Learning Objectives
- Understand Werner's theory and nomenclature of coordination compounds
- Learn about coordination number, ligands, and isomerism
- Study Crystal Field Theory (CFT) and its applications
- Understand bonding, colour, and magnetic properties
- Know the applications of coordination compounds
Key Concepts
Basic Terminology
Central metal atom/ion: The atom to which ligands are bonded.
Ligand: Ion or molecule that donates electron pairs to the central metal. Types by denticity:
- Monodentate: One donor atom (Cl⁻, NH₃, H₂O, CN⁻, CO).
- Bidentate: Two donor atoms (en = ethylenediamine, ox²⁻ = oxalate, bipy).
- Polydentate: Multiple donor atoms (EDTA⁴⁻: hexadentate).
- Ambidentate: Can bind through different atoms (SCN⁻/NCS⁻, NO₂⁻/ONO⁻).
Coordination number (CN): Number of ligand donor atoms bonded to central metal. Common: 4 (tetrahedral/square planar), 6 (octahedral).
Chelation: Bidentate/polydentate ligands form ring structures (chelate rings). More stable than monodentate complexes (chelate effect).
IUPAC Nomenclature
Ligands named first (alphabetical), then metal with oxidation state in Roman numerals.
Anionic ligands end in -o (chlorido, cyanido, hydroxido). Neutral: aqua (H₂O), ammine (NH₃), carbonyl (CO).
Anionic complex: metal name ends in -ate. [Fe(CN)₆]⁴⁻ = hexacyanidoferrate(II).
Cationic/neutral complex: metal name unchanged. [Co(NH₃)₆]³⁺ = hexaamminecobalt(III).
Isomerism
Structural:
- Ionisation isomerism: [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br
- Linkage isomerism: [Co(NH₃)₅NO₂]²⁺ (nitro) vs [Co(NH₃)₅ONO]²⁺ (nitrito)
- Coordination isomerism: [Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆]
Stereoisomerism:
- Geometrical (cis-trans): In square planar MA₂B₂ and octahedral MA₄B₂.
- Optical isomerism: Non-superimposable mirror images. Found in octahedral [Co(en)₃]³⁺.
Crystal Field Theory (CFT)
Ligands are treated as point charges creating an electrostatic field that splits d-orbitals.
Octahedral splitting: d-orbitals split into t₂g (lower, 3 orbitals) and eg (higher, 2 orbitals). Splitting energy = Δ₀ = 10Dq.
Tetrahedral splitting: Splits into e (lower, 2) and t₂ (higher, 3). Δt ≈ 4/9 Δ₀.
Crystal Field Stabilisation Energy (CFSE): Energy gained due to preferential filling of lower orbitals.
Spectrochemical series (increasing Δ): I⁻ < Br⁻ < S²⁻ < Cl⁻ < N₃⁻ < F⁻ < OH⁻ < ox²⁻ < H₂O < NCS⁻ < CH₃CN < py < NH₃ < en < bipy < phen < NO₂⁻ < PPh₃ < CN⁻ < CO < NO⁺
Strong field ligands: CN⁻, CO, NO₂⁻ → large Δ → low spin (paired electrons).
Weak field ligands: I⁻, Br⁻, Cl⁻, F⁻, H₂O → small Δ → high spin (maximum unpaired electrons).
Colour and Magnetism
Colour: Due to d-d transitions. The absorbed wavelength corresponds to Δ. Complementary colour is observed.
Magnetism: Paramagnetic if unpaired electrons present. μ = √(n(n+2)) BM. Strong field → low spin → fewer unpaired electrons → lower μ.
Applications
Extraction (Au with CN⁻), photography (AgBr), electroplating, medicine (cisplatin for cancer), biological (haemoglobin, chlorophyll, vitamin B₁₂), analytical chemistry (EDTA titrations).
Summary
Coordination compounds consist of a central metal ion surrounded by ligands. IUPAC nomenclature follows specific rules. Isomerism (structural and stereo) creates diversity. Crystal Field Theory explains colour and magnetism through d-orbital splitting. The spectrochemical series ranks ligands by splitting ability. These compounds have wide applications in industry, medicine, and biology.
Important Terms
- Ligand: Electron pair donor that bonds to central metal
- Coordination Number: Number of donor atoms bonded to metal
- Chelation: Ring formation by polydentate ligands
- Crystal Field Splitting: Splitting of d-orbitals in ligand field
- Spectrochemical Series: Ligands ranked by splitting ability
- CFSE: Energy stabilisation from preferential orbital filling
Quick Revision
- CN 4: tetrahedral/square planar; CN 6: octahedral
- Octahedral: t₂g (3) + eg (2); Tetrahedral: e (2) + t₂ (3)
- Strong field (CN⁻, CO): low spin; Weak field (Cl⁻, H₂O): high spin
- Δt ≈ 4/9 Δ₀; colour = complementary of absorbed
- EDTA: hexadentate; en: bidentate
- Cisplatin [PtCl₂(NH₃)₂]: anti-cancer drug