Complex Ions, Ligands, Ligand Exchange

This lesson follows the exact 9476 ligand-exchange sequence: define ligand and complex, track the copper(II) complexes formed with water, ammonia and chloride ions, and explain oxygen/carbon monoxide competition in haemoglobin.

Use the Transition Elements hub for the lesson sequence and d-Orbital Splitting and Colour for the orbital explanation behind an exchange colour change.

Definitions (Must Know)

A. Ligand

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

B. Complex

A complex contains a central metal atom or ion surrounded by ligands joined through coordinate bonds. A charged complex is written in square brackets with its overall charge outside, for example [Cu(H₂O)₆]²⁺.

Key Ideas (What Earns Marks)

• The assessed copper(II) sequence uses water, ammonia and chloride ions as ligands.
• In ligand exchange, one ligand replaces another around the same central ion; conserve every ligand, atom and charge in the equation.
• Aqueous Cu²⁺ is represented as the pale-blue hexaaquacopper(II) ion, [Cu(H₂O)₆]²⁺.
• Excess ammonia forms the deep-blue complex [Cu(NH₃)₄(H₂O)₂]²⁺ after the initial pale-blue precipitate dissolves.
• Concentrated chloride ions establish an equilibrium with [CuCl₄]²⁻; the observed colour depends on the mixture of complexes.
• Carbon monoxide and oxygen exchange as ligands at the metal centre in haemoglobin. This is ligand competition, not a redox explanation.

Detailed Explanations

A. Why a complex forms

A ligand supplies both electrons in the new bond. Its lone pair is donated into an available orbital on the electron-deficient metal ion, producing a coordinate bond.

B. Water–ammonia sequence for copper(II)

Adding a little ammonia first supplies enough OH⁻ for a pale-blue copper(II) hydroxide precipitate: [Cu(H₂O)₆]²⁺ + 2OH⁻ → Cu(OH)₂(s) + 6H₂O

In excess ammonia, ligand exchange produces the deep-blue ammine complex: [Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O

Describe the observations in order: pale-blue precipitate first, then dissolution to a deep-blue solution in excess ammonia.

C. Water–chloride ligand exchange

[Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ + 6H₂O

Increasing chloride concentration shifts the equilibrium towards the chloride complex. Dilution shifts it back towards the aqua complex. State that a different ligand environment changes the d-orbital splitting and hence the colour.

D. Oxygen–carbon monoxide exchange in haemoglobin

Oxygen binds reversibly as a ligand at the metal centre. Carbon monoxide competes for the same site and forms a more stable complex, displacing oxygen and reducing the number of sites available for oxygen transport. No detailed biological mechanism is required.

E. Equation audit

For every exchange equation, check the central metal, each ligand, total atoms and overall charge. Brackets identify the complete complex; the charge belongs outside them.

Worked Examples

Common Mistakes

• Describing water, ammonia or chloride as a catalyst rather than a ligand.
• Calling the initial copper(II) hydroxide precipitation ligand exchange.
• Omitting brackets or placing the overall charge on one ligand.
• Giving only a colour word without the species or stage of reagent addition.
• Claiming oxygen/carbon monoxide exchange is a change in the metal oxidation state.

Use the Transition Elements topic check to diagnose whether the error is equation balance, observation order or chemical classification.

Exam Tips

• For a definition, say donates a lone pair and central metal atom or ion.
• In an observation question, write “dropwise” and “in excess” stages separately.
• In an exchange equation, conserve the complete ligand count as well as total charge.
• To distinguish ligand exchange from redox, compare the metal oxidation state before and after.
• For haemoglobin, describe competing ligands and reversible site occupancy; detailed biology is not required.

Mind Stretchers