The following test tubes each contain a different chromium...

The following test tubes each contain a different chromium complex ion. For each complex ion, predict the predominant color of light absorbed. If the complex ions are $\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}^{3+}, \mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}$, and $\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}^{+}$, what is the identity of the complex ion in each test tube? (Hint: Reference the spectrochemical series.)

The following test tubes each contain a different chromium complex ion. For each complex ion, predict the predominant color of light absorbed. If the complex ions are $\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}^{3+}, \mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}$, and
$\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}^{+}$, what is the identity of the complex ion in each test tube? (Hint: Reference the spectrochemical series.)

Key Concepts

Complementary Colors

The color observed in a complex is due to the absorption of light in a certain region of the spectrum, leaving the complementary color to be seen. When a complex absorbs light corresponding to a certain wavelength (determined by the ligand-induced d-orbital splitting), the color that is not absorbed (its complement) is the color perceived by the observer. This principle is used to predict the visual appearance of the complexes.

Electronic (d-d) Transitions

In many transition metal complexes, color arises from d-d transitions where an electron is promoted from a lower energy d orbital to a higher energy one. The energy difference between these orbitals, which depends on the ligand environment, corresponds to a specific photon energy. Consequently, the wavelength of light absorbed (and its complementary color observed) is directly related to these electronic transitions.

Spectrochemical Series

The spectrochemical series orders ligands by their field strength, from weak to strong. Ligands that are strong field (like NH3) cause a larger splitting (a greater ?) than weak field ligands (like H2O or Cl–). As a result, complexes with strong field ligands absorb different wavelengths compared to those with weak field ligands. This ordering is key in predicting and comparing the colors of different coordination complexes.

Crystal Field Theory

This theory explains the energy splitting of d orbitals in transition metal complexes due to the approach of ligands. In an octahedral field, the five d orbitals split into two sets, and the energy gap between these sets (?) determines the wavelengths of light that the complex absorbs. The magnitude of ? is influenced by the nature of the ligands present, leading to differences in color perception.

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