6.3.5 Effect of Ligands on Colour of Complexes

In the previous section, we learned that the color of transition metal complexes arises from d-d electronic transitions and depends on the energy gap (Δ E) between the split d orbitals. Now, we will explore how different ligands affect the size of this energy gap and, consequently, the color of the complex. This is a key concept in coordination chemistry and helps explain why the same metal ion can form complexes of different colors with different ligands.

Main Idea

The main idea is that when ligands approach a transition metal ion, their electric fields interact with the d orbitals of the metal ion, causing the d orbitals to split into two different energy levels. Different ligands have different abilities to split the d orbitals, and they follow a specific order known as the spectrochemical series. A strong field ligand causes a larger splitting (higher Δ E) than a weak field ligand. This change in Δ E changes the wavelength of light absorbed and, therefore, the color observed.

Core Concept

The core concept is that the strength of the ligand field determines the magnitude of the d orbital splitting. Strong-field ligands create a large energy gap, causing the complex to absorb higher energy (shorter wavelength) light. Weak-field ligands create a smaller energy gap, causing the complex to absorb lower energy (longer wavelength) light. The color observed is always the complementary color of the light absorbed.

Detailed Explanation

The Spectrochemical Series

The textbook states that on the basis of their different abilities to split d orbitals, ligands show different field strengths and follow the order:

CN^- > NH_3 > H_2O > OH^- > Cl^- > Br^-

This order is known as the spectrochemical series.

• Strong-field ligands (e.g., CN^-, NH_3) cause a large splitting of the d orbitals.
• Weak-field ligands (e.g., Cl^-, Br^-) cause a small splitting.

A strong field ligand splits d orbitals to a greater extent, hence causing higher splitting energy (Δ E) than a weak field ligand.

How Ligands Affect Color: The Example of Copper(II)

The textbook provides a clear example of how changing the ligand changes the color of a copper(II) complex.

Starting Point: The Hexaaquacopper(II) Ion

When copper(II) ions are dissolved in water, they form the hexaaquacopper(II) complex ion: [Cu(H_2O)_6]²⁺ This complex appears blue. The water ligands are weak-field ligands, causing a relatively small splitting of the d orbitals. The complex absorbs orange light and appears blue (the complementary color).

Adding Ammonia: Ligand Exchange

When ammonia solution is added to the aqueous solution of copper(II) ions, a ligand exchange reaction occurs. Ammonia is a stronger field ligand than water, so it will replace water in the complex. The reaction is: [Cu(H_2O)_6]²⁺_(aq) + 4NH_3_(aq) ⟶ [Cu(NH_3)_4(H_2O)_2]²⁺_(aq) + 4H_2O_(l) - Before: The solution is blue due to [Cu(H_2O)_6]²⁺. - After: The solution becomes deep blue (violet-blue) due to the formation of [Cu(NH_3)_4(H_2O)_2]²⁺.

Why the Color Changes:

The ammonia ligands are stronger-field ligands than water. They cause a larger splitting of the d orbitals (larger Δ E). This means the complex now absorbs light of a higher energy (shorter wavelength). The absorption shifts from orange (in the aqua complex) to a different wavelength (often described as shifting the absorption from blue to violet-blue, as stated in the textbook). Because the wavelength of light absorbed changes, the complementary color observed also changes, resulting in the deep blue color.

The textbook summarizes this by saying: "In this reaction, four ammonia molecules replace four water molecules, shifting absorption from blue to violet-blue."

Summary of Effects

• Strong-field ligands (like NH_3, CN^-): Larger splitting Δ E → absorb higher energy light → different color.
• Weak-field ligands (like H_2O, Cl^-): Smaller splitting Δ E → absorb lower energy light → different color.
• The observed color is always the complementary color of the absorbed light.

