Coordination Chemistry

Learning Objectives

Define a complex ion, ligand, and coordination number.

Describe the type of bond formed between a central metal ion and its ligands.

Identify common ligand types (monodentate, bidentate).

Recognize the importance of coordination compounds in biological systems, such as hemoglobin.

The Chemistry of Complex Ions

Coordination chemistry is the study of compounds containing complex ions. A complex ion consists of a central metal cation bonded to one or more molecules or ions called ligands.

Central Metal Ion: Typically a transition metal cation (e.g., Fe²⁺, Cu²⁺, Ni²⁺). It acts as a Lewis acid (electron pair acceptor).

Ligand: A molecule or ion that has a lone pair of electrons that it can donate to the central metal ion. It acts as a Lewis base (electron pair donor). Examples: H₂O, NH₃, Cl⁻, CN⁻.

Coordination Number: The number of donor atoms from the ligands that are bonded directly to the central metal ion. Common coordination numbers are 4 and 6.

The Coordinate Covalent Bond

The bond between the central metal and the ligand is a coordinate covalent bond. This is a type of covalent bond where both electrons in the shared pair come from the same atom (in this case, the ligand).

Types of Ligands

Monodentate Ligand: A ligand that donates only one electron pair to the central metal (it binds at only one site). 'One-toothed'. Examples: H₂O, NH₃, Cl⁻.

Bidentate Ligand: A ligand that can donate two electron pairs from two different atoms to the central metal. 'Two-toothed'. Example: ethylenediamine (en).

Polydentate Ligand: A ligand that can form two or more bonds to a central metal ion. These are also called chelating agents.

Importance of Coordination Compounds

Coordination compounds are vital in many areas:

Biology: Hemoglobin, the protein that carries oxygen in the blood, is a coordination compound. An iron(II) ion is the central metal, coordinated to a large porphyrin ring ligand. Oxygen acts as another ligand that binds reversibly to the iron. Chlorophyll, essential for photosynthesis, has magnesium as its central metal.

Industry: Used as catalysts, pigments, and in analytical chemistry.

Key Terms

Complex Ion: An ion consisting of a central metal ion bonded to one or more molecules or ions called ligands.
Ligand: An ion or molecule that binds to a central metal atom to form a coordination complex. Ligands act as Lewis bases.
Coordination Number: The number of ligand atoms that are bonded directly to the central metal ion in a complex ion.
Coordinate Covalent Bond: A type of covalent bond in which both shared electrons are provided by one of the atoms (the Lewis base).
Chelating Agent: A substance whose molecules can form several coordinate bonds to a single metal ion.