Lesson Plan: Coordination Compounds

Inorganic Chemistry (Class 12)

Coordination Compounds

This chapter studies complex salts where a central metal is surrounded by ligands. The logic here is very structured: if you know the Spectrochemical Series (Weak vs Strong ligands), you can predict Hybridization, Geometry, Magnetic property, and Colour.

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⚠️ Prerequisites

d-Block Elements: Electronic configuration of transition metal ions (e.g., $Fe^{2+}$ is $3d^6$, not $3d^4 4s^2$).
Redox: Calculating Oxidation State of the central metal.
Bonding: Concept of Hybridization ($sp^3, dsp^2$).

🧠 Study Approach

Algorithmic Thinking: For VBT questions, follow a fixed recipe: Write Config $\to$ Identify Ligand Strength $\to$ Pair electrons (if strong) $\to$ Hybridize. Don't guess!

Study Sequence

1. Werner's Coordination Theory

Primary Valency: Ionizable, satisfies Oxidation State (dotted lines).
Secondary Valency: Non-ionizable, satisfies Coordination Number (solid lines). determines geometry.

2. Definitions & Nomenclature

Ligands (Uni, Di, Polydentate). Chelation (Ring formation = Stability). Denticity.
IUPAC Naming: Cation first, then Anion. Ligands alphabetic. Metal oxidation state in Roman numerals. (e.g., Potassium hexacyanoferrate(II)).

3. Isomerism in Complexes

Structural: Linkage ($NO_2$ vs $ONO$), Solvate ($H_2O$ inside/outside), Ionization, Coordination.
Stereo: Geometrical (Cis/Trans, Fac/Mer). Optical (Enantiomers in chelated complexes).

4. Valence Bond Theory (CRITICAL)

Determining Hybridization and Shape.
CN=4: $sp^3$ (Tetrahedral) vs $dsp^2$ (Square Planar).
CN=6: $sp^3d^2$ (Outer Orbital) vs $d^2sp^3$ (Inner Orbital).
Magnetic behavior (Paramagnetic vs Diamagnetic).

5. Crystal Field Theory (CFT)

Splitting of d-orbitals into $t_{2g}$ and $e_g$.
Crystal Field Splitting Energy ($\Delta_o$):
Strong Ligand $\to$ Large $\Delta_o$ $\to$ Pairing ($low \ spin$).
Weak Ligand $\to$ Small $\Delta_o$ $\to$ No Pairing ($high \ spin$).
Explanation of Colour (d-d transition).

6. Bonding in Metal Carbonyls

Synergic Bonding: $\sigma$-donation (C to Metal) and $\pi$-back donation (Metal to C). This strengthens the M-C bond and weakens the C-O bond.

7. Stability & Applications

Stability Constant ($\beta$). Factors affecting stability (Chelate effect, Charge on metal).
Applications: Hardness of water (EDTA), Cancer treatment (Cis-platin), Photography.

🎯 How to Practice

VBT Workout: Solve for $[Fe(CN)_6]^{3-}$, $[Fe(CN)_6]^{4-}$, $[CoF_6]^{3-}$, and $[Ni(CN)_4]^{2-}$. Determine their hybridization, geometry, and magnetic nature.

IUPAC Drill: Write names for 10 complexes, covering cationic, anionic, and neutral spheres. Remember: metal in anion gets "-ate" suffix (Ferrate, Argentate).

Isomer Drawing: Draw 'cis' and 'trans' forms of $[Co(en)_2Cl_2]^+$. Draw 'fac' and 'mer' forms of $[Co(NH_3)_3(NO_2)_3]$. Visualizing these is essential.

📝 Quick Revision Notes

Spectrochemical Series (Memory)

$I^- < Br^- < Cl^- < F^- < OH^- < H_2O < NH_3 < en < CN^- < CO$
(Halogens: Weakest | CO/CN: Strongest)

VBT Logic (CN=6)

Strong Ligand (CN, CO, NH3): Force pairing $\to$ Inner $d^2sp^3$ (Low spin).
Weak Ligand (Halogens, H2O): No pairing $\to$ Outer $sp^3d^2$ (High spin).

CFSE Relations

$\Delta_t = \frac{4}{9} \Delta_o$
Octahedral: $t_{2g}$ (lower), $e_g$ (higher).
Tetrahedral: $e$ (lower), $t_2$ (higher).

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