Intermediate Level

The color of a transition metal complex depends on the energy gap (Δ E) between the split d orbitals. This gap is determined by the ligand field strength. Ligands can be arranged in the spectrochemical series according to their ability to split the d orbitals: CN^- > NH_3 > H_2O > OH^- > Cl^- > Br^-. Strong-field ligands (like NH_3 and CN^-) cause a large splitting, so the complex absorbs higher energy (shorter wavelength) light. Weak-field ligands (like H_2O and Cl^-) cause a small splitting, so the complex absorbs lower energy (longer wavelength) light. When a ligand exchange reaction occurs, such as replacing water with ammonia in a copper(II) complex, the color changes because the new ligand changes the magnitude of Δ E, thus changing the wavelength of light absorbed and the complementary color observed.

Important Points

• - Spectrochemical Series: A ranking of ligands by their ability to split d orbitals.
• - Order: CN^- > NH_3 > H_2O > OH^- > Cl^- > Br^-.
• - Strong-field ligands: Cause large splitting (higher Δ E).
• - Weak-field ligands: Cause small splitting (lower Δ E).
• - Ligand exchange changes the color of the complex by changing the energy gap.
• - Example: [Cu(H_2O)_6]²⁺ (blue) + NH_3 → [Cu(NH_3)_4(H_2O)_2]²⁺ (deep blue).
• - Reason for color change: Ammonia is a stronger-field ligand than water, causing a larger splitting and changing the wavelength of light absorbed.

Remember

• - Spectrochemical series: CN^- > NH_3 > H_2O > OH^- > Cl^- > Br^-.
• - Strong ligand = larger splitting = higher energy absorbed.
• - Weak ligand = smaller splitting = lower energy absorbed.
• - Color change = change in Δ E.
• - Example: Water is a weaker ligand than ammonia. The copper-ammonia complex is a deeper blue than the copper-water complex.

Common Mistakes

• - Thinking that all ligands cause the same splitting. Different ligands have different field strengths and cause different splittings.
• - Forgetting the spectrochemical series order. Remember that CN^- is the strongest and Br^- is the weakest among the common ligands listed.
• - Confusing the effect of ligand strength on the color. A stronger ligand causes a larger splitting, which means it absorbs higher-energy light. This changes the observed color.
• - Assuming that the color observed is the color absorbed. It is the complementary color.
• - Not recognizing that ligand exchange changes the color. The textbook example clearly shows that adding ammonia to a blue copper solution changes it to a deep blue solution.

Quick Revision

The color of a transition metal complex is determined by the energy gap (Δ E) between the split d orbitals, which in turn depends on the ligand field strength. Ligands can be ranked in the spectrochemical series: CN^- > NH_3 > H_2O > OH^- > Cl^- > Br^-. Strong-field ligands like CN^- and NH_3 cause a large splitting, so the complex absorbs higher energy light. Weak-field ligands like H_2O and Cl^- cause a smaller splitting. When one ligand is exchanged for another, the color of the complex changes because the new ligand alters the magnitude of Δ E and hence the wavelength of light absorbed. For example, adding ammonia to a blue solution of [Cu(H_2O)_6]²⁺ forms [Cu(NH_3)_4(H_2O)_2]²⁺, which is deep blue because ammonia is a stronger-field ligand than water.

60-Second Revision

Ligands affect the color of transition metal complexes by changing the energy gap (Δ E) between the split d orbitals. The spectrochemical series ranks ligands by their field strength: CN^- > NH_3 > H_2O > OH^- > Cl^- > Br^-. Strong-field ligands cause a large splitting and absorb higher-energy light. Weak-field ligands cause a smaller splitting. Changing the ligand (ligand exchange) changes the color. For example, adding ammonia to [Cu(H_2O)_6]²⁺ (blue) forms [Cu(NH_3)_4(H_2O)_2]²⁺, which is deep blue.

Frequently Asked Questions (FAQs)

Q1. What is the spectrochemical series? Answer: The spectrochemical series is a ranking of ligands by their ability to split the d orbitals of a transition metal ion. The order of field strength is: CN^- > NH_3 > H_2O > OH^- > Cl^- > Br^-. Strong-field ligands (like CN^-) cause a large splitting, while weak-field ligands (like Cl^-) cause a small splitting.

Q2. Why does the color of a complex change when a different ligand is added? Answer: The color of a complex depends on the energy gap (Δ E) between the split d orbitals. Different ligands cause different degrees of splitting. When one ligand is replaced by another (ligand exchange), the energy gap changes. This changes the wavelength of light absorbed, and therefore the complementary color observed also changes.

