Classification of Ligands (Part 2)

Ligand Classification by Donor Atoms

Nitrogen-Based Donor Atoms

• Monodentate: Ammonia (NH₃)
• Bidentate: Ethylenediamine (H₂NCH₂CH₂NH₂)

Oxygen-Based Donor Atoms

• Monodentate: Water (H₂O)
• Bidentate: Catechol

Mixed Donor Types

• Example: Glycine (NH₂CH₂COOH) - Nitrogen and Oxygen donors

Other Types of Donor Atoms

• Sulfur-Based Donors
• Phosphorus-Based Donors
• Carbon-Based Donors: Metal-carbon bonds (e.g., Tetracarbonylnickel(0))

Macrocyclic Ligands

• Encircle metal centers with multiple donor atoms.
• Forms: Macrocycles (large cycles) have significant biological roles.
• Examples: Porphyrins, Crown ethers

Tridentate Ligands

• Pyridine Rings: Multi-pyridine ligands provide multiple coordination points.
• Formation Example: Terpyridine (three pyridine rings)
• Example Complex: Ru(terpy)₂²⁺ - Excellent electron transfer properties

N₃-Type Ligands

• Example: Triaza-9-cyclononane (TACN) - Binds metals in trigonal face configurations.
• Application: DNA hydrolysis, catalytic properties

Diethylenetriamine, Triethylenetetramine

• Example: Trion - Forms N₄ ligands
• Reaction Products: Hexadentate ligands with salicylaldehyde
• Cyclisation: Formation of products like TACN chloride when reacting with metal salts

Complex Ligand Structures

• Tetra- and Hexa-dentate Ligands: Bind to metal centers via multiple donor atoms.
• Train Ligand: Tris(2-aminoethyl)amine - Forms trigonal bipyramidal or octahedral geometries.

Macrocyclic Ligands

• Example: Cyclam - 1,4,7,10-tetraazacyclododecane
• Application: Binding transition metals, forming puckered geometries

Crown Ethers and Cryptands

• Crown Ethers: Cyclic ligands with multiple ether groups, bind alkali metals.
• Cryptands: Bicyclic ligands with mixed donor atoms, encapsulate metal ions

Sulfur-Based Ligands

• S-donors: Diethyl dithiocarbamate, etc., bind heavy metals like Cd, Hg
• Application: Chelating therapy for heavy metal poisoning

Pincer-Type Ligands

• Tridentate Ligands: Bind tightly to metal centers in meridional configuration
• Applications: Catalysis, e.g., Ruthenium or Iridium complexes