NEET Weightage: 3-4%

NEET Chemistry — Coordination Compounds Complete Chapter Guide

Coordination Compounds for NEET. Chapter weightage, key concepts, solved PYQs, preparation strategy.

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Tags Coordination Compounds

Chapter Overview & Weightage

Coordination Compounds covers Werner’s theory, IUPAC naming, types of isomerism, Crystal Field Theory (CFT), and Valence Bond Theory (VBT). NEET asks 1-2 questions, primarily on naming and CFT.

This chapter carries 3-4% weightage in NEET with 1-2 questions. IUPAC naming, isomerism, and CFT (crystal field splitting, magnetic properties) are the most tested areas.

Key Concepts You Must Know

Tier 1 (Core)

  • Terminology: central metal, ligands (mono/bi/polydentate), coordination number, coordination sphere
  • IUPAC naming rules: cation first, then anion; ligands alphabetically; metal oxidation state in Roman numerals
  • CFT: crystal field splitting in octahedral (Δo\Delta_o) and tetrahedral (Δt\Delta_t) fields
  • Spectrochemical series: I−^- < Br−^- < Cl−^- < F−^- < OH−^- < H2_2O < NH3_3 < en < CN−^- < CO (increasing field strength)
  • Magnetic properties: paramagnetic (unpaired electrons) vs diamagnetic (no unpaired electrons)

Tier 2 (Frequently tested)

  • Isomerism: geometrical (cis-trans in square planar and octahedral), optical, ionisation, linkage, coordination
  • Common ligands: monodentate (Cl−^-, NH3_3, H2_2O, CN−^-), bidentate (en, ox2−^{2-}), polydentate (EDTA — hexadentate)
  • VBT: hybridisation and geometry prediction

Important Formulas

Octahedral: dd orbitals split into t2gt_{2g} (lower, 3 orbitals) and ege_g (higher, 2 orbitals). Splitting energy = Δo\Delta_o.

Tetrahedral: Split into ee (lower, 2 orbitals) and t2t_2 (higher, 3 orbitals). Δt=49Δo\Delta_t = \frac{4}{9}\Delta_o.

Strong field ligand (CN−^-, CO): large Δo\Delta_o → electrons pair up → low spin → fewer unpaired electrons → diamagnetic or weakly paramagnetic.

Weak field ligand (Cl−^-, F−^-): small Δo\Delta_o → electrons avoid pairing → high spin → more unpaired electrons → paramagnetic.
  1. Cation named first, then anion
  2. Within coordination sphere: ligands listed alphabetically
  3. Anionic ligands end in -o (chlorido, cyano, hydroxido), neutral ligands keep name (aqua, ammine)
  4. Metal name: unchanged in cation complex, ends in -ate in anion complex
  5. Oxidation state in Roman numerals in parentheses

Example: [Co(NH3)4Cl2]Cl[Co(NH_3)_4 Cl_2]Cl → Tetraamminedichloridocobalt(III) chloride

For NEET, know that EDTA is hexadentate (coordination number = 6 for a single EDTA molecule binding to one metal). Also, “en” (ethylenediamine) is bidentate. These specific ligand denticities are directly tested.

Solved Previous Year Questions

PYQ 1 — NEET 2024

Problem: The coordination number and oxidation state of Co in [Co(NH3)5Cl]Cl2[Co(NH_3)_5 Cl]Cl_2 are:

Solution:

Coordination number = 5 (NH3_3) + 1 (Cl inside sphere) = 6

Charge balance: x+0(5)+(−1)+(−1)(2)=0x + 0(5) + (-1) + (-1)(2) = 0 → x−3=0x - 3 = 0 → oxidation state = +3

Answer: CN = 6, OS = +3

PYQ 2 — NEET 2023

Problem: Which of the following is a strong field ligand?

(A) Cl−^- (B) F−^- (C) CN−^- (D) I−^-

Solution:

From the spectrochemical series, CN−^- is a strong field ligand (near the top). It causes large crystal field splitting, leading to low-spin complexes. Halogens (I−^-, Br−^-, Cl−^-, F−^-) are all weak field ligands.

Answer: (C) CN−^-

PYQ 3 — NEET 2022

Problem: How many geometrical isomers are possible for [Pt(NH3)2Cl2][Pt(NH_3)_2 Cl_2] (square planar)?

Solution:

[Pt(NH3)2Cl2][Pt(NH_3)_2Cl_2] in square planar geometry: 2 geometrical isomers — cis (same ligands adjacent) and trans (same ligands opposite).

Answer: 2 (cis and trans)

Expert Strategy

Day 1: IUPAC naming — practice naming 10-15 complexes. The naming rules are mechanical once you learn the pattern.

Day 2: CFT and magnetic properties. Understand strong vs weak field ligands and how they determine spin state. The spectrochemical series needs to be memorised.

Day 3: Isomerism — geometrical isomers in square planar (MA2B2MA_2B_2, MABCDMABCD) and octahedral complexes. Solve PYQs.

Common Traps

Trap 1 — Ligands INSIDE the coordination sphere count for coordination number; ions OUTSIDE don’t. In [Co(NH3)5Cl]Cl2[Co(NH_3)_5Cl]Cl_2, only the 5 NH3_3 and 1 Cl inside the brackets count. The 2 Cl outside are counter-ions.

Trap 2 — Strong field ligand = low spin, NOT high spin. Strong field → large splitting → electrons pair up → fewer unpaired electrons → low spin. Weak field → small splitting → electrons spread out → high spin → more unpaired electrons.

Trap 3 — EDTA is hexadentate, not tetradentate. EDTA has 4 carboxylate oxygen donors and 2 nitrogen donors = 6 donor atoms total. It wraps around the metal ion, occupying all 6 coordination sites.