Werner's Theory
JEE Focus: Primary vs Secondary Valence, Coordination Number, Isomerism
Key Postulates:
- Metals show two types of linkages: Primary valence (ionizable, oxidation state) and Secondary valence (non-ionizable, coordination number)
- Secondary valence is fixed for a metal and determines geometry
- Characteristic spatial arrangements → Coordination polyhedra
Experimental Evidence:
| Compound |
AgCl precipitated |
Coordination Entity |
| CoCl₃·6NH₃ (Yellow) |
3 mol |
[Co(NH₃)₆]³⁺ 3Cl⁻ |
| CoCl₃·5NH₃ (Purple) |
2 mol |
[CoCl(NH₃)₅]²⁺ 2Cl⁻ |
| CoCl₃·4NH₃ (Green/Violet) |
1 mol |
[CoCl₂(NH₃)₄]⁺ Cl⁻ |
Important Definitions
Coordination Entity:
Central metal + attached ligands
Example: [Co(NH₃)₆]³⁺, [Fe(CN)₆]⁴⁻
Ligands Classification:
| Type |
Denticity |
Examples |
| Unidentate |
1 |
NH₃, H₂O, Cl⁻, CO |
| Didentate |
2 |
en (ethane-1,2-diamine), ox²⁻ (oxalate) |
| Polydentate |
>2 |
EDTA⁴⁻ (hexadentate), dien |
| Ambidentate |
1 (two donor atoms) |
NO₂⁻ (N or O), SCN⁻ (S or N) |
Coordination Number (CN):
Number of ligand donor atoms directly bonded to metal
[PtCl₆]²⁻ → CN = 6
[Ni(NH₃)₄]²⁺ → CN = 4
[Fe(C₂O₄)₃]³⁻ → CN = 6 (oxalate is didentate)
Nomenclature Rules
Writing Formulas:
- Central atom first
- Ligands in alphabetical order (ignore prefixes)
- Entire complex in square brackets
- Charge indicated as superscript
Naming Complexes:
- Cation named before anion
- Ligands in alphabetical order before metal
- Anionic ligands end in '-o'
- Oxidation state in Roman numerals
- If complex is anion, metal name ends in '-ate'
Examples:
[Cr(NH₃)₃(H₂O)₃]Cl₃ → triamminetriaquachromium(III) chloride
K₃[Cr(C₂O₄)₃] → potassium trioxalatochromate(III)
[CoCl₂(en)₂]⁺ → dichloridobis(ethane-1,2-diamine)cobalt(III)
Isomerism in Coordination Compounds
Structural Isomerism:
- Linkage: Ambidentate ligands (NO₂⁻ → nitro vs nitrito)
- Coordination: Ligand exchange between cation & anion
- Ionisation: Counter ion exchange ([Co(NH₃)₅SO₄]Br vs [Co(NH₃)₅Br]SO₄)
- Solvate/Hydrate: Water inside/outside coordination sphere
Stereoisomerism:
Geometrical Isomerism:
Square planar [MX₂L₂]: cis & trans isomers
Octahedral [MX₂L₄]: cis & trans isomers
Octahedral [MA₃B₃]: fac & mer isomers
Optical Isomerism:
- Mirror images that are non-superimposable
- Common with chelating ligands
- Example: [Co(en)₃]³⁺ shows d and l forms
NEET Tip: Optical activity requires molecular chirality - no plane of symmetry
Bonding Theories
Valence Bond Theory (VBT):
| Coordination Number |
Hybridization |
Geometry |
Examples |
| 4 |
sp³ |
Tetrahedral |
[NiCl₄]²⁻ |
| 4 |
dsp² |
Square planar |
[Ni(CN)₄]²⁻ |
| 6 |
sp³d² |
Octahedral (outer orbital) |
[CoF₆]³⁻ |
| 6 |
d²sp³ |
Octahedral (inner orbital) |
[Co(NH₃)₆]³⁺ |
Crystal Field Theory (CFT):
Octahedral Splitting: t₂g (lower) and e_g (higher) orbitals
Splitting Energy: Δ₀
Spectrochemical Series:
I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < edta⁴⁻ < NH₃ < en < CN⁻ < CO
JEE Focus: Strong field ligands cause large Δ₀ → low spin complexes
Weak field ligands cause small Δ₀ → high spin complexes
Properties & Applications
Magnetic Properties:
- Paramagnetic: Unpaired electrons
- Diamagnetic: All electrons paired
- Magnetic moment = √[n(n+2)] BM (n = unpaired electrons)
Color in Coordination Compounds:
Due to d-d transitions when Δ matches visible light energy
[Ti(H₂O)₆]³⁺: d¹, absorbs green → appears violet
Color depends on: Metal ion, Oxidation state, Ligands
Important Applications:
- Biological: Chlorophyll (Mg), Hemoglobin (Fe), Vitamin B₁₂ (Co)
- Analytical: EDTA titrations, Qualitative analysis
- Industrial: Catalysts, Electroplating, Photography
- Medical: Cisplatin (anti-cancer), Chelation therapy
- Metallurgical: Extraction of Au, Ag using CN⁻ complexes
JEE/NEET Practice Questions
Question 1:
Which of the following complexes shows geometrical isomerism?
(a) [Co(NH₃)₆]³⁺ (b) [Pt(NH₃)₂Cl₂] (c) [Ni(CO)₄] (d) [Co(en)₃]³⁺
Answer: (b) [Pt(NH₃)₂Cl₂] - Square planar complex
Question 2:
The complex [Fe(CN)₆]³⁻ has one unpaired electron, while [FeF₆]³⁻ has five unpaired electrons. Explain using CFT.
Answer: CN⁻ is strong field ligand (large Δ₀) → low spin complex. F⁻ is weak field ligand (small Δ₀) → high spin complex.
Question 3:
Write IUPAC name: K₃[Fe(C₂O₄)₃]
Answer: Potassium trioxalatoferrate(III)
Exam Strategy:
• Memorize spectrochemical series
• Practice naming and formula writing
• Understand magnetic behavior predictions
• Know common biological coordination compounds