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Ch.21 - Transition Elements and Coordination Chemistry
McMurry - Chemistry 8th Edition
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Tutti i libri di testoMcMurry 8th EditionCh.21 - Transition Elements and Coordination ChemistryProblema 21.105
Capitolo 21, Problema 21.105

The [Cr(H2O)6]3+ ion is violet, and [Cr(CN)6]3- is yellow. Explain this difference using crystal field theory. Use the colors to order H2O and CN- in the spectrochemical series.

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1
Step 1: Understand that the color of a complex ion is due to the absorption of light which promotes an electron from a lower energy d-orbital to a higher energy d-orbital. The color we see is the complementary color of the light absorbed.
Step 2: Recognize that the crystal field theory explains the color of transition metal complexes in terms of the splitting of their d-orbitals into two sets of different energies when ligands approach the metal ion. The difference in energy between these two sets of d-orbitals corresponds to the energy of light absorbed.
Step 3: Realize that the extent of splitting of the d-orbitals, and hence the color of the complex, depends on the nature of the ligands. Ligands that cause a large splitting of the d-orbitals are known as strong-field ligands, while those that cause a small splitting are known as weak-field ligands.
Step 4: Infer from the given colors that [Cr(H2O)6]3+ absorbs light of higher energy (shorter wavelength) than [Cr(CN)6]3-. This is because violet light is the complementary color of yellow light, which is of lower energy (longer wavelength). Therefore, H2O is a weaker field ligand than CN-.
Step 5: Conclude that in the spectrochemical series, which orders ligands according to their ability to split the d-orbitals, H2O comes before CN-.

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Crystal Field Theory

Crystal Field Theory (CFT) explains the electronic structure of transition metal complexes by considering the effect of surrounding ligands on the d-orbitals of the metal ion. When ligands approach the metal ion, they create an electric field that splits the degenerate d-orbitals into different energy levels. The extent of this splitting depends on the nature of the ligands and their arrangement around the metal ion.
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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 produce when coordinating to a metal ion. Strong field ligands, like CN<sup>-</sup>, cause a larger splitting of the d-orbitals, leading to higher energy transitions and different colors. Conversely, weak field ligands, such as H<sub>2</sub>O, result in smaller splitting and lower energy transitions, which can be observed in the color of the complexes.
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Activity Series Chart

Color and d-Orbital Splitting

The color observed in transition metal complexes is a result of the specific wavelengths of light absorbed during electronic transitions between split d-orbitals. The energy difference between these orbitals corresponds to specific colors in the visible spectrum. For example, the violet color of [Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> indicates that it absorbs light in the yellow-green region, while the yellow color of [Cr(CN)<sub>6</sub>]<sup>3-</sup> suggests it absorbs light in the violet region, reflecting the differences in ligand field strength.
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d Orbital Orientations
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