Complex Ions

A complex ion consists of a central metal ion surrounded by ligands. Ligands are molecules or ions that donate a lone pair of electrons to the metal ion, forming a coordinate (dative covalent) bond. The study of complex ions is a central topic in transition metal chemistry and appears extensively at A-Level.

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Key Terminology

• Central metal ion: The transition metal ion at the centre of the complex (e.g., Fe³⁺, Cu²⁺, Co²⁺)
• Ligand: A species (molecule or ion) with at least one lone pair of electrons that it donates to the central metal ion
• Coordination number: The total number of coordinate bonds formed between the ligands and the central metal ion
• Complex ion: The central metal ion together with its ligands, usually carrying an overall charge

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Types of Ligand

Ligands are classified by the number of coordinate bonds they can form with the central metal ion:

Monodentate Ligands

These donate one lone pair and form one coordinate bond per ligand: