Complex Ions and Ligand Coordination HL Only

Full Notes

Transition metal ions form complex ions by accepting lone pairs from ligands via dative covalent bonds.

For Example: Water molecules (H₂O) are able to act as ligands as the oxygen atom can use one its lone pairs electrons to form a co-ordinate bond to a central metal atom or ion.

What is a Ligand?

A ligand is a molecule or ion with a lone pair of electrons that can form a coordinate bond to a central metal ion.

The bond is a dative covalent bond – both electrons in the bond come from the ligand.

What is a Complex Ion?

A complex ion is a species formed when ligands coordinate (bond) to a central metal ion.

Example: [Cu(H₂O)₆]²⁺ is a copper ion surrounded by six water ligands.

The formulas of complex ions are written in square brackets with the overall charge of the complex ion shown as a superscript.

Coordination Number

The coordination number of a complex is the number of coordinate bonds formed with the central metal ion and determines the geometry (shape) of the complex.

Shapes and Bond Angles of Complexes

• Linear: 180° (e.g., [Ag(NH₃)₂]⁺)
• Tetrahedral: ~109.5° (e.g., [CuCl₄]²⁻)
• Square Planar: 90° (e.g., [Pt(NH₃)₂Cl₂])
• Octahedral: 90° (e.g., [Fe(H₂O)₆]³⁺)

Matt’s exam tip

Don’t forget the coordination number of a complex and the number of ligands don’t have to be the same.

How to Deduce the Charge of a Complex Ion

Charge of complex ion = Metal ion charge + total ligand charge

Example 1: [Cu(NH₃)₄]²⁺

• Cu = +2
• NH₃ = 0 (×4 = 0)
• Total = +2 → [Cu(NH₃)₄]²⁺

Example 2: [Fe(CN)₆]³⁻

• Fe = +3
• CN⁻ = –1 (×6 = –6)
• Total = +3 + (–6) = –3 → [Fe(CN)₆]³⁻

Example 3: [CrCl₂(H₂O)₄]⁺

• Cr = +3
• Cl⁻ = –1 (×2 = –2), H₂O = 0 (×4 = 0)
• Total = +3 + (–2) = +1 → [CrCl₂(H₂O)₄]⁺

Summary

• Complex ions form when ligands donate lone pairs to transition metals.
• Ligands may be neutral or negatively charged.
• Coordination number determines the shape of the complex.
• Charge on a complex = metal ion charge + sum of ligand charges.