Overview
This lecture explains complex ions, their structures, bonding, and how different types of ligands interact with metal ions, illustrated by key examples.
Structure of Complex Ions
- A complex ion consists of a central metal ion surrounded by molecules or ions called ligands.
- Ligands attach to the metal ion via coordinate (dative covalent) bonds using lone pairs.
- All ligands act as Lewis bases because they donate electron pairs to the metal ion.
Bonding in Simple Complex Ions
- Metal ions like Al³⁺ and Fe³⁺ use empty orbitals to accept lone pairs from six ligands, forming octahedral complexes.
- The maximum number of coordinate bonds creates the most stable (lowest energy) complex.
- The coordination number counts the number of coordinate bonds, commonly 6 for small ligands like water or ammonia.
Examples of Complex Ions
- [Al(H₂O)₆]³⁺: Aluminum ion with 6 water ligands, coordination number 6.
- [Fe(H₂O)₆]³⁺: Iron(III) ion with 6 water ligands, similar structure and coordination.
- [CuCl₄]²⁻: Copper(II) ion with 4 chloride ligands due to larger ligand size, coordination number 4.
Types of Ligands
- Unidentate ligands form only one bond to the metal ion (e.g., water, ammonia, chloride).
- Bidentate ligands form two bonds using two different lone pairs (e.g., ethylenediamine/en, ethanedioate/oxalate).
- Quadridentate ligands form four bonds to the metal ion (e.g., heme group in hemoglobin).
- Hexadentate ligands form six bonds, wrapping completely around the metal ion (e.g., EDTA⁴⁻).
Multidentate (Polydentate) Ligand Examples
- [Ni(en)₃]²⁺: Nickel(II) ion complexed with three bidentate ethylenediamine ligands (6 coordinate bonds, coordination number 6).
- [Cr(C₂O₄)₃]³⁻: Chromium(III) ion with three bidentate oxalate ligands (coordination number 6).
- Heme in hemoglobin: Iron(II) with a quadridentate ring, plus two more bonds for coordination number 6.
- [Cu(EDTA)]²⁻: Copper(II) ion wrapped by a hexadentate EDTA ligand (coordination number 6).
Biological and Practical Relevance
- Hemoglobin carries oxygen by reversible binding at a coordination site on iron in the heme group.
- Carbon monoxide binds irreversibly to hemoglobin, preventing oxygen transport and causing toxicity.
Key Terms & Definitions
- Complex Ion — a charged species with a central metal ion bonded to surrounding ligands.
- Ligand — a molecule or ion that donates a pair of electrons to a metal ion via a coordinate bond.
- Coordinate (dative covalent) bond — a bond where both electrons come from the same atom (the ligand).
- Coordination Number — the total number of coordinate bonds to the central metal ion.
- Unidentate — ligand forming one bond to the metal ion.
- Bidentate/Polydentate — ligand forming two or more bonds to the metal ion.
- Lewis base — a species that donates an electron pair.
Action Items / Next Steps
- Practice drawing and identifying coordination numbers and ligand types in complex ions.
- Review the electronic structure of common metal ions for understanding bonding in complexes.