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

The following test tubes each contain a different chromium complex ion. For each compound, 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 compound, 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

Spectrochemical Series

The spectrochemical series orders ligands by the strength of the crystal field they generate. This series helps predict the extent of d-orbital splitting in a complex: strong field ligands produce larger splitting, which can result in absorption of light at different wavelengths compared to complexes with weak field ligands.

d-d Transitions and Color in Complexes

The colors observed in transition metal complexes are largely due to d-d transitions, where electrons are excited between split d-orbitals. The specific wavelengths of light absorbed, and thus the complementary colors observed, are determined by the magnitude of the splitting, which depends on the identity and arrangement of the ligands around the metal ion.

Coordination Complexes

Coordination complexes are compounds in which a central metal ion is bonded to surrounding molecules or ions, called ligands, through coordinate covalent bonds. The nature, number, and arrangement of these ligands determine many physical and chemical properties of the complex, including its color.

Crystal Field Theory

Crystal field theory explains the splitting of a metal ion’s d-orbitals when surrounded by ligands. In an octahedral or other geometric arrangement, the ligands create an electrostatic field that raises the energy of some d-orbitals relative to others, leading to specific patterns of energy levels that dictate the wavelengths of light absorbed through electronic transitions.

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