Q3. What happens to the color of a copper(II) solution when ammonia is added? Answer: When ammonia is added to a blue solution of [Cu(H_2O)_6]²⁺, the ammonia molecules replace the water molecules to form [Cu(NH_3)_4(H_2O)_2]²⁺. The solution changes from blue to deep blue (violet-blue) because ammonia is a stronger-field ligand than water, causing a larger splitting of the d orbitals.

Q4. Which ligand is stronger: H_2O or NH_3? Answer: NH_3 is a stronger-field ligand than H_2O. This is evident from the spectrochemical series, where NH_3 is placed before H_2O. The stronger ligand causes a larger splitting of the d orbitals.

Q5. Why does the [Cu(NH_3)_4(H_2O)_2]²⁺ complex have a different color than the [Cu(H_2O)_6]²⁺ complex? Answer: The two complexes have different ligands, which cause different degrees of d orbital splitting. NH_3 is a stronger-field ligand than H_2O, so it causes a larger energy gap (Δ E). This means the complex absorbs light of a different wavelength, resulting in a different observed color.

Q6. What is the relationship between the ligand field strength and the color of a complex? Answer: A stronger-field ligand causes a larger splitting of the d orbitals. This means the complex absorbs higher energy (shorter wavelength) light. A weaker-field ligand causes a smaller splitting, so the complex absorbs lower energy (longer wavelength) light. The observed color is the complementary color of the absorbed light.

Q7. How does the spectrochemical series help predict the color of a complex? Answer: The spectrochemical series ranks ligands by their field strength. By knowing the relative field strengths of the ligands, you can predict whether a complex will have a larger or smaller Δ E, and thus whether it will absorb higher or lower energy light. This helps in predicting the color of the complex.

Q8. What is a ligand exchange reaction? Answer: A ligand exchange reaction (or ligand substitution reaction) is a reaction in which one ligand in a complex is replaced by another ligand. For example, the replacement of water by ammonia in the copper(II) complex: [Cu(H_2O)_6]²⁺ + 4NH_3 → [Cu(NH_3)_4(H_2O)_2]²⁺ + 4H_2O

Q9. Why is the color of the [Cu(NH_3)_4(H_2O)_2]²⁺ complex described as "violet-blue"? Answer: The complex absorbs orange light. The complementary color of orange is blue, but due to the specific energy gap created by the ammonia ligands, the absorption is shifted, and the observed color is a deeper, more intense blue-violet. The textbook describes this as "shifting absorption from blue to violet-blue."

Multiple Choice Questions (MCQs)

Q1. According to the spectrochemical series, which ligand is the strongest? A. H_2O B. NH_3 C. Cl^- D. CN^- Correct Answer: D Explanation: The spectrochemical series lists ligands in order of increasing field strength: Br^- < Cl^- < OH^- < H_2O < NH_3 < CN^-. Therefore, CN^- is the strongest among the given options.

Q2. What happens to the color of a copper(II) solution when ammonia is added? A. It changes from blue to green. B. It changes from blue to deep blue. C. It changes from blue to red. D. It remains blue. Correct Answer: B Explanation: The addition of ammonia replaces water ligands with ammonia, forming a complex that has a deeper blue color due to the stronger field of the ammonia ligand.

• Q3. Which of the following is a weak-field ligand?
• A. CN^-
• B. NH_3
• C. H_2O
• D. Cl^-
• Correct Answer: D
• Explanation: In the spectrochemical series, Cl^- is a weaker-field ligand than CN^-, NH_3, and H_2O.

• Q4. Why does a stronger-field ligand cause a larger splitting of d orbitals?
• A. Because it has a larger size.
• B. Because it interacts more strongly with the d electrons.
• C. Because it is a neutral molecule.
• D. Because it is a cation.
• Correct Answer: B
• Explanation: A stronger-field ligand creates a stronger electric field, which causes greater repulsion with the d electrons and a larger energy splitting.

