Apex Class Logo
APEX CLASS ESTD 2018

IITJEE CHEMISTRY: Crystal Field Splitting (Δo vs Δt), VBT Hybridization & Isomerism

By

Ashish Kumar Bahekar

Published on

in

APEX CLASS • IITJEE Coordination Chemistry Lab

Crystal Field Splitting (Δo vs Δt), VBT Hybridization & Isomerism

Master Octahedral vs. Tetrahedral Splitting, High-Spin vs. Low-Spin d⁶, [NiCl₄]²⁻ vs. [Ni(CN)₄]²⁻, and Synergic Carbonyls!

🔬 Spin-Only Magnetic Moment μ = √[n(n+2)] BM. Select a CFT Scenario:
Free Ion (5 d) Barycenter (Average Energy = 0) eg (Axial: dx²-y², dz²) +0.6 Δo (+⅗ Δo) ↑ t2g (Non-Axial: dxy, dyz, dxz) -0.4 Δo (-⅖ Δo) ↓ Δo (10 Dq) Octahedral CFSE Formula CFSE = [-0.4 p + 0.6 q] Δo + m P • p = electrons in t2g | q = electrons in eg • Why eg is higher? In an octahedron, 6 ligands approach directly ALONG the X, Y, Z axes! • 4th Electron Choice (d⁴ to d⁷): If Δo < P → High Spin (t2g³ eg¹) 🎯 Barycenter Conservation Rule: 2 × (+0.6 Δo) + 3 × (-0.4 Δo) = 0 (Total energy of all 5 d-orbitals stays conserved)! 1. [CoF₆]³⁻ (Co³⁺ = 3d⁶, Weak Field F⁻: Δo < P) ↑ ↑ eg² (2 Unpaired) ↑↓ ↑ ↑ t2g⁴ (2 Unpaired) • Config: t2g⁴ eg² | CFSE = -0.4 Δo • Outer Orbital: sp³d² (Uses 4d) | n = 4 (μ = 4.90 BM) HIGH-SPIN & PARAMAGNETIC! 2. [Co(NH₃)₆]³⁺ (Co³⁺ = 3d⁶, Strong Field: Δo > P) eg⁰ (Empty! Used for d²sp³) ↑↓ ↑↓ ↑↓ t2g⁶ (All Paired!) • Config: t2g⁶ eg⁰ | CFSE = -2.4 Δo + 2P • Inner Orbital: d²sp³ (Uses 3d) | n = 0 (μ = 0 BM) LOW-SPIN & DIAMAGNETIC! 🏆 d¹ to d³ and d⁸ to d¹⁰ have the EXACT SAME configuration in Weak and Strong fields (e.g., [Ni(H₂O)₆]²⁺ & [Ni(NH₃)₆]²⁺ are BOTH sp³d² with 2 unpaired e⁻)! Tetrahedral Barycenter t₂ (Non-Axial: dxy, dyz, dxz) +0.4 Δt (+⅖ Δt) ↑ e (Axial: dx²-y², dz²) -0.6 Δt (-⅗ Δt) ↓ Tetrahedral Splitting Rules Δt = (4 / 9) Δo ≈ 0.44 Δo • CFSEtet = [-0.6 ne + 0.4 nt2] Δt • Why NO Low-Spin Tetrahedral Complexes? Because only 4 ligands approach off-axis, Δt is always smaller than pairing energy (Δt < P)! • Note: Subscript ‘g’ (gerade) is dropped (No inversion center!) 🔺 Notice why ‘g’ is absent in Tetrahedral (e & t₂ instead of eg & t2g): A tetrahedron has NO center of symmetry (inversion center)! 🌈 NCERT Spectrochemical Series (Weak Field Halides → Water → Strong Field N, C Donors) I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < edta⁴⁻ < NH₃ < en < CN⁻ < CO 🧠 Donor Atom Order: Halogen Donors (X⁻) < Oxygen Donors (O) < Nitrogen Donors (N) < Carbon Donors (CN⁻, CO)! 1. Wavelength Absorbed vs. Ligand Strength Δo = h c / λabsorbed ⇒ λabs ∝ 1 / Δo! Stronger Ligand (CN⁻ > NH₃ > H₂O) = SHORTER λabs! 2. Colorless vs. Colored Complexes • d⁰ (Sc³⁺, Ti⁴⁺) & d¹⁰ (Cu⁺, Zn²⁺): COLORLESS (No d-d!) • Exception: MnO₄⁻ (d⁰ Purple) & CrO₄²⁻ (d⁰ Yellow) via LMCT! 🎨 Complementary Color Wheel (“V-B-G-Y-O-R”): [Ti(H₂O)₆]³⁺ (3d¹) absorbs Blue-Green (498 nm) and appears VIOLET-PURPLE!
