Chemistry Inorganic - How to Solve: Coordination Compounds (IUPAC, Hybridisation, Isomerism, CFSE, Magnetic Moment, Organometallic) – IIT JEE Guide

How to Solve: Coordination Compounds (IUPAC, Hybridisation, Isomerism, CFSE, Magnetic Moment, Organometallic) – IIT JEE Guide

Introduction

Mastering coordination compounds unlocks 10-12 marks in IIT JEE (Main + Advanced)—enough to push you from a 90 to a 99+ percentile. These compounds are the backbone of catalysis, medicine (cisplatin), and pigments (Prussian blue), and JEE loves testing them in multi-concept problems (e.g., linking IUPAC naming to magnetic moment or CFSE).

WHAT YOU NEED TO KNOW FIRST

1. Valency vs. Oxidation State – Know how to calculate oxidation states of metals in complexes.
2. VSEPR Theory – Understand how electron pairs arrange around a central atom.
3. Electronic Configuration of d-Block Elements – Recall filling order (Aufbau principle) and exceptions (Cr, Cu).

KEY TERMS & FORMULAS

1. IUPAC Nomenclature Rules

• Order of naming: Cation → Anion (alphabetical order of ligands, then metal).
• Ligand prefixes:
• 1 = mono- (omitted for first ligand)
• 2 = di-
• 3 = tri-
• 4 = tetra-
• 5 = penta-
• 6 = hexa-
• Anionic ligands end in "-o" (e.g., Cl⁻ = chloro, CN⁻ = cyano).
• Neutral ligands keep their name (e.g., NH₃ = ammine, H₂O = aqua).
• Metal oxidation state: Roman numeral in parentheses (e.g., Fe(II)).
• Anionic complex: Metal name ends in "-ate" (e.g., [Fe(CN)₆]⁴⁻ = hexacyanoferrate(II)).

MEMORISE THIS: Common ligand names (e.g., en = ethylenediamine, ox = oxalate).

2. Hybridisation & Geometry

MEMORISE THIS: For d²sp³, inner d-orbitals (3d) are used → strong field ligands (e.g., CN⁻, CO). For sp³d², outer d-orbitals (4d) are used → weak field ligands (e.g., F⁻, H₂O).

3. Isomerism in Coordination Compounds

A. Structural Isomerism

1. Ionisation Isomerism: Exchange of ligand with counter ion (e.g., [Co(NH₃)₅Br]SO₄ vs. [Co(NH₃)₅SO₄]Br).
2. Linkage Isomerism: Ambidentate ligands (e.g., NO₂⁻ → nitro (-NO₂) or nitrito (-ONO)).
3. Coordination Isomerism: Exchange of ligands between cationic and anionic complexes (e.g., [Co(NH₃)₆][Cr(CN)₆] vs. [Cr(NH₃)₆][Co(CN)₆]).
4. Solvate Isomerism: Water as ligand vs. water of crystallisation (e.g., [Cr(H₂O)₆]Cl₃ vs. [Cr(H₂O)₅Cl]Cl₂·H₂O).

B. Stereoisomerism

1. Geometrical Isomerism (cis-trans):
2. Square planar (MA₂B₂): cis (90°) vs. trans (180°).
3. Octahedral (MA₄B₂): cis (90°) vs. trans (180°).
4. Optical Isomerism: Non-superimposable mirror images (e.g., [Co(en)₃]³⁺).

MEMORISE THIS: Octahedral complexes with bidentate ligands (e.g., en, ox) show optical isomerism.

4. Crystal Field Splitting Energy (CFSE)

• Octahedral field: d-orbitals split into t₂g (dxy, dyz, dzx) and eg (dz², dx²-y²).
• Δ₀ (CFSE) = Energy difference between t₂g and eg.
• Strong field ligands (CN⁻, CO) → large Δ₀ → low spin.
• Weak field ligands (I⁻, Br⁻) → small Δ₀ → high spin.
• Tetrahedral field: d-orbitals split into e (dz², dx²-y²) and t₂ (dxy, dyz, dzx).
• Δₜ = (4/9)Δ₀ (always high spin due to smaller splitting).

Formula: - Octahedral CFSE = (-0.4 × n(t₂g) + 0.6 × n(eg)) × Δ₀ + P (pairing energy, if applicable). - Tetrahedral CFSE = (-0.6 × n(e) + 0.4 × n(t₂)) × Δₜ.

