Why do coordination compounds show colour?

Transition metal complexes are coloured because ligands cause d-orbital splitting. When white light hits the complex, photons matching the crystal field splitting energy (Δ) are absorbed, promoting d-electrons to higher d-orbitals. The remaining transmitted/reflected light is the complementary colour we see. For example, [Cu(H₂O)₆]²⁺ absorbs red light and appears blue. Complexes with no d-electrons (Sc³⁺) or filled d-orbitals (Zn²⁺) are colourless.

What is the chelate effect and why are chelate complexes more stable?

The chelate effect is the enhanced stability of complexes formed by polydentate (chelating) ligands compared to equivalent monodentate ligands. Reason: Thermodynamic — the reaction replacing monodentate ligands with chelate ligands is driven by a large positive entropy change (ΔS). Multiple monodentate ligands are released, increasing disorder. Also, if one donor atom of a chelate detaches, it remains geometrically close and easily re-attaches. EDTA (hexadentate) forms extremely stable chelates with Ca²⁺, used to treat heavy metal poisoning.

Ligands, Complex Ions, Bonding, and Nomenclature

Coordination Compounds • Class 12 Chemistry • NCERT • CBSE

Coordination number = number of ligand donor atoms bonded to central metal. EAN rule: Effective atomic number = atomic number of metal − oxidation state + 2×(coordination number). Crystal Field Theory explains colour and magnetism of complexes.

Key Formulas

Oxidation state: charge of complex − sum of ligand charges
EAN = Z − oxidation state + 2(CN)
Spectrochemical series: I⁻ < Br⁻ < Cl⁻ < H₂O < NH₃ < en < CN⁻

Frequently Asked Questions

Why do coordination compounds show colour?: Transition metal complexes are coloured because ligands cause d-orbital splitting. When white light hits the complex, photons matching the crystal field splitting energy (Δ) are absorbed, promoting d-electrons to higher d-orbitals. The remaining transmitted/reflected light is the complementary colour we see. For example, [Cu(H₂O)₆]²⁺ absorbs red light and appears blue. Complexes with no d-electrons (Sc³⁺) or filled d-orbitals (Zn²⁺) are colourless.
What is the chelate effect and why are chelate complexes more stable?: The chelate effect is the enhanced stability of complexes formed by polydentate (chelating) ligands compared to equivalent monodentate ligands. Reason: Thermodynamic — the reaction replacing monodentate ligands with chelate ligands is driven by a large positive entropy change (ΔS). Multiple monodentate ligands are released, increasing disorder. Also, if one donor atom of a chelate detaches, it remains geometrically close and easily re-attaches. EDTA (hexadentate) forms extremely stable chelates with Ca²⁺, used to treat heavy metal poisoning.

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