Ch.21 - Transition Elements and Coordination Chemistry

McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook

McMurry 8th Edition

Ch.21 - Transition Elements and Coordination Chemistry

Problem 21.105

Chapter 21, Problem 21.105

The [Cr(H₂O)₆]³⁺ ion is violet, and [Cr(CN)₆]^3- is yellow. Explain this difference using crystal field theory. Use the colors to order H₂O and CN^- in the spectrochemical series.

Verified step by step guidance

Step 1: Understand that the color of a complex ion is due to the absorption of light which promotes an electron from a lower energy d-orbital to a higher energy d-orbital. The color we see is the complementary color of the light absorbed.

Step 2: Recognize that the crystal field theory explains the color of transition metal complexes in terms of the splitting of their d-orbitals into two sets of different energies when ligands approach the metal ion. The difference in energy between these two sets of d-orbitals corresponds to the energy of light absorbed.

Step 3: Realize that the extent of splitting of the d-orbitals, and hence the color of the complex, depends on the nature of the ligands. Ligands that cause a large splitting of the d-orbitals are known as strong-field ligands, while those that cause a small splitting are known as weak-field ligands.

Step 4: Infer from the given colors that [Cr(H₂O)₆]³⁺ absorbs light of higher energy (shorter wavelength) than [Cr(CN)₆]^3-. This is because violet light is the complementary color of yellow light, which is of lower energy (longer wavelength). Therefore, H₂O is a weaker field ligand than CN^-.

Step 5: Conclude that in the spectrochemical series, which orders ligands according to their ability to split the d-orbitals, H₂O comes before CN^-.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Crystal Field Theory

Crystal Field Theory (CFT) explains the electronic structure of transition metal complexes by considering the effect of surrounding ligands on the d-orbitals of the metal ion. When ligands approach the metal ion, they create an electric field that splits the degenerate d-orbitals into different energy levels. The extent of this splitting depends on the nature of the ligands and their arrangement around the metal ion.

Spectrochemical Series

The spectrochemical series is a list that ranks ligands based on the strength of the field they produce when coordinating to a metal ion. Strong field ligands, like CN-, cause a larger splitting of the d-orbitals, leading to higher energy transitions and different colors. Conversely, weak field ligands, such as H2O, result in smaller splitting and lower energy transitions, which can be observed in the color of the complexes.

Color and d-Orbital Splitting

The color observed in transition metal complexes is a result of the specific wavelengths of light absorbed during electronic transitions between split d-orbitals. The energy difference between these orbitals corresponds to specific colors in the visible spectrum. For example, the violet color of [Cr(H2O)6]3+ indicates that it absorbs light in the yellow-green region, while the yellow color of [Cr(CN)6]3- suggests it absorbs light in the violet region, reflecting the differences in ligand field strength.

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Related Practice

Textbook Question

What is the oxidation state of the metal in each of the complexes?

a. AgCl₂^–

b. [Cr(H2O)₅Cl]²⁺

c. [Co(NCS)₄]^2–

d. [ZrF₈]^4–

e. [Fe(EDTA)(H₂O)]^–

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Textbook Question

Draw the structures of all possible diastereoisomers of an octahedral complex with the formula MA₂B₂C₂. Which of the diastereoisomers, if any, can exist as enantiomers?

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Textbook Question

What is the electron configuration of Co²⁺ and how many unpaired electrons are in the free transition metal ion?

(a) [Ar]3d⁵4s²; 5 unpaired electrons

(b) [Ar]3d⁵4s²; 1 unpaired electron

(d) [Ar]3d⁷; 1 unpaired electron

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Textbook Question

Look at the colors of the isomeric complexes in Figure 21.12, and predict which is the stronger field ligand, nitro (-NO₂) of nitrito (-ONO). Explain.

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Textbook Question

Based on the wavelength of maximum absorption of the cobalt complexes, arrange the following ligands in a spectrochemical series from weakest-field to strongest-field ligand.

(a) Cl^- < NCS^- < H₂O < NH₃

(b) Cl^- < NCS^- < H₂O < NH₃

(d) Cl^- < H₂O < NCS^- < NH₃

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Textbook Question

Based on effective nuclear charge (Z_eff), which ion is the strongest oxidizing agent?

(a) Cu²⁺

(b) Ni²⁺

(d) Mn²⁺

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