Textbook Question
Although Cl- is a weak-field ligand and CN- is a strong field ligand, [CrCl6]3- and [Cr(CN)6]3- exhibit approximately the same amount of paramagnetism. Explain.
Ch.21 - Transition Elements and Coordination Chemistry
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McMurry 8th EditionCh.21 - Transition Elements and Coordination ChemistryProblem 21.120
McMurry 8th EditionCh.21 - Transition Elements and Coordination ChemistryProblem 21.120Chapter 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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03:04The 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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02:40Strong-Field Ligands result in a large Δ and Weak-Field Ligands result in a small Δ.
Related Practice
Textbook Question
Draw a crystal field energy-level diagram, and predict the number of unpaired electrons for each of the following:
(a) [Mn(H2O)6]2+
Textbook Question
Explain why [CoCl4]2- (blue) and [Co(H2O)6]2+ (pink) have different colors. Which complex has its absorption bands at longer wavelengths?
Textbook Question
Predict the crystal field energy-level diagram for a linear ML2 complex that has two ligands along the :
Textbook Question
Predict the crystal field energy-level diagram for a square pyramidal ML5 complex that has two ligands along the axes but only one ligand along the z axis. Your diagram should be intermediate between those for an octahedral ML6 complex and a square planar ML4 complex.
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.
(b) [NiBr4]2- (tetrahedral)