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Ch.21 - Transition Elements and Coordination Chemistry
McMurry - Chemistry 8th Edition
McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook
Chapter 21, Problem 21.8

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 
(c) H2O < Cl- < NH3 < NCS-
(d) Cl- < H2O < NCS- < NH3

Verified step by step guidance
1
Understand the concept of the spectrochemical series, which ranks ligands based on their ability to split the d-orbitals of a metal ion.
Recall that ligands causing a smaller splitting (weak-field ligands) result in longer wavelengths of maximum absorption, while those causing larger splitting (strong-field ligands) result in shorter wavelengths.
Identify the typical order of ligands in the spectrochemical series: Cl^- < H2O < NCS^- < NH3.
Compare the given options with the typical order to determine which one matches the weakest to strongest field ligands.
Select the option that correctly arranges the ligands from weakest-field to strongest-field based on the spectrochemical series.

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Key Concepts

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

Spectrochemical Series

The spectrochemical series is a ranking of 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. This series helps predict the color and magnetic properties of the complexes formed with different ligands.
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Activity Series Chart

Crystal Field Theory

Crystal Field Theory (CFT) explains the electronic structure of transition metal complexes by considering the interaction between the metal ion and surrounding ligands. According to CFT, the presence of ligands causes the degenerate d-orbitals to split into different energy levels, influencing the absorption of light and the resulting color of the complex. Understanding CFT is essential for analyzing how ligands affect the properties of metal complexes.
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The study of ligand-metal interactions helped to form Ligand Field Theory which combines CFT with MO Theory.

Ligand Field Strength

Ligand field strength refers to the ability of a ligand to influence the energy levels of the d-orbitals in a transition metal complex. Strong-field ligands, such as NH3 and CN-, cause a greater splitting of the d-orbitals compared to weak-field ligands like Cl-. This concept is crucial for determining the arrangement of ligands in the spectrochemical series and predicting the behavior of the complexes.
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Strong-Field Ligands result in a large Δ and Weak-Field Ligands result in a small Δ.
Related Practice
Textbook Question

What is a racemic mixture? Does it affect plane-polarized light? Explain.

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Textbook Question

The amount of paramagnetism for a first-series transition metal complex is related approximately to its spin-only magnetic moment. The spin-only value of the magnetic moment in units of Bohr magnetons (BM) is given by sqrt(n(n + 2)), where n is the number of unpaired electrons. Calculate the spin-only value of the magnetic moment for the 2+ ions of the first-series transition metals (except Sc) in octahedral complexes with (a) weak-field ligands and (b) strong-field ligands. For which electron configurations can the magnetic moment distinguish between high-spin and low-spin electron configurations?

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Textbook Question

Draw a crystal field energy-level diagram, assign the electrons to orbitals, and predict the number of unpaired electrons for each of the following.

(a) [Cu(en)3]2+

(b) [FeF6]2-

(c) [Co(en)3]3+ (low spin) 

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Textbook Question

Which of the following complexes are paramagnetic?

(a) [Mn(CN)6]3-

(b) [Zn(NH3)4]2+ (tetrahedral)

(c) [Fe(CN)6]4-

(d) [FeF6]4-

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Textbook Question

What hybrid orbitals are used by the metal ion and how many unpaired electrons are present the complex ion [VCl4]- with tetrahedral geometry?

(a) sp3; 2 unpaired electrons

(b) sp3; 3 unpaired electrons

(c) sp3d2; 3 unpaired electrons

(d) sp3d2; 4 unpaired electrons

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Textbook Question

For each of the following complexes, draw a crystal field energy-level diagram, assign the electrons to orbitals, and predict the number of unpaired electrons. 

(a) [CrF6]3-

(b) [V(H2O)6]3+

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

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