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Ch.21 - Transition Elements and Coordination Chemistry
All textbooksMcMurry 8th EditionCh.21 - Transition Elements and Coordination ChemistryProblem 125
Chapter 21, Problem 125

Based on the colors of their Cr(III) complexes, arrange the following ligands in a spectrochemical series in order of increasing crystal field splitting (∆): acac- (a bidentate ligand), CH3CO2- (acetate), Cl-, H2O, NH3, and urea. The colors of the Cr(III) complexes are red for Cr(acac)3, violet for [Cr(H2O)6]3+, green for [CrCl2(H2O)4]+, green for [Cr(urea)6]3+, yellow for [Cr(NH3)6]3+, and blue-violet for Cr(CH3CO2)3(H2O)3.

Verified step by step guidance
1
Step 1: Understand the concept of the spectrochemical series, which ranks ligands based on their ability to split the d-orbitals of a metal ion in a complex. Ligands that cause a larger splitting (∆) are considered stronger field ligands.
Step 2: Recognize that the color of a complex is related to the wavelength of light absorbed, which corresponds to the energy difference between the split d-orbitals. A complex that absorbs higher energy (shorter wavelength) light will appear as the complementary color of the absorbed light.
Step 3: Identify the colors of the given Cr(III) complexes: red for Cr(acac)3, violet for [Cr(H2O)6]3+, green for [CrCl2(H2O)4]+, green for [Cr(urea)6]3+, yellow for [Cr(NH3)6]3+, and blue-violet for Cr(CH3CO2)3(H2O)3.
Step 4: Use the color wheel to determine the approximate wavelength of light absorbed by each complex. For example, a red complex absorbs green light, which is of higher energy than the light absorbed by a violet complex.
Step 5: Arrange the ligands in order of increasing crystal field splitting (∆) based on the colors of the complexes: Cl- (green), urea (green), CH3CO2- (blue-violet), H2O (violet), acac- (red), NH3 (yellow).

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 how the arrangement of ligands around a central metal ion affects the energy levels of the d-orbitals. 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, denoted as Δ, is influenced by the nature of the ligands and their positions in the spectrochemical series.
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Guided course
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The study of ligand-metal interactions helped to form Ligand Field Theory which combines CFT with MO Theory.

Spectrochemical Series

The spectrochemical series is a list that ranks ligands based on their ability to split the d-orbitals of transition metal complexes. Ligands that produce a larger splitting of the d-orbitals are considered strong field ligands, while those that cause less splitting are weak field ligands. Understanding this series is crucial for predicting the colors and magnetic properties of metal complexes.
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Activity Series Chart

Color and Wavelength

The color observed in transition metal complexes is a result of the specific wavelengths of light absorbed by the d-electrons during electronic transitions. When a complex absorbs light, the color seen is complementary to the color absorbed. By analyzing the colors of the Cr(III) complexes provided, one can infer the relative crystal field splitting and thus arrange the ligands in the spectrochemical series.
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Frequency-Wavelength Relationship
Related Practice
Textbook Question

For each of the following, (i) give the systematic name of the compound and specify the oxidation state of the transition metal, (ii) draw a crystal field energy-level diagram and assign the d electrons to orbitals, (iii) indicate whether the complex is high-spin or low-spin (for d4 - d7 complexes), and (iv) specify the number of unpaired electrons.

(e) [Pt(NH3)4](ClO4)2

Textbook Question

For each of the following, (i) give the systematic name of the compound and specify the oxidation state of the transition metal, (ii) draw a crystal field energy-level diagram and assign the d electrons to orbitals, (iii) indicate whether the complex is high-spin or low-spin (for d4 - d7 complexes), and (iv) specify the number of unpaired electrons.

(f) Na2[Fe(CO)4]

Textbook Question

Give a valence bond description of the bonding in each of the following complexes. Include orbital diagrams for the free metal ion and the metal ion in the complex. Indicate which hybrid orbitals the metal ion uses for bonding, and specify the number of unpaired electrons.

(a) [Ti(H2O)6]3+

Textbook Question

Give a valence bond description of the bonding in each of the following complexes. Include orbital diagrams for the free metal ion and the metal ion in the complex. Indicate which hybrid orbitals the metal ion uses for bonding, and specify the number of unpaired electrons.

(c) [Fe(CN)6]3- (low-spin)

Textbook Question

Give a valence bond description of the bonding in each of the following complexes. Include orbital diagrams for the free metal ion and the metal ion in the complex. Indicate which hybrid orbitals the metal ion uses for bonding, and specify the number of unpaired electrons.

(d) [MnCl6]32 (high-spin)