Ligands (CIE A Level Chemistry)

• A ligand is a molecule or ion that has one or more lone pairs of electrons
• These lone pairs of electrons are donated by the ligand, to form dative covalent bonds to a central metal atom or ion

Examples of ligands table

Ligand name	Ligand formula
Water	H2O
Ammonia	NH3
Chloride	Cl–
Cyanide	CN–
Thiocyanate	SCN–
Ethanedioate (ox)	–COO–COO– C2O42–
1,2-diaminoethane	H2NCH2CH2NH2

Example of a complex formed between a central Fe²⁺ ion and water ligands

The complex of a central Fe²⁺ ion and water ligands is formed with dative covalent bonds

• Different ligands can form different numbers of dative bonds to the central metal ion in a complex.
  • Some ligands can form one dative bond to the central metal ion
  • Other ligands can form two dative bonds, and some can form multiple dative bonds

Monodentate ligands

• Monodentate ligands can form only one dative bond to the central metal ion
• Examples of monodentate ligands are:
  • Water (H₂O) molecules
  • Ammonia (NH₃) molecules
  • Chloride (Cl^-) ions
  • Cyanide (CN^-) ions

Examples of complexes with monodentate ligands

Larger monodentate ligands tend to form octahedral complexes, while smaller monodentate ligands tend to form tetrahedral complexes

Bidentate ligands

• Bidentate ligands can each form two dative bonds to the central metal ion
• This is because each ligand contains two atoms with lone pairs of electrons
• Examples of bidentate ligands are:
  • 1,2-diaminoethane (H₂NCH₂CH₂NH₂) which is also written as ‘en’
  • Ethanedioate ion (C₂O₄^2- ) which is sometimes written as ‘ox’

 Examples of complexes with bidentate ligands

The en and ox bidentate ligands form coordinate bonds through the lone pairs on the nitrogen and oxygen atoms respectively

Polydentate ligands

• Some ligands contain more than two atoms with lone pairs of electrons
• These ligands can form more than two dative bonds to the central metal ion and are said to be polydentate ligands
• An example of a polydentate ligand is EDTA^4- , which is a hexadentate ligand as it forms 6 dative covalent bonds to the central metal ion

Example of a polydentate ligand complex

 

The EDTA^4- ligand forms 6 coordinate bonds to the central metal ion