Which compound is coloured due to charge transfer spectra and not due to \(\mathrm{d}-\mathrm{d}\) transitions? (a) \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) (b) \(\mathrm{KMnO}_{4}\) (c) \(\mathrm{CrO}_{3}\) (d) All of these

In coordination chemistry, the term d-d transitions refers to the process where electrons jump between different levels of the d-orbitals within a metal ion. It's essential to note that this phenomenon is generally observed in transition metals with partially filled d-orbitals. These transitions can cause the compounds to exhibit various colors.

When light is absorbed by a complex, an electron in a lower energy d-orbital may be excited to a higher energy d-orbital. However, not all d-d transitions are allowed by the selection rules, which are based on the conservation of spin and symmetry. Only certain transitions that follow these rules will result in the absorption of light in the visible spectrum and thus impart color to the compound.

This is why compounds with completely filled or empty d-orbitals usually do not show colors resulting from these transitions. To make this concept clearer, consider the analogy of a ladder with rungs representing different energy levels of d-orbitals. An electron can move up to a higher rung (higher energy level) when it absorbs the right amount of energy, and this is what we see as color.

Coordination compounds play a prominent role in influencing color through both d-d transitions and charge transfer spectra. These compounds consist of a central metal atom or ion surrounded by molecules or ions referred to as ligands. The coordination compounds are uniquely structured, which affects their electronic configuration and, subsequently, how they interact with light.

Each ligand can have a different impact on the energy levels of the central metal's d-orbitals, thus altering the wavelengths of light absorbed. This can change the perceived color of the compound. For example, replacing one ligand with another in a coordination complex can result in a completely different color. It's like changing sunglasses; a different lens can filter light differently and produce a different view.

Importance in Technological Applications

Coordination compounds aren't just essential for academic purposes; they have significant real-world applications. They are crucial in many technological applications such as catalysis, biological systems, and material science. Understanding their properties helps in the designing of new materials and drugs, showcasing their indispensable value in both research and industry.

Another intriguing aspect of coordination chemistry is ligand to metal charge transfer (LMCT). This type of charge transfer occurs when an electron is donated from a ligand, which is usually the non-metal part of the complex, to the metal ion. It can result in the absorption of light and the compound exhibiting color.

This is different from d-d transitions, where the electron jump happens within the metal ion's d-orbitals. In LMCT, the energy involved when an electron moves from a ligand to a metal can be significant, often leading to intense colors. Think of it as an energy leap instead of a step on our earlier ladder analogy. It's these leaps that contribute to the vivid colors in many minerals and industrial pigments.

A classic example is the deep red color of mercury sulfide, HgS, where the charge transfer occurs from the sulfide ion to the mercury ion. This concept is critical in understanding the coloration in a wide variety of compounds beyond just simple transition metal complexes, and it guides chemists in synthesizing materials with specific optical properties.