The Cr3+(aq) cation is violet, but Y3+(aq) is colorless. Explain why this difference in color occurs.

Verified step by step guidance

Step 1: Understand the concept of color in transition metal complexes. The color of a transition metal complex is due to the d-d electron transitions within the d-orbitals, which are split in energy in the presence of a ligand field.

Step 2: Recognize that Cr3+ is a transition metal ion. Chromium in the +3 oxidation state has a partially filled d-orbital (specifically, it has 3 electrons in the 3d orbitals), which allows for d-d transitions when light is absorbed, resulting in the violet color.

Step 3: Note that Y3+ is not a transition metal ion. Yttrium in the +3 oxidation state has a completely empty d-orbital (it has no electrons in the 4d orbitals), so there are no d-d transitions possible, leading to it being colorless.

Step 4: Consider the role of ligand field theory. In Cr3+, the presence of ligands causes the d-orbitals to split into different energy levels, allowing for the absorption of specific wavelengths of light, which is perceived as color.

Step 5: Conclude that the difference in color between Cr3+ and Y3+ is due to the presence of d-electrons in Cr3+ that can undergo electronic transitions, whereas Y3+ lacks d-electrons, preventing such transitions and resulting in a colorless appearance.

Key Concepts

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

d-Block Elements and Color

Transition metals, such as chromium (Cr) and yttrium (Y), have partially filled d-orbitals. The color of a transition metal ion in solution is primarily due to the presence of d-electrons, which can absorb specific wavelengths of light. When these d-electrons transition between energy levels, they absorb certain colors, resulting in the observed color of the solution.

Crystal Field Theory

Crystal Field Theory explains how the arrangement of ligands around a metal ion affects the energy levels of its d-orbitals. In the case of Cr3+, the presence of ligands causes a splitting of the d-orbitals, allowing for electronic transitions that absorb visible light, leading to the violet color. In contrast, Y3+ has a full d-subshell, resulting in no such transitions and thus no color.

Ligand Field Strength

The strength of the ligands surrounding a metal ion influences the extent of d-orbital splitting. Strong field ligands cause greater splitting, which can lead to more significant color differences. In the case of Cr3+, the ligands create a suitable environment for electronic transitions, while Y3+ does not exhibit such transitions due to its electronic configuration, resulting in a colorless solution.

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Guided course

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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

Consider the octahedral complex [Co(en)(dien)Cl]2+, where dien = H2NCH2CH2NHCH2CH2NH2, which can be abbreivated

(a) The dien (diethylenetriamine) ligand is a tridentate ligand. Explain what is meant by 'tridentate' and why dien can act as a tridentate ligand.

(b) Draw all possible stereoisomers of [Co(en)(dien)Cl]2+ (dien is a flexible ligand). Which stereoisomers are chiral, and which are achiral?

Textbook Question

The reaction of the octahedral complex Co(NH3)3(NO2)3 with HCl yields a complex [Co(NH3)3(H2O)Cl2]+ in which the two chloride ligands are trans to one another.

(a) Draw the two possible stereoisomers of the starting material [Co(NH3)3(NO2)3]. (All three NO2- ligands are bonded to Co through the N atom.)

(b) Assuming that the NH3 groups remain in place, which of the two starting isomers could give rise to the observed product?

Textbook Question

In octahedral complexes, the choice between high-spin and low-spin electron configurations arises only for d4 - d7 complexes. Explain.

Textbook Question

For each of the following, (i) give the systematic name of the compound and specify the oxidation state of the transition metal, (ii) draw a crystal field energy-level diagram and assign the d electrons to orbitals, (iii) indicate whether the complex is high-spin or low-spin (for d⁴ - d⁷ complexes), and (iv) specify the number of unpaired electrons.

(a) (NH₄)[Cr(H₂O)₆](SO₄)₂