Chapter 6 – Coordination Compounds (JEE Chemistry)
1. Introduction to Coordination Compounds
Coordination compounds are chemical compounds in which a central metal atom or ion
is surrounded by a number of molecules or ions called ligands, which donate
electron pairs to the metal.
Coordination chemistry explains the structure, bonding, colour, magnetic behaviour
and reactivity of many important compounds used in industry and biology.
2. Werner’s Theory of Coordination Compounds
According to Werner’s theory:
- Metal shows two types of valencies: primary and secondary
- Primary valency = oxidation state (ionisable)
- Secondary valency = coordination number (non-ionisable)
- Secondary valencies have fixed spatial arrangement
Example: $[Co(NH_3)_6]Cl_3$
- Primary valency = 3
- Secondary valency = 6
3. Important Terms in Coordination Compounds
| Term | Meaning |
|---|---|
| Central Metal Ion | Metal atom/ion to which ligands are attached |
| Ligand | Ion or molecule donating lone pair |
| Coordination Number | Number of ligands attached |
| Coordination Sphere | Metal + ligands inside square brackets |
4. Types of Ligands
Based on number of donor atoms:
- Monodentate: $NH_3, Cl^-$
- Bidentate: $en$ (ethylenediamine)
- Polydentate: $EDTA^{4-}$
Chelating ligands form more stable complexes (Chelate effect).
5. Nomenclature of Coordination Compounds (IUPAC)
Important rules:
- Name ligands first (alphabetically)
- Then metal name
- Oxidation state in Roman numerals
- Anionic complex → metal ends with “ate”
Example:
$$[Co(NH_3)_5Cl]Cl_2$$
Name: Pentaamminechloridocobalt(III) chloride
6. Oxidation State of Metal Ion
Oxidation state = Charge on complex − sum of ligand charges
Example:
$$[Fe(CN)_6]^{4-}$$
Let oxidation state of Fe = $x$
$$x + 6(-1) = -4 \Rightarrow x = +2$$
7. Coordination Number
Coordination number is the total number of donor atoms bonded to metal ion.
| Coordination Number | Geometry |
|---|---|
| 2 | Linear |
| 4 | Tetrahedral / Square planar |
| 6 | Octahedral |
8. Isomerism in Coordination Compounds
Isomerism arises due to different arrangement of ligands.
Structural Isomerism
- Ionisation isomerism
- Hydrate isomerism
- Linkage isomerism
Stereoisomerism
- Geometrical isomerism
- Optical isomerism
9. Geometrical Isomerism
Occurs due to different spatial arrangement of ligands.
Example:
$$[Pt(NH_3)_2Cl_2]$$
cis-form and trans-form
10. Optical Isomerism
Optical isomers are non-superimposable mirror images.
Complexes with bidentate ligands often show optical isomerism.
11. Bonding in Coordination Compounds
Valence Bond Theory (VBT)
Explains geometry using hybridisation of metal orbitals.
$d^2sp^3$ → octahedral
$sp^3$ → tetrahedral
$dsp^2$ → square planar
12. Crystal Field Theory (CFT)
CFT explains:
- Colour
- Magnetic properties
- Stability of complexes
Splitting in octahedral field:
$$\Delta_o$$
13. Colour of Coordination Compounds
Colour arises due to $d-d$ transitions when electrons absorb visible light.
14. Magnetic Properties
- Paramagnetic → unpaired electrons
- Diamagnetic → no unpaired electrons
15. Stability of Coordination Compounds
Stability increases due to:
- Higher charge on metal
- Smaller metal size
- Chelate effect
16. Applications of Coordination Compounds
- Biological systems (Haemoglobin, Chlorophyll)
- Medicines (Cisplatin)
- Analytical chemistry (EDTA titration)
- Electroplating
17. Common JEE Mistakes
- Wrong oxidation state
- Incorrect ligand naming
- Confusing coordination number
- Ignoring geometry
18. Final Revision Checklist
You have mastered this chapter if you can:
- Name complexes correctly
- Calculate oxidation states
- Predict geometry
- Identify isomerism
- Explain colour and magnetism