Question

Which of the following compounds are coloured due to charge transfer spectra? (a) \(\mathrm{AgNO}_{3}\) (b) \(\mathrm{CuSO}_{4}\) (c) \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) (d) \(\mathrm{KMnO}_{4}\)

Short answer

(c) \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\) and (d) \(\mathrm{KMnO}_{4}\) are colored due to charge transfer spectra.

Step-by-step solution

Understanding Charge Transfer

Charge transfer (CT) spectra occur when an electron transitions between a ligand and a metal or vice versa. For this to result in color, the energy gap involved in this transition must correspond to the visible region of the electromagnetic spectrum.

Analyzing Compound (a) - \( \mathrm{AgNO}_{3} \)

\( \mathrm{AgNO}_{3} \) does not exhibit charge transfer properties. Silver nitrate is typically colorless because it does not involve any electron transition between a ligand and metal that falls in the visible spectrum.

Analyzing Compound (b) - \( \mathrm{CuSO}_{4} \)

\( \mathrm{CuSO}_{4} \) is blue due to d-d electron transitions in the copper ion, rather than charge transfer transitions between ligand and metal, since sulfate ions are not effective charge transfer ligands.

Analyzing Compound (c) - \( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \)

\( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \) is orange because it exhibits charge transfer between oxygen and chromium ions. These transitions fall within the visible light range, and thus, contribute significantly to its color.

Analyzing Compound (d) - \( \mathrm{KMnO}_{4} \)

\( \mathrm{KMnO}_{4} \) is deep purple, largely due to charge transfer between oxo-ligands (oxygen) and the manganese ion. This is a characteristic feature of the compound's color.

Conclusion

Both \( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \) and \( \mathrm{KMnO}_{4} \) are colored compounds due to their charge transfer spectra, as they have electron transitions involving oxygen and their respective metal ions that occur in the visible region.

Key concepts

Color of Compounds

The colors we observe in compounds arise from the way they interact with light, specifically visible light. When a compound displays a particular color, it usually absorbs certain wavelengths of the visible spectrum and reflects or transmits others. The absorbed wavelengths correspond to specific energies required for electronic transitions within the compound.

• Charge Transfer: In some compounds, such as \( \mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7 \) and \( \mathrm{KMnO}_4 \), charge transfer transitions are responsible for their vivid colors. This type of transition occurs when electrons move from a ligand to a metal or vice versa.
• d-d Transitions: On the other hand, compounds like \( \mathrm{CuSO}_4 \) are colored due to d-d transitions, which are different from charge transfers and involve electrons moving between d orbitals of the metal ion.
• Practical Observation: These differences in electronic transitions are why compounds exhibit such a wide variety of colors.

It is fascinating to note how these microscopic electron movements result in the rich and diverse colors that we can observe with our eyes.

Ligand-Metal Electron Transition

Electron transitions between metals and ligands play a crucial role in determining the properties of many chemical compounds. Specifically, ligand to metal charge transfer (LMCT) and metal to ligand charge transfer (MLCT) transitions can substantially influence the color and reactivity of compounds.

• LMCT: In this process, an electron moves from an electron-rich ligand to an electron-poor metal center. This increases the oxidation state of the metal and decreases that of the ligand.
• MLCT: This is essentially the reverse process, where an electron transitions from the metal to the ligand.

The involvement of these transitions in compounds like \( \mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7 \) and \( \mathrm{KMnO}_4 \) results in strong absorption of specific wavelengths of light, contributing to their distinct colors such as orange and purple respectively. Understanding these transitions helps chemists predict not only color but other properties like magnetic behavior and catalytic activity.

Visible Light Spectrum

The visible spectrum is a part of the electromagnetic spectrum that can be detected by the human eye. It ranges approximately from 380 to 750 nanometers in wavelength. This small slice of the vast electromagnetic spectrum is responsible for the immense variety of colors we perceive in nature.

• Wavelengths and Colors: Blue light has shorter wavelengths (around 450 nm), while red has longer wavelengths (around 700 nm). Each color we see corresponds to a specific wavelength range within the visible spectrum.
• Light Absorption and Reflection: When a compound absorbs certain wavelengths, the light that is not absorbed is what we perceive as the compound's color. If a compound absorbs mainly blue light, it often appears orange or red.
• Role in Chemistry: In chemistry, understanding the visible spectrum's role in electronic transitions helps in explaining why certain compounds appear colored. For example, charge transfer transitions in \( \mathrm{KMnO}_4 \) result in the absorption of light in the visible range, leading to its purple appearance.

Thus, the connection between electronic transitions in compounds and the visible spectrum is essential for interpreting and predicting the coloration of different substances.