Spin-Only Moment (μ)
μ = √[ n(n + 2) ] BM
Quick Trick: n=1→1.73 | n=2→2.83 | n=3→3.87 | n=4→4.90 | n=5→5.92
Octahedral vs Tetrahedral
Δt = (4 / 9) Δo
Tetrahedral is ALWAYS High-Spin!
Absorbed Wavelength
λabs ∝ 1 / Ligand Strength
λabs: [Ni(H₂O)₆]²⁺ > [Ni(NH₃)₆]²⁺ > [Ni(en)₃]²⁺
Charge Transfer (LMCT)
MnO₄⁻, CrO₄²⁻, Cr₂O₇²⁻
Intensely colored despite having d⁰!
🧭 Select a High-Frequency JEE VBT, Carbonyl or Stereoisomerism Topic:
1. [NiCl₄]²⁻ (Ni²⁺ = 3d⁸, Weak Cl⁻) 3d⁸: [↑↓][↑↓][↑↓][ ↑ ][ ↑ ] sp³ (Tetrahedral) • 2 Unpaired e⁻ (Paramagnetic) • μ = √[2(2+2)] = 2.83 BM (Cl⁻ cannot pair up 3d⁸ electrons) 2. [Ni(CN)₄]²⁻ (Ni²⁺ = 3d⁸, Strong CN⁻) 3d⁸: [↑↓][↑↓][↑↓][↑↓][ Empty! ] dsp² (Square Planar!) • 0 Unpaired e⁻ (Diamagnetic) • μ = 0 BM (Uses 3dx²-y²) (Strong CN⁻ pairs up 3d⁸ into 4 boxes!) 3. [Ni(CO)₄] (Ni⁰ = 3d⁸ 4s², Strong CO) 3d¹⁰ 4s⁰: [↑↓][↑↓][↑↓][↑↓][↑↓] sp³ (Tetrahedral!) • 0 Unpaired e⁻ (Diamagnetic) • μ = 0 BM (4s² pushed into 3d!) (#1 JEE Trap: sp³ yet Diamagnetic!) 🏆 Memorize This Trio: [NiCl₄]²⁻ = sp³ Para (2 e⁻) | [Ni(CN)₄]²⁻ = dsp² Dia (0 e⁻) | [Ni(CO)₄] = sp³ Dia (0 e⁻)! 1. Co³⁺ (3d⁶) Oxygen-Donor Exception! • Normally H₂O and C₂O₄²⁻ (Oxalate) are Weak Field, • BUT with Co³⁺ (3d⁶), Oxygen donors act as STRONG FIELD! • [Co(H₂O)₆]³⁺ & [Co(C₂O₄)₃]³⁻: Both are LOW-SPIN, Inner Orbital (d²sp³), and DIAMAGNETIC (n = 0, μ = 0 BM)! • Only [CoF₆]³⁻ is High-Spin sp³d² Paramagnetic for Co³⁺! 2. [Cu(NH₃)₄]²⁺ (dsp²) & 4d/5d Metals (Pt²⁺, Pd²⁺) • [Cu(NH₃)₄]²⁺ (Cu²⁺ = 3d⁹): Square Planar (dsp²)! 1 unpaired e⁻ promoted to 4p (n = 1, μ = 1.73 BM) • 4d & 5d Metals (Pt²⁺, Pd²⁺, Au³⁺, Ir³⁺, Rh³⁺): Have huge Δo, so EVERY ligand acts as Strong Field! • [PtCl₄]²⁻ & [PdCl₄]²⁻ (d⁸ with Cl⁻): Both are Square Planar (dsp²) & DIAMAGNETIC! 🚨 JEE Trap: [NiCl₄]²⁻ (3d⁸) is Tetrahedral Paramagnetic (sp³), whereas [PtCl₄]²⁻ and [PdCl₄]²⁻ (5d⁸/4d⁸) are Square Planar Diamagnetic (dsp²)! 