Complex Ions and Ligands

A complex ion consists of a central metal ion surrounded by ligands. This lesson covers the types of ligands, coordination numbers, shapes of complexes, and IUPAC naming conventions (AQA 3.2.5).

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Key Definitions

Complex ion: A central metal ion surrounded by ligands, which are bonded to it by coordinate (dative covalent) bonds.

Ligand: A species (molecule or anion) that donates a lone pair of electrons to the central metal ion to form a coordinate bond.

Coordination number: The total number of coordinate bonds formed between the ligands and the central metal ion.

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Types of Ligands

Monodentate Ligands

A monodentate ligand forms one coordinate bond with the central metal ion (it has one lone pair available for donation, or one lone pair used for bonding).

Ligand	Formula	Lone pair donor atom
Water	H₂O	O
Ammonia	NH₃	N
Chloride	Cl⁻	Cl
Cyanide	CN⁻	C
Hydroxide	OH⁻	O
Thiocyanate	SCN⁻	N or S
Carbon monoxide	CO	C

Bidentate Ligands

A bidentate ligand forms two coordinate bonds with the central metal ion (it has two lone pairs on different atoms that can both donate).

Ligand	Formula	Donor atoms
1,2-diaminoethane (ethylenediamine)	H₂NCH₂CH₂NH₂ (en)	Both N atoms
Ethanedioate (oxalate)	C₂O₄²⁻ (ox)	Two O atoms from different carboxylate groups

Multidentate (Polydentate) Ligands

A multidentate ligand forms more than two coordinate bonds with the central metal ion.

EDTA⁴⁻ (ethylenediaminetetraacetate) is a hexadentate ligand — it forms six coordinate bonds with a single metal ion (through 2 nitrogen atoms and 4 oxygen atoms). This gives it exceptional ability to sequester metal ions.

Key Definition: Chelation is the formation of a complex containing a multidentate ligand. The resulting complex is called a chelate. The word comes from the Greek "chele" meaning claw.

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Coordination Number and Shapes

The coordination number determines the shape of the complex.

Coordination Number 6 — Octahedral

Six coordinate bonds arranged symmetrically around the central metal ion, with bond angles of 90°.

Examples:

• [Fe(H₂O)₆]²⁺ — hexaaquairon(II), green solution
• [Fe(H₂O)₆]³⁺ — hexaaquairon(III), yellow/brown solution
• [Cr(H₂O)₆]³⁺ — hexaaquachromium(III), green solution (sometimes violet)
• [Cu(H₂O)₆]²⁺ — hexaaquacopper(II), blue solution
• [Co(H₂O)₆]²⁺ — hexaaquacobalt(II), pink solution
• [Co(NH₃)₆]³⁺ — hexaamminecobalt(III)
• [Cr(NH₃)₆]³⁺ — hexaamminechromium(III)
• [Fe(CN)₆]³⁻ — hexacyanidoferrate(III)
• [Ni(en)₃]²⁺ — tris(1,2-diaminoethane)nickel(II)
• [CaEDTA]²⁻ — EDTA complex of calcium

Note: Six monodentate ligands, three bidentate ligands, or one hexadentate ligand all give a coordination number of 6 and an octahedral shape.

Coordination Number 4 — Tetrahedral

Four coordinate bonds arranged in a tetrahedral geometry, with bond angles of 109.5°. This is common with larger ligands (especially Cl⁻) because four large ligands cannot fit around the metal ion in a square planar arrangement.