Complex Ions and Ligands

Complex Ions Definitions

• Ligand: A particle with lone pairs of electrons that bonds to a metal ion via a coordinate bond.
• Complex: A metal ion with co-ordinately bonded ligands.
• Coordination number: The number of coordinate bonds from ligands to a metal ion.
• Lewis base: A lone pair donor.
• Lewis acid: A lone pair acceptor.

Complex Ion Formation

• Common ligands include H₂O, NH₃, and Cl⁻ ions, which have lone pairs of electrons.
• Complexes form when ligands use their lone pairs to form coordinate bonds with a metal ion.
• In the example of [CuCl₄]²⁻, four chloride ions act as ligands, each donating a lone pair to the Cu²⁺ ion.
• Ligands act as Lewis bases because they donate lone pairs to transition metals to form coordinate bonds.
• The metal ion acts as a Lewis acid because it accepts lone pairs.

Shapes of Complexes

• Common shapes include linear (2), square planar (4), tetrahedral (4), and octahedral (6).
• Coordination number reflects the number of ligands surrounding the metal ion.
• Linear complexes (e.g., [Ag(NH₃)₂]⁺) are common with Ag⁺ ions, while square planar complexes (e.g., [PtCl₄]²⁻) are common with Pt²⁺ and Ni²⁺ and when ligands are too big for 6 to fit.
• Tetrahedral complexes (e.g., [CuCl₄]²⁻) are common with small ligands (e.g., H₂O, NH₃).
• Octahedral complexes (e.g., [Cu(H₂O)₆]²⁺) are also common with small ligands.

Types of Ligands

• Unidentate ligands: Form one coordinate bond to a metal ion (e.g., Cl⁻, OH⁻, CN⁻, H₂O, NH₃).
• Bidentate ligands: Form two coordinate bonds to a metal ion (e.g., 1,2-diaminoethane, ethanedioate ion (C₂O₄²⁻)).
• Multidentate ligands: Form more than two coordinate bonds to a metal ion (e.g., EDTA⁴⁻, porphyrin).
  • EDTA⁴⁻ can form 6 bonds.
  • Porphyrin can form 4 bonds.

Haemoglobin

• Haemoglobin is a protein containing four iron(II) (Fe²⁺) centres, each with a porphyrin ligand.
• One of the coordination sites in the Haemoglobin is available to bind oxygen.
• Oxygen is transported in blood bound to iron in haemoglobin.
• Cyanide ions (CN⁻) and carbon monoxide (CO) are better ligands than oxygen (O₂), so they bind to haemoglobin in preference to oxygen, preventing oxygen transfer.