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Ch.21 - Transition Elements and Coordination Chemistry
McMurry - Chemistry 8th Edition
McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook
All textbooksMcMurry 8th EditionCh.21 - Transition Elements and Coordination ChemistryProblem 21.120
Chapter 21, Problem 21.120

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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Step 1: Recall that the color of a complex is complementary to the color absorbed by the complex. This is due to the fact that when white light (which contains all colors) shines on the complex, the complex absorbs a certain color and the remaining colors are what we see.
Step 2: The color absorbed by the complex corresponds to a certain energy, which is the energy difference between the ground state and the excited state of the d-electrons in the metal ion. This energy difference is influenced by the field strength of the ligands: stronger field ligands cause a larger energy difference, which corresponds to a shorter wavelength (and thus a color towards the violet end of the spectrum).
Step 3: Look at the colors of the isomeric complexes in Figure 21.12. The complex with the color towards the red end of the spectrum is the one that absorbs a color towards the violet end, and thus has a stronger field ligand.
Step 4: Identify which of the isomers has the color towards the red end of the spectrum. This is the isomer with the stronger field ligand.
Step 5: Therefore, if the isomer with the nitro (-NO2) ligand is towards the red end of the spectrum, then nitro is the stronger field ligand. If the isomer with the nitrito (-ONO) ligand is towards the red end of the spectrum, then nitrito is the stronger field ligand.

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Key Concepts

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

Ligand Field Theory

Ligand Field Theory explains how the presence of ligands around a central metal ion affects the energy levels of the d-orbitals. Strong field ligands cause a larger splitting of these d-orbitals, leading to greater stabilization of low-energy states. This theory is crucial for understanding the color and magnetic properties of coordination complexes.
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Strong-Field Ligands result in a large Δ and Weak-Field Ligands result in a small Δ.

Crystal Field Splitting

Crystal Field Splitting refers to the energy difference between the split d-orbitals in a transition metal complex due to the electrostatic interactions between the metal ion and surrounding ligands. The extent of this splitting is influenced by the nature of the ligands, with strong field ligands causing larger splits, which can affect the color observed in the complex.
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The crystal field splitting pattern for octahedral complexes has the d orbitals on or along the axes as having the higher energy.

Field Strength of Ligands

The field strength of ligands is a measure of their ability to split the d-orbitals of a metal ion. Ligands are classified as strong or weak field based on their ability to cause this splitting. Nitro (-NO2) is generally considered a stronger field ligand than nitrito (-ONO), leading to greater d-orbital splitting and different electronic transitions, which can be observed as distinct colors in 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

The oxalate ion is a bidentate ligand as indicated in Figure 21.8. Would you expect the carbonate ion to be a monodentate or bidentate ligand? Explain your reasoning.

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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

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

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

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