ (3) and upper $e_g$ (2), separated by $\Delta_o$ .

Tetrahedral: $e$ (lower, 2) and $t_2$ (upper, 3), with $\Delta_t = \frac{4}{9}\Delta_o$ .

For octahedral d⁴–d⁷ configurations:

Strong-field ligand (large $\Delta_o$ > pairing energy P): low-spin, electrons pair in $t_{2g}$ .
Weak-field ligand (small $\Delta_o$ < P): high-spin, electrons spread before pairing.

Tetrahedral complexes are almost always high-spin (small $\Delta_t$ ).

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

where $n$ is the number of unpaired electrons.

$\text{EAN} = Z - \text{oxidation state} + 2 \times \text{coordination number}$

Stable complexes often satisfy EAN = noble gas configuration (e.g., 36 for Kr).

Solved Previous Year Questions

PYQ 1 (JEE Main 2024, 4 marks)

Find the spin-only magnetic moment of $[CoF_6]^{3-}$ .

Solution:

Co³⁺ has electron configuration $[Ar]\,3d^6$ . F⁻ is a weak-field ligand (low in spectrochemical series), so the complex is high-spin.

For octahedral high-spin d⁶: 4 unpaired electrons in $t_{2g}^4 e_g^2$ .

$\mu = \sqrt{4 \times 6} = \sqrt{24} \approx 4.9\,\text{BM}$

PYQ 2 (JEE Main 2023, 4 marks)

Number of geometrical isomers of $[Co(NH_3)_3 Cl_3]$ is:

Solution:

Octahedral $MA_3B_3$ has two geometrical isomers: facial (fac) and meridional (mer).

In fac, the three NH₃ occupy one face. In mer, they form a meridian (great circle).

Answer: 2.

PYQ 3 (JEE Advanced 2022)

Among the following, the complex that exhibits optical isomerism is:

(a) $[Pt(NH_3)_2 Cl_2]$ (square planar) (b) $[Co(en)_3]^{3+}$ (octahedral) (c) $[Ni(CO)_4]$ (tetrahedral) (d) $[Pt(NH_3)_4]^{2+}$ (square planar)

Solution:

Optical isomerism requires absence of an internal mirror plane.

(a) Square planar with two pairs of identical ligands — has mirror plane. (b) Octahedral with three bidentate ligands — chiral! Two non-superimposable mirror images (Δ and Λ). (c) Tetrahedral with four identical ligands — has mirror planes. (d) Has a mirror plane.

Answer: (b) $[Co(en)_3]^{3+}$ .

Difficulty Distribution

Easy (nomenclature, EAN, identifying ligand type): 4 marks per Main paper
Medium (geometric isomerism, magnetic moment, CFT splitting diagrams): 8 marks
Hard (optical isomerism, mixed CFT + spectrochemistry): 4 marks in Advanced

Expert Strategy

For magnetic moment questions, the workflow is fixed: 1) find oxidation state of metal, 2) find d-electron count, 3) identify field strength of ligand (strong/weak), 4) draw splitting diagram, 5) count unpaired, 6) plug into $\mu = \sqrt{n(n+2)}$ .

Memorise the spectrochemical series for ligands. Common strong-field: CN⁻, CO, NH₃, en. Common weak-field: F⁻, Cl⁻, Br⁻, I⁻, H₂O (borderline). $H_2O$ is the borderline case worth knowing well.

For octahedral complexes with three bidentate ligands (like $[M(en)_3]$ or $[M(ox)_3]$ ), automatically expect optical isomerism. This is a JEE Advanced favorite.

Common Traps

Trap 1 — Wrong oxidation state. Always start by computing the metal’s oxidation state correctly. Mistakes here cascade into wrong d-electron count, wrong magnetic moment, wrong everything.

Trap 2 — Assuming all octahedral d⁴-d⁷ are low-spin. Field strength of the ligand decides. F⁻ gives high-spin; CN⁻ gives low-spin.

Trap 3 — Confusing geometrical and optical isomerism. Cis-trans is geometrical (different connectivity in 3D). Optical isomerism requires non-superimposability with mirror image.

Trap 4 — Mis-identifying tetrahedral splitting. In tetrahedral, $t_2$ is upper and $e$ is lower — opposite of octahedral. Easy to swap under exam pressure.

Trap 5 — Forgetting that $\Delta_t < \Delta_o$ . Tetrahedral splitting is much smaller, so almost all tetrahedral complexes are high-spin.