6.2.4 Ligands

• 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

Example of a complex formed between a transition metal ion (Fe²⁺) and a ligand (H₂O) by 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

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

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 atom 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