Coordination Compounds
'A coordination compound is a central atom SURROUNDED by a cage of ligands — and the properties are determined by the COMBINATION.'
1. Chapter Overview
Coordination compounds are compounds where a CENTRAL metal atom or ion is surrounded by a number of LIGANDS (ions or molecules that donate electron pairs). Topics include: WERNER'S THEORY (the pioneering theory of coordination), the DEFINITION of coordination compounds (complex ion, central atom, ligands, coordination number, coordination sphere), IUPAC NAMING of coordination compounds, ISOMERISM (structural and stereoisomerism), VALENCE BOND THEORY (VBT) and CRYSTAL FIELD THEORY (CFT), and the APPLICATIONS of coordination compounds.
2. Werner's Theory (1893)
- Alfred Werner proposed that metals have TWO types of valency:
- Primary valency (IONISABLE) : Corresponds to oxidation state. Satisfied by NEGATIVE ions.
- Secondary valency (NON-IONISABLE) : Corresponds to coordination number. Directed in SPACE — determines geometry.
- 'Werner's theory was REMARKABLY prescient — he described coordination geometry decades before X-ray crystallography confirmed it.'
Example: CoCl₃·6NH₃
- Primary valency of Co = +3 (satisfied by 3 Cl⁻).
- Secondary valency of Co = 6 (satisfied by 6 NH₃ ligands).
- Formula: [Co(NH₃)₆]Cl₃ — 3 Cl⁻ are IONISABLE, 6 NH₃ are in the coordination sphere.
3. Key Terminology
| Term | Definition | Example |
|---|---|---|
| Coordination entity | The complex ion (central atom + ligands) | [Fe(CN)₆]³⁻ |
| Central atom/ion | The metal at the centre | Fe³⁺ |
| Ligand | Ion/molecule that DONATES electron pair to central atom | CN⁻, NH₃, H₂O |
| Coordination number | Number of LIGAND ATOMS directly bonded to the central atom | 6 in [Fe(CN)₆]³⁻ |
| Coordination sphere | The central atom + ligands — written in SQUARE BRACKETS | [Fe(CN)₆] |
| Counter ion | Ion outside the coordination sphere | K⁺ in K₃[Fe(CN)₆] |
| Chelate | A ligand that forms a RING by bonding through TWO or more donor atoms | EDTA, oxalate |
4. Types of Ligands
| Type | Number of Donor Atoms | Examples |
|---|---|---|
| Unidentate | 1 | NH₃, H₂O, Cl⁻, CN⁻ |
| Bidentate | 2 | Ethane-1,2-diamine (en), oxalate (ox), bipyridyl |
| Polydentate | 2+ | EDTA (hexadentate — 6 donor atoms) |
| Ambidentate | Can bind through either of TWO atoms | NO₂⁻ (nitro vs nitrito), SCN⁻ (thiocyanato vs isothiocyanato) |
5. IUPAC Naming
Rules
- Cation named FIRST, then anion.
- Ligands named in ALPHABETICAL ORDER (ignoring prefixes).
- ANIONIC ligands end in −o. NEUTRAL ligands keep their name (except: water = aqua, ammonia = ammine, CO = carbonyl, NO = nitrosyl).
- The central metal: CATION → metal name (e.g., chromium). ANIONIC complex → metal name + −ate (e.g., chromate).
- Oxidation state in ROMAN NUMERALS in parentheses.
Examples
- [Co(NH₃)₆]Cl₃: Hexaamminecobalt(III) chloride.
- K₃[Fe(CN)₆]: Potassium hexacyanoferrate(III).
6. Isomerism in Coordination Compounds
Structural Isomerism
| Type | Description | Example |
|---|---|---|
| Linkage isomerism | Ambidentate ligand bonds through DIFFERENT atoms | [Co(NH₃)₅(NO₂)]²⁺ (nitro) vs [Co(NH₃)₅(ONO)]²⁺ (nitrito) |
| Ionisation isomerism | Different ions produced in solution | [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br |
| Hydrate isomerism | Water is inside vs outside coordination sphere | [Cr(H₂O)₆]Cl₃ (violet) vs [Cr(H₂O)₅Cl]Cl₂·H₂O (grey-green) |
Stereoisomerism
- Geometrical (cis-trans) : Different spatial arrangement of same ligands. Common in square planar Pt(NH₃)₂Cl₂ and octahedral complexes.
- Optical isomerism: NON-SUPERIMPOSABLE mirror images (enantiomers). The complex is CHIRAL — like left and right hands.
7. Bonding Theories
Valence Bond Theory (VBT)
- Metal uses HYBRID ORBITALS to accommodate electron pairs from ligands.
- Inner orbital complexes (low spin): Use (n−1)d orbitals for hybridisation. Example: [Co(NH₃)₆]³⁺ (d²sp³).