• Q5. What is the color of the [Cu(H_2O)_6]²⁺ complex?
• A. Green
• B. Blue
• C. Red
• D. Yellow
• Correct Answer: B
• Explanation: The hexaaquacopper(II) ion is blue because it absorbs orange light and transmits the remaining blue light.

• Q6. Which of the following ligands causes the largest splitting of d orbitals?
• A. Br^-
• B. Cl^-
• C. H_2O
• D. NH_3
• Correct Answer: D
• Explanation: Among the given options, NH_3 is the strongest-field ligand and causes the largest splitting.

• Q7. What is the relationship between the field strength of a ligand and the energy gap Δ E?
• A. Stronger field → smaller Δ E.
• B. Stronger field → larger Δ E.
• C. There is no relationship.
• D. The energy gap is independent of the ligand.
• Correct Answer: B
• Explanation: A stronger-field ligand causes a larger splitting of the d orbitals, resulting in a larger energy gap Δ E.

• Q8. When [Cu(H_2O)_6]²⁺ is converted to [Cu(NH_3)_4(H_2O)_2]²⁺, what type of reaction occurs?
• A. Redox reaction
• B. Ligand exchange reaction
• C. Precipitation reaction
• D. Acid-base reaction
• Correct Answer: B
• Explanation: The water ligands are replaced by ammonia ligands, which is a ligand exchange (substitution) reaction.

• Q9. The spectrochemical series is a ranking of:
• A. Metals by their reactivity.
• B. Ligands by their field strength.
• C. Anions by their charge.
• D. Complexes by their color.
• Correct Answer: B
• Explanation: The spectrochemical series ranks ligands by their ability to split the d orbitals (their field strength).

• Q10. If a complex absorbs yellow-green light, what color will it appear?
• A. Red-violet
• B. Blue
• C. Green
• D. Orange
• Correct Answer: A
• Explanation: The complementary color of yellow-green is red-violet.

Multiple Choice Questions (MCQs)

• Q1. According to the spectrochemical series, which ligand is the strongest?
• A. H_2O
• B. NH_3
• C. Cl^-
• D. CN^-
• Correct Answer: D
• Explanation: CN^- is the strongest-field ligand among the options.

• Q2. What happens to the color of a copper(II) solution when ammonia is added?
• A. It changes from blue to green.
• B. It changes from blue to deep blue.
• C. It changes from blue to red.
• D. It remains blue.
• Correct Answer: B
• Explanation: The addition of ammonia replaces water ligands with ammonia, forming a deeper blue complex.

• Q3. Which of the following is a weak-field ligand?
• A. CN^-
• B. NH_3
• C. H_2O
• D. Cl^-
• Correct Answer: D
• Explanation: Cl^- is a weaker-field ligand than CN^-, NH_3, and H_2O.

• Q4. Why does a stronger-field ligand cause a larger splitting of d orbitals?
• A. Because it has a larger size.
• B. Because it interacts more strongly with the d electrons.
• C. Because it is a neutral molecule.
• D. Because it is a cation.
• Correct Answer: B
• Explanation: A stronger-field ligand creates a stronger electric field, causing greater repulsion and a larger splitting.

• Q5. What is the color of the [Cu(H_2O)_6]²⁺ complex?
• A. Green
• B. Blue
• C. Red
• D. Yellow
• Correct Answer: B
• Explanation: The hexaaquacopper(II) ion is blue.

• Q6. Which of the following ligands causes the largest splitting of d orbitals?
• A. Br^-
• B. Cl^-
• C. H_2O
• D. NH_3
• Correct Answer: D
• Explanation: Among the given options, NH_3 is the strongest-field ligand.

• Q7. What is the relationship between the field strength of a ligand and the energy gap Δ E?
• A. Stronger field → smaller Δ E.
• B. Stronger field → larger Δ E.
• C. There is no relationship.
• D. The energy gap is independent of the ligand.
• Correct Answer: B
• Explanation: A stronger-field ligand causes a larger splitting, resulting in a larger Δ E.

• Q8. When [Cu(H_2O)_6]²⁺ is converted to [Cu(NH_3)_4(H_2O)_2]²⁺, what type of reaction occurs?
• A. Redox reaction
• B. Ligand exchange reaction
• C. Precipitation reaction
• D. Acid-base reaction
• Correct Answer: B
• Explanation: The water ligands are replaced by ammonia ligands, which is a ligand exchange reaction.

