Ch.21 - Transition Elements and Coordination Chemistry

McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook

McMurry 8th Edition

Ch.21 - Transition Elements and Coordination Chemistry

Problem 21.108

Chapter 21, Problem 21.108

The Ni²⁺(aq) cation is green, but Zn²⁺(aq) is colorless. Explain.

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Understand the electronic configuration of the metal ions: Ni²⁺ and Zn²⁺. Nickel has an electron configuration that allows for unpaired electrons in its d-orbitals, whereas zinc typically has a full d-orbital with no unpaired electrons.

Recognize the role of unpaired electrons in color: The presence of unpaired electrons in the d-orbitals of a transition metal ion like Ni²⁺ allows for electronic transitions within the d-orbitals when light is absorbed. These transitions can impart color to the compound.

Consider the absence of unpaired electrons in Zn²⁺: Since Zn²⁺ has a full d-orbital with paired electrons, it does not undergo the same electronic transitions in the d-orbitals, and thus does not absorb visible light, appearing colorless.

Relate the absorption of light to color visibility: The specific wavelengths of light absorbed by Ni²⁺ due to its electronic transitions result in the complementary color being observed, which in this case is green.

Summarize the difference in electronic structure and its impact on color: The fundamental difference in the electronic structures of Ni²⁺ and Zn²⁺ explains why Ni²⁺ is colored and Zn²⁺ is colorless. The presence or absence of unpaired electrons in the d-orbitals plays a crucial role in the color properties of these metal ions.

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Key Concepts

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

Transition Metal Ions and Color

Transition metal ions often exhibit color due to the presence of partially filled d-orbitals. When light hits these ions, electrons can be excited from lower to higher energy d-orbitals, absorbing specific wavelengths of light. The color observed is the complementary color of the light absorbed, which is why Ni2+ appears green.

Electronic Configuration of Ions

The electronic configuration of an ion determines its chemical properties, including color. Ni2+ has an electronic configuration of [Ar] 3d8, which allows for d-d transitions that result in color. In contrast, Zn2+ has a full d-orbital configuration of [Ar] 3d10, preventing such transitions and resulting in a colorless solution.

Ligand Field Theory

Ligand field theory explains how the presence of ligands around a metal ion affects its electronic structure and energy levels. In the case of Ni2+, ligands cause splitting of the d-orbitals, allowing for the absorption of visible light. Zn2+, with its filled d-orbitals, does not experience this splitting in a way that leads to color, hence it remains colorless.

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What is the systematic name for each of the following coordination compounds?

(d) Cu(gly)₂

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What is the name of the compound [Fe(H2O)5(SCN)]Cl2?

(a) pentaaquathiocyanatoiron(III) chloride

(b) pentaaquachlorothiocyanato iron(III)

(d) pentaaquathiocyanatoiron(II) chloride

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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⁺

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Draw the structure of all isomers of the octahedral complex [NbX₂Cl₄]^- (X- = NCS^-), and identify those that are linkage isomers.

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Textbook Question

Which of the following complexes can exist as enantiomers? Draw their structures.

(a) [Cr(en)₃]³⁺

(b) cis-[Co(NH₃)Cl]²⁺

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