Coordination Compounds

QUICK NOTES FOR EASY REVISION!

1. WHAT ARE COORDINATION COMPOUNDS?

Compounds in which a central metal atom/ion is surrounded by ions or molecules (ligands) which are held together by coordinate bonds.

General Notation: [M(L)_n]ⁿ⁺

• M = Central metal atom/ion
• L = Ligand (donor atom/molecule)
• n = Charge on the complex

2. IMPORTANT TERMS

Central Metal Atom/Ion	Accepts lone pair of electrons from ligands.
Ligand	Donor species that donates a lone pair to the metal.
Coordinate Bond	Formed by sharing of both electrons from ligand to metal.
Coordination Number	Number of donor atoms bonded to the central metal atom/ion.
Complex Ion	Charged complex species.
Coordination Sphere	Central metal ion plus the ligands directly attached to it.

3. LIGANDS (with examples)

Type	Ligand	Formula	Donor Atom
Monodentate (Ligand binds through one donor atom)	Ammonia	NH3	N
	Water	H2O	O
	Chloride	Cl-	Cl
	Cyanide	CN-	C
	Nitrite	NO2-	N
Pyridine	C5H5N	N
Polydentate (Ligand binds through more than one donor atom)	Ethylenediamine (en)	H2NCH2CH2NH2	N, N
	Oxalate (ox)	C2O42-	O, O
	EDTA4-	(EDTA)	Various (6)
	Carbonate (CO32-)	CO32-	O, O

⭐ Dentacity = Number of donor atoms in a ligand.

4. COORDINATION NUMBER

Number of donor atoms directly bonded to the central metal.

Coordination Number	Geometry	Example
2	Linear	[Ag(NH3)2]+
4	Tetrahedral / Square planar	[Ni(Cl)4]2-
6	Octahedral	[Co(NH3)6]3+
8	Square antiprismatic / Dodecahedral	[Ni(CN)4]2-

💡 Most common coordination number = 4 and 6

5. EXAMPLES OF COORDINATION COMPOUNDS

Formula	Name	Ions Present	Coordination Number
[Co(NH3)6]Cl3	Hexaamminecobalt(III) chloride	Co3+, NH3, Cl-	6
[Cu(NH3)4]SO4	Tetraamminecopper(II) sulfate	Cu2+, NH3, SO42-	4
K3[Fe(CN)6]	Potassium ferricyanide	K+, [Fe(CN)6]3-	6
[Ag(NH3)2]Cl	Diamminesilver(I) chloride	Ag+, NH3, Cl-	2

⭐ Name of complex ion is written first, then the counter ion.

6. IUPAC NAMING OF COMPLEX COMPOUNDS

1. Name the cation or complex ion first.
2. Name the ligands in alphabetical order.
3. Use prefixes to indicate number of ligands:
   • di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6)
4. Use "-ate" for anionic complex.
5. Indicate the oxidation state of metal in Roman numerals.

Examples:

• [Co(NH₃)₆]Cl₃ : Hexaamminecobalt(III) chloride
• K₄[Fe(CN)₆] : Potassium hexacyanoferrate(II)
• [Cu(H₂O)₄]SO₄ : Tetraaquacopper(II) sulfate

7. ISOMERISM IN COORDINATION COMPOUNDS

Structural Isomerism

• Ionization: [Co(NH₃)₅Br]SO₄ ⇌ [Co(NH₃)₅SO₄]Br
• Hydrate: [Cr(H₂O)₆]Cl₃ ⇌ [Cr(H₂O)₅Cl]Cl₂·H₂O
• Linkage: [M-NO₂]²⁺ (nitro) ⇌ [M-ONO]²⁺ (nitrito)

Stereoisomerism

• Geometrical: [Co(NH₃)₄Cl₂]⁺ (cis and trans forms)

• Optical: [Co(en)₃]³⁺ (d and l forms)

8. MAGNETIC PROPERTIES

• Depend on number of unpaired electrons.
• Calculated using formula: μ = √[n(n + 2)] Bohr Magneton (B.M.), where n = number of unpaired electrons.

Examples:

Complex	Unpaired electrons (n)	Magnetic Moment (μ)	Nature
[Ni(CN)4]2-	0	0	Diamagnetic
[Co(NH3)6]3+	0	0	Diamagnetic
[CoCl4]2-	3	3.87	Paramagnetic
[Fe(H2O)6]2+	4	4.90	Paramagnetic

⭐ Low spin complexes → fewer unpaired electrons
High spin complexes → more unpaired electrons

9. COLOR OF COORDINATION COMPOUNDS

• Due to d–d transitions or charge transfer.
• Color depends on:
  • ✅ Nature of metal
  • ✅ Ligands present
  • ✅ Coordination number
  • ✅ Geometry of complex

Examples:

• [Ti(H₂O)₆]³⁺ → Violet
• [Cu(H₂O)₆]²⁺ → Blue
• [Ni(H₂O)₆]²⁺ → Green
• [Fe(CN)₆]^4- → Yellow

10. IMPORTANCE / APPLICATIONS

• Used in analytical chemistry (EDTA in titrations)
• Extraction of metals
• Biological systems (Hemoglobin, Chlorophyll, Vitamin B₁₂)
• Dyes and pigments
• Catalysts in industrial reactions

QUICK REVISION TIPS

• ✓ Learn common ligands and their denticity.
• ✓ Practice writing IUPAC names.
• ✓ Draw structures to understand isomerism.
• ✓ Remember formulas of important complexes.
• ✓ Solve magnetic moment problems regularly.

REMEMBER!

• ⭐ Coordination compounds = Metal + Ligand
• ⭐ Ligand donates lone pair.
• ⭐ Coordinate bond is a dative bond.
• ⭐ Understand structure → Easy to remember!