MEMORISE THIS: Spectrochemical series (strong to weak field): CO > CN⁻ > NO₂⁻ > en > NH₃ > H₂O > OH⁻ > F⁻ > Cl⁻ > Br⁻ > I⁻

5. Magnetic Moment (μ)

• Spin-only formula: μ = √[n(n+2)] BM (where n = number of unpaired electrons).
• For dⁿ configurations:
• High spin: Maximum unpaired electrons.
• Low spin: Minimum unpaired electrons (strong field ligands).

MEMORISE THIS: μ (BM) for common dⁿ configurations: | dⁿ | High Spin (μ) | Low Spin (μ) | |--------|-------------------|------------------| | d¹ | 1.73 | 1.73 | | d² | 2.83 | 2.83 | | d³ | 3.87 | 3.87 | | d⁴ | 4.90 | 2.83 | | d⁵ | 5.92 | 1.73 | | d⁶ | 4.90 | 0 (diamagnetic) | | d⁷ | 3.87 | 1.73 | | d⁸ | 2.83 | 2.83 | | d⁹ | 1.73 | 1.73 |

6. Organometallic Compounds

• Definition: Compounds with metal-carbon bonds (e.g., [Fe(CO)₅], ferrocene).
• 18-Electron Rule: Stable organometallics have 18 valence electrons (like noble gas configuration).
• Counting electrons:
• Metal: Group number (e.g., Fe = 8, Ni = 10).
• Ligands:
  • CO, PR₃ = 2e⁻ each.
  • Alkyl (CH₃⁻), Halides (Cl⁻) = 2e⁻ each.
  • Cyclopentadienyl (Cp⁻) = 6e⁻.
  • NO (linear) = 3e⁻, NO (bent) = 1e⁻.

MEMORISE THIS: Ferrocene ([Fe(Cp)₂]) = 18e⁻ (Fe = 8, 2 × Cp = 10 → 8 + 10 = 18).

STEP-BY-STEP METHOD

Step 1: Identify the Central Metal & Its Oxidation State

• Find the charge on the complex (e.g., [Co(NH₃)₅Cl]Cl₂ → overall charge = +2).
• Assign charges to ligands (NH₃ = 0, Cl⁻ = -1).
• Solve for metal oxidation state: Metal charge + Σ(Ligand charges) = Complex charge.

Step 2: Name the Complex (IUPAC)

1. Cation first, then anion.
2. Ligands in alphabetical order (ignore prefixes like di-, tri-).
3. Metal name:
4. If complex is cationic, use metal name (e.g., cobalt).
5. If complex is anionic, add "-ate" (e.g., ferrate, cuprate).
6. Oxidation state in Roman numerals in parentheses.

Step 3: Determine Hybridisation & Geometry

1. Count coordination number (number of ligand bonds to metal).
2. Check ligand field strength (strong field → low spin → inner orbital (d²sp³)).
3. Assign hybridisation based on coordination number and ligand type.

Step 4: Identify Isomerism (If Applicable)

1. Structural isomerism: Check for ionisation, linkage, coordination, or solvate isomers.
2. Stereoisomerism:
3. Geometrical: Check for cis/trans in square planar or octahedral.
4. Optical: Check for non-superimposable mirror images (common in [M(AA)₃] or [M(AA)₂X₂]).

Step 5: Calculate CFSE

1. Determine dⁿ configuration (e.g., Co³⁺ = d⁶).
2. Check ligand field strength (strong/weak) → high/low spin.
3. Count electrons in t₂g and eg (octahedral) or e and t₂ (tetrahedral).
4. Apply CFSE formula:
5. Octahedral: CFSE = (-0.4 × t₂g + 0.6 × eg) × Δ₀.
6. Tetrahedral: CFSE = (-0.6 × e + 0.4 × t₂) × Δₜ.

Step 6: Calculate Magnetic Moment

1. Count unpaired electrons (n) from dⁿ configuration.
2. Apply spin-only formula: μ = √[n(n+2)] BM.

Step 7: Check 18-Electron Rule (For Organometallics)

1. Count metal valence electrons (group number).
2. Add ligand electrons (2e⁻ per CO, 6e⁻ per Cp⁻, etc.).
3. Total should be 18 for stability.