- Outer orbital complexes (high spin): Use nd orbitals. Example: [CoF₆]³⁻ (sp³d²).
| Coordination Number | Hybridisation | Geometry | Examples |
|---|---|---|---|
| 4 | sp³ | Tetrahedral | [Ni(CO)₄] |
| 4 | dsp² | Square planar | [Ni(CN)₄]²⁻, Pt(NH₃)₂Cl₂ |
| 6 | d²sp³ or sp³d² | Octahedral | [Fe(CN)₆]³⁻ (d²sp³), [FeF₆]³⁻ (sp³d²) |
Crystal Field Theory (CFT)
- 'In CFT, ligands are treated as POINT CHARGES that cause the d-orbitals to SPLIT in energy.'
- Octahedral field: d-orbitals split into t₂g (lower energy: d_xy, d_yz, d_zx) and e_g (higher energy: d_x²−y², d_z²). Δ₀ = crystal field splitting energy.
- Tetrahedral field: Splitting is INVERTED and SMALLER (Δ_t = 4/9 Δ₀).
Crystal Field Stabilisation Energy (CFSE)
- For octahedral: CFSE = (−0.4 × n_t₂g + 0.6 × n_e_g) Δ₀ (+ pairing energy if applicable).
- Low spin vs High spin: For strong field ligands (CN⁻, CO): Δ₀ is LARGE → low spin. For weak field ligands (I⁻, Br⁻, F⁻): Δ₀ is SMALL → high spin.
- Spectrochemical series (increasing Δ₀): I⁻ < Br⁻ < S²⁻ < SCN⁻ < Cl⁻ < NO₃⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NH₃ < en < NO₂⁻ < CN⁻ < CO.
8. Applications of Coordination Compounds
- Biological: Chlorophyll (Mg complex), Haemoglobin (Fe complex), Vitamin B₁₂ (Co complex).
- Medicine: Cisplatin Pt(NH₃)₂Cl₂ — anticancer drug. EDTA for heavy metal poisoning.
- Industrial: Extraction of metals (Ni via [Ni(CO)₄]), purification of metals.
- Analytical: Detection of metal ions (blue colour of [Cu(NH₃)₄]²⁺, blood-red [Fe(SCN)]²⁺).
9. Common Mistakes
- Coordination number vs oxidation state: Coordination number = number of ligand ATOMS bonded (not number of ligands) — e.g., EDTA is hexadentate, CN=6 even with 1 ligand.
- Naming order: Ligands in ALPHABETICAL order, NOT by charge. 'ammine comes before chloro, even though NH₃ is neutral and Cl⁻ is negative.'
- IUPAC name for anionic complexes: [Fe(CN)₆]³⁻ is ferrate(III), NOT iron(III). The suffix −ate is ESSENTIAL for anionic complexes.
- CFT pairing energy: LOW spin occurs when Δ₀ is GREATER than pairing energy. HIGH spin occurs when Δ₀ is SMALLER than pairing energy.
10. CBSE Exam Focus
- Werner's theory — primary and secondary valency
- IUPAC naming of coordination compounds (up to 6 ligands)
- Types of isomerism — structural (linkage, ionisation, hydrate) and stereoisomerism (geometrical, optical)
- VBT — hybridisation, inner vs outer orbital complexes
- CFT — d-orbital splitting in octahedral and tetrahedral fields, CFSE, low spin vs high spin
- Applications — biological, medicinal, industrial
11. Self-Test
Q1: Write the IUPAC name of K₃[Fe(C₂O₄)₃]. A1: Potassium trioxalatoferrate(III). (Oxalate is C₂O₄²⁻, bidentate, so 3 ligands × 2 = CN 6.)
Q2: What type of isomerism is shown by [Co(NH₃)₅(ONO)]²⁺ and [Co(NH₃)₅(NO₂)]²⁺? A2: Linkage isomerism. NO₂⁻ is ambidentate — it can bond through N (nitro) or O (nitrito).
Q3: [Fe(H₂O)₆]³⁺ is paramagnetic with 5 unpaired electrons. [Fe(CN)₆]³⁻ has 1 unpaired electron. Explain. A3: H₂O is a WEAK field ligand — Δ₀ is SMALL, electrons occupy higher energy levels before pairing → HIGH SPIN (5 unpaired e⁻). CN⁻ is a STRONG field ligand — Δ₀ is LARGE, electrons PAIR before occupying higher levels → LOW SPIN (1 unpaired e⁻).
Q4: What is the coordination number of Co in [Co(NH₃)₄Cl₂]⁺? A4: CN = 6 (4 NH₃ + 2 Cl⁻ — each provides 1 donor atom). Octahedral geometry.
Q5: Name one medicinal application of coordination compounds. A5: Cisplatin Pt(NH₃)₂Cl₂ is used as an ANTICANCER drug. It binds to DNA and prevents cell division — particularly effective against testicular and ovarian cancers.
12. Conclusion
Coordination compounds are where CHEMISTRY meets BIOLOGY:
- STRUCTURE: 'A central metal surrounded by ligands — the geometry determines the properties.'
- BONDING: 'VBT tells us about hybridisation. CFT tells us about colour and magnetism.'
- ISOMERISM: 'Same formula, different arrangement — leading to different properties.'
- LIFE: 'Haemoglobin, chlorophyll, vitamin B₁₂ — nature uses coordination chemistry at the MOLECULAR level of life.'
'Coordination compounds are the DIAMONDS of inorganic chemistry — their beauty lies in their structure, colour, and the diversity of their applications.'