• Q9. The spectrochemical series is a ranking of:
• A. Metals by their reactivity.
• B. Ligands by their field strength.
• C. Anions by their charge.
• D. Complexes by their color.
• Correct Answer: B
• Explanation: The spectrochemical series ranks ligands by their field strength.

• Q10. If a complex absorbs yellow-green light, what color will it appear?
• A. Red-violet
• B. Blue
• C. Green
• D. Orange
• Correct Answer: A
• Explanation: The complementary color of yellow-green is red-violet.

Multiple Choice Questions (MCQs)

• Q1. Which ligand is stronger than NH_3 in the spectrochemical series?
• A. H_2O
• B. Cl^-
• C. CN^-
• D. OH^-
• Correct Answer: C
• Explanation: The spectrochemical series order is CN^- > NH_3 > H_2O > OH^- > Cl^-.

• Q2. What is the color of the [Cu(NH_3)_4(H_2O)_2]²⁺ complex?
• A. Pale blue
• B. Deep blue
• C. Green
• D. Colorless
• Correct Answer: B
• Explanation: The ammonia complex of copper is deep blue.

• Q3. Which of the following is a strong-field ligand?
• A. Cl^-
• B. Br^-
• C. NH_3
• D. OH^-
• Correct Answer: C
• Explanation: NH_3 is a strong-field ligand compared to Cl^-, Br^-, and OH^-.

• Q4. What is the effect of a weak-field ligand on the splitting of d orbitals?
• A. It causes a large splitting.
• B. It causes a small splitting.
• C. It causes no splitting.
• D. It causes the d orbitals to degenerate.
• Correct Answer: B
• Explanation: A weak-field ligand causes a smaller splitting of the d orbitals.

• Q5. Which of the following statements is true about the color of a complex?
• A. The color observed is the color absorbed.
• B. The color observed is the complementary color of the color absorbed.
• C. The color is always the same for a given metal ion, regardless of the ligand.
• D. The color is independent of the d orbital splitting.
• Correct Answer: B
• Explanation: The color observed is the complementary color of the light absorbed.

• Q6. What is the color of the [Ti(H_2O)_6]³⁺ complex?
• A. Blue
• B. Green
• C. Red-violet
• D. Orange
• Correct Answer: C
• Explanation: [Ti(H_2O)_6]³⁺ absorbs yellow-green light and appears red-violet.

Q7. Which of the following ligands would cause the largest splitting of d orbitals? A. F^- B. Cl^- C. Br^- D. I^- Correct Answer: A Explanation: Among the halides, the field strength decreases down the group: F^- > Cl^- > Br^- > I^-. So F^- causes the largest splitting.

• Q8. What is the relationship between the ligand field strength and the wavelength of light absorbed?
• A. Stronger ligand → longer wavelength absorbed.
• B. Stronger ligand → shorter wavelength absorbed.
• C. There is no relationship.
• D. The wavelength is independent of the ligand.
• Correct Answer: B
• Explanation: A stronger ligand causes a larger splitting, which means higher energy light is absorbed, corresponding to a shorter wavelength.

• Q9. The spectrochemical series is a list of ligands arranged in order of their:
• A. Size
• B. Charge
• C. Ability to split d orbitals
• D. Color
• Correct Answer: C
• Explanation: The spectrochemical series ranks ligands by their ability to split the d orbitals (their field strength).

• Q10. Why is the [Cu(H_2O)_6]²⁺ complex blue while the [Cu(NH_3)_4(H_2O)_2]²⁺ complex is deep blue?
• A. Because ammonia is a stronger-field ligand and causes a larger splitting.
• B. Because water is a stronger-field ligand and causes a larger splitting.
• C. Because the two complexes have different numbers of d electrons.
• D. Because the two complexes have different metal ions.
• Correct Answer: A
• Explanation: Ammonia is a stronger-field ligand than water, causing a larger splitting and a different absorption wavelength, leading to the deeper blue color.