WORKED EXAMPLES

Example 1 – Basic: IUPAC Naming & Hybridisation

Question: Name [Co(NH₃)₅Cl]Cl₂ and determine its hybridisation.

Step-by-Step Solution:
1. Oxidation state of Co: - Complex charge = +2 (since 2 Cl⁻ outside). - NH₃ = 0, Cl⁻ (inside) = -1. - Co + 5(0) + (-1) = +2 → Co = +3.
2. IUPAC Name: - Cation: [Co(NH₃)₅Cl]²⁺ → Pentaamminechlorocobalt(III). - Anion: Cl⁻ → chloride. - Full name: Pentaamminechlorocobalt(III) chloride.
3. Hybridisation: - Coordination number = 6 (5 NH₃ + 1 Cl). - NH₃ is a strong field ligand → low spin d⁶. - Inner orbital hybridisation: d²sp³ (octahedral).

What we did and why: - We calculated oxidation state to name the complex correctly. - Strong field ligands force d²sp³ hybridisation (inner orbital).

Example 2 – Medium: CFSE & Magnetic Moment

Question: For [Fe(CN)₆]⁴⁻, calculate: (a) CFSE (b) Magnetic moment

Step-by-Step Solution:
1. Oxidation state of Fe: - Complex charge = -4. - CN⁻ = -1 each → 6 × (-1) = -6. - Fe + (-6) = -4 → Fe = +2.
2. dⁿ configuration: Fe²⁺ = d⁶.
3. Ligand field strength: CN⁻ is strong field → low spin.
4. CFSE: - Low spin d⁶: t₂g⁶ eg⁰. - CFSE = (-0.4 × 6 + 0.6 × 0) × Δ₀ = -2.4 Δ₀.
5. Magnetic moment: - Low spin d⁶ → 0 unpaired electrons → μ = 0 BM (diamagnetic).

What we did and why: - Strong field ligands (CN⁻) cause low spin configurations. - CFSE is negative (stabilising), and no unpaired electrons → diamagnetic.

Example 3 – Exam-Style: Integrated Problem

Question: A complex has the formula [Cr(H₂O)₄Cl₂]Cl·2H₂O. (a) Write its IUPAC name. (b) What type of isomerism does it show? (c) Predict its magnetic moment (μ). (d) If H₂O is replaced by NH₃, how does CFSE change?

Step-by-Step Solution:
1. Oxidation state of Cr: - Complex charge = +1 (since 1 Cl⁻ outside). - H₂O = 0, Cl⁻ (inside) = -1 each → 2 × (-1) = -2. - Cr + 4(0) + (-2) = +1 → Cr = +3.
2. IUPAC Name: - Cation: [Cr(H₂O)₄Cl₂]⁺ → Tetraaquadichlorochromium(III). - Anion: Cl⁻ → chloride. - Full name: Tetraaquadichlorochromium(III) chloride dihydrate.
3. Isomerism: - Geometrical isomerism (cis/trans for MA₄B₂). - Solvate isomerism (H₂O as ligand vs. water of crystallisation).
4. Magnetic moment: - Cr³⁺ = d³ → 3 unpaired electrons. - μ = √[3(3+2)] = √15 ≈ 3.87 BM.
5. CFSE change (H₂O → NH₃): - H₂O is weak field → high spin d³ → CFSE = -1.2 Δ₀. - NH₃ is stronger field → still high spin d³ (but Δ₀ increases). - CFSE becomes more negative (more stable).

What we did and why: - We integrated naming, isomerism, magnetic moment, and CFSE—just like JEE. - NH₃ increases Δ₀, making CFSE more negative (greater stabilisation).

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP (Night Before Exam)

"Listen up—this is your 60-second cheat sheet for coordination compounds:
1. Naming: Cation first, ligands alphabetical, metal ends in "-ate" if anionic. Oxidation state in Roman numerals.
2. Hybridisation: Coordination number 6 → octahedral (d²sp³ for strong field, sp³d² for weak). 4 → square planar (dsp²) or tetrahedral (sp³).
3. Isomerism: Check for geometrical (cis/trans), optical (mirror images), and structural (ionisation, linkage).
4. CFSE: Strong field → low spin → more negative CFSE. Weak field → high spin → less negative CFSE.
5. Magnetic moment: Count unpaired electrons → μ = √[n(n+2)] BM.
6. Organometallics: 18-electron rule—count metal + ligand electrons. Now go crush that exam!