Complex Ions, Ligands, Ligand Exchange
Learn and apply Complex Ions, Ligands, Ligand Exchange in the published Chemistry course sequence.
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The core idea
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Complex Ions, Ligands and Ligand Exchange: Orientation
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⁺.
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
Modelled example 1
Oxidation State in a Cyanide Complex
Problem
Study the worked solution
Total ligand charge
Method
Assign six CN⁻ ligands a total charge of -6.Reason
Each cyanide ligand carries charge -1.Working
6(-1) = -6.Match the complex charge
Method
Solve x-6 = -4.Reason
Metal plus ligand charges equal the overall complex charge.Working
x = +2.
Common misconception 2
Charge of a Cobalt Ammine Complex
Learner claim
Account for ligand charge
View solution step by step
Assign ligand charge
Method
Treat all six ammonia ligands as neutral.Reason
Ligand count does not imply ionic charge.Working
6(0) = 0.Write the complex
Method
Keep the cobalt(II) charge.Reason
+ 2 + 0 = +2.Working
[Co(NH₃)₆]²⁺.
Challenge 3
Copper(II) Hydroxide from a Hexaaqua Ion
Equation transfer
Balance charge, hydroxide and water
Hints
Hint 1: solid
Hint 2: ligands
View solution step by step
Form the neutral solid
Method
Combine copper(II) with two hydroxides.Reason
Cu(OH)₂ is charge neutral.Working
Cu²⁺ + 2OH⁻ → Cu(OH)₂(s).Account for aqua ligands
Method
Release six water molecules.Reason
The starting copper species is hexaaqua.Working
[Cu(H₂O)₆]²⁺ + 2OH⁻ → Cu(OH)₂(s) + 6H₂O.
Mind Stretchers
Mind stretcher 1: Following a reversible chloride exchangeExtension
Question. Concentrated chloride solution changes a pale-blue copper(II) solution to a different colour. Adding much water restores the pale-blue colour. Explain both changes and state what evidence would distinguish this process from a redox reaction.
Show Hint
Write the reversible exchange equation, then compare the copper oxidation state before and after the colour change.
Show Answer
Added chloride shifts [Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ + 6H₂O towards the chloride complex. Dilution lowers chloride concentration and shifts the equilibrium back towards pale-blue [Cu(H₂O)₆]²⁺. The copper oxidation state remains +2 on both sides; showing no oxidation-number change distinguishes ligand exchange from redox.
Mind stretcher 2: Competing ligands in haemoglobinExtension
Question. Carbon monoxide binds more strongly than oxygen to the same metal centre in haemoglobin. Explain, using ligand exchange and equilibrium, why even a modest carbon monoxide exposure can reduce oxygen transport.
Show Hint
Treat oxygen and carbon monoxide as competing ligands at the same binding site, not as redox reagents.
Show Answer
Oxygen and carbon monoxide act as ligands competing for the same metal centre. Stronger binding by carbon monoxide shifts ligand exchange towards the carbon monoxide complex, so fewer sites remain available for reversible oxygen binding. Oxygen transport therefore falls even though carbon monoxide is not itself consuming the oxygen in a redox reaction.