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Ch.23 - Transition Metals and Coordination Chemistry
All textbooksBrown 15th EditionCh.23 - Transition Metals and Coordination ChemistryProblem 82
Chapter 23, Problem 82

Complete the exercises below. Solutions of [Co(NH₃)₆]²⁺, [Co(H₂O)₆]²⁺ (both octahedral), and [CoCl₄]²⁻ (tetrahedral) are colored. One is pink, one is blue, and one is yellow. Based on the spectrochemical series and remembering that the energy splitting in tetrahedral complexes is normally much less than that in octahedral ones, assign a color to each complex.

Verified step by step guidance
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Step 1: Understand the spectrochemical series, which ranks ligands based on the strength of the field they produce. Strong field ligands cause a larger splitting of the d-orbitals, while weak field ligands cause a smaller splitting.
Step 2: Identify the ligands in each complex: [Co(NH₃)₆]²⁺ has NH₃ ligands, [Co(H₂O)₆]²⁺ has H₂O ligands, and [CoCl₄]²⁻ has Cl⁻ ligands. According to the spectrochemical series, NH₃ is a stronger field ligand than H₂O, and Cl⁻ is a weak field ligand.
Step 3: Recall that the color observed in a complex is complementary to the color of light absorbed. Strong field ligands (like NH₃) cause larger splitting, leading to absorption of higher energy (shorter wavelength) light, while weak field ligands (like Cl⁻) absorb lower energy (longer wavelength) light.
Step 4: Consider the geometry of the complexes. Octahedral complexes generally have larger splitting than tetrahedral ones. Therefore, [Co(NH₃)₆]²⁺ and [Co(H₂O)₆]²⁺, being octahedral, will have larger splitting compared to [CoCl₄]²⁻, which is tetrahedral.
Step 5: Assign colors based on the above analysis: [Co(NH₃)₆]²⁺, with the strongest field ligand, is likely to be pink (absorbing green light), [Co(H₂O)₆]²⁺ is likely to be blue (absorbing orange light), and [CoCl₄]²⁻, with the weakest field ligand, is likely to be yellow (absorbing violet light).

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. In octahedral complexes, the d-orbitals split into two energy levels (t2g and eg), while in tetrahedral complexes, they split into a different pattern. This theory helps predict the colors of complexes based on the wavelengths of light absorbed during electronic transitions.
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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 the strength of the field they create around a metal ion. Strong field ligands cause greater splitting of d-orbitals, leading to higher energy transitions and different colors. Understanding this series is crucial for predicting the color of coordination complexes based on their ligands and geometry.
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Activity Series Chart

Color and Wavelength Absorption

The color observed in a solution is the result of the wavelengths of light that are transmitted or reflected, while the complementary color is absorbed. For example, if a complex appears blue, it absorbs light in the orange region of the spectrum. By analyzing the colors of the complexes and their expected absorption based on their d-orbital splitting, one can assign the correct colors to each complex.
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Frequency-Wavelength Relationship
Related Practice
Textbook Question

Carbon monoxide, CO, is an important ligand in coordination chemistry. When CO is reacted with nickel metal, the product is [Ni(CO)4] which is a toxic, pale yellow liquid.

a. What is the oxidation number for nickel in this compound?

b. Given that [Ni(CO)4] is a diamagnetic molecule with a tetrahedral geometry, what is the electron configuration of nickel in this compound?

c. Write the name for [Ni(CO)4] using the nomenclature rules for coordination compounds.