1. Synergic Bonding in Metal Carbonyls (M–C≡O) M C O ← σ (LP) π-Back Bond: M(d) → CO(π*) ↘ • M–C Bond Order INCREASES (Shorter M–C bond!) • C–O Bond Order DECREASES (Longer C–O bond!) 2. C–O Bond Length & Stretching Freq (νCO) • More Negative Charge on Metal = Stronger M → π* Back-Bond = Weaker C–O Bond = LONGER C–O Bond Length! • Isoelectronic Carbonyl Series ([V(CO)₆]⁻, [Cr(CO)₆], [Mn(CO)₆]⁺): C–O Length: [V(CO)₆]⁻ > [Cr(CO)₆] > [Mn(CO)₆]⁺ > Free CO • Effective Atomic Number: EAN = Z – O.S. + 2 × (CN) [Fe(CO)₅]: 26 – 0 + 2(5) = 36 (Kr) | [Cr(CO)₆]: 24 + 12 = 36! 🎯 Why electrons enter empty antibonding π* of CO: Filling π* of CO weakens the C≡O triple bond (1.128 Å → ~1.15 Å) and lowers IR stretching νCO! 1. Geometrical Isomer Count Cheat Table • Tetrahedral (sp³): ZERO Geometrical Isomers (All 109.5°)! • Square Planar [MA₂B₂] or [MA₂BC]: 2 (cis & trans) • Square Planar [MABCD]: 3 Geometrical Isomers! 🏆 • Octahedral [MA₄B₂] or [M(en)₂B₂]: 2 (cis & trans) • Octahedral [MA₃B₃]: 2 — Facial (fac) & Meridional (mer)! • Octahedral [MABCDEF]: 15 Geometrical (30 Optical)! 2. Optical Activity (Chiral = No Plane of Symmetry!) • Square Planar Complexes: Almost NEVER optically active (The molecular plane itself acts as a Plane of Symmetry POS!) • [M(en)₃] (e.g., [Co(en)₃]³⁺, [Cr(ox)₃]³⁻): 0 Geometrical, ALWAYS Optically Active (d & l pair)! • [M(en)₂B₂] (e.g., [Co(en)₂Cl₂]⁺): cis-isomer is OPTICALLY ACTIVE; trans-isomer is INACTIVE! 🪞 Total Stereoisomers of [Co(en)₂Cl₂]⁺ = 3 (1 trans + 2 optical enantiomers d-cis and l-cis)!
Inner vs Outer Orbital
d²sp³ (n-1)d vs sp³d² (nd)
d²sp³ is Low-Spin; sp³d² is High-Spin
Ambidentate Ligands
NO₂⁻/ONO⁻ | SCN⁻/NCS⁻
Show Linkage Isomerism!
Chelate Effect Stability
[M(en)₃] > [M(NH₃)₆]
5 & 6-membered rings increase ΔS > 0!
Brown Ring Complex
[Fe(H₂O)₅(NO)]²⁺
Fe is +1 (NO⁺), 3 unpaired e⁻ (3.87 BM)!
🔥 2 Guaranteed JEE Main Coordination Questions:
1. Brown Ring Complex [Fe(H₂O)₅(NO)]SO₄: Here NO exists as NO⁺ (+1), so Iron is in the Fe⁺¹ (3d⁷, 1 electron transferred gives 3 unpaired electrons, μ = 3.87 BM) oxidation state!
2. Werner’s AgNO₃ Precipitation Test: [Co(NH₃)₆]Cl₃ + excess AgNO₃ → 3 moles AgCl↓ (4 ions in solution); [Co(NH₃)₅Cl]Cl₂ → 2 moles AgCl↓; [Co(NH₃)₃Cl₃] → 0 moles AgCl↓ (Non-electrolyte)!

, , ,

Read Next:

Leave a Reply

Apex Class App

Play revision games, read notes, and test your skills anywhere!

🚀

Fresh Updates

Latest notes, lectures & announcements!

View All Posts ➔
⚡

Interactive Zones

🎮
Study Games Arena Play & Revise Concepts
→ 🎯
Study Quiz Arena Test Your Knowledge
→ 🏛️
UPSC Quiz Arena Master Civil Services
→

Discover more from Apex Class

Subscribe now to keep reading and get access to the full archive.

Continue reading

Enable Notifications OK No thanks